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.
Highlights
Anthropogenic PM2.5 is known to play a significant role in atmospheric visibility, human health, and climate
The Multi-resolution Emission Inventory for China (MEIC) (Zhang et al, 2009; Lei et al, 2011; He, 2012; Li et al, 2014) for October 2010 is used as the base emission scenario
The observed chemical concentrations used in the study are from three data sets: (1) the daily mean concentration of gas phase and PM2.5 from the Air Pollution Index (API) database in 10 cities in the North China Plain (NCP); (2) the average hourly concentrations of gasphase pollutants and PM2.5 at 34 monitoring sites in Beijing from the China National Environmental Monitoring Center (CNEMC); and (3) the hourly PM2.5 measured by TEOM and 15 min species concentrations (BC, sulfate, nitrate, ammonium) in PM1 measured in situ by an aerosol chemical speciation monitor (ACSM) at the Beijing Normal University (BNU, blue dot in Fig. 1) from Yang et al (2015)
Summary
Anthropogenic PM2.5 (fine particulate matter with aerodynamic diameters less than 2.5 μm) is known to play a significant role in atmospheric visibility, human health, and climate. Those reactions are not included in current chemical mechanisms (traditional gas-phase or aqueousphase chemistry) in most air quality models. Wang et al (2014) and Zhang et al (2015b) introduced and parameterized the heterogeneous uptake of SO2 on deliquesced aerosols in the GEOS-Chem model and Zheng et al (2015) comprehensively evaluated the effects of heterogeneous chemistry in the CMAQ model Their simulations for the conditions during the 2013 winter showed great improvements when heterogeneous chemistry was included. Using the GEOS-Chem model, Wang et al (2013) and Zhang et al (2015b) concluded that NH3 emission plays a critical role in the SNA simulations in the NCP. We parameterized the SNA relevant heterogeneous reactions in WRF-Chem and conducted simulations in the NCP for a haze period in the autumn of 2014.
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