Simulations of sulfate–nitrate–ammonium (SNA) aerosols during the extreme haze events over northern China in October 2014

Abstract

Abstract. Extreme haze events have occurred frequently over China in recent years. Although many studies have investigated the formation mechanisms associated with PM2.5 for heavily polluted regions in China based on observational data, adequately predicting peak PM2.5 concentrations is still challenging for regional air quality models. In this study, we evaluate the performance of one configuration of the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) and use the model to investigate the sensitivity of heterogeneous reactions on simulated peak sulfate, nitrate, and ammonium concentrations in the vicinity of Beijing during four extreme haze episodes in October 2014 over the North China Plain. The highest observed PM2.5 concentration of 469 µg m−3 occurred in Beijing. Comparisons with observations show that the model reproduced the temporal variability in PM2.5 with the highest PM2.5 values on polluted days (defined as days in which observed PM2.5 is greater than 75 µg m−3), but predictions of sulfate, nitrate, and ammonium were too low on days with the highest observed concentrations. Observational data indicate that the sulfur/nitric oxidation rates are strongly correlated with relative humidity during periods of peak PM2.5; however, the model failed to reproduce the highest PM2.5 concentrations due to missing heterogeneous/aqueous reactions. As the parameterizations of those heterogeneous reactions are not well established yet, estimates of SO2-to-H2SO4 and NO2/NO3-to-HNO3 reaction rates that depend on relative humidity were applied, which improved the simulation of sulfate, nitrate, and ammonium enhancement on polluted days in terms of both concentrations and partitioning among those species. Sensitivity simulations showed that the extremely high heterogeneous reaction rates and also higher emission rates than those reported in the emission inventory were likely important factors contributing to those peak PM2.5 concentrations.

Emission or atmospheric processes? An attempt to attribute the source of large bias of aerosols in eastern China simulated by global climate models

“…Observations and regional air quality modeling with more complex chemistry reveal the importance of sulfate production on mineral dust through gas-phase uptake or heterogeneous reactions in increasing the PM 2.5 concentrations and the mass fractions of secondary inorganic aerosols Huang et al, 2014;Zheng et al, 2015;Dong et al, 2016). The coexistence of NO 2 and SO 2 promotes heterogeneous production of sulfate aerosol under high-relative-humidity conditions Chen et al, 2016;. The aqueous-phase oxidation of SO 2 by NO 2 Cheng et al, 2016) or O 3 (Palout et al, 2016) is efficient at forming sulfate aerosol under high-relativehumidity and NH 3 neutralization conditions.…”

“…Modifications of the gas-phase chemistry and inorganic aerosol treatment in the Community Atmospheric Model version 5 (CAM5) improve the model performance for aerosol mass and AOD , but substantial low biases still exist for East Asia. Most GCMs, including CAM5, do not include the aqueousphase chemistry on preexisting particles, which proves to be important for the formation of the winter haze in northern China He et al, 2014;Huang et al, 2014;Zheng et al, 2015;Chen et al, 2016;Dong et al, 2016;Wang et al, 2016;Cheng et al, 2016). With all the abovementioned uncertainties mingled in the GCMs, it is not clear whether the emission or the aerosol processes are more responsible for the low biases of AOD simulated by GCMs in eastern China.…”

Insight into winter haze formation mechanisms based on aerosol hygroscopicity and effective density measurements

“…Indeed, a large enhancement of particulate sulfate was typically observed during regional haze events in China Wang et al, 2015;Fu et al, 2008;Xie et al, 2015). Currently, the highly elevated sulfate concentration during haze events cannot be fully explained by model simulations (Wang et al, 2014a;Chen et al, 2016). Recently, a significant breakthrough made by Wang et al (2016) has provided a reasonable explanation about the high level of sulfate during haze events.…”

The concentration, source and deposition flux of ammonium and nitrate in atmospheric particles during dust events at a coastal site in northern China

“…In the region, the dry deposition flux of the inorganic nitrogen accounted for more than 75 % for the TDN (Qi et al, 2013). When deposited to the ocean via atmospheric dry deposition, inorganic nitrogen has a great impact on marine productivity due to its bioavailability.…”