Abstract
Abstract. Dry deposition at the Earth's surface is an important sink of atmospheric ozone. Currently, dry deposition of ozone to the ocean surface in atmospheric chemistry models has the largest uncertainty compared to deposition to other surface types, with implications for global tropospheric ozone budget and associated radiative forcing. Most global models assume that the dominant term of surface resistance in the parameterisation of ozone dry deposition velocity at the oceanic surface is constant. There have been recent mechanistic parameterisations for air–sea exchange that account for the simultaneous waterside processes of ozone solubility, molecular diffusion, turbulent transfer, and first-order chemical reaction of ozone with dissolved iodide and other compounds, but there are questions about their performance and consistency. We present a new two-layer parameterisation scheme for the oceanic surface resistance by making the following realistic assumptions: (a) the thickness of the top water layer is of the order of a reaction–diffusion length scale (a few micrometres) within which ozone loss is dominated by chemical reaction and the influence of waterside turbulent transfer is negligible; (b) in the water layer below, both chemical reaction and waterside turbulent transfer act together and are accounted for; and (c) chemical reactivity is present through the depth of the oceanic mixing layer. The new parameterisation has been evaluated against dry deposition velocities from recent open-ocean measurements. It is found that the inclusion of only the aqueous iodide–ozone reaction satisfactorily describes the measurements. In order to better quantify the global dry deposition loss and its interannual variability, modelled 3-hourly ozone deposition velocities are combined with the 3-hourly MACC (Monitoring Atmospheric Composition and Climate) reanalysis ozone for the years 2003–2012. The resulting ozone dry deposition is found to be 98.4 ± 30.0 Tg O3 yr−1 for the ocean and 722.8 ± 87.3 Tg O3 yr−1 globally. The new estimate of the ocean component is approximately a third of the current model estimates. This reduction corresponds to an approximately 20 % decrease in the total global ozone dry deposition, which (with all other components being unchanged) is equivalent to an increase of approximately 5 % in the modelled tropospheric ozone burden and a similar increase in tropospheric ozone lifetime.
Highlights
In the troposphere, the budget of ozone (O3) is determined by its transport from the stratosphere, dry deposition at the Earth’s surface, and chemical production and loss
We consider an alternative two-layer approach in which chemical reaction in the top water layer of depth δm is fast enough such that it dominates over turbulent transfer, with the assumption Kt = 0, and transport is maintained by molecular diffusion (Fig. 2)
Recent mechanistic schemes used to parameterise the oceanic surface resistance take into account the simultaneous effects of ozone solubility in water, waterside molecular diffusion and turbulent transfer, and first-order chemical reaction of ozone with dissolved iodide and other compounds
Summary
The budget of ozone (O3) is determined by its transport from the stratosphere, dry deposition at the Earth’s surface, and chemical production and loss. We consider an alternative two-layer approach in which chemical reaction in the top water layer of depth δm (i.e. the reaction–diffusion sublayer that is embedded within the viscous sublayer) is fast enough such that it dominates over turbulent transfer, with the assumption Kt = 0, and transport is maintained by molecular diffusion (Fig. 2). The thickness of this layer is of the order of the so-called reaction–diffusion length scale lm = (D/a)1/2 for the ozone– iodide reaction in seawater which is typically a few micrometres. The modified Bessel functions that appear in Eq (16) were calculated using the algorithms given in Press et al (1997)
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