Practice question

Acid precipitation is conventionally defined as precipitation below about pH 5.6, the natural acidity expected when carbon dioxide equilibrates with rainwater. A value of 5.2 is certainly acidic and some monitoring or textbook schemes use approximately 5.2 as an operational cutoff, but it is not the standard general threshold. Thus the keyed value is defensible only under a narrower convention and is inconsistent with the workbook's row 16. At organismal level, respiratory surfaces are especially exposed because large volumes of air contact thin, moist epithelia designed for rapid gas exchange. 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.