Sources of Air Pollutants

Every flame is a chemical reaction trying to combine fuel with oxygen. When combustion is complete, carbon in the fuel becomes carbon dioxide and hydrogen becomes water vapor. But real engines and furnaces never burn perfectly. When oxygen is limited or the fuel doesn't fully mix with air, carbon atoms get stuck halfway, forming carbon monoxide instead of carbon dioxide, and unburned carbon particles escape as soot. High flame temperatures also force nitrogen and oxygen from the air itself to react, producing nitrogen oxides, a process that has nothing to do with the fuel and everything to do with heat. Sulfur dioxide comes from sulfur impurities already in the fuel, mostly coal and some oils, oxidizing as the fuel burns.

This is why a car in stop-and-go traffic pollutes more per kilometer than one cruising on a highway. Idling and repeated acceleration from a stop are inefficient: the engine runs cooler and richer during those moments, so combustion is less complete and more CO, soot, and unburned hydrocarbons form for each km actually driven. Steady highway speeds let the engine run at a more efficient, consistent burn.

This source-level thinking sets up the rest of the unit: once you know what comes out of a tailpipe or smokestack, you can track what sunlight and atmospheric chemistry do to it next, forming ozone and smog, and how scrubbers, catalytic converters, and inversions change how much of it actually reaches people.