APES 7.1-7.2: Introduction to Air Pollution and Photochemical Smog

Air pollution

The presence of gases, particles, or other substances in air at concentrations that can harm humans, other organisms, or the environment. It can occur both outdoors and indoors.

Aerosol

A mixture in which solid particles or liquid droplets are dispersed throughout a gas. Atmospheric particulate matter is commonly present as an aerosol.

How do primary and secondary air pollutants differ?

Primary pollutants are emitted directly by a source, such as CO from combustion or SO₂ from burning sulfur-containing fuels. Secondary pollutants form in the atmosphere when primary pollutants react with one another or with atmospheric components; ground-level O₃ is an example.

Which major human activities are important sources of outdoor air pollution?

Major sources include fossil-fuel combustion for electricity and transportation, industrial processes, construction and demolition, agriculture, waste burning, and wildfires influenced by human activity.

Why does indoor air pollution often become severe when solid fuels are used for cooking or heating?

Burning wood, dung, coal, charcoal, or agricultural waste can release CO, NOₓ, soot, and other particles. Poor ventilation allows these pollutants to accumulate to high concentrations indoors.

What are the main combustion products of incomplete fuel combustion that are especially important pollutants?

Incomplete combustion can produce carbon monoxide, soot or black carbon, volatile organic compounds, and particulate matter. Complete combustion of hydrocarbons primarily produces CO₂ and H₂O, although high-temperature combustion can also form NOₓ.

How can agriculture contribute to air pollution beyond smoke from field burning?

Livestock digestion releases methane, while manure and fertilizer use release ammonia. Methane and ammonia can participate in atmospheric reactions that increase ozone or fine particulate matter.

What pollutants can open burning of municipal or electronic waste release?

Open burning can emit soot, CO, VOCs, toxic metals, and hazardous organic compounds such as dioxins and furans. Electronic-waste burning may also release metals and other toxic substances.

Why can transportation emissions contribute to both air pollution and climate change?

Combustion engines release greenhouse gases such as CO₂ and pollutants such as NOₓ, CO, VOCs, and particles. Vehicles also produce non-exhaust particles from tire and brake wear and from road-surface disturbance.

How do electric vehicles affect air pollution compared with gasoline or diesel vehicles?

Electric vehicles have no tailpipe emissions, so they avoid direct exhaust of CO, NOₓ, and combustion particles. They can still produce non-exhaust particles from tires, brakes, and road wear, and their overall emissions depend partly on how electricity is generated.

Ammonia (NH₃)

A gas emitted largely from nitrogen fertilizer use and livestock manure or urine. It can react with atmospheric acids to form fine ammonium particulate matter and can cause eutrophication when deposited into ecosystems.

Carbon dioxide (CO₂)

A greenhouse gas emitted mainly by burning fossil fuels. It is sometimes classified as an air pollutant because it is the principal long-lived greenhouse gas responsible for climate change.

Carbon monoxide (CO)

A colorless, odorless, toxic gas produced primarily by incomplete combustion. It can accumulate in poorly ventilated areas and interfere with oxygen transport in the body, potentially causing unconsciousness or death.

Why is carbon monoxide especially dangerous in enclosed spaces?

CO can be produced continuously by fuel-burning appliances while ventilation is limited, allowing its concentration to rise. Because it is colorless and odorless, people may not detect exposure before serious effects occur.

Ground-level ozone (O₃)

A secondary pollutant formed when NOₓ and volatile organic compounds react in sunlight. It is a major component of photochemical smog and can damage lungs, plants, crops, forests, and materials.

What chemical conditions promote photochemical smog?

Photochemical smog develops when NOₓ and VOCs accumulate in sunlight, especially in warm, stagnant air. The reactions generate secondary pollutants including ground-level O₃ and additional oxidized organic compounds.

Why are ground-level ozone concentrations often highest on hot, sunny afternoons?

Sunlight drives the photochemical reactions that produce O₃ from NOₓ and VOCs, while higher temperatures often accelerate these reactions. Consequently, ozone pollution commonly peaks during hot summer afternoons.

What are the major atmospheric sources of nitrogen oxides (NOₓ)?

NOₓ is produced mainly during high-temperature combustion of fossil fuels, such as in engines and power plants; lightning contributes a smaller natural source. The group includes NO and NO₂.

How is nitrogen dioxide formed from nitric oxide in the atmosphere?

Nitric oxide is oxidized by atmospheric oxidants, commonly represented as $2NO + O_2 \rightarrow 2NO_2$. NO₂ is a toxic, reddish-brown gas and can participate in photochemical smog and acid formation.

Particulate matter (PM)

A mixture of airborne solid particles and liquid droplets rather than gases. Its composition may include soot, dust, sulfate, nitrate, ammonium salts, metals, organic compounds, and biological material.

How are PM₁₀, PM₂.₅, and ultrafine particles distinguished?

PM₁₀ consists of particles with diameters of $10\ \mu\text{m}$ or less, PM₂.₅ consists of particles smaller than or equal to about $2.5\ \mu\text{m}$, and ultrafine particles are about $0.1\ \mu\text{m}$ or smaller. Smaller particles can penetrate more deeply into the respiratory system.

How do natural and human sources of particulate matter commonly differ in particle size?

Natural sources such as dust storms often produce relatively coarse particles. Human combustion and vehicle sources commonly produce finer particles, although both natural and human activities can emit a range of particle sizes.

How do sulfate, nitrate, and ammonium particles form in the atmosphere?

Atmospheric oxidation converts precursor gases into particulate substances: SO₂ can form sulfate, NO₂ can form nitrate, and NH₃ can combine with acidic species to form ammonium salts such as ammonium sulfate or ammonium nitrate.

Why is fine particulate matter generally more harmful than coarse particulate matter?

Fine particles can reach deep lung tissue, and some can cross into the bloodstream and affect other organs. Their large total surface area relative to mass also allows toxic substances to be transported on or within the particles.

Sulfur dioxide (SO₂)

An acidic, corrosive gas produced mainly by burning coal or crude oil containing sulfur compounds, as well as by metal smelting and volcanoes. It can form sulfate particles and sulfuric acid, contributing to acid rain.

Volatile organic compound (VOC)

A carbon-containing compound that readily exists as a gas at room temperature. VOCs can contribute to photochemical smog and ozone formation; examples include methane, benzene, acetone, and toluene.

How do VOCs contribute to ground-level ozone formation?

In sunlight, VOCs react with atmospheric oxidants and participate in reaction cycles that convert NO into NO₂ without being consumed in a simple one-step process. Photolysis of NO₂ then helps generate O atoms that combine with O₂ to form O₃.

What are the six criteria air pollutants commonly regulated under the Clean Air Act?

The six criteria pollutants are carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO₂), ozone (O₃), particulate matter (PM₁₀ and PM₂.₅), and sulfur dioxide (SO₂).

Which criteria pollutants are typically primary, and which is primarily secondary?

CO, SO₂, lead, and much of particulate matter are emitted directly and are therefore primary pollutants. NOₓ is emitted directly but also participates in atmospheric reactions. Ground-level O₃ is a secondary pollutant formed from NOₓ and VOCs in sunlight.

Lead (Pb)

A toxic heavy-metal air pollutant that can occur in particles or other atmospheric forms. Exposure can impair nervous-system development and cause learning disabilities, especially in children.

What is the basic sequence of reactions that produces photochemical smog?

Combustion and other sources release NOₓ and VOCs. Sunlight photolyzes NO₂, producing an oxygen atom that combines with O₂ to form O₃; VOC-driven reactions regenerate NO₂, allowing ozone production to continue and producing other secondary pollutants.