Research

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Key elements and processes in land-atmosphere interactions relevant to pollution- and climate-relevant tropospheric chemistry. From He, Clifton, et al., 2021. Figure illustrated by Cenlin He and Mj Riches.

I use a hierarchy of numerical models and multiscale observations to advance mechanistic understanding and predictive ability of variability and trends in air pollution and impacts on humans, resources, and climate. In particular, I am interested in land-atmosphere interactions and influences on climate- and pollution-relevant tropospheric chemistry. Much of my work has  been on dry deposition of tropospheric ozone.

Dry deposition of ozone in the earth system: controlling processes and local-to-global impacts. Figure illustrated by Simmi Sinha.

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. 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 vegetation and influence local-to-global carbon and water cycling. Nonstomatal deposition, occurring to snow-covered surfaces, leaf cuticles, soil, or bodies of water, is highly uncertain, but an important fraction of the total ozone dry deposition.

Past research priorities & relevant work

Advance understanding of dry deposition

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With one of the longest datasets of ozone eddy covariance fluxes, I show that there are strong interannual variations in ozone deposition velocity, a measure of the efficiency of the ozone removal by the surface (Clifton et al. 2017). While stomatal uptake is often considered the driver of ozone dry deposition, I use process modeling to show that stomatal uptake does not drive the interannual variability in ozone deposition velocity at Harvard Forest (Clifton et al. 2017). Instead, I suggest that the variability is driven by increases in ozone uptake by soil when soil is dry (Clifton et al. 2019). For more information on ozone depositional processes, as well as a current synthesis of modeling and observations, see Clifton, Fiore, Massman, et al. (2020).

Constrain the influence of 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 overemphasis 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 updated the dry deposition scheme in the GFDL model for ozone to be consistent with current understanding, and recently published a paper investigating the effect of dynamic representation of ozone dry deposition on ozone pollution. In Clifton, Paulot, Fiore, et al. (2020), I show that nonstomatal ozone dry deposition is important for ozone pollution, including hemisphere-scale levels and extremes.

Evaluate the importance of meteorology-plant functioning interactions for dry deposition & plant damage

Meteorology and plant functioning are often overlooked in considering how ozone damages ecosystems. Here I used (i) multilayer canopy large eddy simulation (LES) that uniquely resolves turbulence above and inside the forest canopy (Patton et al. 2016) and (ii) the NOAA GFDL global model to investigate interactions between ozone, meteorology, and stomatal aperture as relevant for ozone plant damage.

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In terms of (i), the above figure shows horizontal variability in ozone and leaf uptake in a ~2km x 2km forest under moderately unstable atmospheric boundary layer conditions as simulated by the NCAR LES. In Clifton & Patton (2021), we investigate the effect of these variations in leaf uptake and ozone on the total amount of ozone taken up by the leaf. We find that dry deposition parameterizations can neglect covariations between ozone and uptake due to organized turbulence. See here for the Research Spotlight on this work in AGU Eos!

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In terms of (ii), in Clifton, Lombardozzi, et al., 2020, I use the NOAA GFDL model to show interannual variability (IAV) in stomatal conductance (egs) is key for the cumulative stomatal uptake of ozone, which indicates the amount of ozone entering the leaf over time available to cause physiological damage. In particular, we find that IAV in stomatal conductance is more important than IAV in ozone pollutionMy findings imply that the most ozone damage happens in years when ecosystems are most productive, challenging widely used metrics suggesting that the most ozone damage occurs in the highest ozone years.

 

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