Abstract
Abstract. The heterogeneous reactivity of dinitrogen pentoxide (N2O5) on ambient aerosols plays a key role in the 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 relative humidity (RH), and N2O5 formation and titration with high levels of NO, NOx, 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 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 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.
Highlights
Dinitrogen pentoxide (N2O5) is a nocturnal reactive intermediate in the atmospheric oxidation of nitrogen oxides (NOx), which plays an important role in atmospheric photochemistry and the production of secondary pollutants (e.g., Chang et al, 2011)
At the exit of the flow tube reactor, O3 was measured by a UV photometric analyzer (Thermo, Model 49i) and NO2 was measured by a chemiluminescence NOx analyzer (Thermo, Model 42i) equipped with a blue-light photolytic converter (BLC)
The method uses an aerosol flow tube reactor combined with an iterative box model, to determine the heterogeneous loss rate of synthesized N2O5 on ambient aerosols with consideration of multiple reactions affecting N2O5 in the flow tube
Summary
Dinitrogen pentoxide (N2O5) is a nocturnal reactive intermediate in the atmospheric oxidation of nitrogen oxides (NOx), which plays an important role in atmospheric photochemistry and the production of secondary pollutants (e.g., Chang et al, 2011). This flow tube apparatus was deployed at two urban sites in Boulder and one coastal site in La Jolla to measure γ N2O5 on ambient aerosols (Bertram et al, 2009b; Riedel et al, 2012) They found that the fluctuation of relative humidity (RH) and NO3 reactivity (mainly dominated by NO) could lead to great uncertainty in measured γ N2O5, and they applied some screening criteria, including only data with a RH fluctuation of less than 2 % and NO concentration lower than 750 pptv. It was necessary to adopt these criteria because only first-order loss is considered in the flow tube reactor and other reactions involving ambient NO, NO2, and O3 are not The latter treatment is suitable when ambient concentrations are low and the air mass is relatively stable, but it may be problematic in polluted environments with high fresh NOx emissions, high O3 concentrations, and rapidly changing air mass. Laboratory tests and field deployment of the method are presented to demonstrate its application under conditions with high ambient concentrations of NO2 and O3 and fresh NO emission
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