Aircraft observations of daytime NO3 and N2O5 and their implications for tropospheric chemistry

The nitrate radical (NO3) and dinitrogen pentoxide (N2O5) are normally considered only in the context of nighttime atmospheric chemistry. Although their importance during the day is often small, it is not always negligible. Here, we present daylight observations of both compounds from the NOAA P-3 aircraft taken during the New England Air Quality Study in the summer of 2004. Observed mixing ratios agreed with predictions from a simple, rapidly established daytime steady state, although observed and calculated mixing ratios of NO3 were near the instrumental detection limit. The observations have several implications for tropospheric chemistry during the daytime, including the loss of ozone through photolysis of NO3 to NO + O2, oxidation of biogenic volatile organic compounds (VOC), and conversion of NOx (=NO + NO2) to HNO3 via N2O5 hydrolysis. The magnitude of each process is considered in comparison to its photochemically driven analog. For example, NO3 oxidation of α-pinene in the presence of NOx contributes 10–40% of the total daytime oxidation rate. Hydrolysis of N2O5 increases the daytime conversion rate of NOx to HNO3 by several percent, with a maximum of 13%, relative to OH + NO2. The implications of these observations for daytime NO3 and N2O5 reactions in a variety of locations and seasons are discussed.