An in situ flow tube system for direct measurement of N₂O₅ heterogeneous uptake coefficients in polluted environments

The heterogeneous reactivity of dinitrogen pentoxide (N₂O₅) on ambient aerosols plays a key role in the atmospheric fate of NO_x 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 relative humidity (RH), and N₂O₅ formation and titration with high levels of NO, NO_x, and O3. The overall uncertainty was estimated to be 37%-40% 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 8ppbv 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 and O3 and fresh NO emission. The results demonstrate that the iterative model improved the accuracy of γN₂O₅ calculations in 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.