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

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.

Automated summary

The study reveals a significant contribution of nonstomatal pathways in canopy to the dry deposition of O-3, which is not correctly captured in current models. Stomatal uptake and estimated absorption only explain a small portion of O-3 loss.