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Abstract
Abstract. We conducted simultaneous measurements of concentrations and above-canopy fluxes of isoprene and α-pinene, along with their oxidation products in aerosols in a Larix kaempferi (Japanese larch) forest in summer 2012. Vertical profiles of isoprene showed the maximum concentration near the forest floor with a peak around noon, whereas oxidation products of isoprene, i.e., methacrolein (MACR) and methyl vinyl ketone (MVK), showed higher concentrations near the canopy level of the forest. The vertical profile suggests large emissions of isoprene near the forest floor, likely due to Dryopteris crassirhizoma (a fern species), and the subsequent reaction within the canopy. The concentrations of α-pinene also showed highest values near the forest floor, with maximums in the early morning and late afternoon. The vertical profiles of α-pinene suggest its large emissions from soil and litter in addition to emissions from L. kaempferi leaves at the forest site. Isoprene and its oxidation products in aerosols exhibited similar diurnal variations within the forest canopy, providing evidence of secondary organic aerosol (SOA) formation via oxidation of isoprene most likely emitted from the forest floor. Although high abundance of α-pinene was observed in the morning, its oxidation products in aerosols showed peaks in daytime, due to a time lag between the emission and atmospheric reactions of α-pinene to form SOA. Positive matrix factorization (PMF) analysis indicated that anthropogenic influence is the most important factor contributing to the elevated concentrations of molecular oxidation products of isoprene- (> 64 %) and α-pinene-derived SOA (> 57 %). The combination of the measured fluxes and vertical profiles of biogenic volatile organic compounds (BVOCs) suggests that the inflow of anthropogenic precursors/aerosols likely enhanced the formation of both isoprene SOA and α-pinene SOA within the forest canopy even when the BVOC flux was relatively low. This study highlights the importance of intra-canopy processes that promote biogenic SOA formation in the presence of significant inflow of oxidants together with anthropogenic aerosols and their precursors.
Highlights
Forest vegetation contributes substantially to emissions of a variety of biogenic volatile organic compounds (BVOCs), such as isoprene, monoterpenes, and sesquiterpenes, which are involved in plant growth, reproduction, and defense
The goal of this study is to investigate controlling factors of biogenic secondary OA (BSOA) formation from BVOC in a forest environment located near anthropogenic sources
Isoprene and its oxidation products in aerosols exhibited similar diurnal variations within the forest canopy, providing evidence of secondary organic aerosol (SOA) formation within a timescale of a few hours via oxidation of isoprene emitted from Dryopteris crassirhizoma on the forest floor
Summary
Forest vegetation contributes substantially to emissions of a variety of biogenic volatile organic compounds (BVOCs), such as isoprene, monoterpenes, and sesquiterpenes, which are involved in plant growth, reproduction, and defense. BVOCs have significant effects on insects and other organisms and atmospheric chemistry and physics. Approximately 90 % of annual VOC emissions are derived from biogenic sources, with an estimated amount of about 1000 Tg year−1 (Guenther et al, 2012). The emission strength and composition of BVOCs in the forest environment depend on vegetation types, seasons, and meteorological parameters such as temperature, solar
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