Modeling analysis of the seasonal characteristics of haze formation in Beijing

Abstract. The air quality modeling system RAMS-CMAQ (Regional Atmospheric Modeling System–Community Multiscale Air Quality), coupled with an aerosol optical property scheme, was applied to simulate the meteorological field, major aerosol components (sulfate, nitrate, ammonium, black carbon, organic carbon, dust, and sea salt), and surface visibility over the North China Plain (NCP) in 2011. The modeled results in February and July 2011 were selected and analyzed to obtain an in-depth understanding of the haze formation mechanism in Beijing for different seasons. The simulation results showed that the visibility was below 10 km for most regions of the NCP, and dropped to less than 5 km over the megacities of Beijing and Tianjin, the whole of Hebei Province, and the northwest part of Shandong Province during pollution episodes in February and July. The heavy mass concentration of PM_2.5 ranged from 120 to 300 μg m⁻³ and was concentrated in the areas with low visibility. The haze formation mechanism in Beijing in winter was different from that in summer. The mass concentration of PM_2.5 was higher, and the components more complicated, in winter. While the mass concentration of PM_2.5 in summer was lower than that in winter, the mass concentrations of hygroscopic inorganic salts were comparable with those in winter, and the relative humidity was, as expected, higher. Therefore, the water uptake of hygroscopic aerosols played a key role in summer. Moreover, the analysis showed that the influence of the PM_2.5 mass burden on visibility was very weak when its value was larger than 100 μg m⁻³. Only when the mass burden of PM_2.5 decreased to a certain threshold interval did the visibility increase rapidly. This indicates that, when emission reduction measures are taken to control haze occurrence, the mass burden of PM_2.5 must be cut to below this threshold interval. The relationship between the threshold of haze occurrence and the relative humidity in Beijing was fitted by an exponential function, and the resulting fitting curves could provide a new theoretical basis to understand and control haze formation in Beijing.