Abstract
Abstract. Understanding the fate of ozone within and above forested environments is vital to assessing the anthropogenic impact on ecosystems and air quality at the urban-rural interface. Observed forest-atmosphere exchange of ozone is often much faster than explicable by stomatal uptake alone, suggesting the presence of additional ozone sinks within the canopy. Using the Chemistry of Atmosphere-Forest Exchange (CAFE) model in conjunction with summer noontime observations from the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX-2007), we explore the viability and implications of the hypothesis that ozonolysis of very reactive but yet unidentified biogenic volatile organic compounds (BVOC) can influence the forest-atmosphere exchange of ozone. Non-stomatal processes typically generate 67 % of the observed ozone flux, but reactions of ozone with measured BVOC, including monoterpenes and sesquiterpenes, can account for only 2 % of this flux during the selected timeframe. By incorporating additional emissions and chemistry of a proxy for very reactive VOC (VRVOC) that undergo rapid ozonolysis, we demonstrate that an in-canopy chemical ozone sink of ~2 × 108 molec cm−3 s−1 can close the ozone flux budget. Even in such a case, the 65 min chemical lifetime of ozone is much longer than the canopy residence time of ~2 min, highlighting that chemistry can influence reactive trace gas exchange even when it is "slow" relative to vertical mixing. This level of VRVOC ozonolysis could enhance OH and RO2 production by as much as 1 pptv s−1 and substantially alter their respective vertical profiles depending on the actual product yields. Reaction products would also contribute significantly to the oxidized VOC budget and, by extension, secondary organic aerosol mass. Given the potentially significant ramifications of a chemical ozone flux for both in-canopy chemistry and estimates of ozone deposition, future efforts should focus on quantifying both ozone reactivity and non-stomatal (e.g. cuticular) deposition within the forest.
Highlights
Forest-atmosphere exchange of ozone (O3) influences both atmospheric composition and biosphere productivity, with significant ramifications for air quality, ecosystem health and atmosphere-biosphere-climate feedbacks
Using a 1-D chemical-transport model of a resolved forest canopy, we have demonstrated that emissions of highly reactive but yet unidentified biogenic hydrocarbons can significantly enhance downward ozone fluxes
For the BEARPEX2007 conditions modeled here, the entire non-stomatal component of the measured ozone flux can be explained with a very reactive VOC (VRVOC) reactivity of 2 × 108 molec cm−3 s−1, consistent with previous observational inferences for this forest
Summary
Forest-atmosphere exchange of ozone (O3) influences both atmospheric composition and biosphere productivity, with significant ramifications for air quality, ecosystem health and atmosphere-biosphere-climate feedbacks. Global chemicaltransport models predict a global dry deposition flux of ∼1000 Tg(O3) yr−1, which is equal in magnitude to the sum of inputs from net chemistry (production – loss) and stratosphere-troposphere exchange (Stevenson et al, 2006). A coupled chemistry-climate modeling study by Sitch et al (2007) suggests that indirect radiative forcing associated with the limiting effect of ozone deposition on terrestrial net primary productivity is equal in magnitude to the direct forcing from tropospheric ozone. Stomatal uptake is thought to be the primary pathway for plant damage, leading to a suggested shift in risk assessment criteria. Wolfe et al.: Forest-atmosphere exchange of ozone from concentration-based metrics (e.g. accumulated total exposure) to a stomatal flux-based index (UNECE, 2004; Ashmore, 2005; Matyssek and Innes, 1999). Non-stomatal deposition can be harmful; for example, reactive uptake to cuticular waxes may increase surface wetability and stomatal occlusion (Karnosky et al, 1999)
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