Interdecadal Variation Of Haze Days Over China And The Atmospheric Causes In Recent 50 Years

The present study analyzes the spatial and temporal distribution characteristic of haze days over China and the plausible meteorological causes during 1961 to 2013 using the observation data from 745 national base stations in China.The results show that:Most of haze weather occurs over the region of east China from South China to North China where the haze days exhibit an increasing trend especially in the economically developed and densely-populated areas,for example North China,South China,the Huanghuai and Jianghuai plains.The haze days are fewer in large parts of northeastern China and western China than those in east China.In the meantime,haze days show decreasing tendency in northeastern China.Generally,haze are more frequent in the autumn and winter than those in spring and summer.Also autumn and winter are the seasons that variation of haze days is most significant.Haze occurs more frequently in January and December than the other months.The first mode of EOF reflects a monotonically increasing trend in east China.The second mode shows that the region from South China to Huaihe River and the region from Huaihe River to North China present opposite variation tendency of phase.According to this phenomenon,it is necessary to focus on the region from South China to Huaihe River and the region from Huaihe River to North China.Haze days have an increasing trend year by year over the region from south China to Huaihe River in autumn and winter before 2000.Then the trend is smooth.While in the region from Huaihe River to North China haze days change gently in recently 30 years in the autumn and winter.The study shows the variation of haze days is connected with surface wind speed and relative humidity.Haze days show a significant negative relationship with surface wind speed before 1990 s.The haze days influenced by the variations of east wind in the region from South China to Huaihe River in the autumn,while northeast wind in the winter.For the region from Huaihe River to North Chin haze days is concerned with south wind in the autumn,while haze days have nothing to do with surface wind in the winter.The relativity between haze days and surface wind speed weakened,but haze days have significant relationship with relative humidity after 1990 s.Haze days show a significant negative relationship with relative humidity in the region from South China to Huaihe River in the autumn and winter.Haze days experience an increasing tendency because of the reduced surface wind before 1990 s and the decrease of relative humidity after 1990 s over the region from South China to Huaihe River in the autumn and winter.No marked relationship between haze days and relative humidity over the region from Huaihe River to North China in the autumn.while significant positive correlation in the winter.In comparison,haze days show moderate variations in the region from Huaihe River to North China,which are probably related to the moderate variability in surface speed and relative humidity in the autumn and winter.With the standard that relative humidity at two o ’clock pm less than ninety percent,visibility less than 10 kilometers and eliminate the possibility of precipitation,snowstorm,blowing snow,dust blowing,storm,fly ash,smoke affecting the visibility,haze days are selected accurately.The research results about the spatial and temporal distribution characteristic of haze days can reflect the fact.There are fog and haze in the atmosphere when the relative humidity more than ninety percent.Haze occurs far more frequent than fog.Therefore it is haze plays a leading role in the weather phenomenon which can result in low-visibility.Fog days in the winter are more than those in the autumn.Fog is of common occurrence in the winter.Increasing trend of haze and fog days is more obvious than that of fog days.