Nighttime enhancement of PM2.5 nitrate in ammonia-poor atmospheric conditions in Beijing and Shanghai: Plausible contributions of heterogeneous hydrolysis of N2O5 and HNO3 partitioning

In our previous study, exceptionally high concentrations of nitrate (24-h average of up to 42 μg m−3) were found in ammonium-deficient samples of PM2.5 during summer 2005 near Beijing and Shanghai. The formation of such high levels of nitrate in ammonium-poor PM2.5 cannot be explained by the homogeneous gas-phase reaction mechanism involving ammonia and nitric acid. Hence, we postulated that high levels of nitrate were formed via the heterogeneous hydrolysis of N2O5. In the present study, we further examine the formation of nitrate using hourly measurements of sulfate and nitrate and a multiphase chemistry box model (RACM + CAPRAM). Enhancements of nitrate and the NO3−/NOy ratio were observed at nighttime when sulfate and NOy concentrations remained relatively stable. These enhancements and a concomitant shift in the size of the nitrate concentration peak to a bin larger than that of sulfate peak suggested the possibility that nitrate was formed on pre-existing sulfate aerosol surfaces. A multiphase chemistry box model was set up to simulate the observed enhancement of nitrate at nighttime. The model was constrained by the measured concentrations of sulfate, nitrate, ammonium and O3 and NOy, and estimated acidity and aerosol water content using a thermodynamic model (AIM). Assuming an NO2/NOy ratio in the range of 0.2–0.4 for Beijing and 0.4–0.6 for Shanghai, the model results suggest that the heterogeneous hydrolysis of N2O5 contributed 50%–100% of the nighttime enhancement of nitrate concentration. Sensitivity analysis shows that increasing the aerosol water content significantly favors the partitioning of nitrate in the aerosol phase that was formed via the hydrolysis of N2O5 at nighttime.