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.

Abstract.A three channel broadband cavity enhanced absorption spectroscopy (BBCEAS) instrument has been developed for airborne measurements of atmospheric trace gases involved in night-time oxidation chemistry and air quality. The instrument was deployed on board the Facility for Airborne Atmospheric Measurements BAe 146-301 atmospheric research aircraft during the Role of Nighttime Chemistry in Controlling the Oxidising Capacity of the Atmosphere (RONOCO) measurement campaigns between December 2009 and January 2011. In its present configuration (i.e. specifications of the cavity optics and spectrometers) the instrument is designed to measure NO 3 , N 2 O 5 (by detection of NO 3 after thermal dissociation of N 2 O 5 ), H 2 O and NO 2 by characterising the wavelength dependent optical attenuation within ambient samples by molecular absorption around 662 nm (NO 3 and H 2 O) and 445 nm (NO 2 ). This paper reports novel advancements in BBCEAS instrumentation including a refined method for performing BBCEAS mirror reflectivity calibrations using measurements of the phase delay introduced by the optical cavities to amplitude modulated radiation. Furthermore, a new methodology is introduced for fitting the strong but unresolved transitions of water vapour, which is required for accurate retrieval of water absorption features from the 662 nm absorption band used to measure NO 3 concentrations. The paper also details the first examCorrespondence to: R. L. Jones (rlj1001@cam.ac.uk) ple of airborne measurements of NO 3 , N 2 O 5 and NO 2 over Europe from a flight over the North Sea and Thames Estuary on the night of the 20 July 2010, one of the most polluted days of the RONOCO summertime flying period. As part of this analysis, the performance of the BBCEAS instrument is assessed by comparing airborne NO 2 measurements to those reported concurrently by a photolytic chemiluminescence based detector.

Dinitrogen pentoxide (N2O5) plays a central role in nighttime tropospheric chemistry as its formation and subsequent loss in sink processes limits the potential for tropospheric photochemistry to generate ozone the next day. Since accurate observational data for N2O5 are critical to examine our understanding of this chemistry, it is vital also to evaluate the capabilities of N2O5 measurement techniques through the co-deployment of the available instrumentation. This work compares measurements of N2O5 from two aircraft instruments

Abstract. The RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) aircraft campaign during July 2010 and January 2011 made observations of OH, HO 2 , NO 3 , N 2 O 5 and a number of supporting measurements at night over the UK, and reflects the first simultaneous airborne measurements of these species. We compare the observed concentrations of these short-lived species with those calculated by a box model constrained by the concentrations of the longer lived species using a detailed chemical scheme. OH concentrations were below the limit of detection, consistent with model predictions. The model systematically underpredicts HO 2 by ∼ 200 % and overpredicts NO 3 and N 2 O 5 by around 80 and 50 %, respectively. Cycling between NO 3 and N 2 O 5 is fast and thus we define the NO 3x (NO 3x = NO 3 + N 2 O 5 ) family. Production of NO 3x is overwhelmingly dominated by the reaction of NO 2 with O 3 , whereas its loss is dominated by aerosol uptake of N 2 O 5 , with NO 3 + VOCs (volatile organic compounds) and NO 3 + RO 2 playing smaller roles. The production of HO x and RO x radicals is mainly due to the reaction of NO 3 with VOCs. The loss of these radicals occurs through a combination of HO 2 + RO 2 reactions, heterogeneous processes and production of HNO 3 from OH + NO 2 , with radical propagation primarily achieved through reactions of NO 3 with peroxy radicals. Thus NO 3 at night plays a similar role to both OH and NO during the day in that it both initiates RO x radical production and acts to propagate the tropospheric oxidation chain. Model sensitivity to the N 2 O 5 aerosol uptake coefficient (γ N 2 O 5 ) is discussed and we find that a value of γ N 2 O 5 = 0.05 improves model simulations for NO 3 and N 2 O 5 , but that these improvements are at the expense of model success for HO 2 . Improvements to model simulations for HO 2 , NO 3 and N 2 O 5 can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.