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. The Air Stagnation Index (ASI) is a vital meteorological measure ofthe atmosphere's ability to dilute air pollutants. The original metricadopted by the US National Climatic Data Center (NCDC) is found to be notvery suitable for China, because the decoupling between the upper and loweratmospheric layers results in a weak link between the near-surface airpollution and upper-air wind speed. Therefore, a new threshold for theASI–Boundary-layer air Stagnation Index (BSI) is proposed, consisting ofdaily maximal ventilation in the atmospheric boundary layer, precipitation,and real latent instability. In the present study, the climatologicalfeatures of the BSI are investigated. It shows that the spatial distributionof the BSI is similar to the ASI; that is, annual mean stagnations occur mostoften in the northwestern and southwestern basins, i.e., the Xinjiang andSichuan basins (more than 180 days), and least over plateaus, i.e., theQinghai–Tibet and Yunnan plateaus (less than 40 days). However, the seasonalcycle of the BSI is changed. Stagnation days under the new metric areobserved to be maximal in winter and minimal in summer, which is positivelycorrelated with the air pollution index (API) during 2000–2012. Thecorrelations between the BSI and the concentration of fine particulatematter (PM 2.5 ) during January 2013 and November to Decemberin 2015–2017 of Beijing are also investigated. It shows that the BSI matchesthe day-by-day variation of PM 2.5 concentration very well and is able tocatch the haze episodes.

Climatological study of the Boundary-layer air Stagnation Index for China and its relationship with air pollution