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Secondary organic carbon (SOC) is often a significant portion of organiccarbon (OC) in ambient particulate matter (PM). The levels and seasonalpatterns of SOC in Hong Kong were examined using more than 2000 PM10measurements made over a 4.5-year period (1998–2002) in a network of ten airquality monitoring stations. The positive matrix factorization (PMF) modelwas used to analyze this large data set for source identification andapportioning. SOC was subsequently estimated to be the sum of OC present inthe secondary sources, i.e., secondary sulfate, secondary nitrate, andsecondary organic aerosol. The annual average SOC as estimated by the PMFmethod was 4.25 μg C/m3 while the summer average was1.66 μg C/m3 and the winter average was 7.05 μg C/m3. Incomparison, the method that uses EC as a tracer for primary carbonaceousaerosol sources to derive SOC overestimated SOC by 70–212% for the summersamples and by 4–43% for the winter samples. The overestimation by the ECtracer method resulted from the inability of obtaining a single OC/EC ratiothat represented a mixture of primary sources varying in time and space.We found that SOC and secondary sulfate had synchronous seasonal variationand were correlated in individual seasons, suggesting common factors thatcontrol their formation. Additionally, the presence of SOC was found to beenhanced more than that of secondary sulfate in the winter. We postulate thisto be a combined result of favorable partitioning of semivolatile SOC speciesin the particle phase and more abundant SOC precursors in the winter

Application of positive matrix factorization in estimating aerosol secondary organic carbon in Hong Kong and its relationship with secondary sulfate