Radicals in the marine boundary layer during NEAQS 2004: a model study of day-time and night-time sources and sinks

Abstract

This paper describes a modelling study of several HO_x and NO_x species (OH, HO₂, organic peroxy radicals, NO₃ and N₂O₅) in the marine boundary layer. A model based upon the Master Chemical Mechanism (MCM) was constrained to observations of chemical and physical parameters made onboard the NOAA ship R/V <i>Brown</i> as part of the New England Air Quality Study (NEAQS) in the summer of 2004. The model was used to calculate [OH] and to determine the composition of the peroxy radical pool. Modelled [NO₃] and [N₂O₅] were compared to in-situ measurements by Cavity Ring-Down Spectroscopy. The comparison showed that the model generally overestimated the measurements by 30–50%, on average. <br><br> The model results were analyzed with respect to several chemical and physical parameters, including uptake of NO₃ and N₂O₅ on fog droplets and on aerosol, dry deposition of NO₃ and N₂O₅, gas-phase hydrolysis of N₂O₅ and reactions of NO₃ with NMHCs and peroxy radicals. The results suggest that fog, when present, is an important sink for N₂O₅ via rapid heterogeneous uptake. The comparison between the model and the measurements were consistent with values of the heterogeneous uptake coefficient of N₂O₅ (γ_N₂O₅)&gt;1&times;10^&minus;2, independent of aerosol composition in this marine environment. The analysis of the different loss processes of the nitrate radical showed the important role of the organic peroxy radicals, which accounted for a significant fraction (median: 15%) of NO₃ gas-phase removal, particularly in the presence of high concentrations of dimethyl sulphide (DMS).