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

Abstract

Dry 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.

New Evidence for the Importance of Non‐Stomatal Pathways in Ozone Deposition During Extreme Heat and Dry Anomalies

“…Low relative humidity may reduce cuticular O 3 uptake (Sun et al., 2016; Zhang et al., 2002), or high temperatures may promote cuticular O 3 uptake through faster surface reactions (Cape et al., 2009). In some forests where direct ozonolysis by biogenic volatile organic compounds (BVOC) plays an important role in O 3 uptake, the inferred non‐stomatal uptake could increase as a function of temperature (Kurpius & Goldstein, 2003; Vermeuel et al., 2021; Wolfe et al., 2011). Dry soil may also promote O 3 uptake to soil (Mészáros et al., 2009; Stella et al., 2019; Stella, Loubet, et al., 2011).…”

The combined impact of canopy stability and soil NOx exchange on ozone removal in a temperate deciduous forest

“…The strong vertical gradients in ozone mixing ratios are unlikely to be caused by VOC-ozone reactions, since the contribution of measured VOCs to the total ozone flux at Bosco Fontana is very small (Finco et al, 2018). However, we cannot rule out that unmeasured, highly reactive compounds could have contributed to in-canopy ozone removal (Goldstein et al, 2004;Vermeuel et al, 2021).…”

“…Therefore, these parameterizations do not account for the different photo-chemical regime inside the canopy. Observation-based studies indicate a widely varying contribution of chemical ozone removal by soil NO and BVOCs, that largely depends on site-specific characteristics such as soil and plant type, temperature, soil moisture and vapor pressure deficit (Fares et al, 2012;Finco et al, 2018;Rannik et al, 2012;Vermeuel et al, 2021). In Europe, anthropogenic NO x and VOC sources are decreasing (Kuenen et al, 2022).…”

The Combined Impact of Canopy Stability and Soil NOx Exchange on Ozone Removal in a Temperate Deciduous Forest

“…The strong vertical gradients in ozone mixing ratios are unlikely to be caused by VOC‐ozone reactions, since the contribution of measured VOCs to the total ozone flux at Bosco Fontana is very small (Finco et al., 2018). However, we cannot rule out that unmeasured, highly reactive compounds could have contributed to in‐canopy ozone removal (Goldstein et al., 2004; Vermeuel et al., 2021).…”