Modeling particulate nitrate in China: Current findings and future directions.

Particulate nitrate (pNO) is now becoming the principal component of PM during severe winter haze episodes in many cities of China. To gain a comprehensive understanding of the key factors controlling pNO formation and driving its trends, we reviewed the recent pNO modeling studies which mainly focused on the formation mechanism and recent trends of pNO 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, model-observation biases still exist due to large uncertainties in the parameterization of dinitrogen pentoxide (NO) uptake and ammonia (NH) emissions, insufficient heterogeneous reaction mechanism, and the predicted low sulfate concentrations in current CTMs. The heterogeneous hydrolysis of NO dominates nocturnal pNO formation, however, the contribution to total pNO varies among studies, ranging from 21.0% to 51.6%. Moreover, the continuously increasing PM pNO 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 NH emissions is found to be the most effective control strategy for mitigating pNO pollution in China. This review suggests that more field measurements are needed to constrain the parameterization of heterogeneous NO and nitrogen dioxide (NO) uptake. Future studies are also needed to quantify the relationships of pNO to AOC, O, NOx, and volatile organic compounds (VOCs) in different regions of China under different meteorological conditions. Research on multiple-pollutant control strategies involving NH, NO and VOCs is required to mitigate pNO pollution, especially during severe winter haze events.