In this paper, conventional meteorological observations and observed hourly PM2.5 concentration are analyzed to study the pollution events in Shanghai during the recent three years. Two predominant categories of weather systems, which are defined as cold air transport type and stagnant meteorological situation type, led to severe pollution events in Shanghai. In order to better describe the two types and make a good indication in forecasting operation, the meteorological air pollution index is created, based on the ventilation index and the retention index which respectively reflects atmospheric vertical and horizontal diffusion conditions. The air pollution diffusion index has linear positive correlation with PM2.5 concentration, with a correlation coefficient of 0.525 after a significant test. This index is greatly superior to the single ventilation index or retention index. Moreover, based on historical pollution events, the transport intensity index is modified for the pollution caused by cold air regional transport. The emission parameter in the equation is changed into upstream observed PM2.5 concentration. The backcalculation test suggests that the modified transport intensity index has positive correlation with the Shanghai PM2.5 concentration, which shows a good indication to the pollution events caused by regional transport of cold airs.