Interdecadal Variation of Haze Days over China with Atmospheric Causes in Recent 50 Years

Characteristics of interdecadal variations of haze days over China and plausible meteorological causes during 1961 to 2013 are analyzed, using observations from 745 meteorological stations in China. Results show that most haze weather occurs over the east part of China from South China to North China where haze days exhibit an increasing trend especially in economically developed areas, such as North China, the Huanghuai and Jianghuai Plains. Haze days are fewer in large parts of Northeastern China and Western China, and haze days show decreasing tendency in these regions. Generally, haze days are more frequent in autumn and winter than those in spring and summer. Also, autumn and winter are seasons that variation of haze days is most significant. Haze days occur more frequently in January and December than other months. The first mode of EOF reflects a monotonically increasing trend in the east part of China. The second mode shows that the region from South China to the Huaihe River and the region from the Huaihe River to North China present opposite variation tendency of phase. Focusing on regions from South China to the Huaihe River and from the Huaihe River to North China, haze days show an increasing trend year by year over the region from South China to the Huaihe River in autumn and winter before 2000, and then the trend is smoother. In the region from the Huaihe River to North China, however, haze days change gently in recent 30 years in autumn and winter. The variation of haze days relate to surface wind speed and relative humidity. Haze days show a significant negative relationship with surface wind speed before 1990s. Haze days may be influenced by variations of east wind in the region from South China to the Huaihe River in autumn, while northeast wind in the winter. For region from the Huaihe River to North China, haze days is concerned with south wind in autumn, while haze days have nothing to do with wind in winter. The relativity between haze days and surface wind speed weakens, but haze days have significant relationship with relative humidity after 1990s. Haze days experience an increasing tendency because of the reduced surface wind before 1990s and the decrease of relative humidity after 1990s over the region from South China to the Huaihe River in autumn and winter. In comparison, haze days show moderate variations in the region from the Huaihe River to North China, which are probably related to the moderate variability in surface speed and relative humidity in autumn and winter.