Simultaneous Measurements of O3 and HCOOH Vertical Fluxes Indicate Rapid In‐Canopy Terpene Chemistry Enhances O3 Removal Over Mixed Temperate Forests

Abstract

AbstractDry deposition, the second largest removal process of ozone (O3) in the troposphere, plays a role in controlling the natural variability of surface O3 concentrations. Terrestrial ecosystems remove O3 either through stomatal uptake or nonstomatal processes. In chemical transport models, nonstomatal pathways are roughly constrained and may not correctly capture total O3 loss. To address this, the first simultaneous eddy covariance measurements of O3 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 O3 deposition velocities, vd (O3), ranged between 0.5 and 1.2 cm s−1. Comparison of observed vd (O3) with observationally constrained estimates of stomatal uptake and parameterized estimates of cuticular and soil uptake reveal a large (10%–90%) residual nonstomatal contribution to vd (O3). The residual downward flux of O3 was well correlated with measurements of HCOOH upward flux, suggesting unaccounted for in‐canopy gas‐phase chemistry.