Abstract
Abstract. Forest canopies are primary emission sources of biogenic volatile organic compounds (BVOCs) and have the potential to significantly influence the formation and distribution of secondary organic aerosol (SOA) mass. Biogenically-derived SOA formed as a result of emissions from the widespread forests across the globe may affect air quality in populated areas, degrade atmospheric visibility, and affect climate through direct and indirect forcings. In an effort to better understand the formation of SOA mass from forest emissions, a 1-D column model of the multiphase physical and chemical processes occurring within and just above a vegetative canopy is being developed. An initial, gas-phase-only version of this model, the Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS), includes processes accounting for the emission of BVOCs from the canopy, turbulent vertical transport within and above the canopy and throughout the height of the planetary boundary layer (PBL), near-explicit representation of chemical transformations, mixing with the background atmosphere and bi-directional exchange between the atmosphere and canopy and the atmosphere and forest floor. The model formulation of ACCESS is described in detail and results are presented for an initial application of the modeling system to Walker Branch Watershed, an isoprene-emission-dominated forest canopy in the southeastern United States which has been the focal point for previous chemical and micrometeorological studies. Model results of isoprene profiles and fluxes are found to be consistent with previous measurements made at the simulated site and with other measurements made in and above mixed deciduous forests in the southeastern United States. Sensitivity experiments are presented which explore how canopy concentrations and fluxes of gas-phase precursors of SOA are affected by background anthropogenic nitrogen oxides (NOx). Results from these experiments suggest that the level of ambient NOx influences the pathways by which SOA is formed by affecting the relative magnitudes and fluxes of isoprene oxidation products emitted from the canopy. Future versions of the ACCESS model are planned to be multiphase, including gas- and aerosol-phase chemical and physical processes, to more fully explore these preliminary results.
Highlights
Forests are a critical component of our planet’s global ecosystem, occupying a little more than 30 percent of total land area (Potter, 1999), comprising 50–60 percent of total carbon biomass (Malhi et al, 2002), accounting for 40 percent of the total solar energy captured by green plants (Perry et al, 2008), and generating 50 percent of total terrestrial photosynthesis (Malhi et al, 2002)
For the initial application of ACCESS reported here for a temperate, broadleaf forest in the southeastern US (SE US), isoprene is assumed to be the dominant biogenic volatile organic compounds (BVOCs) emitted from the canopy (Goldstein et al, 2009) and other emitted BVOCs are neglected in this initial analysis
Organic matter (OM) comprises roughly 30–40 percent of total fine particle mass in the SE US (Edgerton et al, 2005; Tanner and Parkhurst, 2000) and evidence has accumulated that isoprene oxidation products are routinely found in PM2.5 samples from this region (Clements and Seinfeld, 2007; Edney et al, 2005)
Summary
Forests are a critical component of our planet’s global ecosystem, occupying a little more than 30 percent of total land area (Potter, 1999), comprising 50–60 percent of total carbon biomass (Malhi et al, 2002), accounting for 40 percent of the total solar energy captured by green plants (Perry et al, 2008), and generating 50 percent of total terrestrial photosynthesis (Malhi et al, 2002). CAFE utilizes a subset of the MCM v3.1 chemical mechanism including reactions for isoprene, monoterpenes, sesquiterpenes and other BVOCs, but no anthropogenic VOCs other than propanal (a total of 2085 reactions of 632 species) In their comparison of BEARPEX 2007 measurements with model results, Wolfe et al (2011a) noted in particular that the model significantly underestimates OH concentrations and requires an unknown enhanced radical recycling mechanism to correct this under prediction. Recent results of Mao et al (2012) indicate that OH concentrations previously measured in many field campaigns (such as BEARPEX 2007) by laser-induced fluorescence in low-pressure detection chambers were likely over estimated by a factor of two, lessening the model under prediction of Wolfe et al (2011b) It is noted in Wolfe et al (2011b) that ozonolysis of very reactive BVOCs within the forest canopy may influence a variety of important chemical pathways, including OH production and fluxes of BVOC oxidation products (and thereby may influence the fluxes of SOA precursor species).
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