Characterizing the dry deposition of ozone in global atmospheric models

Dry deposition of atmospheric oxidants to the Earth's surface or vegetation is important as both a major removal pathway governing their atmospheric abundance and as a key input of oxidants and nutrients to sensitive vegetation surfaces. By linking the atmosphere and biosphere, dry deposition processes contribute to wider climate and Earth system feedbacks which need to be adequately quantified for a full understanding of Earth system responses. In addition, they have immediate policy-relevant implications for air quality, ecosystem health and crop productivity that need to be assessed on local, regional and global scales. In this study we use results from the recent Task Force on Hemispheric Transport of Air Pollution (HTAP) model intercomparison to explore how dry deposition of ozone varies across a large sample of current atmospheric chemistry and transport models. While most models take a similar, resistances-based approach to parameterising dry deposition, there are substantial differences across the models in the magnitude and variability of the annual and monthly ozone deposition fluxes which contribute to the differences in modelled surface ozone and in the global tropospheric ozone budget. The range in annual global ozone deposition flux over 14 HTAP models spans about 30%, but this hides larger spatial and temporal variations in deposition that reveal differences in the parameterization approach taken. We extend this analysis by running sensitivity studies with a single model to determine the importance of meteorological processes and leaf area index, and to attribute the ozone flux by vegetation type. This provides an important first step towards quantifying the uncertainty in ozone dry deposition and permitting more thorough, observation-based evaluation of this important process.