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Average PM2.5 concentrations of similar to 250 mu g m(-3) and peak concentrations of similar to 500 mu g m(-3) were observed in Xi'an, the largest city in Northwest China during an extreme event in January 2013. The source-oriented versions of the Community Multi-scale Air Quality (CMAQ) model with anthropogenic emissions from Emissions Database for Global Atmospheric Research (EDGAR) were used to study the source contributions of six different source categories including energy production, industries, transportation, residential activities, "other" (agriculture, biomass, waste burning, and biogenic sources), and windblown dust to primary and secondary inorganic PM2.5 (nitrate and sulfate) during this episode. The model generally captured the variation and magnitude of PM2.5 concentrations at monitoring sites. The monthly average concentration of the predicted PM2.5 in Xi'an was >200 mu g m(-3), comparing favorably with the measurement of similar to 250 mu g m(-3). Predicted concentrations of elemental carbon (EC), organic aerosol (OA), sulfate, nitrate, and ammonium were 6, 35, 18, 22, and 12 mu g m(-3), respectively. Chemically unresolved PM2.5 components (PM2.5 Other) were similar to 80 mu g m(-3). Industries and residential activities dominated EC, organic carbon (OC) and PM2.5 Other, contributing 85%, 95%, and 83%, respectively. Energy production (mainly coal combustion) was the dominating source for secondary nitrate, contributing 46%. Other local and upwind sources were also important, contributing 43% and 11% of total nitrate, respectively. Primary sulfate was similar to 10 mu g m(-3) in vicinity surrounding point sources. Secondary sulfate from upwind sources was also important with concentrations of similar to 4-5 mu g m(-3). Secondary sulfate formed by SO2 emitted from local sources was dominated by energy production. Based on the contributions of different sources to primary components and secondary nitrate and sulfate, the contributions of different sources to PM2.5 total mass in Xi'an during the extremely polluted months are: energy 5%, industries 58%, transportation 2%, residential activities 16%, dust 4%, and other (including other components, inexplicit sources, and upwind sources) 15%. (C) 2014 Elsevier Ltd. All rights reserved.