Emissions of isoprenoids and oxygenated biogenic volatile organic compounds from a New England mixed forest

摘要

Fluxes of biogenic volatile organic compounds, including isoprene,monoterpenes, and oxygenated VOCs measured above a mixed forest canopy incentral Massachusetts during the 2005 and 2007 growing seasons are reported.Mixing ratios were measured using proton transfer reaction mass spectrometry(PTR-MS) and fluxes computed by the disjunct eddy covariance technique.Isoprene was by far the predominant BVOC emitted at this site, with summermid-day average fluxes of 5.3 and 4.4 mg m?2 hr?1 in 2005 and 2007,respectively. In comparison, mid-day average fluxes of monoterpenes were0.21 and 0.15 mg m?2 hr?1 in each of these years. On short timesscales (days), the diel pattern in emission rate compared well with astandard emission algorithm for isoprene. The general shape of the seasonalcycle and the observed decrease in isoprene emission rate in early Septemberwas, however, not well captured by the model. Monoterpene emission ratesexhibited dependence on light as well as temperature, as determined from theimproved fit to the observations obtained by including a light-dependentterm in the model. The mid-day average flux of methanol from the canopy was0.14 mg m?2 hr?1 in 2005 and 0.19 mg m?2 hr?1 in 2007, butthe maximum flux was observed in spring (29 May 2007), when the flux reached1.0 mg m?2 hr?1. This observation is consistent with enhancedmethanol production during leaf expansion. Summer mid-day fluxes of acetonewere 0.15 mg m?2 hr?1 during a short period in 2005, but only 0.03mg m?2 h?1 averaged over 2007. Episodes of negative fluxes ofoxygenated VOCs, particularly acetone, were observed periodically,especially in 2007. Thus, deposition within the canopy could help explainthe low season-averaged flux of acetone in 2007. Fluxes of species ofbiogenic origin at mass-to-charge ($m/z$) ratios of 73(0.05 mg m?2 hr?1 in 2005; 0.03 mg m?2 hr?1 in 2007) and 153(5 μg m?2 hr?1 in 2007), possibly corresponding to methyl ethyl ketone and anoxygenated terpene or methyl salicylate, respectively, were also observed.