Re-evaluating the variation in trend of haze days in the urban areas of Beijing during a recent 36-year period

Abstract

By using meteorological station data, the inter-annual variability of haze days and its trends are re-evaluated in the urban areas of Beijing during a recent 36-year period. Observations from station 54,511, which is a national reference climatological station in the urban area of Beijing, are not suitable for representing the whole urban area, since it trends oppositely to the surrounding stations. Instead, averaged haze days according to five stations in the urban area of Beijing were selected representatively, illustrating that haze days have a positive trend during the period 1980–2015, and haze occurs more often in autumn and winter than in spring and summer. Notably, the number of haze days has increased more rapidly in summer than in the other three seasons. Severe and persistent haze days exhibit positive trends of 4.1 and 13 days/decade, respectively, during the investigation period, while the corresponding ratios to the total haze days have also increased gradually. The haze in Beijing has also become more severe and drier. Under the weakening East Asian monsoon in winter, there has been a reduction in days of locally strong wind speeds and rain, and an increase in days of weak wind speeds. This has directly contributed to the weakening of the diffusion of pollutants, which would otherwise act to maintain the haze, thus prolonging the duration of haze pollution days in urban areas of Beijing.