Empirical evidence for the low- and high-NOx photochemical regimes of sulfate and nitrate formation

The formation of sulfate and nitrate in eastern North America is chemically linked to the abundance of oxidants and therefore to the emissions of nitrogen oxides (NOx). Depending on conditions, NOx reacts under either of two distinct photochemical regimes, defined by the types and levels of radical production. In the low-NOx regime (typical of summer), nitrogen dioxide (NO2) is readily consumed during nitric acid formation, leaving an excess of radicals that recombine to form peroxides and a highly oxidizing state favorable to sulfate formation. On the other hand, under high-NOx conditions (as in winter), the oxidizing capacity of the atmosphere is reduced because the NO2 rapidly combines with and thereby depletes hydroxyl radicals, producing nitric acid, but few peroxides. The distinction between these two chemical regimes is crucial for interpreting atmospheric deposition data because it determines whether sulfate or nitrate is the dominant acidifying component. Evidence for these regimes is gained from seasonal observations of sulfate and nitrate in rain samples collected at several sites of the National Atmospheric Deposition Program (NADP). A simple modeling exercise elucidates the processes by which sulfate and nitrate are formed under the high- and low-NOx states.