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Discussion papers
https://doi.org/10.5194/acp-2019-629
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-629
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Aug 2019

Submitted as: research article | 15 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

The sensitivity of Southern Ocean aerosols and cloud microphysics to sea spray and sulfate aerosol production in the HadGEM3-GA7.1 chemistry-climate model

Laura E. Revell1, Stefanie Kremser2, Sean Hartery1, Mike Harvey3, Jane P. Mulcahy4, Jonny Williams3, Olaf Morgenstern3, Adrian J. McDonald1, Vidya Varma3, Leroy Bird2, and Alex Schuddeboom1 Laura E. Revell et al.
  • 1School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand
  • 2Bodeker Scientific, Alexandra, New Zealand
  • 3National Institute of Water and Atmospheric Research, Wellington, New Zealand
  • 4Met Office, Exeter, United Kingdom

Abstract. With low concentrations of tropospheric aerosol, the Southern Ocean offers a natural laboratory for studies of aerosol-cloud interactions. Aerosols over the Southern Ocean are produced from biogenic activity in the ocean, which generates sulfate aerosol via dimethylsulfide (DMS) oxidation, and from strong winds and waves that lead to bubble bursting and sea-spray emission. Here we evaluate the representation of Southern Ocean aerosols in the HadGEM3-GA7.1 chemistry-climate model. Compared with aerosol optical depth (AOD) observations from two satellite instruments (the Moderate Resolution Imaging Spectroradiometer, MODIS-Aqua c6.1 and the Multi-angle Imaging Spectroradiometer, MISR), the model simulates too-high AOD during winter and too-low AOD during summer. By switching off DMS emission in the model, we show that sea spray aerosol is the dominant contributor to AOD during winter. In turn, the simulated sea spray aerosol flux depends on near-surface wind speed. By examining MODIS AOD as a function of wind speed from the ERA-Interim reanalysis and comparing it with the model, we show that the sea spray aerosol source function in HadGEM3-GA7.1 overestimates the wind speed dependency. We test a recently-developed sea spray aerosol source function derived from measurements made on a Southern Ocean research voyage in 2018. In this source function the wind speed dependency of the sea spray aerosol flux is less than in the formulation currently implemented in HadGEM3-GA7.1. The new source function leads to good agreement between simulated and observed wintertime AOD over the Southern Ocean, however reveals partially compensating errors in DMS-derived AOD. While previous work has tested assumptions regarding the seawater climatology or sea-air flux of DMS, we test the sensitivity of simulated AOD, cloud condensation nuclei and cloud droplet number concentration to three atmospheric sulfate chemistry schemes. The first scheme adds DMS oxidation by halogens and the other two test a recently-developed sulfate chemistry scheme for the marine troposphere; one tests gas-phase chemistry only while the second adds extra aqueous-phase sulfate reactions. We show how simulated sulfur dioxide and sulfuric acid profiles over the Southern Ocean change as a result, and how the number concentration and particle size of the soluble Aitken, accumulation and coarse aerosol modes are affected. The new DMS chemistry scheme leads to a 20 % increase in the number concentration of cloud condensation nuclei and cloud droplets, which improves agreement with observations. Our results highlight the importance of atmospheric chemistry for simulating aerosols and clouds accurately over the Southern Ocean.

Laura E. Revell et al.
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Laura E. Revell et al.
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Short summary
Aerosols over the Southern Ocean consist primarily of sea salt and sulfate, yet are seasonally biased in our model. We test three sulfate chemistry schemes to investigate DMS oxidation, which forms sulfate aerosol. Simulated cloud droplet number concentrations improve using more complex sulfate chemistry. We also show that a new sea spray aerosol source function, developed from measurements made on a recent Southern Ocean research voyage, improves the model's simulation of aerosol optical depth.
Aerosols over the Southern Ocean consist primarily of sea salt and sulfate, yet are seasonally...
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