Major Air Pollutants

Why This Matters

Air pollutants sit at the intersection of atmospheric chemistry, human health, and environmental policy—three areas you'll be tested on repeatedly in atmospheric science. Understanding these pollutants means grasping photochemical reactions, combustion chemistry, radiative forcing, and biogeochemical cycles all at once. When an exam asks about smog formation or acid deposition, you need to know which pollutants are primary versus secondary, which ones interact to create new problems, and why certain weather conditions make everything worse.

Don't just memorize a list of chemicals and their health effects. Instead, focus on how pollutants form, what atmospheric conditions amplify their impacts, and how they connect to larger systems like climate change and ecosystem degradation. The pollutants below are grouped by their primary atmospheric behavior—this is how exam questions will approach them, and it's how your brain should organize them too.

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Primary Combustion Pollutants

These pollutants are released directly into the atmosphere from burning fossil fuels. Primary pollutants don't require atmospheric transformation—they're harmful the moment they're emitted.

Carbon Monoxide (CO)

• Produced by incomplete combustion—forms when fuel burns without enough oxygen, common in vehicle engines and poorly ventilated heating systems
• Binds to hemoglobin 200-250 times more readily than oxygen, reducing blood's oxygen-carrying capacity and causing hypoxia
• Highest concentrations occur in urban areas during rush hour and in enclosed spaces like garages and tunnels

Nitrogen Oxides (NOx)

• High-temperature combustion creates NOx—vehicles, power plants, and industrial boilers are primary sources because heat forces $N_2$ and $O_2$ to react
• Precursor pollutant that contributes to ground-level ozone formation, particulate matter, and acid rain through atmospheric reactions
• $NO_2$ causes the brown haze visible over cities; it absorbs visible light and serves as a key indicator of urban air quality

Sulfur Dioxide (SO2)

• Released when sulfur-containing fuels burn—coal and heavy oil combustion in power plants and industrial facilities are dominant sources
• Oxidizes in the atmosphere to form sulfuric acid ($H_2SO_4$), a major component of acid rain that damages ecosystems and infrastructure
• Respiratory irritant that triggers bronchoconstriction; effects are immediate and dose-dependent

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Secondary and Photochemical Pollutants

These pollutants form through atmospheric chemical reactions, often requiring sunlight. Secondary pollutants demonstrate how the atmosphere acts as a chemical reactor.

Ground-level Ozone (O3)

• Secondary pollutant formed photochemically—sunlight drives reactions between VOCs and $NO_x$ to produce $O_3$, making it a marker of photochemical smog
• Peak concentrations occur in summer afternoons—warm temperatures and intense sunlight accelerate formation; urban areas downwind of pollution sources are most affected
• Powerful oxidant that damages lung tissue, reduces crop yields, and degrades materials like rubber and plastics

Volatile Organic Compounds (VOCs)

• Ozone precursors that react with $NO_x$ in sunlight—without VOCs, ground-level ozone formation is limited
• Sources span industrial and everyday activities—vehicle exhaust, refineries, paints, solvents, and even vegetation emit VOCs
• Health effects range from acute to chronic—short-term exposure causes headaches and dizziness; long-term exposure to certain VOCs like benzene increases cancer risk

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Particulate Pollutants

Particulate matter includes both primary emissions and secondary particles formed through atmospheric reactions. Size determines how deep particles penetrate into the respiratory system.

Particulate Matter (PM2.5 and PM10)

• Size classification determines health impact—$PM_{2.5}$ (≤2.5 μm) penetrates deep into alveoli and enters the bloodstream; $PM_{10}$ (≤10 μm) deposits in upper airways
• Both primary and secondary sources—direct emissions from combustion and dust, plus secondary formation from $SO_2$, $NO_x$, and ammonia reactions
• Strongest association with mortality of any air pollutant; linked to cardiovascular disease, respiratory illness, and premature death

Ammonia (NH3)

• Agricultural emissions dominate—livestock waste and synthetic fertilizer application release most atmospheric ammonia
• Reacts to form secondary $PM_{2.5}$—combines with $SO_2$ and $NO_x$ oxidation products to create ammonium sulfate and ammonium nitrate particles
• Causes eutrophication when deposited to water bodies, driving algal blooms and oxygen depletion in aquatic ecosystems

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Greenhouse Gases and Climate Pollutants

These pollutants trap outgoing longwave radiation, contributing to radiative forcing and climate change. Their atmospheric lifetimes and global warming potentials vary dramatically.

Methane (CH4)

• Global warming potential ~80× $CO_2$ over 20 years—shorter atmospheric lifetime (~12 years) but much stronger heat-trapping ability per molecule
• Biogenic and anthropogenic sources—enteric fermentation in livestock, rice paddies, landfills, and natural gas leaks all contribute significantly
• Target for near-term climate mitigation—reducing methane yields faster climate benefits than $CO_2$ reductions due to its shorter lifetime

Chlorofluorocarbons (CFCs)

• Destroy stratospheric ozone—UV radiation breaks CFCs apart, releasing chlorine atoms that catalytically destroy thousands of $O_3$ molecules each
• Extremely potent greenhouse gases—some CFCs have global warming potentials thousands of times higher than $CO_2$
• Montreal Protocol success story—international phase-out demonstrates effective environmental policy; ozone layer is slowly recovering

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Toxic and Legacy Pollutants

These pollutants pose severe health risks even at low concentrations, often bioaccumulating in organisms and ecosystems.

Lead (Pb)

• Neurotoxin with no safe exposure level—causes irreversible developmental damage in children, affecting IQ, behavior, and learning
• Leaded gasoline phase-out represents a major regulatory success—blood lead levels dropped dramatically after the 1970s ban
• Legacy contamination persists—soil near highways and older buildings with lead paint continues to expose populations, especially in low-income communities

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Quick Reference Table

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Self-Check Questions

1. Which two pollutants must both be present for ground-level ozone to form, and what additional condition is required?

2. Compare and contrast the atmospheric behavior of $SO_2$ and $CO$—how do their health effects and environmental impacts differ?

3. A city experiences high $PM_{2.5}$ levels despite having few industrial sources. What secondary formation pathways could explain this, and which precursor pollutants would you investigate?

4. Why does methane receive attention for near-term climate mitigation even though $CO_2$ is more abundant? Reference atmospheric lifetime and global warming potential in your answer.

5. An FRQ asks you to explain why summer afternoons in urban areas have the worst air quality. Which pollutants and formation mechanisms would you discuss?