Inter-annual variability of wintertime PM_2.5 chemical composition in Xi'an, China: Evidences of changing source emissions

Chemical characteristics of PM_2.5 in Xi'an in wintertime of 2006, 2008, and 2010 were investigated. Markers of OC2, EC1, and NO₃^-/SO₄^2- ratio were calculated to investigate the changes in PM_2.5 emission sources over the 5-year period. Positive matrix factorization (PMF) model was used to identify and quantify the main sources of PM_2.5 and their contributions. The results showed that coal combustion, motor vehicular emissions, fugitive dust, and secondary inorganic aerosol accounted for more than 80% of PM_2.5 mass. The importance of these major sources to the PM_2.5 mass varied yearly: coal combustion was the largest contributor (31.2%±5.2%), followed by secondary inorganic aerosol (20.9%±5.2%) and motor vehicular emissions (19.3%±4.8%) in 2006; the order was still coal combustion emissions (27.6%±3.4%), secondary inorganic aerosol (23.2%±6.9%), and motor vehicular emissions (20.9%±4.6%) in 2008; while coal combustion emission further decreased (24.1%±3.1%) with fugitive dust (19.4%±5.5%) increasing in 2010. The changes in PM_2.5 chemical compositions and source contributions can be attributed to the social and economic developments in Xi'an, China, including energy structure adjustment, energy consumption, the expansion of civil vehicles, and the increase of urban construction activities.