Wintertime cooling and a potential connection with transported aerosols in Hong Kong during recent decades

As a southeast neighbor of the heavily industrialized and urbanized region of the Pearl River Delta (PRD), Hong Kong, China, has been susceptible to aerosols transported from the PRD in winter when the prevailing flow is from the west. The decadal trend in pollution during recent decades has likely indicated a difference in aerosol loading between winter and summer when different winds dominate. The surface air temperature shows a wintertime cooling trend during recent decades in Hong Kong, particularly during the midday hours. Differently, the surface air temperature shows a summertime warming trend during recent decades in Hong Kong. By examining the distinct winter versus summer trends in midday hour temperature under low cloud cover conditions in Hong Kong from 1979 to 2014, we found a close relationship between the temperature reduction during midday hours and increased aerosol loading in winter but not in summer. The consistency in turning point for both the increase in aerosol loading and the variation in winter-summer temperature differences in the late 1980s also supports the strong association between aerosol and temperature changes. Further analysis shows that there was a significant negative correlation between downwelling solar radiation (DSR) and aerosol loading. Quantitatively, aerosols over Hong Kong during the study period reduced surface DSR and midday temperatures by approximately 30 W m−2 and 2.1 °C for low cloud cover cases.