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

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.