Oxidation of Biogenic Volatile Organic Compound Emissions from Harvard Forest

The contribution of biogenic volatile organic compounds (BVOCs) to regional ozone production and secondary organic aerosol (SOA) formation and growth depends on the emission rates of these compounds and the mechanisms and rates of their subsequent oxidation. Recent findings have suggested that both the range of emitted species and the total rate of emissions of BVOCs from forested ecosystems could be larger than previously thought. Simultaneous in situ observations of biogenic volatile organic compounds (BVOCs) and their oxidation products can be used to identify novel emitted compounds, provide constraints on emission rates, and test proposed oxidation mechanisms. In this study, Proton Transfer Reaction Mass Spectrometry (PTR-MS) was used to obtain fast (ca. 1 Hz) measurements of BVOCs, including isoprene, terpenoids, and oxygenated BVOCs above the canopy at Harvard Forest (Petersham, MA) during the summers of 2005, 2007, and 2008. Fluxes of emitted species were determined using the virtual disjunct eddy covariance method. Concentrations of BVOC oxidation products, including methyl vinyl ketone (MVK) + methacrolein (MACR), and oxygenated terpene derivatives such as nopinone, pinonaldehyde, and others, were measured simultaneously. The MVK + MACR diurnal cycle follows that of isoprene, but with lower daytime maxima and higher nighttime minima, resulting in an average MVK + MACR / isoprene ratio of ca. 0.35 during daylight hours and ca. 1.8 at night. This relationship provides a constraint on the degree of oxidation of the sampled air. Estimated gas-phase concentrations of terpene oxidation products ranged from near the detection limit (several pptv) to approximately 100 pptv. A simple model, together with the measured emissions and concentration data, is used to assess the contributions of emissions, oxidation, and mixing processes to the observed diurnal patterns of BVOCs and their oxidation products and to evaluate current mechanisms describing their production and loss in the gas phase.