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Forest-atmosphere exchange of ozone: sensitivity to very reactive biogenic VOC emissions and implications for in-canopy photochemistry

by G. M. Wolfe, J. A. Thornton, M. McKay, A. H. Goldstein
Atmospheric Chemistry and Physics Discussions ()

Abstract

Understanding the fate of ozone within and above forested environments\nis vital to assessing the anthropogenic impact on ecosystems and air\nquality at the urban-rural interface. Observed forest-atmosphere\nexchange of ozone is often much faster than explicable by stomatal\nuptake alone, suggesting the presence of additional ozone sinks within\nthe canopy. Using the Chemistry of Atmosphere-Forest Exchange (CAFE)\nmodel in conjunction with summer noontime observations from the 2007\nBiosphere Effects on Aerosols and Photochemistry Experiment\n(BEARPEX-2007), we explore the viability and implications of the\nhypothesis that ozonolysis of very reactive but yet unidentified\nbiogenic volatile organic compounds (BVOC) can influence the\nforest-atmosphere exchange of ozone. Non-stomatal processes typically\ngenerate 67% of the observed ozone flux, but reactions of ozone with\nmeasured BVOC, including monoterpenes and sesquiterpenes, can account\nfor only 2% of this flux during the selected timeframe. By\nincorporating additional emissions and chemistry of a proxy for very\nreactive VOC (VRVOC) that undergo rapid ozonolysis, we demonstrate that\nan in-canopy chemical ozone sink of similar to 2 x 10(8) molec cm(-3)\ns(-1) can close the ozone flux budget. Even in such a case, the 65 min\nchemical lifetime of ozone is much longer than the canopy residence time\nof similar to 2 min, highlighting that chemistry can influence reactive\ntrace gas exchange even when it is ``slow{''} relative to vertical\nmixing. This level of VRVOC ozonolysis could enhance OH and RO(2)\nproduction by as much as 1 pptv s(-1) and substantially alter their\nrespective vertical profiles depending on the actual product yields.\nReaction products would also contribute significantly to the oxidized\nVOC budget and, by extension, secondary organic aerosol mass. Given the\npotentially significant ramifications of a chemical ozone flux for both\nin-canopy chemistry and estimates of ozone deposition, future efforts\nshould focus on quantifying both ozone reactivity and non-stomatal (e.g.\ncuticular) deposition within the forest.

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