Biosphere-atmosphere interactions in forest settings have a large impact on the budget and fate of nitrogen oxides (NOx) and ozone, as forests cover over 30% of Earth’s land surface area, where major sources and sinks of these key trace gases are located. Owing to their structure and biological activity, forests affect trace gas transport and chemistry both within and above the canopy. Factors such as turbulence, surface deposition, soil emission, and gas-phase oxidation chemistry must be considered when evaluating canopy-scale NOx and ozone fluxes. Interactions involving these processes result in canopy-scale bi-directional exchange of NOx. This might be further affected by leaf-level bi-directional NOx exchange characterized by the compensation point, the ambient NOx mixing ratio above which NOx is taken up by leaves and below which NOx is emitted by leaves. During the summer 2016 Program for Research on Oxidants: PHotochemistry, Emissions, and Transport campaign at the University of Michigan Biological Station, we conducted leaf-level gas exchange experiments on white pine (Pinus strobus), bigtooth aspen (Populus grandidentata), red maple (Acer rubrum), and red oak (Quercus rubra), all dominant tree species of the forest surrounding the campaign site. Known amounts of NO, NO2, or ozone were added to a pair of branch and blank enclosures. Measurements of these gases were made continuously in a sequence of inlet and outlet air from the branch enclosure followed by the blank enclosure. We also measured PAR, ambient and enclosure temperatures and moisture, leaf temperatures and wetness, and CO2 within the enclosure. Initial analyses show that the NOx mixing ratio differences before and after the enclosure have a small but clear correlation to the input NOx mixing ratio. Further analysis is required to examine the dependence of these differences on the micro-environment of the enclosures before conclusions can be made on the existence and magnitude of compensation points for each tree type. These results, combined with the concurrently observed NOx and ozone vertical gradients, will be further analyzed using a Multi-Layer Canopy Chemistry Exchange Model for assessing the effect of leaf-level exchange on the NOx and ozone dynamics at this forest site.

Dry deposition is an important ozone sink that impacts ecosystem carbon and water cycling. Ozone dry deposition in forests is regulated by vertical transport, stomatal uptake, and non-stomatal processes including chemical removal. However, accurate descriptions of these processes in deposition parameterizations are hindered by sparse observational constraints on individual sink terms. Here we quantify the contribution of canopy-atmosphere turbulent exchange and chemical ozone removal by soil-emitted nitric oxide (NO) to ozone deposition in a North-Italian broadleaf deciduous forest. We apply a multi-layer canopy exchange model to interpret campaign observations of nitrogen oxides (NOx=NO+NO2) and ozone exchange above and inside the forest canopy. Two state-of-science parameterizations of in-canopy vertical diffusivity, based on above-canopy wind speed or stability, do not reproduce the observed exchange suppressed by canopy-top radiative heating, resulting in overestimated dry deposition velocities of 10-19\% during daytime. Applying observation-derived vertical diffusivities in our simulations largely resolves this overestimation. Soil emissions are an important NOx source despite the observed high background NOx levels. Soil NOx emissions decrease the gradient between canopy and surface layer NOx mixing ratios, which suppresses simulated NOx deposition by 80% compared to a sensitivity simulation without soil emissions. However, a sensitivity analysis shows that the enhanced chemical ozone sink by reaction with soil-emitted NO is offset by increased vertical ozone transport from aloft and suppressed dry deposition. Our results highlight the need for targeted observations of non-stomatal ozone removal and turbulence-resolving deposition simulations to improve quantification and model representation of forest ozone deposition.