A revised global ozone dry deposition estimate based on a new two-layer parameterisation for air–sea exchange and the multi-year MACC composition reanalysis
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...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/4329/2018/acp-18-4329-2018.pdf |
| Summary: | 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 O<sub>3</sub> yr<sup>−1</sup> for the ocean and
722.8 ± 87.3 Tg O<sub>3</sub> yr<sup>−1</sup> 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. |
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| ISSN: | 1680-7316 1680-7324 |