A primary sink of air pollutants and their precursors is dry deposition. Dry deposition estimates differ across chemical transport models, yet an understanding of the model spread is incomplete. Here, we introduce Activity 2 of the Air Quality Model Evaluation International Initiative Phase 4 (AQMEII4). We examine 18 dry deposition schemes from regional and global chemical transport models as well as standalone models used for impact assessments or process understanding. We configure the schemes as single-point models at eight Northern Hemisphere locations with observed ozone fluxes. Single-point models are driven by a common set of site-specific meteorological and environmental conditions. Five of eight sites have at least 3 years and up to 12 years of ozone fluxes. The interquartile range across models in multiyear mean ozone deposition velocities ranges from a factor of 1.2 to 1.9 annually across sites and tends to be highest during winter compared with summer. No model is within 50% of observed multiyear averages across all sites and seasons, but some models perform well for some sites and seasons. For the first time, we demonstrate how contributions from depositional pathways vary across models. Models can disagree with respect to relative contributions from the pathways, even when they predict similar deposition velocities, or agree with respect to the relative contributions but predict different deposition velocities. Both stomatal and nonstomatal uptake contribute to the large model spread across sites. Our findings are the beginning of results from AQMEII4 Activity 2, which brings scientists who model air quality and dry deposition together with scientists who measure ozone fluxes to evaluate and improve dry deposition schemes in the chemical transport models used for research, planning, and regulatory purposes.
Bibliographical noteFunding Information:
Borden Forest Research Station is funded by Environment and Climate Change Canada. Easter Bush measurements were funded by the European Union – the GREENGRASS project (grant no. EC EVK2-CT2001-00105), the NitroEurope Integrated Project (project no. 017841), and CarboEurope (grant no. GOCE-CT-2003-505572); the UK DEFRA (grant no. 1/3/201) “Effects of Ground-Level Ozone on Vegetation in the UK”; and the UK NERC Core national capability. For Ramat Hanadiv, we acknowledge the Israel Science Foundation (grant no. 1787/15) and the Joseph H. and Belle R. Braun Senior Lectureship in Agriculture to Eran Tas. Harvard Forest observations were supported in part by the United States Department of Energy's Office of Science (BER) and the National Science Foundation Long-Term Ecological Research program. Olivia E. Clifton was supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Institute for Space Studies, administered by Oak Ridge Associated Universities under contract with NASA. Christopher D. Holmes was supported by the National Science Foundation (grant no. 1848372). Ivan Mammarella and Timo Vesala were supported by the Academy of Finland Flagship funding (grant no. 337549) and ICOS-Finland (via the University of Helsinki) funding. László Horváth and Tamás Weidinger were partly supported by the National Research, Development and Innovation Office (project no. K138176), ÉCLAIRE (project no. 282910), and the FAIR Network of micrometeorological measurements (grant no. CA20108). DOSE runs performed by Lisa Emberson and Sam Bland were partly supported by a grant (grant no. NE/V02020X/1) from the Future of UK Treescapes research program, funded by the UKRI.
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