Practice question

Photochemical smog develops when volatile organic compounds react through radical chemistry with nitrogen oxides in sunlight. Nitrogen dioxide photolysis releases oxygen atoms that form ozone, while reactions involving VOC-derived peroxy radicals regenerate nitrogen dioxide without consuming ozone. The resulting mixture includes ozone, aldehydes, and peroxyacyl nitrates rather than being a simple accumulation of primary emissions. Effective management combines cleaner fuels, emission standards, monitoring, urban planning, and protection of populations with high exposure or physiological vulnerability. The distinction is ecologically important because emission control must target either the directly released precursor or the product formed after atmospheric transport and reaction. Exposure depends on concentration, duration, ventilation, weather, and individual susceptibility, so a pollutant's presence alone does not fully predict biological harm. Atmospheric inversions can trap contaminants near the surface, while wind, turbulence, precipitation, and chemical transformation determine their eventual distribution and removal. At organismal level, respiratory surfaces are especially exposed because large volumes of air contact thin, moist epithelia designed for rapid gas exchange.