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.