NO3 Induced Nighttime Air Chemistry

In this study the role of nitrate radical (NO3) is evaluated and its effects on the transformation and removal of atmospheric compounds. This study provides the first continuous measurements of nitrate radical over a multi-day period in an arid urban location over the United States using a Differential Optical Absorbance Spectrometry (DOAS). The nitrate radical plays an important role in the nighttime chemistry of the troposphere. Formed through the reaction of NO2 with O3, nitrate reacts rapidly to oxidize aldehydes and alkenes, and converts reactive nitrogen oxides to unreactive nitric acid. Averaged over 24 hours, NO3 may be as important as daytime HO for direct oxidation of alkenes and aldehydes. The formation of NO3 can be an indirect sink of reactive nitrogen oxides if it reacts with NO2 to form dinitrogen pentoxide (N2O5). If liquid water coated surfaces are present than the N2O5 will be converted to nitric acid and thus reduce N2O5 and consequently NO3 mixing ratios. Measurements made under arid conditions allow NO3 mixing ratios to be observed under conditions of lower relative humidity then possible elsewhere. Arid conditions are a new chemical regime for NO3 because its conversion to N2O5, which in turn reacts with liquid water, placing a significant limit on its lifetime. The Differential Optical Absorbance Spectrometry (DOAS) was also used to measure mixing ratios of NO2, HCHO and HONO in addition to instrumentation measuring ozone, volatile organic compounds (including aldehydes), nitrogen oxides, SO2, SO42-, aerosol size and number density, and meteorological variables under desert conditions during the summer of 2008. Preliminary results are presented.