Simultaneous Measurements of O<sub>3</sub> and HCOOH Vertical Fluxes Indicate Rapid In‐Canopy Terpene Chemistry Enhances O<sub>3</sub> Removal Over Mixed Temperate Forests
Michael P. Vermeuel, Patricia Cleary, Ankur R. Desai, Timothy H. Bertram
Abstract
Abstract Dry deposition, the second largest removal process of ozone (O 3 ) in the troposphere, plays a role in controlling the natural variability of surface O 3 concentrations. Terrestrial ecosystems remove O 3 either through stomatal uptake or nonstomatal processes. In chemical transport models, nonstomatal pathways are roughly constrained and may not correctly capture total O 3 loss. To address this, the first simultaneous eddy covariance measurements of O 3 and formic acid (HCOOH), a tracer of in‐canopy oxidation of biogenic terpenes, were made in a mixed temperate forest in Northern Wisconsin. Daytime maximum O 3 deposition velocities, v d (O 3 ), ranged between 0.5 and 1.2 cm s −1 . Comparison of observed v d (O 3 ) with observationally constrained estimates of stomatal uptake and parameterized estimates of cuticular and soil uptake reveal a large (10%–90%) residual nonstomatal contribution to v d (O 3 ). The residual downward flux of O 3 was well correlated with measurements of HCOOH upward flux, suggesting unaccounted for in‐canopy gas‐phase chemistry.