A yearlong study of water-soluble organic carbon in Beijing I: Sources and its primary vs. secondary nature

Sources and properties of water-soluble organic carbon (WSOC) were investigated based on fine particulate matter (PM2.5) samples collected in Beijing during a thirteen month campaign. The WSOC to OC ratios averaged 45.9% annually and were substantially higher in summer compared with the other seasons. WSOC exhibited strong correlation with secondary components such as secondary organic aerosol estimated by the elemental carbon (EC)-tracer method and inorganic ions (e.g., sulfate and nitrate), whereas the correlation between WSOC and EC was much weaker, suggesting that WSOC should be dominated by secondary species. Moreover, the trend of the WSOC to EC ratio was found to coincide with that of relative humidity during winter, spring and fall. High WSOC/EC ratio in February indicates high humidity could enhance the formation potential of WSOC in winter. Sources of WSOC were further investigated by a receptor model (Positive Matrix Factorization model). The apportionment results suggested that biomass burning contributed about 40% of WSOC while about 54% of WSOC was associated with oxalate and sulfate, whereas a primary factor was responsible for only 6% of WSOC also demonstrating that primary emissions are not the main source of WSOC.