Describe the layered atmosphere, distinguish primary from secondary air pollutants, and identify the six criteria pollutants regulated under the Clean Air Act.
The atmosphere is layered: the troposphere (roughly the bottom 10-12 km, where weather happens), the stratosphere above it (home to the protective ozone layer, covered next week), then the mesosphere and thermosphere further out. By volume, the atmosphere is about 78% nitrogen, 21% oxygen, and under 1% argon and trace gases, including the CO2 you traced through the carbon cycle in Week 7.
Primary pollutants are emitted directly from a source -- carbon monoxide from a tailpipe, sulfur dioxide from a smokestack. Secondary pollutants form afterward through chemical reactions in the atmosphere; ground-level (tropospheric) ozone and smog are the clearest example, forming when nitrogen oxides and volatile organic compounds react in sunlight, which is why smog gets worse on hot, sunny, still days. The US Clean Air Act of 1970 -- passed with overwhelming bipartisan support and credited with sharply reducing pollution even as the economy grew -- regulates six "criteria pollutants": particulate matter (PM2.5 and PM10, named for their size in micrometers), ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.
Mastery looks like: They can correctly classify a pollutant as primary or secondary, list the six criteria pollutants, and read a given AQI scale correctly.
Common stumbles: Assuming all pollution is emitted directly (missing secondary/reaction-formed pollutants); treating "ozone" as always meaning the same thing regardless of altitude.