Tropospheric ozone is an air pollutant — harmful to both humans and vegetation — and a potent greenhouse gas, as well as the main source of the hydroxyl radical, which controls how long reactive greenhouse gases and toxic pollutants stay in the atmosphere. While the last thirty years of atmospheric chemistry research on tropospheric ozone has focused on the role of sources, my research focuses on the role of one of the main sinks, dry deposition. Dry deposition of ozone happens when turbulence transports ozone to the surface and surface-mediated chemistry removes ozone from the atmosphere. Ozone depositing through plant stomata (the small pores on leaves used for gas exchange) can be injurious to the plant and influence local-to-global carbon and water cycling. There are also nonstomatal deposition pathways, such as ozone uptake to snow-covered surfaces, leaf cuticles, soil, and bodies of water. Nonstomatal deposition is highly uncertain, but an important fraction of the total ozone dry deposition.
Current research priorities and relevant work
Advance understanding of dry deposition of ozone
Using one of the longest datasets of ozone eddy covariance fluxes, I show strong interannual variations in ozone deposition velocity at Harvard Forest (Clifton et al., 2017). Ozone deposition velocity is a measure of the efficiency of the removal of ozone by the surface. I use observation-driven process modeling to show that the year-to-year variations are not caused by stomatal uptake, which is most often considered as the driver of variations in ozone deposition velocity, but rather increases in ozone uptake by soil when soil is dry (Clifton et al., 2019). Even though stomatal and cuticular uptake do not drive interannual variability in ozone deposition velocity at Harvard Forest, I find that they are important contributors to day-to-day and/or spatial variability in ozone deposition velocity (Clifton et al., 2019) — here, I use the ozone fluxes from Harvard Forest and short-term ozone flux datasets from nearby forests in the northeastern USA. I am also currently leading a review on the processes contributing to dry deposition of ozone, synthesizing knowledge from atmospheric chemistry, boundary-layer meteorology, and ecology. I will submit this paper to Reviews of Geophysics by the end of October 2019.
Constrain the influence of ozone dry deposition on air pollution
As illustrated by the above figure, the strong observed interannual variability in ozone deposition velocity at Harvard Forest is not simulated by a global chemical transport model using the widely-used Wesely (1989) dry deposition scheme. Given that simulated ambient ozone concentrations are sensitive to ozone deposition velocities, my findings suggest that using atmospheric chemistry models that employ the Wesely scheme to interpret observed year-to-year changes in ozone pollution may lead to a model over- or under-emphasis of the role of ozone precursor emissions.
The NOAA GFDL global chemistry-climate model now has dry deposition of some reactive trace gases and aerosols simulated in the land component of the model (Paulot et al., 2018), allowing for stomatal and nonstomatal deposition to be coupled to terrestrial carbon and water cycling, land use, and vegetation dynamics. I edited the dry deposition scheme for ozone to be consistent with current understanding, and will soon submit a paper investigating the effect of such a dynamic representation of ozone dry deposition on ozone pollution.
Evaluate the importance of meteorology-plant functioning connections for ozone plant damage
Meteorology (in particular, turbulence) and plant functioning are often overlooked in considering how ozone damages ecosystems. Here, I am using (i) a multilayer canopy LES model that uniquely resolves turbulence above and inside the forest canopy (Patton et al., 2016) and (ii) the new version of the global NOAA GFDL model as described above in order to investigate interactions between ozone concentrations, meteorology, and stomatal aperture as relevant for ozone plant damage.