An in-situ flow-tube system for direct measurement of N<sub>2</sub>O<sub>5</sub> heterogeneous uptake coefficients in polluted environments

Abstract. The heterogeneous reactivity of dinitrogen pentoxide (N2O5) on ambient aerosols plays a key role in atmospheric fate of NOx and formation of secondary pollutants. To better understand the reactive uptake of N2O5 on complex ambient aerosols, an in-situ experimental approach to direct measurement of N2O5 uptake coefficient (γN2O5) was developed for application in environments with high, variable ambient precursors. The method utilizes an aerosol flow-tube reactor coupled with an iterative chemical box model to derive γN2O5 from the depletion of synthetically generated N2O5 when mixed with ambient aerosols. Laboratory tests and model simulations were performed to characterize the system and the factors affecting γN2O5, including mean residence time, wall loss variability with RH, and N2O5 formation and titration with high levels of NO/NOx/O3. The overall uncertainty was estimated to be 9 %–17 % at γN2O5 of 0.03 for RH varying from 20 % to 70 %. The results indicate that this flow tube coupled with the iterative model method could be buffered to NO concentrations below 8 ppbv and against air mass fluctuations switching between aerosol and non-aerosol modes. The system was then deployed in the field to test its applicability under conditions of high ambient NO2/O3 and fresh NO emission. The results demonstrate that the iterative model improved the accuracy of γN2O5 calculations under polluted environments, and thus support the further field deployment of the system to study the impacts of heterogeneous N2O5 reactivity on photochemistry and aerosol formation.