Links Between The Large-scale Circulation And Daily Air Quality Over Central-eastern China During Winter

Rapid economic progress and urbanization in China have introduced a number of environmental challenges,including sharp increases in the concentrations of fine particulates PM2.5 in the atmosphere.Moreover,widespread persistent haze events have occurred more frequently in central and eastern China.As an important factor affecting China's wintertime climate,variations in large-scale atmospheric circulation also have important impacts on air quality.Here,I examine the relationships between day-to-day variations in the large-scale atmospheric circulation and the occurrence,intensity,and duration of haze during several recent winters,beginning with a focused examination of Beijing in 2013-2017 and then evaluating the extent to which the findings can be applied to other urban areas in central-eastern China in 2015-2017.Composite analysis for Beijing indicated that severe pollution preferentially occurred when the East Asian Winter Monsoon(EAWM)and Siberian High(SH)were weak,but rarely occurred when the EAWM or SH were strong.This preferential occurrence was even more pronounced for extended haze events.Correlations between EAWM and SH indices and PM2.5 concentrations in Beijing at event scales were-0.75 and-0.59,respectively.The EAWM and SH indices were likewise negatively correlated with daily-average PM2.5 concentrations in urban areas through much of central-eastern China.Variations in the EAWM and SH were also cross-correlated,but with substantial independent variability and multiple indications that these indices influence PM2.5 in central-eastern China via different mechanisms.Whereas relationships between air quality and the SH are best understood through ventilation and dispersion of boundary-layer pollution,the EAWM is more strongly correlated with relative humidity,and thus particulate formation.As such,the SH index is a more effective metric for distinguishing clean days(PM2.5<75 ?g m-3)from polluted days(PM2.5 ? 75 ?g m-3),while the PV-based EAWM index is more effective for distinguishing between moderate(75 ?g m-3 ?PM2.5<150 ?g m-3)and heavy(PM2.5? 150 ?g m-3)boundary layer pollution.A simple large-scale circulation index that combines weighted EAWM and SH indices is proposed,which outperforms the atmospheric stagnation index(ASI)even after tuning conditional ASI thresholds for air quality in Beijing.Extension of this approach to 106 cities in China reveals that similar combined EAWM-SH indices,optimized and cross-validated for each location,are effective over abroad swath of central-eastern China.The 106 selected cities are mainly distributed around the North China Plain,the Northeast Plain,the Yangtze Plain,and the Sichuan Basin.These cities are consistently among the most polluted in China.The average accuracy of the index for severe haze days is 77%,with values for the most heavily polluted cities greater than 80%.Among the two indices,the PV-based EAWM index plays the dominant role,with more explanatory power over a larger geographical domain than the SH index.This study supports other recent results showing that large-scale circulation systems influence air quality in central-eastern China at daily to weekly timescales.Following the implementation of more stringent controls on coal in 2017,the air quality in northern China was greatly improved in winter 2017-2018 relative to the previous several years.Examination of circulation patterns from the 2017-2018 winter shows that both the coal control measures and favorable large-scale circulation conditions jointly promoted the improvement in air quality in Beijing.I therefore suggest that large-scale meteorological factors should be considered when developing and evaluating air pollution control measures in central-eastern China.