Abstract: Based on the hourly visibility and relative humidity as well as wind speed data in 2011, particulate sample data from November 16 to December 13, 2011, the relationship between visibility and chemical composition of PM_2.5 was analyzed in Tianjin. The results indicate that visibility has a negative correlation with the mass concentration of the particulate, and PM_2.5 and relative humidity plays a major role in the poor visibility. Atmospheric visibility also has a significantly negative correlation with the mass concentration of the total carbon (TC). The SO₄^2-, NO₃^-, OC and EC are the key factors influencing the visibility, especially the SO₄^2-, then the OC and EC, while the effect of the NO₃^- is relatively small. The simulations of the backward trajectory and analysis of the mixed layer height have shown that the heavy PM_2.5 pollution during the observational period is mainly caused by meteorological conditions.

Key words: Atmospheric visibility, Relative humidity, PM_2.5, Water-soluble iron, Carbon composition