Seasonal variations of stable carbon isotopic composition and biogenic tracer compounds of water-soluble organic aerosols in a deciduous forest

To investigate the seasonal changes in biogenic water-soluble organic carbon (WSOC) aerosols in a boreal forest, aerosol samples were collected continuously in the canopy of a deciduous forest in northern Japan during 2009-2010. Stable carbon isotopic composition of WSOC (delta C-13(WSOC)) in total suspended particulate matter (TSP) exhibited a distinct seasonal cycle, with lower values from June through September (-25.5 +/- 0.5 parts per thousand). This cycle follows the net CO2 exchange between the forest ecosystem and the atmosphere, indicating that delta C-13(WSOC) likely reflects the biological activity at the forest site. WSOC concentrations showed the highest values in early summer and autumn. Positive matrix factorization (PMF) analysis indicated that the factor in which biogenic secondary organic aerosols (BSOAs) dominated accounted for similar to 40% of the highest concentrations of WSOC, where BSOAs mostly consisted of alpha-/beta-pinene SOA. In addition, primary biological aerosol particles (PBAPs) made similar contributions (similar to 57 %) to the WSOC near the forest floor in early summer. This finding indicates that the production of both primary and secondary WSOC aerosols is important during the growing season in a deciduous forest. The methanesulfonic acid (MSA) maximum was also found in early summer and had a distinct vertical gradient with larger concentrations near the forest floor. Together with the similar vertical gradients found for WSOC and delta C-13(WSOC) as well as the alpha-/beta-pinene SOA tracers, our results indicate that the forest floor, including ground vegetation and soil, acts as a significant source of WSOC in TSP within a forest canopy at the study site.