The nitrate radical (N03) is a strong radical oxidant formed in the oxidation of N02 by 03:
3 OTHER ATMOSPHERIC OXIDANTS 41
A detailed review of its atmospheric chemistry is given by Wayne (1991). During the daytime, N03 photolyzes on a time scale of l min to return N02:
At night the lifetime of N03 is much longer. In high-NOt regions such as the eastern United States, N03 accumulates to concentrations of 10 to 100 parts per trillion by volume (pptv) during the nighttime hours (Wayne, 1991). At these concentrations, N03 can provide an important sink for some unsaturated hydrocarbons including isoprene and terpenes (03 is also an important oxidant for these compounds). Measurements in relatively polluted marine air over the North Sea indicate a mean nighttime N03 concentration of about lOpptv; at this concentration, N03 represents a major sink for biogenic dimethylsulfide (Carslaw et al., 1997). Nighttime accumulation of N03 is in general limited by equilibrium with N205, followed by hydrolysis of N205 in aerosols:
At low temperatures (T < 280 K) the N03/N205 equilibrium is shifted far to the right; thus N03 is important only in the warm lower troposphere.
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