An air stagnation index to qualify extreme haze events in northern China

Stagnation weather affects atmospheric diffusion ability, and hence causes the occurrence of haze events, which have been happening frequently in northern China (NC). This work puts forward an air stagnation index (ASI(TS)) to characterize the stagnation weather in NC, in which the processes of ventilation, vertical diffusion, and wet deposition potency are concerned. ASI(TS) can be applied to analyze air stagnation conditions with daily to monthly time scale. It is shown that the ASI(TS) and particulate matter smaller than 2.5 m in diameter (PM2.5) concentrations own similar lognormal probability distribution functions on both daily and monthly time scales. And the correlation analyses between the ASI(TS) and PM2.5 concentrations indicate that the ASI(TS) can reflect the monthly and daily variations in PM2.5 concentrations in NC. In addition, ASI(TS) could be used as a leading predictor of haze events since correlation coefficients of ASI(TS) leading PM2.5 concentrations by 1 day were significant and were larger than simultaneous correlation coefficients in almost all areas in NC. The robust relationship between ASI(TS) and PM2.5 concentrations exists possibly because the index can reflect the activities of synoptic systems. ASI(TS) could be a useful statistical indicator for variations in PM2.5 concentrations and haze events, and a good tool in analyzing the relationship between climate change and long-term variations in haze in NC.