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

Abstract. The reactivity dinitrogen pentoxide( N 2 O 5 ) on aerosols plays a key role in the atmospheric fate of NO x and formation secondary pollutants. To better understand thereactive uptake N 2 O 5 on complex aerosols, an in situexperimental approach to direct measurement N 2 O 5 uptakecoefficient ( γ N 2 O 5 ) was developed for application inenvironments with high, variable precursors. The method utilizes anaerosol flow tube reactor coupled with an iterative chemical box model toderive γ N 2 O 5 from the depletion syntheticallygenerated N 2 O 5 when mixed with aerosols. Laboratorytests and model simulations were performed to characterize the system and thefactors affecting γ N 2 O 5 , including mean residence time,wall loss variability with relative humidity (RH), and N 2 O 5 formation and titrationwith high levels NO, NO x , and O 3 . The overall uncertainty wasestimated to be 37 %–40 % at γ N 2 O 5 0.03 for RHvarying from 20 % to 70 %. The results indicate that this flow tubecoupled with the iterative model method could be buffered to NOconcentrations below 8 ppbv and against air mass fluctuations switchingbetween aerosol and non-aerosol modes. The system was then deployed in thefield to test its applicability under conditions high ambient NO 2 and O 3 and fresh NO emission. The results demonstrate that theiterative model improved the accuracy γ N 2 O 5 calculations in polluted environments and thus support the further fielddeployment the system to study the impacts heterogeneous N 2 O 5 reactivity on photochemistry and aerosol formation.