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Comments on the oxidation of NO2 to nitrate—day and night

1983; Elsevier BV; Volume: 17; Issue: 2 Linguagem: Inglês

10.1016/0004-6981(83)90057-4

1878-2442

L. Willard Richards,

Atmospheric Ozone and Climate

It has long been suspected that there are two important mechanisms for the formation of atmospheric paniculate nitrates. Here it is suggested that different mechanisms operate in the day and night. Formation of nitrate (gaseous nitric acid and paniculate nitrate) in the daytime appears to be predominantly caused by the hydroxyl radical, but other reaction pathways also participate. This reaction becomes much less important at night because the hydroxyl radical is primarily formed photochemically. At night, ozone aloft reacts with NO2 to form NO3, which rapidly reacts with NO2 to form N2O5. The N2O5 reacts with water in droplets or adsorbed on surfaces to form nitrate, sometimes in large particles. This pathway is unimportant in the daytime because N2O5 is in equilibrium with NO3, which is photolyzed as well as rapidly destroyed by NO, which in turn is present whenever there is NOx and sunlight. These mechanisms can account for the observation of high concentrations of large particle nitrate in the morning as well as nitrate in smaller particles in the afternoon in Los Angeles. They also predict that NOx is very rapidly oxidized to nitrate aloft at night by the ozone that is typically present, and this has been observed in a power plant plume. Since NOx is typically oxidized to nitrate more rapidly than SO2 is oxidized to sulfate in the daytime, and NOx oxidation continues aloft at night while SO2 oxidation typically does not, the daily average rate of oxidation of NOx to nitrate is much greater than that for the oxidation of SO2 to sulfate, especially in the winter. Because gaseous nitric acid is removed from the atmosphere much more rapidly than paniculate sulfate, and some fine particle nitrates can volatilize to release nitric acid, it is expected that the geographic region over which it is possible for NOx emissions to have an impact on regional haze and deposition chemistry is much smaller than for sulfur dioxide emissions.