Air quality chemistry: pollutants, smog, and the Air Quality Index

What clean air is made of

Clean, dry air is remarkably consistent: about 78% nitrogen (N2), 21% oxygen (O2), roughly 1% argon, and trace amounts of carbon dioxide, water vapor, and other gases. Everything else — the pollutants — is present in tiny fractions, measured in parts per million or billion.

That is what makes air pollution a chemistry problem: amounts too small to see can still change how the air behaves and how our bodies respond to it. Air quality science tracks these trace additions and their sources.

Carbon monoxide: the product of incomplete burning

Carbon monoxide (CO) forms when fuels burn without enough oxygen — in car engines, faulty heaters, and wildfires. It is colorless and odorless, which makes it especially dangerous.

CO harms by binding to hemoglobin in red blood cells far more strongly than oxygen does, blocking oxygen transport through the body. Outdoors it disperses, but in enclosed spaces with a running engine or malfunctioning fuel-burning appliance it can accumulate to lethal levels — the reason working carbon monoxide detectors matter in homes.

Nitrogen oxides and sulfur dioxide

At the high temperatures inside engines and power plants, nitrogen and oxygen from the air itself react to form nitrogen oxides (NOx) — mainly nitric oxide (NO) and nitrogen dioxide (NO2). NO2 is a reddish-brown gas that contributes to the brownish haze over congested cities.

Sulfur dioxide (SO2) comes mostly from burning coal and oil that contain sulfur. In the atmosphere it can convert to sulfuric acid, a major contributor to acid rain, which damages buildings, forests, and lakes. Scrubbers on power plants and low-sulfur fuels have cut SO2 emissions substantially in many countries.

Particulate matter: pollution you can almost see

Particulate matter (PM) is the term for tiny solid and liquid particles suspended in air — dust, soot, smoke, and droplets formed from gases. Regulators classify it by size:

• PM10: particles up to 10 micrometers across (dust, pollen fragments).
• PM2.5: particles up to 2.5 micrometers across — about 30 times thinner than a human hair.

The smaller the particle, the deeper it can travel into the lungs, which is why PM2.5 is tracked so closely. Major sources include vehicle exhaust, wood smoke, wildfires, construction dust, and industrial processes.

Ground-level ozone and smog

Ozone (O3) high in the stratosphere shields us from ultraviolet light — but at ground level it is a pollutant. Ground-level ozone is not usually emitted directly; it forms when nitrogen oxides and volatile organic compounds (VOCs) react in strong sunlight.

That is why smog peaks on hot, sunny, stagnant afternoons: the ingredients (traffic and industrial emissions) plus the energy (sunlight) plus still air that lets it all accumulate. The word 'smog' originally meant smoke-plus-fog, but modern photochemical smog is mostly this sun-driven ozone chemistry.

Reading the Air Quality Index

The Air Quality Index (AQI) translates pollutant measurements into a single 0-to-500 scale with color-coded categories, so anyone can judge the air at a glance. Higher numbers mean worse air.

The U.S. EPA categories run from Good (0–50, green) through Moderate (51–100, yellow) and Unhealthy for Sensitive Groups (101–150, orange) up to Unhealthy (151–200, red), Very Unhealthy (201–300, purple), and Hazardous (301–500, maroon). The reported AQI is driven by whichever pollutant scores worst that day — often ozone in summer or PM2.5 during wildfire season.

Many countries use similar index systems with their own breakpoints; the core idea is the same: convert chemistry measurements into plain-language guidance.

What shapes the air where you live

Local air quality is chemistry plus meteorology:

• Wind disperses pollutants; temperature inversions (warm air trapping cooler air below) hold them near the ground.
• Traffic patterns set the daily rhythm of NOx and PM near roads.
• Wildfires can send PM2.5 hundreds of miles, spiking AQI far from the flames.
• Rain washes many particles and soluble gases out of the air, which is why air often smells fresher after a storm.

Forecasters combine emissions data, chemistry models, and weather to predict AQI a day or two ahead — the same kind of modeling used for weather itself.

Common mistakes

Real-world applications

• Operating city air-monitoring networks and issuing AQI forecasts

• Designing catalytic converters and scrubbers that cut vehicle and plant emissions

• Setting and enforcing emissions standards for industry and transport

• Studying how wildfire smoke travels and affects distant communities

• Planning cities with traffic and green space to reduce exposure