Direct measurement of N2O5 heterogeneous uptake coefficients on atmospheric aerosols in southwestern China and evaluation of current parameterizations

Abstract

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is a critical process in assessing NOx fate and secondary pollutant formation. However, accurate quantification of the N2O5 uptake coefficient (γ (N2O5)) in ambient conditions is a challenging problem that can cause unpredictable uncertainties in the predictions of air quality models. Here, the γ (N2O5) values were directly measured using an improved in situ aerosol flow tube system at a site located in a highland region in southwestern China to investigate influencing factors and the performance of current γ (N2O5) parameterizations under this typical environmental condition. The nocturnal mean γ (N2O5) value ranged from 0.0018 to 0.12 with an average of 0.023±0.021. The relationship between the measured γ (N2O5) and impacting factors was consistent with previous laboratory results, except for aerosol chloride. The aerosol water significantly promoted N2O5 uptake, while particulate organics and nitrate showed suppression effects. We found that several parameterizations can capture the median of measured values, whereas none of the 10 parameterizations were able to reproduce the variabilities and showed poor correlations (R2Combining double low line0.00-0.09). Elevated biases of predictions specifically occurred at high aerosol liquid water content (ALWC) (> 35 M) and low ALWC (< 25 M) levels with an underestimation of-37 % to-1 % and an overestimation of 34 % to 189 %, respectively. Such differences between the measured and parameterized γ (N2O5) would lead to a biased estimation (-77 % to 74 %) of the particulate nitrate production potential. Our findings suggest the need for more direct field quantifications of γ (N2O5) and laboratory measurements under extreme ALWC conditions to re-evaluate the response coefficients between γ (N2O5) and aerosol chemical compositions in parameterizations.