Abstract
Abstract. 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 (δ13CWSOC) in total suspended particulate matter (TSP) exhibited a distinct seasonal cycle, with lower values from June through September (−25.5±0.5 ‰). This cycle follows the net CO2 exchange between the forest ecosystem and the atmosphere, indicating that δ13CWSOC 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 ~40 % of the highest concentrations of WSOC, where BSOAs 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. 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 δ13CWSOC as well as the α-/β-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.
Highlights
Forest ecosystems act as a major sink of atmospheric CO2 and a source of atmospheric organic aerosols (Kavouras et al, 1998; Kulmala et al, 2004; Tunved et al, 2006)
This paper presents the seasonal changes in biogenic water-soluble organic carbon (WSOC) in the canopy of a deciduous forest based on the aerosol samples collected in 2009–2010
Positive matrix factorization (PMF) analysis indicated that factors in which biogenic secondary organic aerosols (BSOAs) dominated (∼40 %) and primary biological aerosol particles (PBAPs) dominated (∼57 %) made similar contributions to the maximum WSOC near the forest floor in early summer
Summary
Forest ecosystems act as a major sink of atmospheric CO2 and a source of atmospheric organic aerosols (Kavouras et al, 1998; Kulmala et al, 2004; Tunved et al, 2006). The relative importance of the canopy floor within forests is not well understood in terms of WSOC formation. The WSOC-specific δ13C analysis, combined with biogenic molecular markers, allows for source apportionment of aerosol WSOC in forested areas. We present seasonal changes in biogenic WSOC within the canopy of a broadleaf deciduous forest over an 18-month period. We present the relative contributions of potential sources to WSOC formation in the forest canopy using δ13CWSOC, BSOA tracers, and PBAP tracers. On the basis of these data sets, we discuss the relative importance of the forest floor in WSOC formation at a deciduous forest site
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