An experimental technique for the direct measurement of N2O5 reactivity on ambient particles

An experimental approach for the direct measurement of trace gas reactivity on ambient aerosol particles has been developed. The method utilizes a newly designed entrained aerosol flow reactor coupled to a custombuilt chemical ionization mass spectrometer. The experimental method is described via application to the measurement of the N2O5 reaction probability, gamma(N2O5). Laboratory investigations on well characterized aerosol particles show that measurements of gamma(N2O5) observed with this technique are in agreement with previous observations, using conventional flow tube methods, to within +/- 20% at atmospherically relevant particle surface area concentrations (0-1000 mu m(2) cm-(3)). Uncertainty in the measured gamma(N2O5) is discussed in the context of fluctuations in potential ambient biases ( e. g., temperature, relative humidity and trace gas loadings). Under ambient operating conditions we estimate a single-point uncertainty in gamma(N2O5) that ranges between +/- (1.3 x 10(-2) + 0.2 x gamma(N2O5)), and +/- (1.3 x 10(-3) + 0.2 x gamma(N2O5)) for particle surface area concentrations of 100 to 1000 mu m(2) cm(-3), respectively. Examples from both laboratory investigations and field observations are included alongside discussion of future applications for the reactivity measurement and optimal deployment locations and conditions.