Numerical Simulation of a Heavy Fog-Haze Episode over the North China Plain in January 2013

A heavy fog-haze episode occurred over the North China Plain during 11-14 January 2013. To investigate the reason for its formation, the Weather Research and Forecasting (WRF)-Chem model was applied to simulate the temporal and spatial distribution features of air pollutants during this fog-haze episode. Compared with observations, the model can reasonably predict temporal and spatial variation of visibility, meteorological conditions such as temperature, relative humidity (RH), wind, and precipitation, and the surface concentration of PM2.5 (particle matter 2.5 mm or less in aerodynamic diameter) in January 2013 over the North China Plain. The model results show that during the study period, high PM2.5 persisted over southern and eastern parts of Hebei Province and the Tianjin and Beijing areas with the highest daily average value ranging from approximately 400－500 μg m^-3. The analysis of historical climate data from 1951 to 2013 indicated a large positive anomaly (20%－40%) of RH and a negative anomaly (－1 m s^-1) of wind speed during the study period. The sounding data also showed the presence of temperature inversion layers over Beijing with a difference of approximately 5 ℃ and high RH maintained at 80%－90% under 1 km. The model results show that during the study period, water vapor was transported with southerly or easterly wind over the near-surface layer and that dust aerosol was transported with northwesterly wind over the upper layer (850 hPa) to the North China Plain. Such meteorological conditions are favorable for the hygroscopic growth and accumulation of aerosols. The balance analysis of nitrate shows that over the Beijing area, a significant increase in chemistry and transport during the nighttime of 10-14 January 2013, contributed to nitrate concentration and the formation of heavy fog-haze.