One relevant loss process for atmospheric gases is dry deposition where the compounds are transported to the surface and deposit there. In particular for ground-level ozone, the uptake to vegetation plays an important role responsible for 20 % of the total loss of tropospheric ozone. Tropospheric ozone is a secondary produced air pollutant harmful for humans and plants which plays an important role for the tropospheric chemistry. Therefore, an accurate representation in models is important. However, most chemistry models show an overestimation of tropospheric ozone with respect to observations. Thereby, the parametrization of dry deposition to vegetation represents a major source of uncertainty for the global tropospheric ozone budget and might account for this mismatch with observations. The global numerical chemistry and climate simulation system used in this study, the Modular Earth Submodel System (MESSy2 - www.messy-interface.org) linked to ECHAM5 as atmospheric circulation model (EMAC), is no exception. The simulation system describes atmospheric processes in the lower and middle atmosphere and their interaction with ocean, land and human influences.
Like many global models, EMAC employs a “resistances in series” scheme within the dry deposition submodel where each resistor represents one compartment considered for the uptake of trace gases. The major surface deposition occurs via plant pores (stomata) which is hardly sensitive to meteorology depending only on solar radiation and thus lacks important dependencies according to the current knowledge. For non-stomatal deposition, however, EMAC uses a simplified high resistance which makes this pathway negligible.
Hence, in this study, the parametrization of ozone dry deposition has been revised based on established schemes. We applied two empirical adjustment factors to predict stomatal responses to temperature and vapour pressure deficit. This enables stomata to react to weather extremes which become more frequent with climate warming. A further development is the implementation of an explicit formulation of the uptake at leaf surfaces (cuticle) which depends on surface wetness, humidity and vegetation density. Next, the soil moisture availability function for plants, which control the stomatal response to the available soil moisture in the model, has been critically reviewed. In regions where the model shows a strong soil dry bias, e.g. Amazon basin in dry season, the function has been modified in order to avoid stomatal closure.
The modifications increase the global dry deposition flux of ozone by 6 % which leads to less ground-level ozone, regionally up to 20 %. This change arises mainly from the inclusion of cuticular uptake which is favourable at moist and humid surfaces. The impact on stomatal uptake varies in sign and magnitude dependent on local meteorology whereas the temperature stress factor dampens the stomatal uptake and the drought factor led to a more dominant and various response.
Comparing simulated dry deposition velocities and fluxes with data at four experimental sites, where ozone deposition is measured with micrometeorological techniques, give further insights to the local different mechanisms. Overall, the comparison shows a more realistic representation of ozone dry deposition regarding the revised scheme limited by the simulated local meteorology. These changes might contribute to an improved model representation of tropospheric ozone and lower the mismatch with observations.
You can read the whole paper here: Emmerichs, T., Kerkweg, A., Ouwersloot, H., Fares, S., Mammarella, I., and Taraborrelli, D. (2021): A revised dry deposition scheme for land–atmosphere exchange of trace gases in ECHAM/MESSy v2.54, Geosci. Model Dev., 14, 495–519, https://doi.org/10.5194/gmd-14-495-2021.
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