Geoscientific Instrumentation Methods and Data Systems Chemical characterization and source apportionment of PM 2.5 in Beijing: seasonal perspective

Abstract

In this study, 121 daily PM 2.5 (aerosol particle with aerodynamic diameter less than 2.5 µm) samples were collected from an urban site in Beijing in four months be-tween April 2009 and January 2010 representing the four seasons. The samples were determined for various com-positions, including elements, ions, and organic/elemental carbon. Various approaches, such as chemical mass bal-ance, positive matrix factorization (PMF), trajectory cluster-ing, and potential source contribution function (PSCF), were employed for characterizing aerosol speciation, identifying likely sources, and apportioning contributions from each likely source. Our results have shown distinctive seasonal-ity for various aerosol speciations associated with PM 2.5 in Beijing. Soil dust waxes in the spring and wanes in the sum-mer. Regarding the secondary aerosol components, inorganic and organic species may behave in different manners. The former preferentially forms in the hot and humid summer via photochemical reactions, although their precursor gases, such as SO 2 and NO x , are emitted much more in winter. The latter seems to favorably form in the cold and dry winter. Synoptic meteorological and climate conditions can over-whelm the emission pattern in the formation of secondary aerosols. The PMF model identified six main sources: soil dust, coal combustion, biomass burning, traffic and waste in-cineration emission, industrial pollution, and secondary in-organic aerosol. Each of these sources has an annual mean contribution of 16, 14, 13, 3, 28, and 26 %, respectively, to PM 2.5 . However, the relative contributions of these identi-fied sources significantly vary with changing seasons. The results of trajectory clustering and the PSCF method demon-strated that regional sources could be crucial contributors to PM pollution in Beijing. In conclusion, we have unraveled some complex aspects of the pollution sources and forma-tion processes of PM 2.5 in Beijing. To our knowledge, this is the first systematic study that comprehensively explores the chemical characterizations and source apportionments of PM 2.5 aerosol speciation in Beijing by applying multiple ap-proaches based on a completely seasonal perspective.