Direct measurement of N.sub.2O.sub.5 heterogeneous uptake coefficients on ambient aerosols via an aerosol flow tube system: design, characterization and performance

An improved aerosol flow tube system coupled with a detailed box model was developed to measure N.sub.2 O.sub.5 heterogeneous uptake coefficients (γ(N.sub.2 O.sub.5 )) on ambient aerosols directly. This system features sequential measurements of N.sub.2 O.sub.5 concentrations at both the entrance and exit of the flow tube to ensure an accurate retrieval of N.sub.2 O.sub.5 loss in the flow tube. Simulation and laboratory tests demonstrate that this flow tube system is able to overcome the interference from side reactions led by varying reactants (e.g., NO.sub.2, O.sub.3 and NO) and to improve the robustness of results with the assistance of the box model method. Factors related to γ(N.sub.2 O.sub.5) derivation - including particle transmission efficiency, mean residence time in the flow tube and the wall loss coefficient of N.sub.2 O.sub.5 - were extensively characterized for normal operating conditions. The measured γ(N.sub.2 O.sub.5) on (NH.sub.4).sub.2 SO.sub.4 model aerosols were in good agreement with literature values over a range of relative humidity (RH). The detection limit of γ(N.sub.2 O.sub.5) was estimated to be 0.0016 at a low aerosol surface concentration (S.sub.a) condition of 200 µm.sup.2 cm.sup.-3 . Given the instrument uncertainties and potential fluctuation of air mass between successive sampling modes, we estimate the overall uncertainty of γ(N.sub.2 O.sub.5) as ranging from 16 % to 43 % for different ambient conditions. This flow tube system was then successfully deployed for field observations at an urban site of Beijing that is influenced by anthropogenic emissions. The performance in field observations demonstrates that the current setup of this system is capable of obtaining robust γ(N.sub.2 O.sub.5) amid the switch of air mass.