Abstract
Abstract. High concentrations of ozone in ambient air are hazardous not only to humans but to the ecosystem in general. The impact of ozone damage on vegetation and agricultural plants in combination with advancing climate change may affect food security in the future. While the future scenarios in themselves are uncertain, there are limiting factors constraining the accuracy of surface ozone modeling also at present: the distribution and amount of ozone precursors and ozone-depleting substances, the stratosphere–troposphere exchange, as well as scavenging processes. Removal of any substance through gravitational settling or by uptake by plants and soil is referred to as dry deposition. The process of dry deposition is important for predicting surface ozone concentrations and understanding the observed amount and increase of tropospheric background ozone. The conceptual dry deposition velocities are calculated following a resistance-analogous approach, wherein aerodynamic, quasi-laminar, and canopy resistance are key components, but these are hard to measure explicitly. We present an update of the dry deposition scheme implemented in Oslo CTM3. We change from a purely empirical dry deposition parameterization to a more process-based one which takes the state of the atmosphere and vegetation into account. We examine the sensitivity of the scheme to various parameters, e.g., the stomatal conductance-based description of the canopy resistance and the choice of ozone surface resistance, and evaluate the resulting modeled ozone dry deposition with respect to observations and multi-model studies. Individual dry deposition velocities are now available for each land surface type and agree generally well with observations. We also estimate the impact on the modeled ozone concentrations at the surface. We show that the global annual total ozone dry deposition decreases with respect to the previous model version (−37 %), leading to an increase in surface ozone of more than 100 % in some regions. While high sensitivity to changes in dry deposition to vegetation is found in the tropics and the Northern Hemisphere, the largest impact on global scales is associated with the choice of prescribed ozone surface resistance over the ocean and deserts.
Highlights
Ozone is an important trace gas for all life forms on Earth
From the MODIS category “barren or sparsely vegetated”, everything poleward from 60◦ is defined as tundra, while everything equatorward is categorized as desert
We focus on changes in ozone total dry deposition OD3 D, dry deposition velocities vDOD3, concentrations in the lowermost model level [O3](p0), and tropospheric burden tropO3
Summary
Ozone is an important trace gas for all life forms on Earth. Depending on the place of its occurrence, it has either a positive or negative connotation. Typical concentrations of surface ozone range from 10 ppb over the tropical Pacific to 100 ppb in the downwind areas of highly emitting sources The number of long-term observations (starting before the 1950s) is limited and restricted to mainly European sites Most of these have indicated a doubling of tropospheric ozone since the 1950s The process of dry deposition is important for predicting surface ozone concentrations and understanding the observed amount and increase of tropospheric background ozone.
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