09. Atmospheric Pollution and Air Quality

Atmospheric Structure and Composition

Air pollution occurs when atmospheric contaminants reach concentrations that damage health, ecosystems, materials, visibility, or climate. The atmosphere is mostly nitrogen and oxygen, but small amounts of trace gases and particles can have substantial effects.

In dry air, nitrogen makes up approximately $78.1\%$, oxygen $20.9\%$, and argon $0.9\%$. Water vapor varies widely, from nearly $0\%$ in very dry air to roughly $4\%$ in warm, humid conditions. Important trace substances include carbon dioxide, methane, nitrous oxide, ozone, aerosols, and particulate matter.

The lowest atmospheric layer is the troposphere. It contains the weather, most human activity, and most air pollution that directly affects people. Above it, the stratosphere contains the ozone layer. Ozone in the stratosphere absorbs harmful ultraviolet radiation, while ozone near the ground is a respiratory irritant and pollutant.

Takeaway: Atmospheric composition is dominated by a few gases, but trace gases and particles can strongly influence health, climate, and chemical reactions.

Pollution Sources and Pollutant Types

Air pollutants come from both natural and human activities. Natural sources include volcanic eruptions, wildfires, dust storms, sea spray, vegetation, and decomposition. Human sources include vehicles, power plants, industrial facilities, construction, mining, agriculture, solvent use, landfills, residential combustion, and indoor activities.

Sources may also be classified by how emissions enter the atmosphere:

• Stationary sources include power plants, factories, and furnaces.

• Mobile sources include cars, trucks, ships, and airplanes.

• Area sources consist of many small sources distributed across a region, such as household heating, solvent use, and agricultural emissions.

A Primary pollutant is emitted directly. Examples include carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter, lead, and volatile organic compounds. A Secondary pollutant forms later through atmospheric reactions. Ground-level ozone, sulfuric acid, nitric acid, sulfate particles, and nitrate particles are examples.

This distinction matters because controlling pollution may require reducing direct emissions, preventing atmospheric reactions, or using both approaches.

Particles, Mixing, and Temperature Inversions

Particulate matter consists of suspended solid particles and liquid droplets. Its composition may include dust, soot, metals, sulfates, nitrates, organic compounds, and biological material.

Particle size affects where particles travel in the respiratory system:

• $PM_{10}$ includes particles with aerodynamic diameters of $10$ micrometers or less. These particles can enter the nose and upper respiratory tract.

• $PM_{2.5}$ includes particles with diameters of $2.5$ micrometers or less. These fine particles can penetrate deeply into the lungs and may enter the bloodstream.

• Ultrafine particles are smaller than $0.1$ micrometer and are often produced by combustion.

Particles may be emitted directly or formed when gases such as $SO_2$, $NO_x$, ammonia, and volatile organic compounds react in the atmosphere.

Normally, air warmed by the ground rises and allows pollutants to mix vertically. During a Temperature inversion, warmer air lies above cooler surface air. The warm layer acts as a lid, trapping pollution near the ground. Valleys, calm weather, nighttime, and early morning conditions can increase the likelihood of an inversion.

Takeaway: Particle size affects health risk, while atmospheric mixing determines how concentrated pollutants become near the surface.

Smog and Ground-Level Ozone

Photochemical smog develops when nitrogen oxides and volatile organic compounds react in sunlight. A simplified sequence is:

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Volatile organic compounds participate in additional reactions that help convert nitrogen monoxide back to nitrogen dioxide without consuming as much ozone. As a result, ground-level ozone can accumulate. It is a secondary pollutant and the principal harmful component of photochemical smog.

Ground-level ozone irritates the respiratory system, worsens asthma, reduces lung function, damages plant tissues, and can reduce crop productivity. Ozone and its precursors may travel hundreds of miles, affecting rural and downwind areas.

Industrial or sulfurous smog is associated with coal combustion, sulfur dioxide, smoke, particulate matter, cool temperatures, and humid conditions. The same chemical compound can therefore have different effects by location: stratospheric ozone protects living organisms, whereas tropospheric ozone harms health and vegetation.

Takeaway: Sunlight-driven reactions involving $NO_x$ and volatile organic compounds create harmful ozone near the surface, while ozone higher in the atmosphere provides protection.

Acid Deposition and Ecosystem Effects

Acid deposition includes wet deposition and dry deposition. Wet deposition is acidic rain, snow, fog, or hail. Dry deposition occurs when acidic gases and particles settle onto surfaces without precipitation.

The main precursors are sulfur dioxide and nitrogen oxides. They can be transformed through atmospheric reactions involving oxygen, water, and other chemicals:

Normal rain is slightly acidic because carbon dioxide dissolves in water and forms carbonic acid. Acid deposition becomes more damaging when sulfuric and nitric acids substantially lower the $pH$ of precipitation, soils, or surface waters.

Possible effects include:

• Lower $pH$ in lakes and streams

• Loss of calcium and magnesium from soils

• Mobilization of toxic aluminum ions

• Stress on forests, especially at high elevations or in nutrient-poor soils

• Reduced biodiversity in sensitive aquatic ecosystems

• Corrosion of buildings, monuments, paint, and metals

• Addition of biologically available nitrogen that can contribute to eutrophication

Limestone-rich soils and bedrock generally have greater buffering capacity than thin soils over granite. Reducing sulfur dioxide and nitrogen oxide emissions, using low-sulfur fuels, improving combustion, and installing flue-gas desulfurization systems can reduce acid deposition.

Indoor Air Quality and Exposure Reduction

People often spend most of their time indoors, so pollutant concentrations inside buildings can sometimes exceed outdoor concentrations. Indoor air pollution can originate from combustion, building materials, consumer products, biological growth, soil gases, and outdoor air entering a building.

Important indoor pollutants include:

• Carbon monoxide, produced by incomplete combustion, which binds to hemoglobin and reduces the blood’s oxygen-carrying capacity

• Radon, a radioactive gas from uranium in soil and rock that can enter through cracks and foundations

• Particulate matter from tobacco smoke, cooking, fireplaces, candles, and biomass burning

• Volatile organic compounds from paints, solvents, adhesives, furnishings, cleaning products, and air fresheners

• Mold and biological allergens promoted by persistent moisture and poor ventilation

• Asbestos and lead dust associated with some older building materials and deteriorating paint

• Formaldehyde emitted by some pressed-wood products, adhesives, and furnishings

Household burning of wood, charcoal, coal, kerosene, crop waste, or animal dung can cause substantial exposure to smoke and other pollutants. Associated health risks include respiratory and cardiovascular disease, childhood lower-respiratory infections, chronic obstructive pulmonary disease, stroke, and lung cancer.

The most effective control hierarchy is:

1. Control sources, such as eliminating indoor smoking, repairing combustion appliances, selecting low-emission products, and testing for radon.

2. Improve ventilation by exhausting pollutants outdoors and bringing in clean outdoor air when conditions permit.

3. Use appropriate, well-maintained filtration and air-cleaning devices.

4. Control moisture by repairing leaks, managing humidity, and removing mold-contaminated materials.

Takeaway: Preventing pollutants at their source is usually more effective than attempting to remove them after they accumulate indoors.

Regulation and Emission Controls

The Clean Air Act provides a framework for regulating air pollution in the United States. The Environmental Protection Agency establishes National Ambient Air Quality Standards for six widespread criteria pollutants:

• Carbon monoxide

• Lead

• Nitrogen dioxide

• Ozone

• Particulate matter, including $PM_{2.5}$ and $PM_{10}$

• Sulfur dioxide

Primary standards protect public health, including sensitive groups such as children, older adults, and people with asthma. Secondary standards protect public welfare, including crops, forests, animals, buildings, visibility, and ecosystems.

Regulatory approaches include emission standards, performance standards, technology standards, state implementation plans, monitoring, permitting, vehicle standards, cleaner fuels, and cap-and-trade programs. Standards may be reviewed as scientific evidence changes.

Engineering controls apply different mechanisms:

• Electrostatic precipitators use electrical charges to remove particles from exhaust.

• Baghouse filters capture particles in fabric filter bags.

• Scrubbers use liquid sprays to remove sulfur dioxide and other gases or particles.

• Catalytic converters transform carbon monoxide, nitrogen oxides, and unburned hydrocarbons into less harmful substances.

• Low-nitrogen-oxide burners reduce nitrogen oxide formation during combustion.

• Activated-carbon filters capture some volatile organic compounds and mercury compounds.

• Leak detection and repair reduces volatile organic compound and methane emissions.

The broadest strategy is pollution prevention: improve energy efficiency, use lower-emission energy, expand renewable electricity, improve public transportation, walk or cycle, electrify vehicles, control agricultural and landfill emissions, reduce open burning, and design buildings with clean ventilation and efficient appliances.

Monitoring, Mitigation, and Key Relationships

Air-quality monitoring networks measure pollution concentrations and help communicate risks through the Air Quality Index (AQI). The AQI supports decisions such as limiting strenuous outdoor activity during pollution episodes and protecting people who are especially sensitive.

The main relationships are:

• $NO_x + \text{volatile organic compounds} + \text{sunlight}$ can produce photochemical smog and ground-level ozone.

• $SO_2 + NO_x + \text{atmospheric water}$ can lead to sulfuric and nitric acids and acid deposition.

• Incomplete combustion can produce carbon monoxide and soot-containing particulate matter.

• Temperature inversions reduce atmospheric mixing and increase local pollution concentrations.

• Stratospheric ozone protects life, while tropospheric ozone harms respiratory health.

A practical pollution-control hierarchy is source reduction first, followed by cleaner technology, capture or treatment, and environmental cleanup when necessary. Individuals can also reduce exposure by avoiding tobacco smoke and indoor combustion, using suitable air cleaners, improving indoor air quality, and adjusting outdoor activity during pollution events.

Final takeaway: Air quality reflects the interaction of emissions, atmospheric chemistry, weather, exposure, and regulation. Effective protection combines prevention at the source with monitoring, engineering controls, cleaner energy, and informed public-health decisions.