Atmospheric pollution by fine particulates and mercury (Hg) in emerging economies is a serious environmental concern. Here, we present Hg concentrations and isotope compositions of 24-hour integrated fine particulate matter (PM2.5) samples collected during January 2014 at four large Chinese cities (three inland cities: Beijing, Changchun and Chengdu; one coastal city: Hong Kong), with an aim of identifying the potential Hg sources and transformation processes. Mean concentrations of PM2.5 (171 +/- 62 mu g m(-3)) and PM2.5-bound Hg (1.3 +/- 1.1 ng m(-3)) in Chengdu were the highest. Overall, PM2.5 samples in Chinese inland cities were characterized by moderately negative delta Hg-202 (-1.08 +/- 0.64 parts per thousand, 1 sigma), slightly negative Delta Hg-199 (- 0.13 +/- 0.28 parts per thousand, 1 sigma) and insignificant Delta Hg-200 (0.03 +/- 0.05 parts per thousand, 1 sigma). On average, delta Hg-202 of PM2.5 was the highest in Chengdu (- 0.74 +/- 0.67 parts per thousand, 1 sigma), followed by Beijing (- 1.11 +/- 0.26 parts per thousand, 1 sigma) and Changchun (- 1.60 +/- 0.45 parts per thousand, 1 sigma). PM2.5 from Beijing showed the most negative Delta Hg-199 (- 0.31 +/- 0.40 parts per thousand, 1 sigma) that was significantly lower than Changchun (- 0.12 +/- 0.21 parts per thousand, 1 sigma) and Chengdu (- 0.02 +/- 0.15 parts per thousand, 1 sigma). Coal combustion and cement production were identified to be the dominant sources of PM2.5-bound Hg in these cities, with additional Hg sources from non-ferrous metal smelting in Chengdu. Besides, Hg emissions from biomass burning were evident in specific days for each city. We found that source materials and isotope fractionation during emission processes could not fully explain the observed Hg isotope compositions in PM2.5, especially the large negative Delta Hg-199 values ( < -0.3 parts per thousand) in Beijing. The near-unity slope of Delta Hg-199 vs. Delta Hg-201 in PM2.5 samples from each studied city indicates that Hg n in the fine aerosols was likely photo-reduced to different degrees in the atmosphere following emission from sources.