Abstract: Visibility reduction is an important feature of air pollution,which has its own distinct characteristics. There are significant differences in visibility among different cities. The meteorological data observed by 753 stations during the period of 1957-2015 and provided by National Meteorological Center of China Meteorological Administration were used to diagnose the spatiotemporal variation of visibility at different levels in China. Results showed that the days of visibility at level 1 and level 2 in the east of “Hu Huanyong Line” were in a significant decrease trend,those at level 0 in the same region were in an increase or a decrease trend,and the increase trend was dominant in the west; the change trend of days of visibility at level 3,4 and 5 was not significant at the most stations,and there was a decrease trend at a few stations. With the increase of visibility level,the stations with the decrease of days of visibility became less and less. The annual days of low visibility were on the rise. Temporally,the days of low visibility were on the rise and occurred in all seasons,and absolutely,their increase was the highest in winter but the lowest in summer,and their proportion was in an increase in summer and autumn. Spatially,the days of low visibility were increased from the east edge of croppingnomadic ecotone to the southeast coastal area,especially in winter,and the increase trend was not significant in summer. The time series correlation analysis revealed that there was a positive correlation between the days of low visibility in China and the total energy production,car ownership,urbanization rate,gross domestic product (GDP) and the second industry,which passed the test of 0.01 significance level. The spatial correlation analysis revealed that there was a high spatially negative correlation between the days of low visibility and the countylevel population density and average wind speed,which passed the test of 0.01 significance level. Therefore,the rapid urbanization and industrialization as well as the decrease of average wind speed may be the main causes of the significant increase of days of low visibility in China.

Key words: low visibility, spatiotemporal pattern, urbanization, industrialization, wind speed, seasonal variation, China