Abstract
Atmospheric aerosols impact the Earth's climate system, directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei (CCN) from regional to global scale. Fine aerosols that abundantly contain organic material also influence air quality and thus affect human health. Water-soluble organic carbon (WSOC) that is considered to be associated with a major fraction of secondary organic aerosol can significantly alter the hygroscopic behavior and enhance the capability of particles to act as CCN and thereby play an important role in climatic impacts. Although anthropogenic emissions are significant in Asia and high aerosol loadings are prevalent in the Asian atmosphere, relative contributions of different sources, particularly fossil and biospheric carbon, to organic aerosols are poorly understood. In this study, we measured radiocarbon (δ14C), a unique tracer to distinguish fossil and modern carbon, and stable carbon (δ13C) isotopic ratios in total carbon (TC) and WSOC in atmospheric aerosols from Sapporo, northern Japan (43.07°N, 141.36°E) over one-year period (2 September 2009 to 5 October 2010). The percent of modern carbon (pMC) ranged from 27% to 85% (ave. 57%) in TC and 58% to 114% (ave. 82%) in WSOC, with elevated levels in spring and lowest levels in winter, whereas δ13C values varied between -25.60/00 and -23.10/00 (ave. -24.8) in TC and -26.70/00 and -21.20/00 (ave. -24.20/00) in WSOC, with a peak in winter. This study clearly demonstrates that WSOC is more enriched with modern carbon than TC in northern Japan. Our findings, followed by air mass trajectory analyses, reveal that fossil fuel combustion and biomass burning are two major sources of organic aerosols in winter whereas emissions of primary biogenic aerosols and the secondary organic aerosol formation due to the photo-oxidation of biogenic volatile organic compounds are important in spring (and also summer).
