Abstract

Abstract. Aerosol chemical composition was found to influence nighttime atmospheric chemistry during a series of airborne measurements in northwestern Europe in summer conditions, which has implications for regional air quality and climate. The uptake of dinitrogen pentoxide, γ (N2O5), to particle surfaces was found to be modulated by the amount of water content and ammonium nitrate present in the aerosol. The conditions prevalent in this study suggest that the net uptake rate of N2O5 to atmospheric aerosols was relatively efficient compared to previous studies, with γ (N2O5) values in the range 0.01–0.03. This is likely a consequence of the elevated relative humidity in the region, which promotes greater aerosol water content. Increased nitrate concentrations relative to particulate water were found to suppress N2O5 uptake. The results presented here contrast with previous ambient studies of N2O5 uptake, which have generally taken place in low-nitrate environments in the USA. Comparison of the N2O5 uptake derived from the measurements with a parameterised scheme that is based on the ratio of particulate water to nitrate yielded reasonably good agreement in terms of the magnitude and variation in uptake, provided the effect of chloride was neglected. An additional suppression of the parameterised uptake is likely required to fully capture the variation in N2O5 uptake, which could be achieved via the known suppression by organic aerosol. However, existing parameterisations representing the suppression by organic aerosol were unable to fully represent the variation in N2O5 uptake. These results provide important ambient measurement constraint on our ability to predict N2O5 uptake in regional and global aerosol models. N2O5 uptake is a potentially important source of nitrate aerosol and a sink of the nitrate radical, which is the main nocturnal oxidant in the atmosphere. The results further highlight the importance of ammonium nitrate in northwestern Europe as a key component of atmospheric composition in the region.

Highlights

  • Aerosols play an important role in nighttime atmospheric chemistry, as they provide an effective sink for reactive oxidised nitrogen via the uptake of N2O5 to particle surfaces (e.g. Chang et al, 2011)

  • The influence of aerosol chemical composition on N2O5 uptake has been studied based on airborne measurements during nighttime conditions in NW Europe

  • Aerosol water content and ammonium nitrate were found to be the major controls on N2O5, with a suppression of γ (N2O5) in regions containing elevated nitrate concentrations

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Summary

Introduction

Aerosols play an important role in nighttime atmospheric chemistry, as they provide an effective sink for reactive oxidised nitrogen via the uptake of N2O5 to particle surfaces (e.g. Chang et al, 2011). Aerosols play an important role in nighttime atmospheric chemistry, as they provide an effective sink for reactive oxidised nitrogen via the uptake of N2O5 to particle surfaces Chang et al, 2011). N2O5 maintains an equilibrium with the nitrate radical, NO3 NO3 serves as one of the most chemically important species in the nocturnal atmosphere by acting as the main oxidant Atmospheric aerosols can serve as an indirect loss of NO3, via heterogeneous uptake of N2O5 to their surfaces, resulting in its removal from the gas phase: NO2 + O3 → NO3 + O2 (R1) NO3 + NO2 → N2O5 (R2) N2O5 → NO2 + NO3 (R3). Net: 2NO2 + O3 + H2O(het) → 2HNO3 + O2.

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