Observations of biogenic volatile organic compounds over a mixed temperate forest during the summer to autumn transition

Abstract. The exchange of trace gases between the biosphere and the atmosphere is an important process that controls both chemical and physical properties of the atmosphere with implications for air quality and climate change. The terrestrial biosphere is a major source of reactive biogenic volatile organic compounds (BVOC) that govern atmospheric concentrations of the hydroxy radical (OH) and ozone (O₃). The oxidation of BVOC leads to the production of low-volatility products that can undergo homogenous nucleation or condense onto existing particles leading to formation and growth of secondary organic aerosol (SOA). Over forests, the net surface-atmosphere exchange of BVOC depends on the unique physiochemical properties of individual compounds as well as the mean physical conditions of the forest canopy that control surface emissions (e.g., temperature, sunlight, leaf area) and loss processes (e.g., uptake through stomata, surface adhesion). Here, we present measurements of BVOC mixing ratios and vertical fluxes over a mixed temperate forest in Northern Wisconsin during broadleaf senescence occurring in the summer-autumn transition. We use these observations to better understand the effects of changes in canopy conditions on net BVOC exchange. The BVOC investigated here include the terpenoids isoprene (C₅H₈), monoterpene hydrocarbons (MT; C₁₀H₁₆), a monoterpene oxide (C₁₀H₁₆O) and sesquiterpenes (SQT; C₁₅H₂₄), as well as a subset of MT oxidation products and dimethyl sulfide (DMS). During this period, MTs were primarily composed of α-pinene, β-pinene, and camphene, where α-pinene and camphene were dominant during the first half of September and β-pinene thereafter. We observed enhanced net MT emissions following the onset of leaf senescence, suggesting that senescence and abscission may be significant controls governing late season MT emissions in this ecosystem. We describe the impact of this MT emissions enhancement and shift in speciation on the potential to form highly oxygenated organic molecules (HOM). The calculated production rates of HOM and H₂SO₄, constrained by terpene and DMS concentrations, suggest that biogenic aerosol formation and growth in this region should be dominated by secondary organics rather than sulfate. Further, we show that models using parameterized MT emissions likely underestimate HOM production, and thus aerosol growth and formation, during early autumn in this region. Further measurements of forest-atmosphere BVOC exchange during seasonal transitions as well as measurements of DMS in temperate regions are needed to effectively predict the effects of canopy changes on reactive carbon cycling and the relative contributions to aerosol production.