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European Geosciences Union, Atmospheric Chemistry and Physics, 5(17), p. 3749-3767, 2017

DOI: 10.5194/acp-17-3749-2017

European Geosciences Union, Atmospheric Chemistry and Physics Discussions, p. 1-40

DOI: 10.5194/acp-2016-844

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An improved parameterisation of ozone dry deposition to the ocean and its impact in a global climate-chemistry model

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

Schemes used to parameterise ozone dry deposition velocity at the oceanic surface mainly differ in terms of how the dominant term of surface resistance is parameterised. We examine three such schemes and test them in a global climate-chemistry model that incorporates meteorological nudging and monthly-varying reactive-gas emissions. The default scheme invokes the commonly used assumption that the water surface resistance is constant. The other two schemes, named the one-layer and two-layer reactivity schemes, include the simultaneous influence on the water surface resistance of ozone solubility in water, waterside molecular diffusion and turbulent transfer, and a first-order chemical reaction of ozone with dissolved iodide. Unlike the one-layer scheme, the two-layer scheme can indirectly control the degree of interaction between chemical reaction and turbulent transfer through the specification of a surface reactive layer thickness. A comparison is made of the modelled deposition velocity dependencies on sea-surface temperature (SST) and wind speed with recently reported cruise based observations. The default scheme overestimates the observed deposition velocities by a factor of 2 to 4 when the chemical reaction is slow (e.g. under colder SSTs in the Southern Ocean). The default scheme has almost no temperature, wind-speed and latitudinal variations in contrast with the observations. The one-layer scheme provides noticeably better variations, but it overestimates deposition velocity by a factor of 2 to 3 due to an enhancement of the interaction between chemical reaction and turbulent transfer. The two-layer scheme with a surface reactive layer thickness specification of 2.5 microns, which is approximately equal to the reacto-diffusive length scale of the ozone-iodide reaction, is able to simulate the field measurements most closely, with respect to absolute values as well as SST and wind-speed dependence. The two-layer scheme yields the largest deposition velocities in the tropics. The global oceanic deposition of ozone determined using this scheme is approximately 80 Tg yr −1 . This amount is 12 % of the modelled total global ozone deposition, is almost half of the original oceanic deposition obtained using the default scheme, and corresponds to a 10 % decrease in the original estimate of the total global ozone deposition. The figure of 12 % is much lower than the previously reported modelled estimate of oceanic deposition being roughly one third of total deposition. Deposition parameterisation influences the predicted ozone mixing ratios, especially in the Southern Hemisphere. For the latitudes 45–70° S, the two-layer scheme improves the prediction of ozone observed at an altitude of 1 km by 7 % and that within the altitude range 1–6 km by 5 % compared to the original scheme.