Modeling particulate nitrate in China: Current findings and future directions

Particulate nitrate (pNO(3)) is now becoming the principal component of PM2.5 during severe winter haze episodes in many cities of China. To gain a comprehensive understanding of the key factors controlling pNO(3) formation and driving its trends, we reviewed the recent pNO(3) modeling studies which mainly focused on the formation mechanism and recent trends of pNO(3) as well as its responses to emission controls in China. The results indicate that although recent chemical transport models (CTMs) can reasonably capture the spatial-temporal variations of pNO(3), model-observation biases still exist due to large uncertainties in the parameterization of dinitrogen pentoxide (N2O5) uptake and ammonia (NH3) emissions, insufficient heterogeneous reaction mechanism, and the predicted low sulfate concentrations in current CTMs. The heterogeneous hydrolysis of N2O5 dominates nocturnal pNO(3) formation, however, the contribution to total pNO(3) varies among studies, ranging from 21.0% to 51.6%. Moreover, the continuously increasing PM2.5 pNO(3) fraction in recent years is mainly due to the decreased sulfur dioxide emissions, the enhanced atmospheric oxidation capacity (AOC), and the weakened nitrate deposition. Reducing NH3 emissions is found to be the most effective control strategy for mitigating pNO(3) pollution in China. This review suggests that more field measurements are needed to constrain the parameterization of heterogeneous N(2)O(5 )and nitrogen dioxide (NO2) uptake. Future studies are also needed to quantify the relationships of pNO(3) to AOC, O-3, NOx, and volatile organic compounds (VOCs) in different regions of China under different meteorological conditions. Research on multiple-pollutant control strategies involving NH3, NOX, and VOCs is required to mitigate pNO(3) pollution, especially during severe winter haze events.

Automated summary

This article reviews recent modeling studies on the formation mechanism, trends, and response to emission controls of particulate nitrate (pNO(3)) in China. The results show that while chemical transport models can capture the spatial-temporal variations of pNO(3), there are still uncertainties and biases due to parameterization and model limitations. Heterogeneous hydrolysis of N2O5 dominates nocturnal pNO(3) formation, but its contribution varies among studies. The increasing pNO(3) fraction in recent years is mainly attributed to reduced SO2 emissions, enhanced atmospheric oxidation capacity, and weakened nitrate deposition. Reducing NH3 emissions is found to be the most effective control strategy. More field measurements and research are needed to understand the relationships of pNO(3) with other factors.