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

摘要

To investigate the seasonal changes in biogenic water-soluble organic carbon(WSOC) aerosols in a boreal forest, aerosol samples were collectedcontinuously in the canopy of a deciduous forest in northern Japan during2009–2010. Stable carbon isotopic composition of WSOC(δ13C_WSOC) in total suspended particulate matter (TSP) exhibited adistinct seasonal cycle, with lower values from June through September(?25.5±0.5 ‰). This cycle follows the net CO₂ exchange betweenthe forest ecosystem and the atmosphere, indicating thatδ13C_WSOC likely reflects the biological activity at the forestsite. WSOC concentrations showed the highest values in early summer andautumn. Positive matrix factorization (PMF) analysis indicated that thefactor in which biogenic secondary organic aerosols (BSOAs) dominatedaccounted for ~40 % of the highest concentrations of WSOC, whereBSOAs mostly consisted of α-/β-pinene SOA. In addition,primary biological aerosol particles (PBAPs) made similar contributions(~57 %) to the WSOC near the forest floor in early summer. Thisfinding indicates that the production of both primary and secondary WSOCaerosols is important during the growing season in a deciduous forest. Themethanesulfonic acid (MSA) maximum was also found in early summer and had adistinct vertical gradient with larger concentrations near the forest floor.Together with the similar vertical gradients found for WSOC andδ13C_WSOC as well as the α-/β-pinene SOA tracers,our results indicate that the forest floor, including ground vegetation andsoil, acts as a significant source of WSOC in TSP within a forest canopy atthe study site.