Have you ever wondered what is in the air around us, even inside a perfectly clean room? Beyond visible dust, indoor air contains microscopic biological particles called bioaerosols. These include bacteria, viruses, fungal spores, pollen and microbial fragments. Although many are harmless, some can affect human health and indoor air quality.

Is the air around us really clean?

Imagine sitting in a classroom, library or bedroom. The surroundings look clean, the windows are closed and everything seems normal. But the air around you may hold thousands of microscopic biological particles that cannot be seen with the naked eye.

This invisible biological world is an important part of our indoor environment. We often link air pollution with vehicle exhaust, industrial smoke and outdoor dust. However, indoor air can also contain biological contaminants that come from people, animals, plants, building materials and the surrounding environment. These airborne biological particles are together known as bioaerosols.

Recent research, including a 2026 review published in Atmospheric Environment, highlights the need to study the characteristics of bioaerosols, their health risks, the techniques used to sample them and the challenges in analysing them.

What are bioaerosols?

Bioaerosols are tiny airborne particles of biological origin. They can be whole microorganisms, fragments of organisms or substances produced by living things. They are broadly classified into two groups:

• Viable bioaerosols are living microorganisms, such as bacteria and fungi, that can stay biologically active under suitable conditions. Infectious viruses are also studied as bioaerosols, although they need host cells to multiply.
• Non-viable bioaerosols include dead microbial cells, pollen, biological fragments, endotoxins and other substances of biological origin.

Characteristics and sources

The size of a bioaerosol particle controls how long it stays suspended in the air, how far it travels and where it settles in our respiratory tract when we breathe it in.

Major indoor sources include breathing, coughing and sneezing, pets, carpets, damp building materials and ventilation systems. Biological particles from outdoors also enter buildings through windows and air exchange. Moisture build-up, in particular, encourages the growth of fungi (mould).

Health effects

Exposure to certain bioaerosols is linked to allergic rhinitis, worsening of asthma, irritation of the airways and infections. Microbial components such as endotoxins (from bacterial cell walls) and β-glucans (from fungal cell walls) can trigger inflammation.

Health effects depend on which microbes are present, how concentrated they are, how long a person is exposed and how sensitive that person is. Children, older adults and people with weak immunity may face greater risks from some biological agents.

Challenges and control measures

Monitoring bioaerosols is not easy. Samplers differ in how efficiently they collect particles, microbes can lose viability during sampling, conditions keep changing, and there are no universal health-based exposure limits. Helpful preventive steps include good ventilation, HEPA filtration, moisture control, regular cleaning and proper maintenance of air-conditioning (HVAC) systems.

The air we cannot see matters

Indoor bioaerosols affect both the quality of our indoor environment and our respiratory health. Understanding their sources, biological characteristics, sampling methods and health effects is essential for assessing exposure properly. Standardised monitoring methods and better indoor air management can help reduce the possible health risks.

A room may look spotless, but its air tells a much deeper story. Understanding that invisible story is the first step towards healthier indoor spaces.

References and further readings

• Naik, A. K. and Zabłocka-Godlewska, E. (2026). Bioaerosols in indoor environments: characteristic, health risk, sampling, analysis methodologies and challenges – a review. Atmospheric Environment, 373, 121934.
• World Health Organization (2009). WHO Guidelines for Indoor Air Quality: Dampness and Mould. WHO Regional Office for Europe, Copenhagen.
• Srikanth, P., Sudharsanam, S. and Steinberg, R. (2008). Bio-aerosols in indoor environment: composition, health effects and analysis. Indian Journal of Medical Microbiology, 26(4), 302–312.
• United States Environmental Protection Agency. Care for Your Air: A Guide to Indoor Air Quality.
• WellAir. Bioaerosols explained: what they are, where they come from, and why they matter for IAQ. wellairsolutions.com (source of Figure 1).

In 1970 the United States passed the Clean Air Act. It required the Environmental Protection Agency to prepare a detailed scientific "criteria document" for each widespread pollutant that harms health or welfare, and then to set a national limit for it. The pollutants covered by these documents came to be called criteria pollutants. The US list has six: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter and sulphur dioxide.

India follows the same idea. Under the Air (Prevention and Control of Pollution) Act, 1981, the Central Pollution Control Board sets the National Ambient Air Quality Standards (NAAQS). The present NAAQS were notified on 18 November 2009 and cover 12 pollutants. NET questions refer to them as the criteria pollutants under NAAQS.

India’s twelve NAAQS pollutants

The numerical limits (annual and 24-hour values, and the separate limits for ecologically sensitive areas) are covered in Module 4. This lesson looks at each pollutant as a substance.

The six gases

Particles and what they carry

Related terms

Several group names appear around the twelve, and NET often tests whether you know exactly what each includes.

Why these twelve?

A pollutant gets a national standard when it is common (many sources, found almost everywhere), harmful at the levels people actually breathe, and measurable by an agreed method. That is why CO and PM are on the list, while very toxic but less widespread chemicals such as dioxins are controlled through emission limits for particular industries instead.

In practice, urban air quality in India is decided mainly by PM₁₀, PM_2.5, NO₂, SO₂, CO and ozone. Of these, particulate matter exceeds the standard most often, and so it decides most AQI values. The AQI is covered in Module 4.

Quick revision

• The term criteria pollutant comes from the US Clean Air Act of 1970. The US has six: CO, Pb, NO₂, O₃, PM and SO₂.
• India’s NAAQS were notified on 18 November 2009 by the CPCB under the Air Act, 1981, and cover 12 pollutants.
• The twelve: SO₂, NO₂, PM₁₀, PM_2.5, O₃, Pb, CO, NH₃, benzene, BaP, As and Ni.
• Units: µg/m³ for most; mg/m³ for CO; ng/m³ for BaP, arsenic and nickel.
• CO₂, cadmium and mercury are not NAAQS pollutants.
• CO binds haemoglobin about 200–250 times more strongly than oxygen.
• NO₂ is reddish-brown; NO, CO and SO₂ are colourless.
• NO_x = NO + NO₂. NO_y = NO_x + HNO₃ + PAN + NO₃ + N₂O₅ and other oxidised nitrogen.
• Ozone is the only one of the twelve that is never emitted directly.
• NH₃ is the main alkaline gas in air; it turns acids into ammonium sulphate and nitrate.
• Benzene is a Group 1 carcinogen that causes leukaemia, and the most volatile of the BTEX compounds.
• BaP is a PAH (C₂₀H₁₂, five rings), a Group 1 carcinogen and the marker for PAHs; household wood and coal burning is its main source.
• Larger PAHs are semi-volatile, not VOCs. Methane is excluded from NMHC.
• Raoult’s law: pᵢ = xᵢ Pᵢ°. In an equimolar BTEX mixture, benzene has the highest partial pressure.

One more point about wording. NH₃ is a NAAQS pollutant in India but is not one of the original six criteria pollutants. When a question asks for the criteria pollutants and no option includes every gas listed, choose the option with the classic gases (SO₂, NO₂, O₃ and CO).

References

1. CPCB (2009). National Ambient Air Quality Standards. Gazette of India, notification of 18 November 2009.
2. US EPA. Criteria Air Pollutants and the NAAQS Table. epa.gov.
3. WHO (2021). WHO Global Air Quality Guidelines. World Health Organization, Geneva.
4. IARC Monographs on the Identification of Carcinogenic Hazards to Humans: benzene (vol. 120), benzo[a]pyrene (vol. 100F), arsenic and nickel (vol. 100C).
5. Seinfeld, J. H. and Pandis, S. N. (2016). Atmospheric Chemistry and Physics, 3rd ed. Wiley. Chapter 2.

On a clear winter morning in Delhi, smoke from a chimney or a roadside fire often spreads out in a flat grey sheet instead of rising. By early afternoon the same smoke climbs and breaks up. To understand this we need to look at the lowest part of the atmosphere, the planetary boundary layer, and read it using potential temperature rather than plain temperature.

Two parts of the lower sky

The troposphere has two quite different parts. The planetary boundary layer (PBL) is the bottom layer that feels the ground: surface heating, friction from buildings and trees, and every exhaust pipe and chimney. Turbulence in it mixes heat, moisture and pollutants up and down. Above it is the free atmosphere, which hardly feels the surface.

Between them sits a stable layer called the capping inversion. It traps surface turbulence and pollutants below it. So whatever we emit near the ground, smoke, NO_x or water vapour, mostly stays in the PBL, and the depth of the PBL decides how much air is available to dilute it.

Why potential temperature?

Rising air expands and cools by about 9.8 °C for every kilometre, even if no heat is added or lost. So temperature falling with height does not tell us whether air will mix. Potential temperature removes this effect. It is the temperature a parcel of dry air would have if we brought it down, without adding heat, to a standard pressure of 1000 hPa.

• Potential temperature falls with height: unstable. Warm parcels keep rising and the air churns.
• Potential temperature stays the same with height: neutral. This is the well-mixed layer of a sunny afternoon.
• Potential temperature rises with height: stable. Parcels are pushed back, so vertical mixing stops. Meteorologists call any such layer an inversion, even when the temperature itself is not increasing.

A day in Delhi’s boundary layer

After sunrise the ground heats the air, and a mixed layer with nearly constant potential temperature grows upward, eating into the stable air above. After sunset the ground cools fast, a thin stable layer forms near the surface with potential temperature rising with height, and the old mixed air above becomes a residual layer that still holds the day’s pollution.

Winter keeps this layer shallow. Lidar measurements over New Delhi in 2008–09 found a mean daytime PBL height of about 695 m in winter, against about 1025 m in the monsoon. At night it can be far shallower. In a 2018 winter field study in Delhi, the PBL at times fell below 100 m. Ceilometer data from Lodi Road showed that in December and January, each 100 m rise in the afternoon mixing height went with about 13–14 µg/m³ less PM_2.5.

Reading a plume from the potential temperature profile

The classic plume shapes from textbooks are simply the potential temperature profile made visible.

Fumigation is the one to watch in winter. Smoke released into stable air at night stays aloft. When the morning mixed layer grows up to it, the plume is suddenly pulled down to the ground, so pollution near the surface can jump for an hour or two after sunrise.

What this means

The same emissions can give clean or choking air depending on how deep the boundary layer is. This is why Delhi’s worst air comes on calm, clear winter nights and mornings. Daily forecasts of mixing height and ventilation could help plan activities such as construction work or waste burning for hours of better mixing. Early morning walks are best moved to later in the day on smoggy days. More ceilometers and lidars across the Indo-Gangetic Plain would help us measure the lid and not just the pollution beneath it.

References and further readings

• Stull, R. B. (1988). An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers.
• American Meteorological Society. Glossary of Meteorology: ‘Capping inversion’ and ‘Potential temperature’.
• Nakoudi, K. et al. (2019). Planetary boundary layer height by means of lidar and numerical simulations over New Delhi, India. Atmospheric Measurement Techniques, 12, 2595–2610.
• Murthy, B. S. et al. (2020). Impact of mixing layer height on air quality in winter. Journal of Atmospheric and Solar-Terrestrial Physics, 197, 105157.
• Gunthe, S. S. et al. Planetary boundary layer height modulates aerosol–water vapour interactions during winter in the megacity of Delhi (ESS Open Archive preprint).
• Cimbala, J. M. Meteorology and plume behaviour, lecture slides, ME 433 Air Pollution Control, Penn State University.

On a late October morning in Delhi, the sun comes up as a pale orange disc and the smell of smoke hangs in the street. By two in the afternoon the same sky looks a little clearer. Then, after sunset, the haze settles back in. Nothing much changed in what we burned. What changed was the air above us. In this post we look at the boundary layer, the part of the atmosphere that “breathes” in and out every day.

What is the boundary layer?

The boundary layer is the lowest part of the atmosphere, the part that feels the ground. It is where we live and where almost all our pollution is released. The top of the boundary layer acts like a lid. Whatever we emit gets mixed below that lid, so a lower lid means the same smoke is squeezed into less air.

A day in the life of the lid

The lid moves because the ground heats and cools.

• Morning: Sunlight warms the ground, and the ground warms the air touching it. Warm bubbles of air (thermals) rise and stir the layer, and the lid starts to climb.
• Afternoon: Convection is strongest. Air from the ground can reach the top of the mixed layer in about ten to twenty minutes, so pollution is spread through a deep layer.
• Evening: At sunset the stirring stops. The air mixed during the day is left floating aloft as a residual layer, cut off from the ground.
• Night: The ground loses heat fast and cools the air just above it. This makes a thin, stable layer, often with a temperature inversion, where cooler air sits under warmer air. Smoke from evening traffic, cooking and heating stays near the ground.

How big is the change over Delhi?

The swing is large. Model estimates for Delhi by UrbanEmissions.info put the average mixing height at about 500 m on winter days and only about 100 m on winter nights, against about 2,000 m on summer days. Lidar measurements near Delhi found the winter daily maximum at about 1,000 m around midday. A simple box model makes the point clear: if emissions stay the same and the mixing height is halved, the concentration doubles.

A ceilometer study at Lodi Road in the winter of 2017–18 found that during daytime mixing, PM_2.5 fell by about 14 µg/m³ for every 100 m rise in mixing height in December. Forecasters combine mixing height with wind speed into a ventilation index (mixing height × mean wind). Values below about 6,000 m²/s are taken as poor for clearing pollution.

What this means for us

The boundary layer is not a source of pollution, but it sets how much air we get to dilute it in. That is why the same emissions give a “poor” day in March and a “severe” night in November. Late evening and early morning are usually the worst times for outdoor exercise in winter; mid-afternoon is often better. Emissions released at night, from burning waste or heavy vehicles, matter more than the same emissions at noon. Better measurements of the layer itself, with ceilometers, lidars and drones, would also sharpen forecasts. Understanding the daily breath of the air helps us plan around it.

References and further readings

• Stull, R. B. (1988). An Introduction to Boundary Layer Meteorology. Kluwer Academic.
• Penn State University, METEO 300. 11.2 A Day in the Life of the Boundary Layer.
• Nakoudi, K. et al. (2019). Planetary boundary layer height by means of lidar and numerical simulations over New Delhi, India. Atmospheric Measurement Techniques, 12, 2595–2610.
• Murthy, B. S. et al. (2020). Impact of mixing layer height on air quality in winter. Journal of Atmospheric and Solar-Terrestrial Physics, 197, 105157.
• UrbanEmissions.info (2015). Delhi: emissions vs meteorology.
• Down To Earth (7 Nov 2022). Delhi air to remain ‘very poor’ this week. Here’s why the problem isn’t going away.

By late October, many of us in Delhi start checking the AQI before stepping out. When the news compares our air with other famous smog cities, Los Angeles and Beijing often come up. All three have had very dirty skies, but the smog in each is made in a different way. In this post we look at three things that shape every smog: what is burned, how much sunlight there is, and how the weather and land hold the air in place.

Los Angeles: a smog made by sunshine

Los Angeles smog is photochemical. Cars give out nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Strong sunlight splits NO₂, and the free oxygen atoms build ozone (O₃). The land helps too. Mountains such as the San Gabriels sit around the basin, and a layer of warm air above acts like a lid on the cooler, polluted air below. In 1970 the highest one-hour ozone reading reached 0.58 ppm, nearly five times the 0.12 ppm health standard adopted later that decade. So LA’s worst days were hot, bright afternoons.

Beijing: a smog made by winter heating

Beijing’s worst smog came in winter, and coal was at the centre of it. Homes and boilers burned coal for heat, giving soot and SO₂ on cold, calm days. The city is surrounded by mountains on three sides, so stagnant air could not escape easily. Over the past decade, about 1.3 million households switched from coal to gas or electricity for heating. Beijing’s yearly average PM_2.5 came down to 32 µg/m³ in 2023, a fall of about 64% from 2013.

Delhi: a smog made by many hands

Delhi has no single big fuel to point at. A synthesis by the Commission for Air Quality Management (CAQM), reported in January 2026, put the winter shares like this:

• Secondary particles formed in the air from gases: about 27%
• Transport: 23%
• Biomass burning, including crop residue and waste: 20%
• Dust: 15%, and industry: 9%

The biggest piece is not emitted directly at all. Gases such as SO₂, NO_x and ammonia (NH₃) react in the cold, humid air to form tiny sulphate and nitrate particles. Delhi also sits in the Indo-Gangetic Plain, with the Himalayas to the north. An earlier TERI–ARAI study (2016–17) found that only about 36% of Delhi’s winter PM_2.5 came from sources inside the city. The rest drifts in from the wider region.

Weather: the lid on the bowl

In all three cities, weather decides how bad a day gets. In Delhi, the mixing layer, the height up to which pollution can spread, is around 1 km in summer but shrinks to a few hundred metres on winter nights. The same smoke is squeezed into far less air. Weak winds after the monsoon add to it. LA and Beijing have mountains on their sides; Delhi has a whole plain of sources upwind.

What this means for us

Los Angeles cleaned up mainly by working on vehicles over many years, and Beijing by changing how homes are heated. Delhi’s mix suggests it needs many steps at once, and across the whole region, not just inside the city. Cutting the gases that form secondary particles, including ammonia from farms, matters as much as cutting smoke we can see. Good local measurements of what is in our air are the first step.

References and further readings

• Commission for Air Quality Management report on Delhi’s winter pollution, as reported by Health Policy Watch, 20 January 2026.
• TERI and ARAI (2018). Source Apportionment of PM2.5 and PM10 of Delhi NCR for Identification of Major Sources.
• University of Reading Research Blog. How Delhi’s geography and weather shape its air pollution.
• Bending the Curve: Climate Change Solutions, Section 9.1, Air Quality as the Genesis for Climate Policy (LibreTexts).
• Beijing Municipal Government. Beijing’s PM2.5 average reaches 32 µg/m³ in 2023 (January 2024); Measures and achievements of air pollution control in Beijing (2023).
• Seinfeld, J. H. and Pandis, S. N. Atmospheric Chemistry and Physics.

On a clear October morning in Delhi, a plane climbs out of the airport and in a few minutes it is above the haze, in deep blue sky. Above it, the sky is not one big block of air. It is stacked in layers, and the strange thing is how temperature behaves as we go up. It falls, then rises, then falls, then rises again. This post explains why.

Four layers, named by temperature

Scientists split the atmosphere by the way temperature changes with height. Typical heights are:

• Troposphere: from the ground to about 10 km. Temperature drops as we go up.
• Stratosphere: up to about 50 km. Temperature rises with height.
• Mesosphere: up to about 85 km. It cools again, to about −90 °C near the top, the coldest place in the atmosphere.
• Thermosphere: above that, with a top somewhere between 500 and 1,000 km. It can reach 500 to 2,000 °C or more.

The turning points are called the tropopause, stratopause and mesopause.

The troposphere: heated from below

Sunlight passes through the air and warms the ground first. The ground then warms the air touching it. On average, temperature falls by about 6.5 °C for every kilometre we climb. This layer holds roughly 80% of the mass of the atmosphere and about 99% of its water vapour, so nearly all clouds, rain and weather happen here.

The top of the troposphere is not at the same height everywhere. It is about 18 km over the equator, about 12 km in the mid-latitudes and only about 8 km over the poles. Balloon data from Gadanki in Andhra Pradesh show the tropopause near 17 km, at around 190 K (about −83 °C).

The stratosphere: an ozone heater

Above the tropopause, the air starts getting warmer. The reason is ozone. O₃ molecules absorb the Sun’s ultraviolet (UV) light, which splits them into O₂ and an oxygen atom. These quickly join again to make ozone, and the absorbed energy ends up as heat. So the stratosphere is heated from above. Warm air sitting over cooler air is stable, which is why this layer is calm and why long-haul jets like to fly near its base.

The mesosphere and thermosphere

Higher up, there is too little ozone to do much heating, while carbon dioxide gives off heat to space. So the mesosphere cools with height. Most meteors burn up here. In the thermosphere, oxygen and nitrogen absorb the Sun’s X-rays and far-UV light, so temperature climbs again. But the air is so thin that it would still feel freezing cold to us. Auroras glow in this layer.

A surprising fact: as CO₂ increases, the ground warms but these upper layers cool. NASA’s SABER instrument found the upper atmosphere at its coldest since the Industrial Age during recent solar minima.

What this means for our air

The tropopause works like a ceiling for most of our pollution. Smoke, dust and gases from vehicles, factories and crop fires mostly stay in the troposphere, and on winter nights they are held even lower, close to the ground. That is the topic of our next post. Knowing these layers helps us read weather balloon data and forecast how far smoke will spread. It also shows that protecting ozone high up and cutting pollution near the ground are two different jobs, and both deserve care.

References and further readings

• UCAR Center for Science Education. Layers of Earth’s Atmosphere.
• NOAA NESDIS. Layers of the Atmosphere.
• Gervais, B. The Layered Atmosphere (Section 1.3), Macmillan Learning physical geography textbook.
• NASA Goddard. SABER observations of cooling in the mesosphere and lower thermosphere (2022).
• Annamalai et al. Extreme cold point tropopauses over Gadanki from radiosonde data, 2006–2014 (MST15 workshop abstract).
• Seinfeld, J. H. and Pandis, S. N. Atmospheric Chemistry and Physics, Chapter 1.

On a smoggy October morning in Delhi, two air quality apps can show two very different numbers for the same street. A report gives ozone in ppb, a government table gives it in µg/m³, and one app shows AQI 154 while another shows 100. None of them has to be wrong. In this post we will see what these units mean, how to move between them, and why apps disagree.

Two ways to say how much

For a gas, we can count molecules or we can weigh them. ppb (parts per billion) counts: 1 ppb of ozone means that out of every billion molecules of air, one is an ozone molecule. Chemists call this a mixing ratio. It suits reaction chemistry, because reactions happen between molecules, not grams.

µg/m³ (micrograms per cubic metre) weighs: it tells us how many millionths of a gram of the pollutant sit in one cubic metre of air. Health studies and most legal standards use it, because the dose our lungs receive is a mass.

India's National Ambient Air Quality Standards (2009) give almost everything in µg/m³, and carbon monoxide in mg/m³. The US EPA lists ozone, NO₂, SO₂ and CO in ppm or ppb, but particles in µg/m³. The WHO 2021 guidelines use µg/m³ throughout.

The conversion, in words

The link between the two is the ideal gas law. At 25 °C and 1 atmosphere, one mole of any gas fills about 24.45 litres. So:

µg/m³ = ppb × molar mass (g/mol) ÷ 24.45

In words: a heavier molecule weighs more for the same count. Ozone (O₃, 48 g/mol) gives 1 ppb ≈ 1.96 µg/m³. For NO₂ (46 g/mol) it is about 1.88, and for SO₂ (64 g/mol) about 2.62. For CO, 1 ppm ≈ 1.15 mg/m³.

Try it on real standards. India's 8-hour ozone limit of 100 µg/m³ is about 51 ppb, while the US 8-hour standard of 70 ppb is about 137 µg/m³.

Two cautions:

• Temperature matters. On a 10 °C winter morning, a mole of gas fills about 23.2 litres, not 24.45, so the same ppb gives roughly 5% more µg/m³.
• PM_2.5 and PM₁₀ cannot be put in ppb. They are tiny bits of solid and liquid of mixed chemistry, with no single molar mass, so they are only reported as mass per volume.

Why apps show different AQI numbers

The AQI is not a unit at all. It is a 0 to 500 scale built on top of concentrations, and each country draws its own breakpoints. India's National AQI, launched in 2014, calls a 24-hour PM_2.5 of 31–60 µg/m³ "Satisfactory". The US EPA scale, revised in 2024, calls 55.5–125.4 µg/m³ "Unhealthy".

So a PM_2.5 level of 60 µg/m³ is AQI 100 on the Indian scale and about 154 on the US scale. Some global apps show the US scale. Some also use low-cost sensors instead of the monitors at official stations, and the Indian scale stops at 500 while some apps go above it.

What this means for us

When you read an air quality number, ask three things: which pollutant, which unit, and which averaging time. If it is an AQI, also ask which country's scale. It helps to note the concentration itself, not only the index, and to convert to one unit before comparing a city, a standard or a study. This small habit makes news reports and app alerts much easier to read.

References and further readings

• Ministry of Environment and Forests, India (2009). National Ambient Air Quality Standards, notification of 18 November 2009 (via UNEP LEAP).
• US EPA. NAAQS Table, Criteria Air Pollutants.
• US EPA (2024). Final updates to the Air Quality Index (AQI) for particulate matter: fact sheet.
• CPCB National Air Quality Index (2014), summarised in Arthapedia, Indian Economic Service.
• WHO (2021). WHO Global Air Quality Guidelines (levels summarised by DieselNet, Sept 2021).
• Markes International. How do I convert ppm to mg/m³ and ppb to µg/m³? (conversion calculator).
• BBC News (2025). Why India caps pollution reading at 500 (republished by The Star, 16 Nov 2025).
• Seinfeld, J. H. and Pandis, S. N. Atmospheric Chemistry and Physics, Chapter 1 (units of concentration).

On an October morning in Delhi the air can look still and grey. But even on the calmest day, the air above us is busy. Molecules are made and broken every second, and most of that work is done by just five small players: ozone (O₃), the hydroxyl radical (OH), the hydroperoxyl radical (HO₂), nitric oxide (NO) and nitrogen dioxide (NO₂). In this post we meet each of them and see how they work as one team.

Ozone, the starter

Near the ground, ozone it is a pollutant that harms lungs and crops, and India’s air quality standard allows 100 µg/m³ averaged over 8 hours. But ozone also has a quieter job. When sunlight strikes an ozone molecule, it breaks into O₂ and an excited oxygen atom. If that atom meets a water molecule, the two together make two OH radicals:

O₃ + sunlight → O₂ + O^•, then O^• + H₂O → 2 OH

In words: ozone, light and water vapour together give birth to the most important cleaner in the air.

OH and HO₂, the cleaners

OH is often called the detergent of the atmosphere. It is so reactive that it lives for only about a second, and there is very little of it, roughly 0.01 to 1 part per trillion in daytime air. Yet it is what removes most of the carbon monoxide, methane and many other gases we put into the air. A CO molecule survives about three months before OH catches it; methane survives about a decade.

When OH attacks CO or a volatile organic compound (VOC), it usually ends up as HO₂. HO₂ is calmer, but it is not wasted. When it meets NO, it hands back an OH. Because the two keep turning into each other, chemists treat them as one family called HO_x.

NO and NO₂, the switch

Vehicles, power plants and fires release mostly NO, which slowly becomes NO₂, a brown, irritating gas. Together they are called NO_x. In bright sun, NO₂ is split in a few minutes into NO and a free oxygen atom, and that atom joins O₂ to make ozone. NO then reacts with ozone and becomes NO₂ again. On their own, these two steps just go round in a circle and make no extra ozone. This balance was first worked out by Philip Leighton.

The circle breaks when HO₂ (or a similar radical made from VOCs) turns NO into NO₂ without using up any ozone. Each time this happens, one extra ozone molecule is left behind. This is how ozone piles up on sunny afternoons.

What this means for Delhi

• Near busy roads, fresh NO from traffic reacts with ozone, so ozone there can drop after sunset and in the early morning. This can hide how much ozone the city makes by afternoon.
• A study using measurements taken in Delhi in the summer and winter of 2018 found that ozone formation here is limited by VOCs. In their model, cutting NO_x alone raised ozone, while cutting VOCs lowered it.
• The same study found that cutting particles alone could also raise ozone, especially in winter, because clearer air lets more sunlight drive the loop.

So these five molecules teach a simple lesson: in the air, everything is connected. Clean air plans tend to work best when particles, NO_x and VOCs are tackled together, and when we measure VOCs and ozone as carefully as we now measure PM_2.5.

References and further readings

• Seinfeld, J. H. and Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Wiley, Chapter 6.
• Finlayson-Pitts, B. J. and Pitts, J. N. Chemistry of the Upper and Lower Atmosphere. Academic Press, 2000.
• NIWA (2008). Detergent of the atmosphere. Water & Atmosphere 16(1).
• Brune, W. H. Chapter 4: Tropospheric chemistry. METEO 532 course notes, Penn State University.
• Chen, Y., Beig, G. et al. (2021). Avoiding high ozone pollution in Delhi, India. Faraday Discussions, 226. doi:10.1039/d0fd00079e
• CPCB (2009). National Ambient Air Quality Standards, Notification of 18 November 2009.

Stand in a wheat field near a mine or a busy highway on a dry morning and you will notice something: the leaves look dull, almost dusty. That film is made of aerosols, tiny solid and liquid particles floating in the air, and it is not just a cosmetic problem.

What exactly are aerosols?

Aerosols include mineral dust, soot, smoke from burning crop residue, vehicle exhaust particles and industrial emissions. Some are coarse enough to see settling on a surface. Others are so fine that you only notice them as haze.

How they affect the plant?

First, there is the light. Aerosols scatter and absorb sunlight, so less of it reaches the leaves. Wheat depends on steady sunlight during its growing season, and a hazy sky can quietly lower the energy available for photosynthesis.

Second, there is the leaf surface. When particles settle on leaves, they can partly block the stomata, the tiny pores a plant uses to take in carbon dioxide and release water vapour. A dusty leaf also heats up differently and can lose chlorophyll over time.

Third, there is chemistry. Some particles carry acidic or toxic compounds that can damage leaf tissue, and plants under this stress often show changes in pigments, leaf pH, water content and ascorbic acid. Scientists combine these into the Air Pollution Tolerance Index (APTI) to judge how well a plant copes.

Not every effect is the same

The damage depends on how much dust there is, what it is made of, and the weather. Rain washes leaves clean, while a dry spell lets particles pile up. Interestingly, a little scattered light can sometimes help crops, so the story is not one-directional. That is why field studies matter more than assumptions.

Why this matters?

Wheat is a staple for millions of people, especially in South Asia, where aerosol levels are often high during the wheat season. If polluted air weakens the crop even slightly, the effect adds up across millions of hectares. Air quality is a farming issue, not only a health issue.

What can be done?

Controlling dust at mining and construction sites, reducing crop residue burning, and monitoring air quality near farmland would all help. We also need more long-term studies linking particle levels to grain yield.

Final thought

We usually protect crops by looking after soil and water. Aerosols remind us that the air matters just as much.

References/further readings

• Burney, J., & Ramanathan, V. (2014). Recent climate and air pollution impacts on Indian agriculture. Proceedings of the National Academy of Sciences, 111(46), 16319–16324.
• Rashki, A., Hatami, Z., Rezvani Moghaddam, P., & Nasiri Mahallati, M. (2025). Desert dust deposition: Impacts on physiological responses, chlorophyll pigments, and stomatal conductance in wheat and cowpea. Desert, 30(2), 310–332. 
• Singh, S. K., Rao, D. N., Agrawal, M., Pandey, J., & Narayan, D. (1991). Air pollution tolerance index of plants. Journal of Environmental Management, 32(1), 45–55.
• Hirano, T., Kiyota, M., & Aiga, I. (1995). Physical effects of dust on leaf physiology of cucumber and kidney bean plants. Environmental Pollution, 89(3), 255–261. 

On a clear October morning in the hills, or by the sea in Goa, the air feels fresh and empty. In Delhi the same month brings haze. It is easy to think that clean air has no chemistry in it at all. In fact, the air over a quiet ocean or a deep forest is always busy. In this post we look at what is going on there, far from any car or chimney.

The ocean breathes out sulphur

Tiny floating plants in the sea, called phytoplankton, make a sulphur compound called dimethyl sulphide (DMS). It gives the seaside its smell. A global study by Lana and colleagues in 2011 estimated that the oceans release about 28 million tonnes (28 Tg) of sulphur as DMS every year.

Once in the air, DMS is slowly oxidised. Part of it ends up as sulphate, the same kind of particle that forms from SO₂ in polluted air. Over the open sea these small particles act as seeds on which cloud droplets form. So even the cleanest marine air makes its own haze particles.

Forests breathe out isoprene

Trees, especially broadleaf trees in the tropics, give off a hydrocarbon called isoprene (C₅H₈) on warm, sunny days. Using the MEGAN model, Guenther and colleagues estimated about 600 million tonnes of isoprene a year, with tropical broadleaf trees giving almost half of it.

These are volatile organic compounds (VOCs), the same family as gases in car exhaust. When they are oxidised, some of the products condense into tiny liquid droplets called secondary organic aerosol. A 2010 study led by Scot Martin and Ulrich Pöschl found that over the pristine Amazon there were only several hundred particles in each cubic centimetre of air, against tens of thousands in big industrial cities. Most were organic droplets made from plant gases.

The air’s own detergent

What drives all this? Mostly one small molecule, the hydroxyl radical (OH). It forms in three steps:

• Ultraviolet sunlight splits ozone (O₃) and frees an excited oxygen atom.
• That oxygen atom reacts with water vapour (H₂O) to give two OH radicals.
• OH then attacks methane, carbon monoxide, isoprene, DMS and many other gases.

A single OH radical lasts less than one second, so it never builds up. Yet it decides how long most gases stay in the air. Methane, for example, survives about nine years before OH removes it, NASA notes. Scientists once thought forests, with so much isoprene, would use up their OH. Aircraft measurements over the Amazon, reported by Jos Lelieveld’s group in 2008, found much more OH than expected. Isoprene chemistry seems to recycle OH, though the exact path is still debated.

What this means for our air

The chemistry that keeps forest and ocean air clean is the same chemistry at work over Delhi. Here, OH meets far more NO_x, SO₂ and VOCs from vehicles, industry and burning, and turns them into ozone and fine particles. In winter, weaker sunlight also means less OH, so gases linger longer near the ground.

So clean air is not air without chemistry. It is air where natural emissions and the air’s own cleaning system stay in balance. Measuring background air at clean sites in the Himalaya or on our coasts gives us a baseline to compare city air against, and shows how much of our winter haze we add ourselves.

References and further readings

• Lana, A. et al. (2011). An updated climatology of surface dimethylsulfide concentrations and emission fluxes in the global ocean. Global Biogeochemical Cycles, 25, GB1004.
• Guenther, A. et al. (2006). Estimates of global terrestrial isoprene emissions using MEGAN. Atmospheric Chemistry and Physics, 6, 3181–3210.
• Lelieveld, J. et al. (2008). Atmospheric oxidation capacity sustained by a tropical forest. Nature, 452, 737–740.
• Harvard SEAS (2010). Air archaeologists isolate pristine aerosol particles.
• NASA. Greenhouse gas ‘detergent’ recycles itself in atmosphere.
• Seinfeld, J. H. and Pandis, S. N. Atmospheric Chemistry and Physics.

Air pollution remains one of the most pressing environmental challenges worldwide. Fine particulate matter, particularly PM_2.5 and PM₁₀, poses significant concerns because of its potential impacts on human health, ecosystems, and environmental quality. 

In the Philippines, monitoring data from the Environmental Management Bureau (EMB) provided valuable insights into particulate matter concentrations and changing air quality conditions. Examining these data from September 2 onward may help identify pollution patterns in selected monitoring areas and strengthen public awareness of the need for science-based interventions.

Endemism and the Cultural Value of Ziziphus talanai

Can Nature-Based Solutions Help?

References and Further readings

• Gaglio, M., Pace, R., Muresan, A. N., Grote, R., Castaldelli, G., Calfapietra, C., & Fano, E. A. (2022). Species-specific efficiency in PM2. 5 removal by urban trees: From leaf measurements to improved modeling estimates. Science of The Total Environment, 844, 157131.
• Matandirotya, N. R., Mahed, G., Manjeru, P., Mathe, P., & Donkor, F. K. (2025). A Perspective on Nature-Based Solutions to Climate Change Adaptation and Mitigation. In Handbook of Nature-Based Solutions to Mitigation and Adaptation to Climate Change (pp. 2657-2674). 
• Sharma, A., & Malaviya, P. (2026). PM2. 5 interactions with vegetation across scales, deposition dynamics, physiological responses and air quality mitigation. Discover Environment, 4(1), 577.

When we think of plastic pollution, we often imagine oceans filled with plastic bottles or beaches covered in waste. But what if plastic pollution is also present in the air we breathe? Airborne microplastics are emerging as an environmental concern, raising important questions about their effects on human health and ecosystems.

What Are Microplastics?

Sources of Airborne Microplastics

• Synthetic clothing: Releases microscopic fibres during wear and handling.
• Vehicle tyres: Tyre wear generates particles that can enter the atmosphere.
• Plastic waste: Weathering and fragmentation release smaller particles.
• Industrial activities: Plastic manufacturing and processing can release particles into the environment.
• Road dust: Previously deposited plastic particles can become airborne again.

Impact on Human Health

Microplastics can enter our bodies through inhalation. Researchers are investigating their potential to cause inflammation, oxidative stress and respiratory problems. However, the long-term health effects of everyday exposure are still not fully understood.

Environmental Impacts

Microplastics and Climate Change

Prevention and Control Measures

• Reduce unnecessary single-use plastics.
• Encourage sustainable textile production.
• Improve plastic waste collection and recycling.
• Develop technologies to capture airborne particles.
• Strengthen environmental monitoring and public awareness.

Conclusion

After all, clean air should mean more than air that simply looks clean.

References/further readings

• World Health Organization (WHO, 2022). Dietary and Inhalation Exposure to Nano- and Microplastic Particles and Potential Implications for Human Health. 
• Allen, S., Allen, D., Phoenix, V. R., Le Roux, G., Durántez Jiménez, P., Simonneau, A., ... & Galop, D. (2019). Atmospheric transport and deposition of microplastics in a remote mountain catchment. Nature geoscience, 12(5), 339-344. 
• Huang, Y., Qing, X., Wang, W., Han, G., & Wang, J. (2020). Mini-review on current studies of airborne microplastics: Analytical methods, occurrence, sources, fate and potential risk to human beings. TrAC Trends in Analytical Chemistry, 125, 115821. 
• Zhao, X., Zhou, Y., Liang, C., Song, J., Yu, S., Liao, G., ... & Wu, C. (2023). Airborne microplastics: Occurrence, sources, fate, risks and mitigation. Science of the Total Environment, 858, 159943. 

As October turns to November, the sky over north India slowly loses its blue. We call it smog, a word made from smoke and fog more than a hundred years ago. But the two most famous smogs in history, London in 1952 and Los Angeles in the 1940s, were not the same thing at all. Their chemistry was almost opposite.

London, December 1952

For five days in December 1952, a thick yellow-black fog sat over London. People could not see their own feet, buses stopped and cinemas closed because the screen could not be seen. About 4,000 deaths were counted in the weeks around it, and later studies put the total closer to 10,000–12,000.

The cause was coal. Homes and power stations burned cheap, sulphur-rich coal, and a cold, still high-pressure system trapped the smoke near the ground. Under this inversion, SO₂ dissolved in fog droplets and was oxidised to sulphuric acid (H₂SO₄). This is called reducing or sulphurous smog: cold, wet, night-time, full of soot and acid.

Los Angeles, from 1943

In July 1943, a brown haze hit Los Angeles so suddenly that people first thought it was a gas attack. Eyes burned and visibility dropped to a few blocks. But this smog came on warm, sunny afternoons, and coal was not to blame.

In 1952 the chemist Arie Haagen-Smit showed what was going on. Sunlight splits nitrogen dioxide (NO₂) into NO and a free oxygen atom, which joins O₂ to make ozone (O₃). With volatile organic compounds (VOCs) from car exhaust and petrol around, the cycle keeps going and builds up ozone, peroxyacetyl nitrate (PAN) and fine organic particles. This is photochemical or oxidising smog: warm, dry, daytime, and made in the air itself.

What each city did

• London passed the Clean Air Act in 1956, created smoke-free zones and moved homes away from open coal fires.
• California set up air pollution control districts and later required catalytic converters on cars, which cut NO_x and VOCs together.

Why this matters for us

North India’s winter haze is a mix of both. Coal and biomass burning give SO₂, soot and organic smoke, as in London. Vehicles give NO_x and VOCs, as in Los Angeles, and ozone often stays high in the afternoon even in winter. Cold nights and shallow mixing layers trap all of it, while ammonia from farms turns the acids into sulphate and nitrate particles.

So one source alone cannot be blamed for our smog. Both stories also show that the air can be cleaned once we know which chemistry we are dealing with. London took one law and a change of fuel; Los Angeles took decades of steady work on vehicles. The first step in both cases was to measure and understand what was in the air.

References and further readings

• UK Met Office. The Great Smog of 1952.
• Bell, M. L. and Davis, D. L. (2001). Reassessment of the lethal London fog of 1952. Environmental Health Perspectives, 109(Suppl 3).
• Haagen-Smit, A. J. (1952). Chemistry and physiology of Los Angeles smog. Industrial & Engineering Chemistry, 44(6).
• South Coast AQMD. The Southland’s war on smog: fifty years of progress toward clean air.
• Seinfeld, J. H. and Pandis, S. N. Atmospheric Chemistry and Physics, Chapter 5.

Stand beside a busy road in Delhi at eight in the morning and you breathe plenty of NO, CO and fresh soot. Go to a park 20 km downwind at three in the afternoon and most of the NO has gone, but ozone is at its highest for the day. Nothing in the park emits ozone. The gases from the traffic travelled with the wind for several hours in strong sunlight and turned into something else.

That is the difference between a primary and a secondary pollutant. It matters for control: a primary pollutant can be caught at the chimney or the exhaust pipe, but a secondary pollutant has no chimney. The only way to reduce it is to cut the gases it forms from.

The two definitions

How the main secondary pollutants form

Ground-level ozone

Sunlight splits NO₂, and the free oxygen atom joins an O₂ molecule to make ozone. On its own, NO would destroy that ozone again. VOCs prevent this: their peroxy radicals turn NO back into NO₂ without using up ozone, so ozone builds up through the day.

NO₂ + sunlight (λ < 420 nm) → NO + O

O + O₂ + M → O₃   (M is N₂ or O₂, which carries away the extra energy)

NO + O₃ → NO₂ + O₂   (fresh NO removes ozone)

RO₂ + NO → RO + NO₂   (VOC radicals convert NO to NO₂, so O₃ accumulates)

This explains a pattern that surprises many students. Ozone is often lower right next to a busy road, where fresh NO removes it, and higher in suburbs and villages downwind. It peaks on sunny afternoons. The full chemistry, including PAN, comes in Module 7.

Sulphate from SO₂

• In the gas phase, SO₂ reacts with OH and goes on to form H₂SO₄. In strong sunlight roughly 1–2% of the SO₂ is converted each hour.
• Inside cloud and fog droplets the conversion is much faster: dissolved SO₂ is oxidised by H₂O₂ and O₃ to sulphate.
• The sulphuric acid is then neutralised by ammonia: H₂SO₄ + 2NH₃ → (NH₄)₂SO₄. Ammonium sulphate is the classic secondary aerosol.

Nitrate from NO_x

• By day, NO₂ reacts with OH to give HNO₃ (nitric acid).
• At night, NO₂ + O₃ gives NO₃, which combines with NO₂ to form N₂O₅. On wet particle surfaces N₂O₅ + H₂O → 2HNO₃.
• HNO₃ + NH₃ ⇌ NH₄NO₃. Ammonium nitrate is semi-volatile: it stays as particles in cold, humid winter air and turns back into gas in summer heat. That is why nitrate is a winter pollutant in north India.

Secondary organic aerosol

VOCs from vehicles, solvents, burning and plants (isoprene and terpenes) are oxidised by OH, O₃ and NO₃. The products are less volatile than the original gases and condense onto existing particles. This secondary organic aerosol can be a large part of the organic carbon in PM_2.5.

Pollutants that are hard to classify

Aerosols in a two-by-two grid

Particles can be sorted in two ways at the same time: natural or human, and primary or secondary. NET has asked this grid directly.

Delhi’s winter PM_2.5

In a Delhi winter a large part of PM_2.5 is secondary: ammonium sulphate, ammonium nitrate and secondary organic aerosol. The IIT Kanpur source study (2016) put secondary particles at roughly a quarter to a third of winter PM_2.5. Their precursors come from a wide area: SO₂ from coal power plants, NO_x from traffic and NH₃ from farms across the Indo-Gangetic Plain.

This has a clear lesson for policy. Controlling Delhi’s own dust and traffic soot helps, but the secondary share falls only when SO₂, NO_x and NH₃ fall across the whole region. That is why the National Clean Air Programme speaks of airsheds and not only of cities.

Quick revision

• Primary pollutant: emitted directly from an identifiable source. Secondary pollutant: formed in the air from precursors.
• Typical primary pollutants: SO₂, NO, CO, NH₃, VOCs, black carbon, fly ash, dust, Cl₂.
• Typical secondary pollutants: O₃, PAN, H₂SO₄ and sulphate, HNO₃ and nitrate, ammonium salts, SOA.
• Ozone is a greenhouse gas and a NAAQS pollutant that is not emitted directly into the troposphere.
• Ozone forms when sunlight splits NO₂ (λ < 420 nm) and O + O₂ → O₃. Fresh NO removes ozone near roads.
• Ozone peaks on sunny afternoons and is often higher downwind and in suburbs than at traffic junctions.
• The OH radical starts most of the reactions that make secondary pollutants.
• SO₂ turns into sulphate faster in cloud and fog droplets (with H₂O₂ and O₃) than in the gas phase.
• Ammonium nitrate is semi-volatile: particles in cold winter air, gas in summer heat.
• Engines emit NO_x mostly as NO, about 90–95%.
• Aerosol grid: soil dust is primary natural; biogenic organic aerosol is secondary natural; soot is primary anthropogenic; sulphate from power-plant SO₂ is secondary anthropogenic.
• Source markers: Si, Al, Ca, Fe for crustal dust; V and Ni for fuel oil; K⁺ and levoglucosan for biomass burning.
• Secondary pollutants are controlled only by cutting their precursors, often across a whole airshed.

Watch for option lists that mix "SO₂" with "sulphates", or "NO" with "nitrates". The gas is primary and the salt is secondary. And if ozone appears as a primary pollutant from vehicles, that option is wrong: vehicles emit only its precursors.

References

1. Seinfeld, J. H. and Pandis, S. N. (2016). Atmospheric Chemistry and Physics, 3rd ed. Wiley. Chapters 6, 7 and 14.
2. Jacob, D. J. (1999). Introduction to Atmospheric Chemistry. Princeton University Press. Chapters 11 and 12.
3. IIT Kanpur (2016). Comprehensive Study on Air Pollution and Green House Gases (GHGs) in Delhi. Report for the Department of Environment, Government of NCT of Delhi.
4. MoEFCC (2019). National Clean Air Programme (NCAP). Government of India.

The paddy harvest has started, and so has the daily fire count. By 13 October, Punjab had recorded 174 stubble-burning cases this season, 48 more than on the same date last year, with 51 in a single day. In Delhi, the first week of October gave an average AQI of 171, the city’s worst start to October in three years. Every year these fire numbers decide how worried we should be. But there is a problem with how they are counted.

How a satellite sees a fire

Most fire counts in India come from two instruments on NASA and NOAA satellites: MODIS (about 1 km pixels) and VIIRS (375 m pixels). A burning field is much hotter than the land around it, so it gives off far more radiation near 4 µm in the infrared. When one pixel looks much brighter at 4 µm than its neighbours, and than its own 11 µm signal, the computer flags it as an ‘active fire’.

The catch is timing. These satellites are in polar orbit, so they cross Punjab only at fixed times, roughly between 10:30 am and 1:30 pm. A field that burns at 5 pm and is ash by night is never seen.

The fires moved to the evening

Geostationary satellites stay over one spot and take a picture every 10–15 minutes. Their pixels are coarse, but they see the whole day. Scientists at ISRO’s Space Applications Centre used them and found that peak burning in Punjab and Haryana moved from about 1:30 pm in 2020 to about 5 pm in 2024. iFOREST found that more than 90% of large Punjab fires in 2024 and 2025 burned after 3 pm, compared with just 3% in 2021.

So the official counts fell by more than 90% after 2021, but the burnt area mapped by Sentinel-2 tells a different story. In Punjab it fell from 31,447 km² in 2022 to about 20,000 km² in 2025, a drop of roughly a third. Burning did come down, but much less than the counts suggest.

Why evening fires matter more for our air

Evening timing is not just a counting problem. After sunset the boundary layer, the mixed layer of air near the ground, shrinks from over a kilometre to a few hundred metres, and winds drop. Smoke released at 5 pm is trapped in this shallow layer overnight. Model studies suggest evening fires can build up more PM_2.5 at night than the same fires at midday. NASA scientists estimate that on bad days stubble smoke can be 40–70% of Delhi’s PM_2.5, though only 20–30% over a season.

What could help

• Report burnt area along with fire counts, every week of the season.
• Add geostationary data, from INSAT-3D/3DR and others, to see fires through the whole day.
• Use evening fire timing in Delhi’s air-quality forecasts, so source shares are not underestimated.
• Keep ground checks and drones for the late afternoon, which states have now started.

None of this is about catching farmers. It is about knowing the real size of the problem. Good data is the first step to good solutions, whether that is happy seeders, residue markets or help for small farmers who have little time between harvest and sowing.

References and further readings

• Singh, N. et al. (2025). Current Science, 24 November 2025 (ISRO Space Applications Centre study on the changing time of farm fires).
• iFOREST (December 2025). Stubble Burning Status Report 2025.
• NASA Earth Observatory (15 December 2025). New Timing for Stubble Burning in India.
• Giglio, L., Schroeder, W. and Justice, C. O. (2016). The collection 6 MODIS active fire detection algorithm and fire products. Remote Sensing of Environment, 178.
• Rozana Spokesman (16 October 2026). Punjab logs nearly 70% spike in farm fires in a week.
• The Patriot (19 October 2026). Delhi gets off to its most polluted October start in three years.

The Union Budget came out in February, and for most people the numbers have long been forgotten. But as the air turns hazy again this October, it is a good time to ask a simple question: where does India’s clean-air money actually go, and is it going to the right places?

What the budget gave

• The Control of Pollution scheme, which funds the National Clean Air Programme (NCAP), got ₹1,091 crore for 2026–27. That is about 16% less than last year’s revised ₹1,300 crore.
• The Commission for Air Quality Management (CAQM) got ₹35 crore, and the Central Pollution Control Board ₹123 crore.

Since 2019, NCAP and Finance Commission grants have released more than ₹11,000 crore to about 130 cities. About two-thirds of what they spent went to one thing: road dust. Vehicles got 14%, waste and biomass burning 11%, and industry and household fuel only about 1% each.

Why most of the money went to dust

• The target is PM₁₀. NCAP measures success as a fall in PM₁₀, and dust is the biggest source of these coarse particles. Under the Finance Commission grants, 70% of a city’s score (and later all of it) depends on cutting PM₁₀ and adding "good air" days.
• Roads are what cities control. Municipal bodies run roads, not power plants, industries or farms outside the city. Sweepers, sprinklers and paving are within their reach.
• Results show up quickly. A street can be washed today; a fuel change in industry or homes takes years.
• Many cities did not know their sources. Only about 49 of 130 cities had finished source apportionment studies, so spending often defaulted to the most visible problem.

How well does dust control work?

The science gives a mixed answer. Studies of street washing in Europe found PM₁₀ near roads fell by roughly 7–18% on the day of washing, but road dust built up again within a day or two. Mechanical sweeping showed no clear short-term benefit, and some vacuum sweepers can even throw fine dust back into the air. Lasting gains come from preventing dust in the first place: paving road edges, covering trucks, managing construction waste and planting along roads. India has paved over 18,000 km of road under NCAP, which is the more durable part of this spending.

The bigger limit is size. Dust is mostly coarse PM₁₀. In winter, when the air is worst, most of the harmful PM_2.5 comes from combustion and from gases that turn into particles in the air. Spending results also vary: some cities used almost all their funds and still saw pollution rise, while others spent less and met their targets. Money alone does not guarantee cleaner air.

The chemistry problem

More than half of India’s PM_2.5 is secondary. It forms over hours to days from SO₂ (coal), NOx (vehicles and power plants) and ammonia (fertiliser and livestock), and these gases can travel hundreds of kilometres. No amount of sweeping can remove them.

Areas that need more attention

• PM_2.5 as the main target, alongside PM₁₀, so that spending follows the particles that harm health most.
• Composition and sources: chemical speciation of PM_2.5, and up-to-date source studies for every city.
• Combustion sources: cleaner fuels in small industries and brick kilns, SO₂ controls at power plants, and steady use of clean cooking fuel at home.
• Waste and open burning, which are cheap to prevent and important in winter.
• Ammonia from agriculture, through better fertiliser use and manure management.
• Airsheds: planning across cities and states that share the same air, and help for smaller towns nearby.

None of this means dust control was wasted. It addressed a real and visible problem. But with winter starting, the next rupee may do more good where the fine particles come from.

References and further readings

• Down To Earth (1 February 2026). Budget 2026-27: Air quality management outlay down 10%; pollution control also sees dip.
• ResGov (2025). Trends in Finances for the National Clean Air Programme.
• Centre for Science and Environment (2024). National Clean Air Programme: An Agenda for Reform.
• Down To Earth (31 March 2026). Why India’s pollution crisis persists despite funds and policies.
• Amato, F. et al. AIRUSE Technical Guide to Reduce Road Dust Emissions in Southern Europe.
• World Bank (3 June 2026). Catalyzing Clean Air in India.
• CREA (2025). Tracing the Hazy Air 2025: Progress Report on the National Clean Air Programme.

In June 1991, Mount Pinatubo in the Philippines threw about 15–20 million tonnes of SO₂ into the stratosphere. The sulphate haze spread around the world and the average surface temperature dropped by roughly 0.5 °C for more than a year. Compare that with a winter morning in Delhi: no volcano anywhere, yet the smog is thick enough to hide the next building. Most of it comes from vehicles, power plants, brick kilns, crop fires and household burning.

Both are air pollution, but the first is natural and the second is anthropogenic, that is, caused by people.

Natural or anthropogenic?

A natural source releases pollutants without any human involvement: volcanoes, dust storms, sea spray, lightning, wetlands, forests, and fires started by lightning. An anthropogenic source exists because of human activity: burning fuel, industry, transport, agriculture, construction, waste and cooking.

When a case is not obvious, ask whether the emission would still happen if people were not there. If it would, the source is natural.

On a global scale nature emits more of several pollutants by mass, especially particles and VOCs. But natural emissions are spread over oceans, deserts and forests, while human emissions are concentrated in towns and cities. That is why, in the air people actually breathe, anthropogenic sources usually dominate. Natural does not mean harmless, though. A dust storm can push PM₁₀ in Delhi above 500 µg/m³ for a day, and smoke from a wildfire is as bad for the lungs as smoke from a chimney.

Natural sources

Two more natural contributions are worth knowing. Stratospheric intrusions bring ozone-rich air down towards the ground, and natural NO_x and VOCs produce a little ozone even in clean air. Together they set a background level that no emission control can remove.

Anthropogenic sources

Human sources can be grouped in three ways, and NET asks about all three.

By whether the source moves

• Stationary sources stay in one place: power plants, factories, refineries, brick kilns and homes.
• Mobile sources move: cars, trucks, two-wheelers, trains, aircraft and ships.

By the shape of the source

Dispersion models (Module 5) treat each shape with a different equation.

By sector

Emission inventories and policy documents count sources by sector.

An emission inventory lists how much of each pollutant comes from each source in a given area and year. It is built from the pollutants, the source categories and the activity data (how much fuel of which type is burnt, how many vehicle-kilometres are driven and so on), multiplied by an emission factor:

Emission = Activity × Emission factor × (1 − control efficiency)

Combustion

Burning fuel is the largest human source of air pollution. Almost every combustion source gives CO₂, CO, NO_x and particles; the rest depends on what is in the fuel.

Among fossil fuels, natural gas (CNG) is the cleanest. It has almost no sulphur or ash, and because it has the most hydrogen per carbon atom it gives the least CO₂ per unit of energy. It still produces thermal NO_x, like any hot flame. LPG (propane and butane) contains practically no nitrogen. Coal is the dirtiest fuel, bringing sulphur, ash, nitrogen and trace metals, although it gives off very little ammonia.

Global totals versus city air

Worldwide, nature emits most of the particles and VOCs, while people emit most of the SO₂, NO_x, methane and ammonia. The shares in Figure 2 are rounded from published global budgets (IPCC AR6, the Global Methane Budget and standard textbooks) and should be read as approximate.

Human sources still decide the quality of the air we breathe, for three reasons.

1. Location. Sea salt is released over the open ocean and much of the dust over empty desert, but traffic and industry emit right where people live.
2. Particle size. Natural particles are mostly coarse (PM₁₀) and settle within hours or days. Combustion particles are fine (PM_2.5 and smaller), stay in the air for about a week and reach deep into the lungs.
3. Chemistry. Human NO_x, SO₂, NH₃ and VOCs react in the air to form secondary pollutants such as ozone, sulphate, nitrate and ammonium. In Indian cities these secondary particles are a large part of winter PM_2.5.

Global budgets are usually given in teragrams per year. One teragram (Tg) is 10¹² g, or one million tonnes.

Cases that are easy to get wrong

The difficult questions are rarely about volcanoes or car exhausts. They are about sources that look natural but are not, or the other way round.

Ozone is a separate case altogether. Near the ground no source, natural or human, emits it directly; it forms in the air from NO_x and VOCs in sunlight. Its precursors have sources, but ozone itself is a secondary pollutant. That is the subject of the next lesson.

Delhi through the year

The main sources of Delhi’s pollution change with the season.

Weather is not a source, but it controls how much of each source we breathe. A cold, shallow mixing layer in winter can concentrate the same emissions several times more than a windy summer day; Module 5 covers this.

Quick revision

• Largest aerosol sources by mass worldwide: sea salt, then mineral dust.
• Largest natural methane source: natural wetlands. Main human sources: livestock, fossil fuels, landfills and rice paddies.
• Largest natural VOC source: vegetation, mainly isoprene. Monoterpenes include α-pinene, limonene and ocimene.
• Natural NO_x comes from lightning and soils; human NO_x from fuel combustion, emitted mostly as NO.
• Volcanic gas is mostly water vapour, then CO₂, then SO₂. Volcanoes do not emit NO_x in any important amount.
• Coal-fired thermal power plants are India’s largest source of SO₂ and mercury.
• The ocean is the main natural source of methyl bromide and DMS. CFC-11 has no natural source.
• DMS → sulphate → cloud droplets is the CLAW hypothesis (1987).
• Radon is a natural, radioactive, inert indoor pollutant: Ra-226 → Rn-222 by alpha decay.
• Pinatubo (1991) cooled the Earth by about 0.5 °C through stratospheric sulphate.
• Forest and grass fires are the natural source of PAHs; waste incineration releases dioxins and furans.
• Natural gas is the cleanest fossil fuel, LPG has the least nitrogen, and hydrogen still gives thermal NO_x in air.
• Mass of SO₂ = 2 × mass of sulphur burnt.
• By shape: point (stack), line (highway), area (many small sources), fugitive (no stack). By motion: stationary and mobile.
• Ozone has no direct source near the ground; it is a secondary pollutant.

A common trick: a question asks for the "major source" without saying whether it means globally or in cities. Look at the options. If sea salt or dust is among them, the question means global emissions by mass; if vehicles or industry are, it means urban air.

References

1. Seinfeld, J. H. and Pandis, S. N. (2016). Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 3rd ed. Wiley. Chapters 2 and 8.
2. IPCC (2021). Climate Change 2021: The Physical Science Basis. Working Group I, Chapter 6: Short-lived climate forcers.
3. Saunois, M. et al. (2020). The Global Methane Budget 2000–2017. Earth System Science Data, 12, 1561–1623.
4. Charlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655–661.
5. IIT Kanpur (2016). Comprehensive Study on Air Pollution and Green House Gases (GHGs) in Delhi. Report for the Department of Environment, Government of NCT of Delhi.
6. de Nevers, N. (2010). Air Pollution Control Engineering, 2nd ed. Waveland Press.

Step outside at noon and the sunlight on your face is also doing chemistry. Light arrives in tiny packets of energy called photons. Sometimes a molecule absorbs a photon with more energy than the bond holding two of its atoms together. When that happens, the bond snaps. Chemists call this photolysis, meaning “splitting by light”.

Energy decides what breaks

A photon’s energy depends on its wavelength (E = hc/λ): the shorter the wavelength, the stronger the punch. Splitting oxygen (O₂) needs ultraviolet light shorter than about 242 nm. Nitrogen dioxide (NO₂) breaks more easily: violet and near-ultraviolet light up to about 420 nm is enough.

This difference matters because the ozone layer absorbs almost all sunlight shorter than about 290 nm. So the toughest molecules, such as O₂ and CFCs, can only be split high up in the stratosphere. Near the ground, only molecules that absorb longer wavelengths come apart.

Three jobs photolysis does

It builds the ozone layer. In the stratosphere, UV light splits O₂ into two oxygen atoms. Each atom then joins another O₂ to form ozone (O₃). The same harsh UV breaks CFCs and frees chlorine atoms, which destroy ozone.

It makes smog. In a city, sunlight splits NO₂ into NO and an oxygen atom, which joins O₂ to make ground-level ozone. At noon, an NO₂ molecule lasts only about two minutes before light breaks it.

It cleans the air. UV-B light splits ozone and releases an energetic oxygen atom, O(¹D). That atom reacts with water vapour to make the hydroxyl radical (•OH), the atmosphere’s detergent. Early in the day, nitrous acid (HONO) that built up overnight is also split, giving a quick burst of •OH.

Sunrise in polluted air

Some gases pile up in the dark and disappear at dawn. Chlorine gas (Cl₂) from industry and plastic waste burning can build up in Delhi’s night-time air. Within an hour or two of sunrise, light splits it into chlorine atoms. These atoms are extremely reactive: they attack hydrocarbons and help form ozone. 

How fast any molecule breaks depends on three things: how much sunlight there is, how strongly the molecule absorbs it, and how often an absorbed photon actually breaks a bond. That is why a hazy winter day, with dimmer light, slows the whole chemistry of the city’s air.

References

• Finlayson-Pitts, B. J., & Pitts Jr, J. N. (1999). Chemistry of the upper and lower atmosphere: theory, experiments, and applications. Elsevier.
• Jacob, D. J. (1999). Introduction to atmospheric chemistry. Princeton university press.

We have been taught many times about the importance of the ozone layer, which protects Earth from the destructive ultraviolet rays. However, it seems that ozone could also be hazardous, and its effects are dependent on the location. Ozone acts as a shield in the stratosphere and as a pollutant near the Earth’s surface.

How Is Ground-Level Ozone Formed?

While vehicle exhaust fumes or smoky industries emit pollution directly into the atmosphere, the production of ground-level ozone takes place due to chemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight.

First, the action of sunlight decomposes the nitrogen dioxide:

NO₂ + sunlight → NO + O

A further reaction results in the creation of ozone:

O + O₂ + M → O₃ + M

where M is any molecule carrying away extra energy of the chemical reaction. Chemical reactions of VOCs promote the accumulation of ozone in the atmosphere in sunny weather.

Why Is It Invisible?

Sometimes people tend to believe that the air is clean because there is no pollution in their sight. However, this does not have to be true, as ground-level ozone is a colorless substance at concentrations present in ambient outdoor air and could influence air quality without any visible haze.

Ground-Level Ozone’s Impact on Human Health

Breathing should provide oxygen and support health, but ozone can complicate the breathing process. Ozone can irritate our airways, trigger coughing, affect breathing, worsen bronchial diseases, and cause other harmful effects. Vulnerability might depend on the age and physical activity of the individual.

An Invisible Menace to Indian Agriculture

Ground-level ozone could affect not only humans but also our plants. According to a 2024 study by Dewan & Lakhani, published in Science of the Total Environment, there was an 11.3% relative wheat yield loss attributed to ozone pollution in the Indo-Gangetic Plain (IGP) region in 2021, including high losses in Uttar Pradesh. Ground-level ozone enters the plant leaf through microscopic pores called stomata, damages plants, and inhibits photosynthesis, which makes it a significant problem for Indian agriculture.

Clear Skies ≠ Clean Air

It is customary for people to determine the degree of air pollution based on factors like visible pollution (smoke or dust) or a murky skyline. It must be known that there are certain types of pollution that may not be visible but could have adverse effects on human beings and nature around us. Ground-level ozone is an example of such pollution.

And sometimes, what looks like clear air actually isn’t.

References

• U.S. Environmental Protection Agency. Ground-Level Ozone Basics. 
• U.S. Environmental Protection Agency. Health Effects of Ozone Pollution. 
• U.S. Environmental Protection Agency. Ecosystem Effects of Ozone Pollution. 
• Dewan, S., & Lakhani, A. (2024). Impact of ozone pollution on crop yield, human health, and associated economic costs in the Indo-Gangetic plains. Science of the Total Environment, 945, 173820. 

Picture a winter morning in Delhi with PM_2.5 at 63 µg/m³. The CPCB shows an AQI of 108: Moderate. A global app shows 156: Unhealthy. Same air, same instrument, different verdict. Why?

An AQI is a translation

An Air Quality Index turns pollutant concentrations into a simple 0–500 number with colours. But each country decides where “good” ends and “unhealthy” begins. Those cut-offs, called breakpoints, are policy choices, not laws of nature.

Where India and the US differ

• Breakpoints. India’s AQI calls PM_2.5 “Satisfactory” up to 60 µg/m³, in line with India’s 24-hour air quality standard. The US now calls anything above 35.4 µg/m³ “Unhealthy for sensitive groups”, and above 55.4 “Unhealthy”.
• Averaging time. India’s official AQI uses 24-hour averages for particles, while many apps show the latest hour, which can be far higher at rush hour.
• Pollutants counted. India’s index can include eight pollutants, among them ammonia and lead. The final number is set by the worst one, usually PM_2.5 or PM₁₀ in Delhi.

The rest of the world

The same 63 µg/m³ is 85, “Good” on China’s scale, which is more relaxed than India’s. In the UK it reads 8 out of 10, “High”, a scale where 10 is the top. The air hasn’t changed, only the scale.

Which number should you trust?

For official decisions such as GRAP stages, school closures and health advisories, use your country’s own index. In India, that’s the CPCB.
To compare cities across countries, look at the raw PM_2.5 in µg/m³, which means the same everywhere.
For your health, remember the WHO’s guideline: a 24-hour average of 15 µg/m³. Most Indian cities are above it for much of the year, whatever colour the app shows.

The takeaway: a “Moderate” day in India isn’t necessarily a safe one. It means moderate by Indian standards.

VayuVarta’s Air now page shows each city on its own country’s scale, with the µg/m³ value alongside.

Every day we release carbon monoxide, methane, sulphur dioxide and thousands of other gases. The sky doesn’t fill up with them because the atmosphere has its own cleaner: the hydroxyl radical, •OH.

Made from sunlight

•OH is one oxygen atom and one hydrogen atom with an unpaired electron, which makes it fiercely reactive. UV sunlight splits ozone, and the energetic oxygen atom it releases reacts with water vapour to form two •OH radicals. No sun means almost no •OH, which is why it nearly vanishes at night.

Short life, huge job

Each •OH radical lasts less than a second, and there’s only about one for every 25 trillion air molecules. Every reaction is instant, but because •OH is so scarce, a pollutant molecule may wait a long time before meeting one. That wait sets how long each gas survives:

NO₂: about a day
SO₂: about one to two weeks
Carbon monoxide: about two months
Methane: about ten years

Detergent leaves residue

The cleaning isn’t spotless. SO₂ becomes sulphate, NO₂ becomes nitrate, and reactive organic gases form ozone and organic particles. Much of Delhi’s PM_2.5 is what’s left behind by this cleaning.

Why winter is slower

In winter the sun is lower, the days are shorter, haze blocks UV and the air is drier. All of these mean less •OH. Emissions don’t slow down, so pollutants linger in a shallow, still layer of air.

The takeaway: the atmosphere cleans itself only as fast as the sun allows. In winter, cutting emissions matters most.

References

• Levy, H. (1971). Normal atmosphere: large radical and formaldehyde concentrations predicted. Science, 173, 141–143.
• Prinn, R. G. et al. (2001). Evidence for substantial variations of atmospheric hydroxyl radicals in the past two decades. Science, 292, 1882–1888.
• Lelieveld, J. et al. (2016). Global tropospheric hydroxyl distribution, budget and reactivity. Atmospheric Chemistry and Physics, 16, 12477–12493.

Every winter, Delhi switches on its emergency pollution plan: the Graded Response Action Plan, or GRAP. On 28 September 2026, the Commission for Air Quality Management (CAQM) revised it again. So what changed, and will it help?

What GRAP is

GRAP is a set of actions that switch on in four stages as the air gets worse:

What’s new this year

• Diesel generator rules now start at Stage I. These cover generator size, cleaner fuel kits and pollution-control devices. Earlier, many of them began only at Stage II.
• Only cleaner buses can enter Delhi from Stage I. Inter-city buses must run on CNG, be electric, or meet BS-VI standards.
• More construction sites are covered. Large sites outside city limits, such as industrial zones and highway corridors, must now register online and be checked for dust.
• The car curbs reach further. The Stage III ban on old cars now also applies in Sonipat and in the border areas of Ghaziabad, Faridabad and Gautam Buddha Nagar.
• The citizen’s charter grew from 22 actions to 34.

The chemistry problem

GRAP mostly reacts once the AQI has already climbed. Forecasts help switch stages on a few days early, but the chemistry works on a slower clock. A large share of Delhi’s winter PM_2.5 is secondary: sulphate, nitrate, ammonium and chloride salts that form in the air over one to several days from NOx, SO₂, NH₃ and HCl. By the time Stage III begins, the precursor gases released earlier are already turning into particles. Moving diesel generator and bus rules to Stage I is a real step forward, because it cuts NOx before the build-up starts. But in a still, foggy week, it may not be enough on its own.

What GRAP still leaves out

• Crop-residue burning in Punjab and Haryana, which on peak days can be one of the largest single sources of Delhi’s PM_2.5.
• Regional transport: studies estimate that a large share of Delhi’s winter pollution comes from outside the city.
• Ammonia from fertiliser and livestock, which is the base that neutralises nitric and hydrochloric acids into particles. Without ammonia controls, cutting NOx removes only part of the problem.

Citizen’s checklist

• Stage I: Don’t burn leaves or waste. Get your vehicle’s PUC (pollution check) done. Report dust from construction sites on the Sameer app.
• Stage II: Use the metro or bus, or share a ride. Avoid using diesel generators at home.
• Stage III: Work from home if you can. Avoid exercising outdoors in the early morning. Use N95 masks outdoors.
• Stage IV: Keep children and elderly people indoors. Use a purifier if you have one, and close windows during the morning and evening peaks.

References and further readings

• CAQM. Revised Graded Response Action Plan for Delhi-NCR, 28 September 2026. Reported in ThePrint, “Winter pollution playbook updated. CAQM tightens GRAP 2026 for Delhi.”
• AIR News (2025). “CAQM tightens GRAP for Delhi-NCR,” 22 November 2025.
• TERI & ARAI (2018). Source Apportionment of PM2.5 & PM10 of Delhi NCR for Identification of Major Sources. Department of Heavy Industry, Government of India.

Shake out a fleece jacket in a sunny room and you’ll see a cloud of fibres drifting in the light. Some of them are plastic. And some of them end up in our lungs.

Where airborne plastic comes from

Into the body

What we know, and what we don’t

What you can do now

• Dust with a damp cloth and vacuum regularly; dry sweeping throws fibres back into the air.
• Air out rooms when outdoor air is clean.
• Choose natural fibres such as cotton and wool where you can.
• Don’t burn waste. Burning plastic releases both particles and toxic gases.

Further readings

• Dris, R., Gasperi, J., Mirande, C., Mandin, C., Guerrouache, M., Langlois, V., & Tassin, B. (2017). A first overview of textile fibers, including microplastics, in indoor and outdoor environments. Environmental pollution, 221, 453-458. 
• Kole, P. J., Löhr, A. J., Van Belleghem, F. G., & Ragas, A. M. (2017). Wear and tear of tyres: a stealthy source of microplastics in the environment. International journal of environmental research and public health, 14(10), 1265. 
• Dris, R., Gasperi, J., Saad, M., Mirande, C., & Tassin, B. (2016). Synthetic fibers in atmospheric fallout: a source of microplastics in the environment?. Marine pollution bulletin, 104(1-2), 290-293. 
• Brahney, J., Hallerud, M., Heim, E., Hahnenberger, M., & Sukumaran, S. (2020). Plastic rain in protected areas of the United States. Science, 368(6496), 1257-1260. 
• Amato-Lourenço, L. F., Carvalho-Oliveira, R., Júnior, G. R., dos Santos Galvão, L., Ando, R. A., & Mauad, T. (2021). Presence of airborne microplastics in human lung tissue. Journal of hazardous materials, 416, 126124. 

As the paddy harvest ends in north-west India, thousands of fields are cleared by fire. A few days later, Delhi’s sky turns grey, and the argument begins. Is it the farms, the traffic or the firecrackers?
The answer isn’t a matter of opinion. Smoke carries chemical fingerprints, and atmospheric chemists know how to read them.

Fingerprint 1: potassium

Fingerprint 2: levoglucosan

Fingerprint 3: chloride

Satellites tell us where; chemistry tells us what arrived

Why the fingerprints matter

Further readings

• Andreae, M. O. (1983). Soot carbon and excess fine potassium: Long-range transport of combustion-derived aerosols. Science, 220(4602), 1148-1151. 
• Simoneit, B. R., Schauer, J. J., Nolte, C. G., Oros, D. R., Elias, V. O., Fraser, M. P., ... & Cass, G. R. (1999). Levoglucosan, a tracer for cellulose in biomass burning and atmospheric particles. Atmospheric Environment, 33(2), 173-182. 
• Cusworth, D. H., Mickley, L. J., Sulprizio, M. P., Liu, T., Marlier, M. E., DeFries, R. S., ... & Gupta, P. (2018). Quantifying the influence of agricultural fires in northwest India on urban air pollution in Delhi, India. Environmental Research Letters, 13(4), 044018. 

Rain can be a detective, but only if the evidence is handled carefully. A few minutes of carelessness can change a sample’s chemistry more than a week of pollution. So rainwater chemistry follows a strict routine, from rooftop to final number.

Catch only the rain

Measure fast, then stop the clock

Count the ions

Check your own work

Turn numbers into stories

• Volume-weighted means stop a light drizzle counting as much as a downpour.
• Sea-salt corrections use sodium as a marker of sea spray. Whatever chloride or sulphate is left over must come from land-based sources such as burning or industry.
• Neutralisation factors show which substances cancel acidity. In India, calcium from dust usually leads, followed by ammonium.

Why the routine matters

Beyond the major ions

• Trace metals such as lead, zinc and nickel, measured by ICP-MS, point to traffic, industry and waste burning.
• Stable isotopes of water (δ¹⁸O and δD) reveal where the rain’s moisture came from: the Arabian Sea, the Bay of Bengal or recycled local water.
• Organic acids, dissolved organic carbon and even microplastics show what else rain is quietly washing out of the sky.

References

• Keene, W. C., Pszenny, A. A., Galloway, J. N., & Hawley, M. E. (1986). Sea‐salt corrections and interpretation of constituent ratios in marine precipitation. Journal of Geophysical Research: Atmospheres, 91(D6), 6647-6658. 
• Possanzini, M., Buttini, P., & Di Palo, V. (1988). Characterization of a rural area in terms of dry and wet deposition. Science of the Total Environment, 74, 111-120. 
• Kulshrestha, U. C., L. Granat, Magnuz Engardt, and H. Rodhe. "Review of precipitation monitoring studies in India—a search for regional patterns." Atmospheric Environment 39, no. 38 (2005): 7403-7419. 

The first heavy shower of the monsoon feels like a reset. The haze lifts and the air smells fresh. But every drop has fallen through kilometres of city air, and it carries a record of what was up there.

Clean rain is not neutral

Even perfectly clean rain is slightly acidic. Carbon dioxide dissolves in droplets to form weak carbonic acid, so pure rain settles at a pH of about 5.6, not 7. Rain below 5.6 has picked up extra acids. Rain above it has picked up something that neutralises them.

How rain picks up pollution

Inside clouds, droplets form on tiny particles, so each drop starts out carrying sulphate, nitrate, salt or dust. Below the cloud, falling drops sweep up more particles and gases. This is why the first rain after a dry spell is the most concentrated: it washes out weeks of build-up.

The main ingredients

• Acids: sulphate from coal burning and industry, and nitrate from vehicles and combustion.
• Neutralisers: calcium and magnesium from soil and dust, and ammonium from fertilisers and livestock.
• Salts: sodium and chloride, mostly from sea spray near the coast.

Why Indian rain is often not acidic

Acid rain once damaged lakes and forests in Europe and North America, where rain often had a pH in the low 4s. In Delhi and much of north India, though, most rain samples have a pH above 5.6, frequently 6 to 7.

The acids are there, but they are outweighed by calcium-rich dust, which neutralises them like an antacid tablet. That protects soils and lakes for now, but it hides a risk. The dust is itself a pollutant, and if it declines while sulphur and nitrogen emissions keep rising, rain could slowly turn more acidic.

Chloride: a clue far from the sea

Sea salt brings chloride and sodium in a nearly fixed ratio. In Delhi, chloride often arrives in excess of that ratio, pointing to local sources such as burning waste, plastics and biomass. Rain chemistry is detective work.

Rain moves pollution, it doesn't destroy it

What rain removes from the air lands on soil, rivers and rooftops. Some of it, such as nitrogen, acts as a fertiliser, but too much of it can disturb lakes and forests. It's also why rooftop harvesting systems divert the first flush of rain away.

A record in every drop

Collected carefully and analysed for pH and major ions, rainwater gives one of the clearest long-term records of a city's air. So when the monsoon clears the sky, remember that the pollution hasn't vanished. It's in the puddles at your feet.

References

• Seinfeld, J. H., & Pandis, S. N. (1998). From air pollution to climate change. Atmospheric chemistry and physics, 1326. 
• Likens, G. E., & Bormann, F. H. (1974). Acid rain: a serious regional environmental problem. Science, 184(4142), 1176-1179. 
• Kulshrestha, U. C., Granat, L., Engardt, M., & Rodhe, H. (2005). Review of precipitation monitoring studies in India—a search for regional patterns. Atmospheric Environment, 39(38), 7403-7419.  
• Kulshrestha, U. C., Kulshrestha, M. J., Sekar, R., Sastry, G. S. R., & Vairamani, M. (2003). Chemical characteristics of rainwater at an urban site of south-central India. Atmospheric Environment, 37(21), 3019-3026.  
• Keene, W. C., Pszenny, A. A., Galloway, J. N., & Hawley, M. E. (1986). Sea‐salt corrections and interpretation of constituent ratios in marine precipitation. Journal of Geophysical Research: Atmospheres, 91(D6), 6647-6658. 

When the AQI app says "PM_2.5: 180", it's telling you the mass of fine particles in a cubic metre of air. It says nothing about what those particles are made of. That matters, because a particle of soot, a droplet of ammonium nitrate and a grain of dust behave very differently in the air and in our bodies.

So what is Delhi actually breathing?

Looking at the smallest particles

Some of the most detailed answers come from the Delhi Aerosol study, which measured particles smaller than one micrometre (PM₁) at IIT Delhi, hour by hour, for over a year. These are the finest particles inside PM_2.5, small enough to reach deep into the lungs.

The researchers used an instrument that pulls air into a vacuum, vaporises the particles and identifies the molecules they're made of. The results paint a clear picture.

Ingredient 1: organic matter, more than half

The biggest single ingredient is organic aerosol: carbon-based material containing hydrogen, oxygen and sometimes nitrogen. In the Delhi study, organics made up more than half of the PM₁ mass in every season and at every time of day, about 54% on average.

Organic aerosol comes from two very different routes:

• Primary organics, emitted directly as particles: exhaust from vehicles, smoke from burning wood, crop residue and rubbish, and cooking fumes. A follow-up study found that these primary sources dominated the very worst pollution episodes, when PM₁ went above 200 µg/m³.
• Secondary organics, which form in the air when gases from vehicles, burning and even plants react and condense into particles. Over a whole season, these secondary organics made up the larger share.

Ingredient 2: the salts made in the air

Next come inorganic salts: ammonium nitrate, ammonium sulphate and ammonium chloride. None of these are emitted as particles. They form in the air from gases:

• nitrogen oxides from vehicles and power plants turn into nitrate;
• sulphur dioxide from coal burning turns into sulphate;
• ammonia from farms and waste supplies the ammonium that holds them together as a salt;
• hydrogen chloride from burning waste and other sources adds chloride.

In Delhi's cooler months, ammonium, nitrate and chloride were each about 10% of PM₁. Cold, humid air helps these salts form and stay in the particle phase, which is one reason winter air is so much worse. As the earlier post on winter smog explained, nitrate in particular is very sensitive to temperature.

Delhi's chloride is unusual. At times it reached 50-100 µg/m³ in an hourly average, among the highest levels measured anywhere in the world. Most cities far from the sea have very little.

Ingredient 3: black carbon

Black carbon, or soot, is pure, dark carbon from incomplete burning in diesel engines, brick kilns and open fires. It made up about 5% of PM₁ in winter. That sounds small, but black carbon absorbs sunlight strongly, warms the air around it and carries other harmful compounds on its surface.

Ingredient 4: dust

Dust from roads, construction and dry land is mostly in larger particles, so it shows up more in PM₁₀ and in the coarser part of PM_2.5. It matters most in the dry, windy months before the monsoon, and much less in the fine particles of a calm winter night.

The big lesson: most of it is made in the air

Add it all up, and a striking fact emerges. In winter and spring, secondary material, formed in the air from gases, made up roughly 50-70% of Delhi's PM₁. In the warmer months, it was up to 60-80%.

This changes how we should think about solutions:

• Cutting smoke and dust is not enough. Much of the problem is made later, in the air, from gases like ammonia, nitrogen oxides and sulphur dioxide.

• Distant sources count. Gases can travel and react for hours before becoming particles over Delhi.

• Ammonia deserves more attention. It's the partner that turns acid gases into particle salts, and most of it comes from agriculture, not cities.

The same number, different air

Two days with the same PM_2.5 reading can be made of very different things: one dominated by fresh smoke from fires, another by salts formed overnight in fog. Scientists are still learning which ingredients harm health the most. Studying what's inside the particles, not just how much there is, is how we find out.

The next time your app says "PM_2.5: 180", picture it as a recipe: mostly organic matter, a good helping of salts made in the air, a dash of soot, and a little dust. Each comes from somewhere different, and each needs a different fix.

References

1. Kulshrestha, U.C., Saxena, A., Kumar, N. et al. Chemical Composition and Association of Size-Differentiated Aerosols at a Suburban Site in a Semi-Arid Tract of India. Journal of Atmospheric Chemistry 29, 109–118 (1998). https://doi.org/10.1023/A:1005796400044

2. Gani, S., Bhandari, S., Seraj, S., et al. (2019). Submicron aerosol composition in the world's most polluted megacity: the Delhi Aerosol Supersite study. Atmospheric Chemistry and Physics, 19, 6843–6859. https://doi.org/10.5194/acp-19-6843-2019   

3. Panda, U., Dey, S., Sharma, A. et al. Exploring the chemical composition and processes of submicron aerosols in Delhi using aerosol chemical speciation monitor driven factor analysis. Sci Rep 15, 14383 (2025). https://doi.org/10.1038/s41598-025-99245-9   

Every winter, Delhi's air becomes the main story in the city. For a few weeks, everyone talks about particulate matter, AQI numbers and stubble fires. Then the monsoon washes the sky clean, and the conversation fades until the next November.

Inside the lab, the conversation never stops. We measure, filter, extract, titrate and model the air all year round. We find things that are surprising, worrying, and sometimes beautiful. But most of it stays inside journals and conference halls.

VayuVarta is my attempt to change that, one post at a time.

What I will write about

Much of my own work is on chlorine in the lower atmosphere: where particulate chloride comes from, how it turns into reactive gases, and what that does to ozone and other oxidants. I will also write about particulate matter, rainwater chemistry, emission inventories, and the instruments and code that make the measurements possible.

Some posts will be explainers. Some will be field notes from sampling campaigns. Some will be about data and methods, for students who are just starting out in this field. And some will ask harder questions about who bears the burden of polluted air.

The air is shared, so knowledge about it should be shared too.

How to read this blog

I will keep the chemistry honest and the language plain. When I simplify, I will say so. When a claim comes from a study, I will link it. If you are a student, a journalist, a policymaker or simply someone who breathes Delhi's air, I hope you find something useful here.

Thank you for reading.

Every year, around late October, the same thing happens. The sky over Delhi turns a flat, milky grey, the sun becomes a pale disc, and air-quality apps slide from orange to red to maroon. Most people blame stubble burning or Diwali crackers, and both play a part. But the bigger story is chemistry and weather working together.
The air in winter is not just dirtier. It behaves differently.

A lid on the city

Gases that become particles

• Ammonia (NH₃), mostly from fertilisers, livestock and waste

Water makes it worse

And then the smoke arrives

What this means

• Seasonal measures alone are not enough. Because winter weather makes every emission count for more, cutting pollution all year round makes winter far less severe.
• Ammonia matters, even though it is rarely discussed. It comes mostly from farms, not cities, yet it is a key partner in forming winter particles.
• Burning anything, anywhere, adds up. Crop residue, garbage and plastic release both particles and gases like HCl that turn into more particles later.

Further reading

• https://thediplomat.com/2025/12/jayalakshmi-k-on-why-delhi-becomes-a-toxic-gas-chamber-every-winter/
• https://www.dw.com/en/india-delhi-air-pollution-coronavirus/a-55546113

• Mishra & Kulshrestha, 2020. https://aaqr.org/articles/aaqr-19-12-oa-0622