Critical role of meteorological conditions in a persistent haze episode in the Guanzhong basin, China

- The synoptic situations have the dominant role in heavy haze formation in the basin.
- Low-level inversion, weak wind, and high relative humidity contribute to the event.
- The emissions need to be mitigated by more than 91% to meet the excellent level.

In the present study, the critical role of the meteorological condition in a persistent extreme haze episode that occurred in Guanzhong basin of China on December 16 to 25, 2013 has been investigated. Analyses of the large-scale meteorological conditions on 850 hPa during the episode have been performed using the NCEP FNL data set, indicating that synoptic situations generally facilitate the accumulation of pollutants either in horizontal or vertical directions in the basin. The FLEXPART model has been utilized to illustrate the pollutant transport patterns during the episode, further showing the dominant role of synoptic conditions in accumulation of pollutants in the basin. Detailed meteorological conditions, such as temperature inversion, and low-level horizontal wind speed also contribute to the extreme haze episode. In addition, the WRF-CHEM model has been used to evaluate the responses of the surface PM2.5 level to the emission mitigation. Generally, the predicted PM2.5 spatial patterns and temporal variations agree well with the observations at the ambient monitoring sites. Sensitivity studies show that the emissions in the basin need to be mitigated by more than 91% to meet the excellent level of the China National Air Quality Standard under the extremely unfavorable meteorological conditions, demonstrating that it is imperative to implement stringent controls on emissions to improve the air quality.

The response of the average PM2.5 level in the Guanzhong basin, China to the anthropogenic emission reduction during the period from December 16 to 26, 2013. Blue line denotes the control simulation; green, red, and brown lines represent simulations with 50%, 75%, and 87.5% reductions of anthropogenic emissions, respectively; Black line is the simulation without anthropogenic emissions. The figure shows that the emissions in the basin need to be mitigated by more than 90% to meet the excellent level of the China National Air Quality Standard under the extremely unfavorable meteorological conditions.131