reported that averaged over 16 urban sites over the Beijing-Tianjin-Hebei area in northern China, the yearly haze days increased from 46 days in 1986 to 78 days in 2007. Owing to the lack of a long-term monitoring network for aerosols in China, haze days are identified as days with visibility less than 10 km and relative humidity less than 90% based on observations, which separates haze from fog assuming that fog occurs when relative humidity is equal to or larger than 90%. Many studies have reported that eastern China has experienced an increasing trend of haze days during recent decades. Wang et al., reported that emissions from northern Hebei and Beijing-Tianjin were two major regional contributors to PM 2.5 pollution in Shijiazhuang in January 2013. found that the transport of pollutants from the southern to northern regions of the North China Plain leads to heavy PM 2.5 loading in Beijing. The transport of pollutants from upwind areas is also an important contributor to local high aerosol concentrations. In addition to the high emissions resulting from accelerated urbanization and rapid economic growth, stable synoptic meteorological conditions, such as strong temperature inversion in the lower troposphere, weak surface wind speed, and descending air in the planetary boundary layer, accumulate pollutants in the shallow layer and produce high concentrations of pollutants within the source regions. The formation of winter haze over eastern China is associated with relatively high anthropogenic aerosol emissions and unusual meteorological conditions. Increased aerosol loading in recent decades has also contributed to the decrease in sunshine duration by 1.0% decade −1 over 1954–1998 in China, trend in radiative forcing by −6.6 W m −2 decade −1 over 1961–2000 in China, and trend of cooling in surface air temperature by 0.1 K decade −1 from 1953 to 1997 over the Sichuan Basin. The increase in aerosol concentrations over eastern China further influences the air quality in downwind areas, such as the North Pacific, through long-range transport of aerosols. showed that as a result of the increase in aerosol pollution, the deaths attributable to ambient particulate matter pollution have increased by about 10% worldwide over 1990–2010. High aerosol concentrations during haze days also have serious adverse effects on human health and ecosystems. During haze days, atmospheric visibility is less than 10 km because of the scattering and absorption of solar radiation by aerosols, which results in traffic jams and flight cancelations. In recent years, eastern China has frequently experienced persistent and severe winter haze pollution episodes with high aerosol concentrations, which have affected half of the 1.3 billion people in China. Among the meteorological parameters, the weakening of winds by −0.09 m s −1 decade −1 over 1985–2005 was found to be the dominant factor leading to the decadal increase in winter aerosol concentrations and haze days over eastern China during recent decades. The increasing trend was only 1.8 (☑.5) µg m −3 decade −1 when variations in meteorological parameters alone were considered. Considering variations in both anthropogenic emissions and meteorological parameters, the model simulated an increase in winter surface-layer PM 2.5 concentrations of 10.5 (☖.2) µg m −3 decade −1 over eastern China. The GEOS-Chem model captured the increasing trend of winter PM 2.5 concentrations for 1985–2005, with concentrations averaged over eastern China increasing from 16.1 µg m −3 in 1985 to 38.4 µg m −3 in 2005. Observed winter haze days averaged over eastern China (105–122.5☎, 20–45°N) increased from 21 days in 1980 to 42 days in 2014 and from 22 to 30 days between 19. The increase in winter haze over eastern China in recent decades due to variations in meteorological parameters and anthropogenic emissions was quantified using observed atmospheric visibility from the National Climatic Data Center Global Summary of Day database for 1980–2014 and simulated PM 2.5 concentrations for 1985–2005 from the Goddard Earth-Observing System (GEOS) chemical transport model (GEOS-Chem).
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