Extreme haze episodes repeatedly shrouded Beijing during the winter of 2012-2013, causing major environmental and health problems. To better understand these extreme events, we performed a model-assisted analysis of the hourly observation data of PM2.5 and its major chemical compositions. The synthetic analysis shows that (1) the severe winter haze was driven by stable synoptic meteorological conditions over northeastern China, and not by an abrupt increase in anthropogenic emissions. (2) Secondary species, including organics, sulfate, nitrate, and ammonium, were the major constituents of PM2.5 during this period. (3) Due to the dimming effect of high loading of aerosol particles, gaseous oxidant concentrations decreased significantly, suggesting a reduced production of secondary aerosols through gas-phase reactions. Surprisingly, the observational data reveals an enhanced production rate of secondary aerosols, suggesting an important contribution from other formation pathways, most likely heterogeneous reactions. These reactions appeared to be more efficient in producing secondary inorganics aerosols than organic aerosols resulting in a strongly elevated fraction of inorganics during heavily polluted periods. (4) Moreover, we found that high aerosol concentration was a regional phenomenon. The accumulation process of aerosol particles occurred successively from cities southeast of Beijing. The apparent sharp increase in PM2.5 concentration of up to several hundred mu g m(-3) per hour recorded in Beijing represented rapid recovery from an interruption to the continuous pollution accumulation over the region, rather than purely local chemical production. This suggests that regional transport of pollutants played an important role during these severe pollution events

Abstract. Severe regional haze pollution events occurred in eastern and central China in January 2013, which had adverse effects on the environment and public health. Extremely high levels of particulate matter with aerodynamic diameter of 2.5 µm or less (PM 2.5 ) with dominant components of sulfate and nitrate are responsible for the haze pollution. Although heterogeneous chemistry is thought to play an important role in the production of sulfate and nitrate during haze episodes, few studies have comprehensively evaluated the effect of heterogeneous chemistry on haze formation in China by using the 3-D models due to of a lack of treatments for heterogeneous reactions in most climate and chemical transport models. In this work, the WRF-CMAQ model with newly added heterogeneous reactions is applied to East Asia to evaluate the impacts of heterogeneous chemistry and the meteorological anomaly during January 2013 on regional haze formation. As the parameterization of heterogeneous reactions on different types of particles is not well established yet, we arbitrarily selected the uptake coefficients from reactions on dust particles and then conducted several sensitivity runs to find the value that can best match observations. The revised CMAQ with heterogeneous chemistry not only captures the magnitude and temporal variation of sulfate and nitrate, but also reproduces the enhancement of relative contribution of sulfate and nitrate to PM 2.5 mass from clean days to polluted haze days. These results indicate the significant role of heterogeneous chemistry in regional haze formation and improve the understanding of the haze formation mechanisms during the January 2013 episode.

A regional haze with daily PM 2.5 (fine particulate matters with diameters less than 2.5 μm)exceeding 500 μg/m 3 lasted for several days in January 2013 over North China, offering an opportunity to evaluate models. Observations show that inorganic aerosols (sulfate, nitrate, and ammonium) are the largest contributor to PM 2.5 during the haze period, while sulfate shows the largest enhancement ratio of 5.4 from the clean to haze period. The nested-grid GEOS-Chem model reproduces the distribution of PM 2.5 and simulates up to 364 μg/m 3 of daily maximum PM 2.5 . Yet on average, the model is a factor of 3 and 4 lower in PM 2.5 and fails to capture the large sulfate enhancement from the clean to haze period. A doubling of SO 2 emissions over North China, along with daily meteorology corrections, would be required to reconcile model results with surface SO 2 observations, but it is not sufficient to explain the model discrepancy in sulfate. Heterogeneous uptake of SO 2 on deliquesced aerosols is proposed as an additional source of sulfate under high-relative humidity conditions during the haze period. Parameterizing this process in the model improves the simulated spatial distribution and results in a 70% increase of sulfate enhancement ratio and a 120% increase in sulfate fraction in PM 2.5 . Combined adjustments in emissions, meteorology, and sulfate chemistry lead to higher sulfate by a factor of 3 and 50% higher PM 2.5 , significantly reducing the model's low bias during the haze.