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Abstract The regional difference and meteorological condition over the Yangtze River Delta (YRD) region during a low visibility episode (from 20 February to 22 February 2014) was investigated by using NCEP reanalysis data, MICAPS meteorological data and air quality data from the Ministry of Environmental Protection. Weather situation shows that the surface of the YRD region was under the control of high pressure, surface wind speed was low, which was favorable for the accumulation of pollutants as well as the formation and maintain of low visibility process (fog or haze). The YRD region was divided into 3sub-regions according to the temporal and spatial distributions of fog and haze. Region I covered the most part of Jiangsu province, which was affected by both fog and haze; region II included Shanghai and its surrounding, which was affected by haze; region III covered the most part of Zhejiang province, which was affected by fog. Factors that influence visibility in region I included that stabilized stratification in the lower tropospheric (thermodynamic factors), aerosol hygroscopic growth under high relative humidity conditions (humidity factor), and weak vertical and horizontal diffusion (dynamic factors). However, pollution factors had little effect on visibility in region I. High mass concentration of PM2.5 was the most important factor that influence visibility in region II. Thermodynamic factors, humidity factor, and dynamic factors had little effects on visibility. Thermodynamic factors, which influences visibility in region III, was represented by stabilized stratification in the lower tropospheric layer and temperature inversion in the surface layer. Humidity factor in region III was represented by aerosol hygroscopic growth under high relative humidity condition. Dynamic factors in region III was characterized by low boundary layer height and weakvertical diffusion speed. The explained variance of meteorological factors in each region showed that humidity factor and dynamic factors were more important in region I while humidity factor was the most important in region III..
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Received: 09 March 2015
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