Atmospheric Pollution

Overview of Atmospheric Pollution

Atmospheric pollution refers to the introduction of substances or forms of energy into the air that lead to harmful changes in its composition or properties. In environmental chemistry, the focus is on:

• What the main air pollutants are
• How they are formed (often from specific chemical reactions)
• How they transform in the atmosphere
• Their impacts on health, ecosystems, and materials
• How chemistry helps us understand and reduce them

Here, the emphasis is on the chemistry of polluted air, not on general environmental topics already treated elsewhere.

Primary vs. Secondary Air Pollutants

A central distinction in atmospheric chemistry is between primary and secondary pollutants:

• Primary pollutants: emitted directly into the atmosphere from a source.
• Secondary pollutants: formed in the atmosphere by chemical reactions involving primary pollutants.

Typical primary pollutants (from natural and human-made sources) include:

• Carbon monoxide: CO
• Sulfur dioxide: SO$_2$
• Nitrogen oxides: often summarized as NO$_x$ (mainly NO and NO$_2$)
• Volatile organic compounds (VOCs), e.g. hydrocarbons
• Particulate matter (aerosols, soot)
• Certain metals and metal compounds (e.g. Pb compounds)

Secondary pollutants include:

• Ozone in the lower atmosphere (tropospheric O$_3$)
• Nitric acid: HNO$_3$
• Sulfuric acid: H$_2$SO$_4$
• Peroxyacetyl nitrate (PAN) and related compounds
• Secondary inorganic and organic aerosols formed from gaseous precursors

The chemical links between primary and secondary pollutants are at the heart of atmospheric pollution chemistry.

Major Atmospheric Pollutants and Their Sources

Sulfur Dioxide and Sulfur Oxides (SO$_x$)

Sources:

• Combustion of sulfur-containing fossil fuels (coal, heavy oil)
• Certain industrial processes (e.g. metal smelting)
• Volcanic activity (natural source)

The main emitted species is SO$_2$; in the atmosphere, it can be oxidized to sulfur trioxide:

$$
\text{SO}_2 + \tfrac{1}{2}\,\text{O}_2 \rightarrow \text{SO}_3
$$

This oxidation proceeds via more complex pathways often involving radicals and catalysts like metal particles. SO$_3$ reacts quickly with water:

$$
\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4
$$

H$_2$SO$_4$ contributes to acid deposition and forms sulfate aerosols.

Nitrogen Oxides (NO, NO$_2$; NO$_x$)

Sources:

• High-temperature combustion in engines and power plants (thermal NO)
• Combustion of nitrogen-containing fuels (fuel NO)
• Lightning and some microbial processes (natural)

Typical primary emission:

• Nitric oxide: NO

In air, NO is readily oxidized:

$$
2\,\text{NO} + \text{O}_2 \rightarrow 2\,\text{NO}_2
$$

NO$_2$ is both a pollutant and a key participant in photochemical smog formation and acid rain chemistry.

Carbon Monoxide (CO) and Carbon Dioxide (CO$_2$)

Carbon monoxide (CO) is formed mainly by incomplete combustion of carbon-containing fuels:

$$
\text{C} + \tfrac{1}{2}\,\text{O}_2 \rightarrow \text{CO}
$$

or from incomplete oxidation of hydrocarbons. CO is toxic and also participates in atmospheric radical chemistry (e.g. reaction with OH radicals).

Carbon dioxide (CO$_2$) is the product of complete combustion:

$$
\text{C} + \text{O}_2 \rightarrow \text{CO}_2
$$

and is the dominant anthropogenic greenhouse gas. In the context of atmospheric pollution, CO$_2$ is crucial for climate change, but it is not toxic at typical ambient levels.

Volatile Organic Compounds (VOCs)

VOCs are organic molecules that evaporate easily into the air, including:

• Hydrocarbons from fuel evaporation and combustion
• Solvent vapors (paints, cleaners)
• Biogenic emissions from plants (e.g. isoprene, terpenes)

In the atmosphere, VOCs are oxidized, often initiated by OH radicals, ozone, or NO$_3$ radicals, leading to a variety of oxygenated products and secondary organic aerosols (SOA).

Particulate Matter (PM) and Aerosols

Particulate matter includes solid and liquid particles suspended in air, such as:

• Soot (elemental carbon from incomplete combustion)
• Dust (mineral particles)
• Secondary inorganic particles (sulfates, nitrates, ammonium salts)
• Secondary organic aerosols (oxidation products of VOCs)

Particles are often categorized by size:

• PM$_{10}$: particles with diameter $< 10\,\mu\text{m}$
• PM$_{2.5}$: particles with diameter $< 2.5\,\mu\text{m}$

Smaller particles penetrate deeper into the respiratory system and have significant health and climate impacts (e.g. scattering and absorption of radiation, serving as cloud condensation nuclei).

Chemical Transformation Processes in the Atmosphere

Photochemical Reactions and Tropospheric Ozone Formation

In unpolluted air, ozone (O$_3$) concentration near the ground is relatively low. In polluted urban air with NO$_x$ and VOCs, complex photochemistry leads to elevated O$_3$ levels (photochemical smog).

A simplified key step is the photolysis of NO$_2$:

$$
\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}
$$

where $h\nu$ denotes a photon of ultraviolet or visible light. The atomic oxygen then reacts:

$$
\text{O} + \text{O}_2 + \text{M} \rightarrow \text{O}_3 + \text{M}
$$

Here, M represents any third body (e.g. N$_2$, O$_2$) carrying away excess energy.

NO can react with ozone:

$$
\text{NO} + \text{O}_3 \rightarrow \text{NO}_2 + \text{O}_2
$$

This cycle alone does not lead to a net ozone increase, because O$_3$ formed is consumed again. Net ozone formation requires reactions that remove NO without consuming O$_3$. VOCs provide such pathways: their oxidation produces peroxy radicals (RO$_2\cdot$), which oxidize NO to NO$_2$ without destroying ozone. For example (schematically):

$$
\text{RO}_2\cdot + \text{NO} \rightarrow \text{RO}\cdot + \text{NO}_2
$$

As NO is converted back to NO$_2$, more NO$_2$ is available for photolysis and O$_3$ production, leading to a net build-up of ozone and other oxidants.

Formation of Acid Rain Components

Acid deposition is largely due to the atmospheric formation of strong acids from SO$_2$ and NO$_x$.

Oxidation of Sulfur Dioxide

In the gas phase, SO$_2$ can react with OH radicals:

$$
\text{SO}_2 + \text{OH}\cdot \rightarrow \text{HSO}_3\cdot
$$

followed by further oxidation to sulfuric acid. Within cloud droplets and aerosols, SO$_2$ (dissolved as sulfite/bisulfite) is oxidized, for example by hydrogen peroxide:

$$
\text{HSO}_3^- + \text{H}_2\text{O}_2 \rightarrow \text{HSO}_4^- + \text{H}_2\text{O}
$$

Overall, these processes yield sulfuric acid:

$$
\text{SO}_2 + \tfrac{1}{2}\,\text{O}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4
$$

Oxidation of Nitrogen Oxides

NO$_2$ also participates in acid formation. In the atmosphere:

$$
4\,\text{NO}_2 + 2\,\text{H}_2\text{O} + \text{O}_2 \rightarrow 4\,\text{HNO}_3
$$

The nitric acid formed is highly water soluble and can be washed out by precipitation or be deposited as dry deposition on surfaces.

Secondary Aerosol Formation

Gaseous substances can transform into particle-phase material via:

• Neutralization reactions, e.g.:

$$
\text{H}_2\text{SO}_4 + 2\,\text{NH}_3 \rightarrow (\text{NH}_4)_2\text{SO}_4
$$

where ammonia from agriculture neutralizes sulfuric acid, forming ammonium sulfate particles.

• Condensation of low-volatility organic compounds formed by oxidation of VOCs, leading to secondary organic aerosols (SOA).

These processes increase particulate matter and affect visibility, climate, and health.

Smog Types

Classical (Reducing) Smog

Also called "London smog," this type is associated with:

• High emissions of SO$_2$ and smoke (particles)
• Cool, humid, stagnant air

The air becomes loaded with SO$_2$, sulfuric acid aerosols, and soot. Chemically, oxidation of SO$_2$ and water uptake by particles are important. This smog is especially corrosive and was historically linked to severe health crises.

Photochemical (Oxidizing) Smog

Also known as "Los Angeles smog," it forms under:

• Strong sunlight
• High NO$_x$ and VOC emissions (e.g. traffic)
• Stable, warm air masses

Main chemical features:

• Formation of O$_3$ via NO$_2$ photolysis and subsequent reactions
• Formation of oxidants such as peroxyacetyl nitrate (PAN) and other peroxides
• Increased levels of reactive radicals and oxygenated organic compounds

PAN, for example, is formed from NO$_2$ and organic peroxyacetyl radicals (from VOC oxidation). It is an eye irritant and a reservoir for NO$_x$.

Local, Regional, and Global Aspects

Atmospheric pollutants differ in how far they are transported and how long they persist:

• Local scale: high NO$_2$, CO, and short-lived VOCs in urban street canyons, affected strongly by traffic emissions and local meteorology.
• Regional scale: acid rain, sulfate and nitrate aerosols, and ozone episodes can spread hundreds of kilometers from the sources.
• Global scale: long-lived greenhouse gases (CO$_2$, CH$_4$, N$_2$O) and some persistent organic pollutants and aerosols influence the global climate and remote ecosystems.

Chemical lifetimes in the atmosphere (seconds to years) and physical processes (transport, dispersion, dry and wet deposition) determine how pollution spreads and where its effects are felt.

Effects of Atmospheric Pollution

Effects on Human Health

From a chemical standpoint, health impacts are linked to reactivity, solubility, and particle size:

• Ozone and oxidants: strong oxidizing agents that attack lipids and proteins in lung tissue.
• NO$_2$ and SO$_2$: irritating gases; SO$_2$ is water-soluble and affects upper airways; NO$_2$ penetrates deeper.
• Fine particles (PM$_{2.5}$): can carry toxic compounds (e.g. metals, organics) and reach deep lung regions.
• CO: binds to hemoglobin, forming carboxyhemoglobin and reducing oxygen transport.

These interactions are grounded in chemical affinities and reaction pathways between pollutants and biological molecules.

Effects on Materials

Key chemical processes include:

• Acid-induced corrosion:
• Metals (e.g. iron) corroded by acidic deposition and wet films containing H$_2$SO$_4$ and HNO$_3$.
• Limestone and marble (calcium carbonate) react with acids:

$$
\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{CO}_2 + \text{H}_2\text{O}
$$

• Oxidation of organic materials (paints, polymers) by ozone and other oxidants.

Effects on Ecosystems

Atmospheric deposition alters the chemistry of soils and waters:

• Acidification: increased H$^+$ input changes solubility and speciation of nutrients and toxic metals (e.g. aluminum). For instance, acidification can mobilize Al$^{3+}$ from minerals, impacting aquatic life and roots.
• Nutrient inputs: deposition of nitrogen compounds (e.g. NH$_4^+$, NO$_3^-$) contributes to eutrophication and disturbs nutrient balances.

These effects are governed by chemical equilibria and reaction kinetics in soils and waters.

Chemical Strategies for Reducing Atmospheric Pollution

Chemistry underpins both the understanding and mitigation of atmospheric pollution. Some typical strategies:

• Desulfurization of fuels and flue gases:
• Removal of sulfur from fuels prior to combustion.
• Flue gas desulfurization (e.g. reaction of SO$_2$ with CaCO$_3$/CaO to form CaSO$_4$).
• Reduction of NO$_x$ emissions:
• Catalytic converters in vehicles, using reduction and oxidation catalysts to convert NO, CO, and hydrocarbons into N$_2$, CO$_2$, and H$_2$O.
• Selective catalytic reduction (SCR) in power plants, where NH$_3$ or urea reduces NO$_x$ to N$_2$:

$$
4\,\text{NO} + 4\,\text{NH}_3 + \text{O}_2 \rightarrow 4\,\text{N}_2 + 6\,\text{H}_2\text{O}
$$

• Control of VOC emissions:
• Use of less volatile and less reactive solvents.
• Oxidation of VOC-containing exhaust gases (thermal or catalytic oxidation).
• Particulate removal:
• Filtration, electrostatic precipitators, and other physical-chemical separation methods.

These measures rely on specific chemical reactions and separations designed to convert harmful substances into less harmful ones or to prevent their formation in the first place.

Summary

Atmospheric pollution is fundamentally a chemical problem:

• Pollutants are distinct chemical species with characteristic sources, transformation pathways, and sinks.
• Sunlight-driven (photochemical) reactions, oxidation by radicals, and gas–particle conversions shape the composition of the air.
• The resulting mixture of gases and particles affects climate, health, materials, and ecosystems.

Understanding the chemistry of atmospheric pollution is essential for diagnosing air quality problems and developing effective, chemically informed strategies to mitigate them.