8.2 Atmospheric chemistry and air quality

Air Pollutants

Types and Characteristics of Air Pollutants

Air pollutants are substances in the atmosphere that harm human health or the environment. They can exist as solid particles, liquid droplets, or gases, and they fall into two categories:

• Primary pollutants are emitted directly from a source (like exhaust from a tailpipe).
• Secondary pollutants form in the atmosphere when primary pollutants undergo chemical reactions (like ground-level ozone forming from other gases reacting in sunlight).

This distinction matters because controlling secondary pollutants means targeting their precursors, not the pollutant itself.

Particulate matter (PM) is a mixture of solid particles and liquid droplets suspended in the air. It's classified by size:

• PM10: particles with a diameter of 10 μm or less (think dust and pollen)
• PM2.5: particles with a diameter of 2.5 μm or less (think soot and smoke from combustion)

PM2.5 is the more dangerous category because those smaller particles can penetrate deep into lung tissue and even enter the bloodstream. Sources include vehicle emissions, industrial activity, and wildfires.

Ozone ($O_3$) is a highly reactive molecule made of three oxygen atoms. Where it exists determines whether it helps or harms:

• In the stratosphere, ozone forms a protective layer that absorbs harmful ultraviolet radiation.
• At ground level, ozone is a pollutant and a major component of photochemical smog. It irritates the respiratory system and worsens conditions like asthma.

Ground-level ozone is a secondary pollutant, meaning it isn't emitted directly. It forms when nitrogen oxides and VOCs react in the presence of sunlight.

Nitrogen and Sulfur Compounds

Nitrogen oxides ($NO_x$) refer to nitric oxide ($NO$) and nitrogen dioxide ($NO_2$). They form during high-temperature combustion in vehicle engines and power plants. $NO_x$ contributes to both ground-level ozone formation and acid rain, making it a key precursor for multiple pollution problems.

Sulfur dioxide ($SO_2$) is a colorless gas with a sharp, pungent smell. It comes primarily from burning sulfur-containing fossil fuels like coal and oil in power plants and industrial facilities. $SO_2$ contributes to acid rain and causes respiratory irritation, especially for people with asthma.

Volatile organic compounds (VOCs) are organic chemicals that evaporate easily at room temperature. Common examples include benzene, toluene, and xylene. You encounter VOC sources regularly: paints, solvents, cleaning products, gasoline, and vehicle exhaust all release them. VOCs are important in atmospheric chemistry because they react with $NO_x$ in sunlight to produce ground-level ozone and photochemical smog. Some VOCs, like benzene, are also directly toxic and carcinogenic.

Air Quality and Pollution Effects

Smog and the Air Quality Index

Smog is visible air pollution that forms a thick, hazy layer in the lower atmosphere. There are two main types, and they form through very different processes:

• Sulfurous smog (London-type smog) forms from a mixture of smoke and $SO_2$, typically in cold, humid conditions. It was responsible for London's Great Smog of 1952, which killed thousands.
• Photochemical smog (Los Angeles-type smog) forms when $NO_x$ and VOCs react in the presence of sunlight, producing ground-level ozone and other secondary pollutants. It's most common in sunny cities with heavy traffic.

The Air Quality Index (AQI) is the standard scale used to communicate pollution levels to the public. It ranges from 0 to 500, with higher values meaning worse air quality and greater health risk. The AQI tracks five major pollutants: ground-level ozone, particulate matter, carbon monoxide, $SO_2$, and $NO_2$. An AQI above 100 is considered unhealthy for sensitive groups, and above 300 is hazardous for everyone.

Acid Rain and Atmospheric Lifetime

Acid rain is any precipitation with a pH below 5.6 (normal rain is slightly acidic at about 5.6 due to dissolved $CO_2$). It forms when $NO_x$ and $SO_2$ react with water vapor, oxygen, and other chemicals in the atmosphere to produce nitric acid and sulfuric acid. These acids then fall as rain, snow, or fog.

The effects are serious: acid rain lowers the pH of lakes and streams (harming aquatic life), leaches nutrients from forest soils, and corrodes buildings, monuments, and infrastructure.

Atmospheric lifetime is the average time a pollutant remains in the atmosphere before being removed through chemical reactions or deposition. This concept explains why some pollutants cause local problems while others have global reach:

• $SO_2$ has a lifetime of roughly 1–2 days, so its effects (like acid rain) tend to stay regional.
• Methane ($CH_4$) has a lifetime of about 12 years, allowing it to spread globally and contribute to climate change over long periods.

Pollutants with longer atmospheric lifetimes are harder to manage because they accumulate and travel far from their sources.