Nighttime radical chemistry in the San Joaquin Valley

Differential optical absorption spectroscopy (DOAS) was used to measure the concentrations of the nitrate radical (N03) and nitrogen dioxide (N02), as part of the San Joaquin Valley Air Quality Study in central California. During 27 nights of measurements in July and August, 1990, the N03 concentration was found to be highly variable with a maximum of 80 parts per trillion by volume (ppt). The average nighttime N03 concentration profile, taken from 15 nights of continuous measurements, exhibits a maximum of 31 ppt about an hour after sunset, and then decreases slowly to sunrise. These concentrations of NO3 indicate that the nighttime oxidation of many organic compounds may be at least as fast as their oxidation by the hydroxyl radical (OH) during daytime. The atmospheric lifetime of N03 was less than 10 min, with an average value of about 3 min. This short lifetime is most likely caused by the heterogeneous loss of nitrogen pentoxide (N2O5) onto moist aerosols, supplemented by the reactions of N03 with olefinic hydrocarbons. These pathways make a significant contribution to the removal of nitrogen oxides from the lower troposphere.