Characteristics of PM 2.5 Chemical Compositions and Their Effect on Atmospheric Visibility in Urban Beijing, China during the Heating Season

Beijing, which is the capital of China, suffers from severe Fine Particles (PM 2.5 ) pollution during the heating season. In order to take measures to control the PM 2.5 pollution and improve the atmospheric environmental quality, daily PM 2.5 samples were collected at an urban site from 15 November to 31 December 2016, characteristics of PM 2.5 chemical compositions and their effect on atmospheric visibility were analyzed. It was found that the daily average mass concentrations of PM 2.5 ranged from 7.64 to 383.00 μg m −3 , with an average concentration of 114.17 μg m −3 . On average, the Organic Carbon (OC) and Elemental Carbon (EC) contributed 21.39% and 5.21% to PM 2.5 , respectively. Secondary inorganic ions (SNA: SO 4 2 − + NO 3 − + NH 4 + ) dominated the Water-Soluble Inorganic Ions (WSIIs) and they accounted for 47.09% of PM 2.5 . The mass concentrations of NH 4 + , NO 3 − and SO 4 2− during the highly polluted period were 8.08, 8.88 and 6.85 times greater, respectively, than during the clean period, which contributed most to the serious PM 2.5 pollution through the secondary transformation of NO 2 , SO 2 and NH 3 . During the highly polluted period, NH 4 NO 3 contributed most to the reconstruction extinction coefficient ( b′ ext ), accounting for 35.7%, followed by (NH 4 ) 2 SO 4 (34.44%) and Organic Matter (OM: 15.24%). The acidity of PM 2.5 in Beijing was weakly acid. Acidity of PM 2.5 and relatively high humidity could aggravate PM 2.5 pollution and visibility impairment by promoting the generation of secondary aerosol. Local motor vehicles contributed the most to NO 3 − , OC, and visibility impairment in urban Beijing. Other sources of pollution in the area surrounding urban Beijing, including coal burning, agricultural sources, and industrial sources in the Hebei, Shandong, and Henan provinces, released large amounts of SO 2 , NH 3 , and NO 2 . These, which were transformed into SO 4 2− , NH 4 + , and NO 3 − during the transmission process, respectively, and had a great impact on atmospheric visibility impairment.