Abstract:A case of sustained low horizontal visibility occurred in Tianjin during January 2009. From the perspective of the meteorological factors, the evolution process of air pollution and air backward trajectory, this event was mainly caused by fine particles in haze weather conditions. The result indicated that fine particle mass concentration was the main influence factor of horizontal visibility. The increase of the ratio of PM2.5 to PM10 resulted in the persistent air pollution and the continuous declined visibility, which was affected in the calm weather conditions under the control of southwest warm and moist air flow. Simulations of the backward trajectory model HYSPLIT have indicated that the aerosol pollutions come from southwestern region and surrounding areas, and cold waves are the important clearance mechanism of regional fine particle pollution in Tianjin. The cyclical change of visibility is caused by the enrichment of fine particle and the quick removal of air pollution, and is also affected by the periodic weather systems.