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

Abstract. 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 custom-built chemical ionization mass spectrometer. The experimental method is described via application to the measurement of the N₂O₅ reaction probability, γ (N₂O₅). Laboratory investigations on well characterized aerosol particles show that measurements of γ (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 μm² cm⁻³). Uncertainty in the measured γ (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 γ (N₂O₅) that ranges between ± (1.3×10^-2 + 0.2×γ (N₂O₅)), and ± (1.3×10^-3 + 0.2×γ (N₂O₅)) for particle surface area concentrations of 100 to 1000 μm² 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.