An Assessment of the Ozone Flux Budget over a Temperate Coniferous Forest Using Comprehensive Observations and 1D Vertical Canopy Modeling

Dry deposition is the second-largest sink of tropospheric ozone (O₃) and has an important influence on surface O₃ concentrations. Terrestrial ecosystems remove O₃ through a combination of stomatal and nonstomatal processes, with the latter less understood and simply parameterized in models. Dry deposition traditionally refers to the biophysical removal of a gas near the Earth’s surface, but flux observations also include chemical O₃ destruction involving reactions with, e.g., nitric oxide (NO) and volatile organic compounds (VOC) within forest canopies. This chemical loss is typically unaccounted-for in data interpretation and model parameterizations. Here, we leverage highly detailed and vertically resolved measurements of O₃, VOC, nitrogen oxides (NO_x = NO +NO₂), carbon dioxide, and water vapor concentration and flux gradients over a temperate coniferous forest to determine the relative contributions of stomatal and nonstomatal O₃ losses and thereby calculate its budget. We employed two high-resolution mass spectrometers to provide comprehensive VOC measurements across both mass spectra and used the data to constrain two O₃ models: 1) a bulk canopy treatment of dry deposition and chemical loss, and 2) a 1D multi-layered biosphere-atmosphere trace gas exchange model (Multi-Layer Canopy CHemistry and Exchange Model; MLC-CHEM).

We show that observed O₃ fluxes at this site were on average 2x higher than explained by dry deposition alone. The chemical loss based on the comprehensive in-canopy VOC and NO_x observations does not account for this missing O₃ sink. Both models predict similar O₃ flux magnitudes and relative contributions by dry deposition and chemistry, with chemistry making up 15% of the simulated daytime O₃ flux. We also discuss how measurements of VOC and their oxidation products inform current understanding of in-canopy loss processes.