Efficiency of N2O5 heterogeneous uptake over Los Angeles during CalNex 2010

The nighttime nitrogen oxides, NO3 and N2O5, are important in numerous atmospheric chemical cycles that affect regional air quality. Key to understanding these cycles is the heterogeneous reaction of N2O5 on aerosol, which governs the rate of nighttime NOx loss, halogen activation through nighttime ClNO2 production and the availability of NO3 as an oxidant for highly reactive VOCs. Previous field investigations have found the efficiency of this heterogeneous uptake to be highly variable. Here we present an analysis of nighttime P-3 aircraft data over Los Angeles and adjacent desert and marine areas during CalNex 2010 to understand the factors that govern N2O5 heterogeneous uptake. The analysis will focus on the role of aerosol composition, relative humidity and marine stratus clouds. It will also illustrate the variability with altitude within the Los Angeles basin, where there were clear differences in nighttime chemistry between the shallow nocturnal boundary layer and the residual layer above it. This variability has implications for halogen activation through ClNO2 formation, which has recently been shown to be important in Los Angeles. Finally, the presentation will compare N2O5 heterogeneous chemistry during CalNex with findings from other recent field intensives.