The contribution of chemical sinks to ozone fluxes over a temperate coniferous forest

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: stomatal O3 loss can be well-predicted through routine CO2 and H2O flux observations, but nonstomatal loss is less understood and is thus only roughly parameterized in models. In particular, the chemical destruction of ozone within forest canopies (one key nonstomatal process) is typically unaccounted-for in data interpretation and model parameterizations. The degree to which these in-canopy reactions affect local-to-regional O3 fluxes and concentrations is therefore not well known. Here, we assess the contribution of in-canopy chemistry to the total observed O3 fluxes over and within a ponderosa pine plantation during August-September 2021. To this end we utilize a novel and comprehensive ensemble of above- and within-canopy concentration and flux observations collected via simultaneous proton-transfer reaction mass spectrometry (PTRMS) and iodide chemical ionization mass spectrometry (ICIMS). Eddy covariance fluxes of O3 and biogenic volatile organic compounds reveal increased nonstomatal downward O3 fluxes concurrent with enhanced upward fluxes of monoterpenes (MT), sesquiterpenes (SQT), and MT oxidation products (C10H16O, C10H14O, C9H14O). Highly-resolved vertical gradient measurements for O3, reactive volatile organic compounds (VOCs), and VOC oxidation products further highlight the importance of in-canopy loss processes. We combine this new dataset with the GEOS-Chem chemical transport model to evaluate current representations of canopy-level O3 deposition and VOC ozonolysis.