Sunshine dimming and brightening in Chinese cities (1955-2011) was driven by air pollution rather than clouds

Sunshine hours in 42 big cities across China declined at a rate of 0.26 h d(-1) decade(-1) from 1955 to 1989. The decreasing trend in sunshine hours levels off from 1990 to 2011, with a marginal decline of 0.02 h d(-1) decade(-1). Since 1990, there has been a recovery of sunshine hours on average by 0.19 h d(-1) decade(-1) in a third of the country, especially South China. In the other two-thirds of the country, sunshine hours have continued to decrease by 0.13 h d(-1) decade(-1) on average. For spring and winter seasons, sunshine hours rebounded in 1990 to 2011 across the country by 0.27 and 0.08 h d(-1) decade(-1), respectively. Total cloud cover (TCC) and air pollution index (API)-2 potential driving factors-were selected as likely candidates for explaining the change in the sunshine trend. Based on Grey Relational Analysis (GRA), for cities and seasons with increasing sunshine hours, the prime driver of recent sunshine hour recovery is API rather than TCC. Annual trends in sunshine hours and TCC of the 42 cities and 42 nearby counties were compared for 1955 to 2011. There is a growing gap in sunshine trends between cities and counties from similar to 1978 onwards. By contrast, TCC trends remained very similar. It is therefore most likely that the best explanation for recent stabilization of sunshine hours in the 42 big cities across China is the decline in API. In general, cities with increasing sunshine hours have good air quality and frequent cloud events.