An aircraft based three channel broadband cavity enhanced absorption spectrometer for simultaneous measurements of NO₃, N₂O₅ and NO₂

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 NO₃, N₂O₅ (by detection of NO₃ after thermal dissociation of N₂O₅), H₂O and NO₂ by characterising the wavelength dependent optical attenuation within ambient samples by molecular absorption around 662 nm (NO₃ and H₂O) and 445 nm (NO₂). 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 NO₃ concentrations. The paper also details the first example of airborne measurements of NO₃, N₂O₅ and NO₂ 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 NO₂ measurements to those reported concurrently by a photolytic chemiluminescence based detector.