Nighttime nitrate radical chemistry at Appledore Island, Maine during the 2004 International Consortium for Atmospheric Research on Transport and Transformation

[1] Trace gases including nitrogen dioxide (NO2), nitrate radical (NO3), ozone (O3), and a suite of volatile organic compounds (VOCs) were measured within the New England coastal marine boundary layer on Appledore Island (AI), Maine, USA as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign. These measurements, together with local meteorological records and published kinetic data were used to investigate nighttime NO3 chemistry at AI during the period of 8–28 July 2004. Among the VOCs, isoprene, monoterpenes and dimethylsulfide (DMS) were the dominant NO3 reactants; on average, DMS accounted for 51 ± 34% of the total reactivity. For three case studies, NO3 mixing ratios were calculated from measured parameters with resultant uncertainties of 30%. Discrepancies with measured NO3 appeared to result primarily from input parameter variability and exclusion of heterogeneous dinitrogen pentoxide (N2O5) chemistry. We indirectly determined that nighttime NO3 and NOx (=NO + NO2) removal via N2O5 chemistry (gas-phase + heterogeneous) was on average 51–54% and 63–66% of the total respectively. Our analysis suggested that the minimum average NO3 and NOx removal via heterogeneous N2O5 chemistry was 10% of the total. Reducing gas-phase N2O5 reactivity in accord with Brown et al. (2006a) increased the importance of heterogeneous N2O5 chemistry substantially. It is plausible that the latter pathway was often comparable to gas-phase removal of NO3 and NOx. Overall, 24 h-averaged NOx removal was 11 ppbv, with nighttime chemical pathways contributing 50%.