Influence of aerosol chemical composition on N2O5 uptake: Airborne regional measurements in North-Western Europe

Aerosol particles play an important role in night-time atmospheric chemistry as they provide an effective sink for reactive oxidised nitrogen. The heterogeneous uptake rate of dinitrogen pentoxide, N₂O₅, to atmospheric aerosols is known to be highly modulated by their chemical composition. Consequently, aerosols may significantly impact the cycling of reactive nitrogen during the night-time, which can also have consequences for day-time atmospheric chemistry. Previous ambient airborne measurements in the North-Eastern USA have shown that uptake rates are enhanced in the presence of acidified sulphate aerosols. While sulphate aerosol is still an important component of the aerosol burden in North-Western Europe, its contribution is often outweighed by that of ammonium nitrate and organic matter. Furthermore, sulphate is often present in its neutralised form due to the abundance of ammonia sources in the region. North-Western Europe therefore provides a contrasting chemical environment to compare with previous studies. This paper presents analysis of the role played by aerosol chemical composition in determining the composition of the troposphere. In-situ measurements of aerosol properties made onboard the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft will be presented. An Aerodyne Aerosol Mass Spectrometer (AMS) measured the size resolved chemical composition of non-refractory particulate matter. These measurements were complemented by a suite of gas phase instrumentation, which measured inorganic and organic acids, reactive nitrogen species and volatile organic compounds. The measurements were conducted as part of the RONOCO (ROle of Night-time chemistry in controlling the Oxidising Capacity of the atmOsphere) project. Science flights were conducted in the UK region during summer 2010 and winter 2011. Daytime measurements were also conducted in order to contrast with the night-time missions during the study periods. A large range of pollution environments were studied, including relatively clean conditions and several major pollution episodes. Uptake of N₂O₅ to aerosol particles appeared to be modulated by the ambient size distribution, which is a confluence of the physical, chemical and hygroscopic properties of the particles. The role of ammonium nitrate in this system was particularly complex, as it increases particle size via water uptake, which will increase N₂O₅ uptake but ammonium nitrate itself will suppress N₂O₅ uptake. These competing effects driven by the aerosol chemical composition have significant ramifications for the cycling of reactive nitrogen in the night-time atmosphere. Such cycling has implications for local ecosystems via the perturbation of the nitrogen cycle, regional air quality and climate change.