Practice question

Nitric oxide contributes indirectly to acid precipitation after atmospheric oxidation. It rapidly reacts to form nitrogen dioxide, which is further converted through radical chemistry to nitric acid; nitrate then returns in rain, snow, particles, or dry deposition. Nitric oxide itself is therefore a precursor rather than the acid deposited. Ammonia can modify acidity by neutralisation and ammonium formation, but the listed combustion-related nitrogen oxide best fits. 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.