Heterogeneous N 2 O 5 reactions on atmospheric aerosols at four Chinese sites: improving model representation of uptake parameters

Heterogeneous reactivity of N2O5 on aerosols is a critical parameter in assessing NOx fate, nitrate production, and particulate chloride activation. Accurate measurement of its uptake coefficient (γN2O5) and representation in air quality models are challenging, especially in the polluted environment. With an in situ aerosol flow-tube system, the γN2O5 was directly measured on ambient aerosols at two rural sites in northern and southern China. The results were analyzed together with the γN2O5 derived from previous field studies in China to obtain a holistic picture of N2O5 uptake and the influencing factors under various climatic and chemical conditions. The field-derived or measured γN2O5 was generally promoted by the aerosol water content and suppressed by particle nitrate. Significant discrepancies were found between the measured γN2O5 and that estimated from laboratory-determined parameterizations. An observation-based empirical parameterization was derived in the present work, which better reproduced the mean value and variability of the observed γN2O5. Incorporating this new parameterization into a regional air quality model (WRF-CMAQ) has improved the simulation of N2O5, nitrogen oxides, and secondary nitrate in the polluted regions of China.