EC-27. Measurement and Model Evaluation of N2O5 Heterogeneous Chemistry in the Upper Troposphere and Lower Stratosphere

The chemistry of dinitrogen pentoxide (N2O5) and nitryl chloride (ClNO2) strongly influences the abundance and distribution of nitrogen oxides (NO + NO2), ozone (O3) and chlorine atoms (Cl) in the atmosphere. N2O5 and ClNO2 have been extensively studied in the lower troposphere, which has revealed a large range in the N2O5 heterogeneous reaction probability (γ(N2O5)) and the yield of ClNO2 (φ(ClNO2)) from N2O5 heterogeneous uptake, dependent on aerosol particle composition and meteorological conditions. In the stratosphere, heterogeneous uptake of N2O5 regulates partitioning of reactive nitrogen with subsequent impacts on ozone chemistry. Stratospheric ClNO2 production is unconstrained by in-situ observations but it may act as a source of reactive halogens, contributing to ozone destruction cycles. Here we present in-situ measurements of N2O5 and ClNO2 mixing ratios in the upper troposphere and lower stratosphere (UT/LS) made from the NASA DC-8 aircraft during the Atmospheric Tomography (ATom) campaign, which sampled the global atmosphere from 0.2 to 14 km altitude over the Atlantic and Pacific Oceans during 2017 and 2018. Mixing ratios of N2O5 above 7 km during ATom ranged from below the instrument detection limit (<1 ppt for 1 s measurements) to approximately 30 pptv, while ClNO2 ranged from below the detection limit to approximately 7 pptv. Both molecules show an apparent enhancement in stratospheric air. We use observationally constrained box modelling of the ATom measurements to derive γ(N2O5) and explore the correlation of γ(N2O5) with aerosol water content and composition. Finally, we will describe a newly developed chemical ionization mass spectrometer for stratospheric sampling which will enable measurements of N2O5, ClNO2, and other reactive halogen species at altitudes of up to 19 km.