In-Canopy Chemistry, Emissions, Deposition, and Surface Reactivity Compete to Drive Bidirectional Forest-Atmosphere Exchange of VOC Oxidation Products

Abstract

Dry deposition is an important sink of oxygenated volatile organic compounds (OVOCs) in forest ecosystems. In the summer of 2021, we measured concentration gradients and exchange velocities of oxidation products of isoprene and 3methyl-3-buten-2-ol (MBO) from a Colorado Ponderosa pine forest as part of the Flux Closure Study (FluCS). MBO oxidation products exhibited bidirectional exchange over the forest. Vertical gradients of MBO oxidation products reveal in-canopy chemical production as a daytime source, whereas air transported from the urban outflow of the front range creates periods of enhanced deposition. Differences between our observed deposition velocities over the arid, sparse pine forest and those from a previous study over a temperate, dense mixed forest suggest that ecosystem type may impact deposition rates in ways not currently captured by GEOS-Chem. We show that a previously inferred increased OVOC solubility threshold on leaf cuticles is not likely to explain the observed rapid rates of deposition but instead suggest that peroxides/epoxides could undergo reactive uptake to broadleaf vegetation while organic nitrates could undergo reactive uptake to pine needles. We point to the need to understand the role of reactive OVOC uptake and its potential implications for bidirectional ecosystem-atmosphere exchange.