Chemical characterization and source apportionment of PM2.5 in Beijing: seasonal perspective

摘要

In this study, 121 daily PM (aerosol particle with aerodynamicdiameter less than 2.5 μm) samples were collected from an urban site inBeijing in four months between April 2009 and January 2010 representing thefour seasons. The samples were determined for various compositions,including elements, ions, and organic/elemental carbon. Various approaches,such as chemical mass balance, positive matrix factorization (PMF),trajectory clustering, and potential source contribution function (PSCF),were employed for characterizing aerosol speciation, identifying likelysources, and apportioning contributions from each likely source. Our resultshave shown distinctive seasonality for various aerosol speciationsassociated with PM in Beijing. Soil dust waxes in the spring andwanes in the summer. Regarding the secondary aerosol components, inorganicand organic species may behave in different manners. The formerpreferentially forms in the hot and humid summer via photochemicalreactions, although their precursor gases, such as SO and NO,are emitted much more in winter. The latter seems to favorably form in thecold and dry winter. Synoptic meteorological and climate conditions canoverwhelm the emission pattern in the formation of secondary aerosols. ThePMF model identified six main sources: soil dust, coal combustion, biomassburning, traffic and waste incineration emission, industrial pollution, andsecondary inorganic aerosol. Each of these sources has an annual meancontribution of 16, 14, 13, 3, 28, and 26%,respectively, to PM. However, the relative contributions of theseidentified sources significantly vary with changing seasons. The results oftrajectory clustering and the PSCF method demonstrated that regional sourcescould be crucial contributors to PM pollution in Beijing. In conclusion, wehave unraveled some complex aspects of the pollution sources and formationprocesses of PM in Beijing. To our knowledge, this is thefirst systematic study that comprehensively explores the chemicalcharacterizations and source apportionments of PM aerosol speciationin Beijing by applying multiple approaches based on a completely seasonalperspective.