Progressively narrow the gap of PM_(2.5) pollution characteristics at urban and suburban sites in a megacity of Sichuan Basin, China

Nowadays,the fine particle pollution is still severe in some megacities of China,especially in the Sichuan Basin,southwestern China.In order to understand the causes,sources,and impacts of fine particles,we collected PM_(2.5)samples and analyzed their chemical composition in typical months from July 2018 to May 2019 at an urban and a suburban (background) site of Chengdu,a megacity in this region.The daily average concentrations of PM_(2.5)ranged from5.6-102.3μg/m^(3)and 4.3-110.4μg/m^(3)at each site.Secondary inorganics and organic matters were the major components in PM_(2.5)at both sites.The proportion of nitrate in PM_(2.5)has exceeded sulfate and become the primary inorganic component.SO_(2)was easier to transform into sulfate in urban areas because of Mn-catalytic heterogeneous reactions.In contrast,NO_(2)was easily converted in suburbs with high aerosol water content.Furthermore,organic carbon in urban was much greater than that in rural,other than elemental carbon.Element Cr and As were the key cancer risk drivers.The main sources of PM_(2.5)in urban and suburban areas were all secondary aerosols (42.9%,32.1%),combustion (16.0%,25.2%) and vehicle emission (15.2%,19.2%).From clean period to pollution period,the contributions from combustion and secondary aerosols increased markedly.In addition to tightening vehicle controls,urban areas need to restrict emissions from steel smelters,and suburbs need to minimize coal and biomass combustion in autumn and winter.