NIGHTTIME RADICAL CHEMISTRY IN THE SAN-JOAQUIN VALLEY

Differential optical absorption spectroscopy (DOAS) was used to measur e the concentrations of the nitrate radical (NO3) and nitrogen dioxide (NO2), as part of the San Joaquin Valley Air Quality Study in central California. During 27 nights of measurements in July and August, 1990 , the NO3 concentration was found to be highly variable with a maximum of 80 parts per trillion by volume (ppt). The average nighttime NO3 c oncentration profile, taken from 15 nights of continuous measurements, exhibits a maximum of 31 ppt about an hour after sunset, and then dec reases slowly to sunrise. These concentrations of NO3 indicate that th e nighttime oxidation of many organic compounds may be at least as fas t as their oxidation by the hydroxyl radical (OH) during daytime. The atmospheric lifetime of NO3 was less than 10 min, with an average valu e of about 3 min. This short lifetime is most likely caused by the het erogeneous loss of nitrogen pentoxide (N2O5) onto moist aerosols, supp lemented by the reactions of NO3 with olefinic hydrocarbons. These pat hways make a significant contribution to the removal of nitrogen oxide s from the lower troposphere.