Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 4 2009 10.5194/acpd-9-14315-2009 http://www.atmos-chem-phys-discuss.net/9/14315/2009/ http://www.atmos-chem-phys-discuss.net/9/14315/2009/acpd-9-14315-2009.html http://www.atmos-chem-phys-discuss.net/9/14315/2009/acpd-9-14315-2009.pdf 14315 14359 2009-07-01 Determination of OM/OC ratios and specific attenuation coefficients (SAC) in ambient fine PM at a rural site in southern Ontario: implications for emission sources, particle aging, and radiative forcing T. W. Chan L. Huang lin.huang@ec.gc.ca W. R. Leaitch S. Sharma J. R. Brook J. G. Slowik J. P. D. Abbatt P. C. Brickell J. Liggio S.-M. Li H. Moosmüller ATSD/STB, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada Department of Chemistry, University of Toronto, Lash Miller Chemical Laboratories, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada, 89512-1095, USA now at: ASTD/STB, Environment Canada, 335 River Road, Ottawa, Ontario, K1A 0H3, Canada Ambient particulate matter (PM) samples were collected on quartz filters at a rural site in southern Ontario during intensive studies in 2005 and 2007. The concentrations of organic carbon (OC), pyrolysis organic carbon (POC), and elemental carbon (EC) were determined by thermal analysis. These results were compared to the organic aerosol mass concentration (OM) measured by an Aerodyne Aerosol Mass Spectrometer (AMS) and to the particle absorption coefficient (<i>b</i>_asp) obtained from a Radiance Research Particle Soot Absorption Photometer (PSAP). The total organic mass to organic carbon ratios (OM/OC) and specific attenuation coefficients (SAC) were also derived. According to the results, the POC mass is proportional to the approximated oxygen mass in the aerosols and OM/OC ratios can be estimated directly from thermal measurements. The study also suggests that the air masses from the south, with relatively low OC/EC ratios, high EC, sulphate contents and OM/OC ratios, were originated from urban and industrial emissions and subsequently experienced photo-oxidations in the atmosphere, implying that the oxygenated organics could come from both primary and secondary sources. Whereas the air masses from the north, with relatively high OC/EC ratios, low EC, sulphate contents and OM/OC ratios, were dominant by the background clean air with relatively larger contributions from biogenic emissions. <br><br> The mean SAC derived from the 2005 and 2007 studies are 4.9 m² g^&minus;1 and 3.8 m² g^&minus;1, respectively. 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