Effect of the strengthened western Pacific subtropical high on summer visibility decrease over eastern China

Daily visibility (Vis) data from 354 meteorological stations are used to evaluate the seasonal Vis trends over China from 1973 to 2009, which show that Vis decline is most distinct over eastern China in summer, with a trend of
1.4 km per decade or
34% in the 37 years. This rapid decline of summertime Vis is found to be partially associated with the intensification and westward extension of the western Pacific subtropical high (WPSH) that has occurred in recent years. Such a change in the strength and breadth of WPSH has resulted in more days with stable, hot, and humid weather, which is favorable for Vis decrease. Analysis through decomposing the trend of Vis into contributions respectively from days with and without WPSH-type meteorological conditions further shows that the intensification of WPSH leads to more episodes of Vis impairment and amplifies the effect of increased aerosol and precursor emissions on the summertime Vis decline in eastern China. Consequently, the difference of Vis between summer and winter follows a declining trend, although the Vis level in winter is still lower than that in summer. The result of this study underscores the importance of considering not only the variation of anthropogenic emissions, but also the change of climate and synoptic systems in the prediction and regulation of air quality and visibility.