Based on the online measurements from December 25,2017 to January 16,2018,we investigated the chemical composition and sources of PM_2.5 in Yangjiang,a South China coastal city.PM_2.5 in Yangjiang mainly consisted of OM,NO^-₃,SO^2-₄,NH⁺₄ and EC,with proportions of 32.75%,25.59%,16.41%,12.37% and 4.82% in mass fraction,respectively.In two pollution events,the mass concentration of NO^-₃ increased to over 6 times of that during clean period,much higher than the increments of other components.The mass fractions of NO^-₃ in two polluted events increased to 29.38% and 30.81%,over twice of that in clean period.Source apportionment of PM_2.5 were performed by Positive Matrix Factorization,which pointed out that secondary formation was the main source/process (51.41%) for PM_2.5 in winter in Yangjiang.The secondary formation from NO_x,contributing 27.18% to the total PM_2.5 mass concentration,was the dominating secondary source.Vehicle exhausts were the biggest primary source (15.11%).In the pollution events,the contribution of the secondary formation from NO_x sharply increased from 11.85% to 33.15% and 36.96%,which should be considered as the dominating source of PM_2.5 pollution in winter of Yangjiang.This study revealed that,as a coastal city,Yangjiang exhibited similar characteristics to those big cities and mega-cities in terms of the PM_2.5 sources,i.e.,dominated by the secondary formation.It suggested that more effort for pollution control should be put into the reduction of nitrates as well as their precursor NO_x,meanwhile a persistent control of vehicles in Yangjiang city is recommended.