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Abstract Dry deposition is the second largest tropospheric ozone (O3) sink and occurs through stomatal and nonstomatal pathways. Current O3uptake predictions are limited by the simplistic big‐leaf schemes commonly used in chemical transport models (CTMs) to parameterize deposition. Such schemes fail to reproduce observed O3fluxes over terrestrial ecosystems, highlighting the need for more realistic treatment of surface‐atmosphere exchange in CTMs. We address this need by linking a resolved canopy model (1D Multi‐Layer Canopy CHemistry and Exchange Model, MLC‐CHEM) to the GEOS‐Chem CTM and use this new framework to simulate O3fluxes over three north temperate forests. We compare results with in situ measurements from four field studies and with standalone, observationally constrained MLC‐CHEM runs to test current knowledge of O3deposition and its drivers. We show that GEOS‐Chem overpredicts observed O3fluxes across all four studies by up to 2×, whereas the resolved‐canopy models capture observed diel profiles of O3deposition and in‐canopy concentrations to within 10%. Relative humidity and solar irradiance are strong O3flux drivers over these forests, and uncertainties in those fields provide the largest remaining source of model deposition biases. Flux partitioning analysis shows that: (a) nonstomatal loss accounts for 60% of O3deposition on average; (b) in‐canopy chemistry makes only a small contribution to total O3fluxes; and (c) the CTM big‐leaf treatment overestimates O3‐driven stomatal loss and plant phytotoxicity in these temperate forests by up to 7×. Results motivate the application of fully online vertically explicit canopy schemes in CTMs for improved O3predictions.
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Simultaneous Measurements of O 3 and HCOOH Vertical Fluxes Indicate Rapid In‐Canopy Terpene Chemistry Enhances O 3 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 O3concentrations. Terrestrial ecosystems remove O3either through stomatal uptake or nonstomatal processes. In chemical transport models, nonstomatal pathways are roughly constrained and may not correctly capture total O3loss. To address this, the first simultaneous eddy covariance measurements of O3and formic acid (HCOOH), a tracer of in‐canopy oxidation of biogenic terpenes, were made in a mixed temperate forest in Northern Wisconsin. Daytime maximum O3deposition velocities,vd(O3), ranged between 0.5 and 1.2 cm s−1. Comparison of observedvd(O3) with observationally constrained estimates of stomatal uptake and parameterized estimates of cuticular and soil uptake reveal a large (10%–90%) residual nonstomatal contribution tovd(O3). The residual downward flux of O3was well correlated with measurements of HCOOH upward flux, suggesting unaccounted for in‐canopy gas‐phase chemistry.
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- Award ID(s):
- 1822420
- PAR ID:
- 10384657
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 48
- Issue:
- 3
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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