Practice question

Unpolluted rain is mildly acidic because dissolved carbon dioxide forms carbonic acid, giving an expected pH near 5.6 rather than 7. Precipitation below approximately 5.6 is conventionally described as acid rain, usually after sulfuric and nitric acids add stronger acidity. Because pH is logarithmic, a fall of one pH unit represents a tenfold increase in hydrogen-ion activity. Sound classification links the source, atmospheric fate, exposure route, and mechanism of toxicity rather than relying only on the pollutant's name. 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.