Temporal and spatial variation of haze during 1951-2005 in Chinese mainland

Haze is an important indicator of air quality and, on the other hand, fog has definite meaning indicative of weather. With the rapid expansion of economic scale and acceleration of urbanization, hazy weather is getting more and more serious in cities across Chinese mainland. Longterm trends and variation of haze have been obtained by studying meteorological data from 743 surface weather stations in Chinese mainland for the period between 1951 and 2005. In terms of geographical distribution, hazy days were relatively few across the nation from 1956 to 1980 with the exception in the Sichuan Basin and the southern Xinjiang where haze exceeds 50 days per year. The number of haze days increased dramatically since 1980s, and surpass 100 days over most of the eastern part of Chinese mainland and more than 150 days in metropolitan areas at the end of last century, indicating strong linkage between haze days and economic activities. The top 10 cities with the most haze days are as follows, Shenyang, Xingtai, Chongqing, Benxi, Xi’an, Chengdu, Suining, Laohekou, Hetian, Qiemo, Minfeng, and Neijiang. Most of these cities are located in central Liaoning, the Sichuan Basin, the North China Plain, the Guanzhong Plain and the southern part of Xinjiang where is less affected by the sand storm. On the national scale, haze weather is more frequently observed in December and January than the other months, which account for 30% of the annual hazy days. September is the month with the least number of haze days, only contributes 5% to the annual hazy days. The weather stations that show an increasing trend of haze days are mainly located in the eastern and southern parts of the China, which are among the most developed zones in economy and industry. The weather stations in which a decreasing trend of haze days is observed are mainly located in the northeastern and northwestern China, due to the relatively laggard economy and industry. The northeastern China is one of the earlier regions to industrialize in China, but the achievements of recent years in industrial structure adjustment and environmental improvement have contributed to the gradual decreasing haze days in this region.