Chemical composition and seasonal variations of PM_(2.5) in an urban environment in Kunming, SW China: Importance of prevailing westerlies in cold season

摘要

Kunming, a Chinese southwestern tourist city which has not large local pollution sources, has found to have an increasing tendency of haze pollution in recent years. But the pollution sources are unclear. In order to identify them, daily PM2.5 samples (n = 346) were collected from September 2017 to August 2018 in the urban area. And the major water-soluble inorganic ions (WSIIs) were determined to better understand the chemical characteristics, source categories and potential region of sources. Our study showed that the mass concentration of PM2.5 in Kunming ranged from 7.61 to 91.83 mu g m(-3), with an annual average value of 33.59 +/- 15.71 mu g m(-3). Positive matrix factorization (PMF) model identified five factors including secondary aerosol (the contributions of 36.3%), coal combustion (26.0%), biomass burning (19.2%), dust (12.5%) and sea salt (6.0%). And coal combustion played a leading role in the source contribution of PM2.5 in winter while biomass combustion was dominant in spring. Being located between two severe haze zones in the world, northern-central China and north of South Asia, and affected by India monsoon and East Asia monsoon in summer and prevailing westerlies in winter, we found that air masses from South Asia (especially India) contained pollutants could be brought to Kunming by prevailing westerlies in winter. In spring, however, the sources of PM2.5 in Kunming were mainly affected by biomass burning from South Asia and Southeast Asia when prevailing westerlies gradually weakened.