Temporal-spatial Variations Of Visibility In Jiangsu And Influencing Factors

Visibility, which is an indicator of atmospheric transparency, is defined as the distance at which the contrast of a target equals the contrast threshold of the human eye. From the recent 30 years, the global visibility was decreasing quickly. The reduction of visibility is due to the suspended particles in the atmosphere which scatter and absorb light. High concentration of PM is the main reason to the reduction of visibility. The reduction of visibility has become an important indicator of ambient air quality in urban areas. As a rapidly developing and newly industrialized province, Jiangsu has attracted considerable attention from all around China. The analysis of Jiangsu visibility and its impact factors will do some help to improve urban environment and perfect energy conservation and the outline of emissions reduction policy.In this paper, observational data from more than 70 automatic meteorological stations in year 2012 are utilized to investigate the temporal-spatial variation patterns of visibility in Jiangsu. Various meteorological data (e.g., relative humidity, wind direction and wind speed) are combined with satellite data (e.g., AOD, aerosol optical depth) to assess their impacts on the temporal-spatial variations in visibility. BP neural network (Back Propagation Neural Network) model and HYSPLIT trajectory model were used to investigate a low visibility event. The results show:(1) Annual averaged visibility in Jiangsu is 6858 meters. The space distribution of visibility is different, the east part is higher and the west is lower. This distribution has relationship with distribution of land and sea and the concentration of air pollution. The diurnal visibility variation presents low visibility in the morning and high in the afternoon. But north, middle and south parts has some difference. The diurnal variation is affected by meteorological elements and air pollution.(2)Seasonal averaged visibility in Jiangsu is 6171meters,7352 meters,6510 meters and 5873 meters respectively. Visibility is highest in summer, and the following is autumn, spring and the lowest is in winter. North-High and Low-South appear in spring and the pattern is reversed in summer. In contrast, East-High and West-Low represent the distributions in autumn and winter. In fact, the change of monsoon, precipitation and the distribution of air pollution are the main factors which influence the seasonal visibility change in Jiangsu. Haze is the most important factor affecting visibility, mist is the second, and fog is the last in Jiangsu,2012. From the statistical analysis, visibility has a negative relationship with relative humidity and AOD, but a positive relationship with wind. Weather conditions can affect visibility, and air pollution concentration is the most important factor to visibility.(3)By analyzing a low visibility process in Jiangsu, we found that due to sea breeze, the distribution of visibility was North-low and South-high in earlier process and East-high and West-low in later period. From surface to 500hPa, the atmosphere was stable and wind speed was low. This situation is benefit for accumulation of pollutants. In later period, visibility was becoming better, because the cold air blew away the air pollution. In this event, PM2.5 is the most important factor to visibility, the second are RH and wind, the last is temperature and pressure. Source of pollution in Jiangsu is mainly from the northeast China, north China and local region of Jiangsu. The airflow, which sank, caused thermal inversion and stable stratification, so the air pollution could not move away. Finally, visibility decreased.