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Gas-fired boilers are the main stationary sources of NOx in Beijing.However,the understanding of gas-fired boilers is limited.In the present study,the emission characteristics of NOx,SO2,and CO from gas-fired boilers in Beijing were established using category-specific emission factors (EFs) from field measurements.To obtain category-specific EFs,boilers were classified through influence analysis.Factors such as combustion mode,boiler type,and installed capacity were considered critical for establishing EFs because they play significant roles in pollutant formation.The EFs for NOx,CO,and SO2 ranged from 1.42-6.86 g m3,0.05-0.67 g m-3 and 0.03-0.48 g m-3.The emissions of NOx,SO2,and CO for gas-fired boilers in Beijing were 11121 t,468 t,and 222 t in 2014,respectively.The emissions were spatially allocated into grid cells with a resolution of 1 km x 1 km,and the results indicated that top emitters were in central Beijing.The uncertainties were quantified using a Monte Carlo simulation.The results indicated high uncertainties in CO (-157% to 154%) and SO2 (-127% to 182%)emissions,and relatively low uncertainties (-34% to 34%) in NOx emission.Furthermore,approximately 61.2% and 96.8% of the monitored chamber combustion boilers (CCBs)met the standard limits for NOx and SO2,respectively.Concerning NOx,low-NOx burners and NOx emission control measures are urgently needed for implementing of stricter standards.Adopting terminal control measures is unnecessary for SO2,although its concentration occasionally exceeds standard limits,because reduction of its concentration can be achieved thorough control of the sulfur content of natural gas at a stable low level.Furthermore,the atmospheric combustion boilers (ACBs) should be substituted with CCBs,because ACBs have a higher emission despite lower gross installed capacity.The results of this study will enable in understanding and controlling emissions from gas-fired boilers in Beijing.