Heavy pollution episodes, transport pathways and potential sources of PM 2.5 during the winter of 2013 in Chengdu (China).

Air mass concentration data from 8 environmental quality monitoring sites and meteorological data of Chengdu from 1 December 2013 to 28 February 2014 were used in this study. Chengdu suffered five continuous heavy pollutions during this winter due to the basin terrain and the meteorological conditions of low wind speed, low precipitation and high relative humidity. Analysing the hourly resolution time series of pollutants' concentrations, variation of PM2.5 in the urban area followed a growing "saw-tooth cycle" pattern during the heavy pollution, with a daily cycle of bimodal distribution. The massive letting-off of fireworks within a short period of time on the Eve of the Lunar New Year under the unfavourable diffusion conditions resulted in an extreme pollution event. The sharply rising Longmen-Qionglai Mountains to the west of the Sichuan Basin not only acted as a huge barrier to block the air mass from the east but also favoured the formation of a local circulation. The cluster analysis of back trajectories revealed that up to 77% of them came from the inner part of the Basin. Combining the concentration data of PM2.5 with air mass back trajectories, a potential source contribution function (PSCF) model and a concentration-weighted trajectory (CWT) method were used to evaluate the transport pathways and sources over PM2.5 of Chengdu, revealing that the main potential sources of PM2.5 were located in southeast cities and the western margin of the Sichuan Basin. The result provided advice for the government to take measures in improving air quality.