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Emissions of isoprenoids and oxygenated biogenic volatile organic compounds

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Atmos. Chem. Phys., 11, 4807–4831, 2011www.atmos-chem-phys.net/11/4807/2011/doi:10.5194/acp-11-4807-2011© Author(s) 2011. CC Attribution 3.0 License.AtmosphericChemistryand PhysicsEmissions of isoprenoids and oxygenated biogenic volatile organiccompounds from a New England mixed forestK. A. McKinney1, B. H. Lee2, A. Vasta1, T. V. Pho1,*, and J. W. Munger21Department of Chemistry, Amherst College, Amherst, Massachusetts, USA2Department of Earth and Planetary Sciences and School of Engineering and Applied Sciences, Harvard University,Cambridge, Massachusetts, USA*now at: Department of Chemistry and Biochemistry, University of California, Santa Barbara, California, USAReceived: 29 September 2010 – Published in Atmos. Chem. Phys. Discuss.: 22 November 2010Revised: 31 March 2011 – Accepted: 12 April 2011 – Published: 24 May 2011Abstract. Fluxes of biogenic volatile organic compounds,including isoprene, monoterpenes, and oxygenated VOCsmeasured above a mixed forest canopy in central Mas-sachusetts during the 2005 and 2007 growing seasons are re-ported. Mixing ratios were measured using proton transferreaction mass spectrometry (PTR-MS) and fluxes computedby the disjunct eddy covariance technique. Isoprene was byfar the predominant BVOC emitted at this site, with sum-mer mid-day average fluxes of 5.3 and 4.4 mg m−2hr−1in2005 and 2007, respectively. In comparison, mid-day aver-age fluxes of monoterpenes were 0.21 and 0.15mg m−2hr−1in each of these years. On short times scales (days), thediel pattern in emission rate compared well with a standardemission algorithm for isoprene. The general shape of theseasonal cycle and the observed decrease in isoprene emis-sion rate in early September was, however, not well cap-tured by the model. Monoterpene emission rates exhibiteddependence on light as well as temperature, as determinedfrom the improved fit to the observations obtained by includ-ing a light-dependent term in the model. The mid-day aver-age flux of methanol from the canopy was 0.14mg m−2hr−1in 2005 and 0.19 mgm−2hr−1in 2007, but the maximumflux was observed in spring (29 May 2007), when the fluxreached 1.0 mg m−2hr−1. This observation is consistent withenhanced methanol production during leaf expansion. Sum-mer mid-day fluxes of acetone were 0.15 mgm−2hr−1dur-ing a short period in 2005, but only 0.03 mgm−2hr−1aver-aged over 2007. Episodes of negative fluxes of oxygenatedCorrespondence to: K. A. McKinney([email protected])VOCs, particularly acetone, were observed periodically, es-pecially in 2007. Thus, deposition within the canopy couldhelp explain the low season-averaged flux of acetone in 2007.Fluxes of species of biogenic origin at mass-to-charge (m/z)ratios of 73 (0.05 mgm−2hr−1in 2005; 0.03mg m−2hr−1in 2007) and 153 (5µg m−2hr−1in 2007), possibly corre-sponding to methyl ethyl ketone and an oxygenated terpeneor methyl salicylate, respectively, were also observed.1 IntroductionVolatile organic compounds (VOCs) play a critical role inmaintaining the oxidant balance of the lower atmosphere,acting as either a source or sink of oxidants, depending onthe local photochemical conditions. In the presence of NOx,VOC oxidation can enhance ozone levels, contributing topoor air quality, and form peroxyacyl nitrates (PANs), re-sulting in the transport of NOxto remote areas (Atkinson,2000). At the same time, oxidation of VOCs can yield re-action products with reduced vapor pressures, which thencondense to form secondary organic aerosol (SOA), givingrise to negative health effects and impacts on cloud forma-tion and the atmospheric radiative balance. Although theseprocesses occur for VOCs of both anthropogenic and bio-genic origin, several factors contribute to making biogenicvolatile organic compounds (BVOCs) the dominant contrib-utors to VOC chemistry throughout most of the troposphere(Fehsenfeld et al., 1992; Chameides et al., 1988). With a to-tal source strength of ca. 1150 Tg C yr−1, global emissions ofBVOCs outweigh those of anthropogenic VOCs by a factorof 10 (Guenther et al., 1995). Biogenic compounds also tendPublished by Copernicus Publications on behalf of the European Geosciences Union.4808 K. A. McKinney et al.: Emissions of isoprenoids and oxygenated biogenic volatile organic compoundsto be more reactive than their anthropogenic counterparts andto more readily form SOA (Atkinson and Arey, 1998; Griffinet al., 1999; Kanakidou et al., 2005; Kavouras et al., 1998;Helmig et al., 2006). Hence, a comprehensive understandingof the processes regulating BVOC concentrations is of fun-damental importance in modeling the chemistry of the tro-posphere and predicting its response to global environmentalchange (Kulmala et al., 2004; Lathiere et al., 2006; Pe˜nuelasand Llusia, 2003; Pe˜nuelas and Staudt, 2010).The major classes of BVOCs (and their relative con-tributions to the total budget) are the isoprenoids, con-sisting of isoprene (35–45 %), monoterpenes (11–25 %),sesquiterpenes and functionalized terpenes (minor), oxy-genated VOCs (20–30%), and other hydrocarbons (<10 %)(Guenther et al., 1995, 2000). Together, fluxes of these com-pounds can represent reemission of a few percent of the pho-tosynthetically fixed carbon (Guenther, 2002; Kesselmeier etal., 2002). The physiological purposes served by these com-pounds vary greatly with chemical species, ranging from pro-tection against high temperatures or oxidative stress, to plantsignaling, to regulation of metabolism, and many are as yetnot well understood (Sharkey et al., 2008; Kesselmeier andStaudt, 1999; Fall, 2003). This variety is reflected in the de-pendence of emission rates on a number of physicochemical,physiological and environmental factors, including radiation,temperature, phenology, water and nutrient availability, en-zyme activity, plant species and physical structure, solubil-ity and vapor pressure (Kesselmeier and Staudt, 1999; Ni-inemets et al., 2004; Laothawornkitkul et al., 2009).Emission models attempt to capture the most importantof these dependencies, using data from leaf- and canopy-scale measurements of emissions under controlled and am-bient conditions to formulate functions describing observedemissions patterns. Among emission models, those for iso-prene and monoterpenes are the most robust and well charac-terized, though shortcomings do exist (Guenther et al., 1993,2006; Grote and Niinemets, 2008; Keenan et al., 2009). Thisis in part due to the fact


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