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Effects of Declining Arctic Sea Ice on Biogenic Sulfate Aerosols
Reference
Sharma, S., Chan, E., Ishizawa, M., Toom-Sauntry, D., Gong, S.L., Li, S.M., Tarasick, D.W., Leaitch, W.R., Norman, A., Quinn, P.K., Bates, T.S., Levasseur, M., Barrie, L.A. and Maenhaut, W. 2012. Influence of transport and ocean ice extent on biogenic aerosol sulfur in the Arctic atmosphere. Journal of Geophysical Research 117: 10.1029/2011JD017074.

Background
The authors indicate that throughout the Arctic Ocean, open-water phytoplankton and ice algae contribute to the production of dimethylsulfoniopropionate (DMSP), which is the precursor of dimethyl sulfide (DMS); and they say that "some DMS near the ocean surface diffuses into the atmosphere or is ventilated by wave action," where it is oxidized to produce sulfate and methanesulfonate (MSA). Subsequently, they say that MSA may condense onto atmospheric aerosol particles, while SO2 may react with existing particles or be further oxidized to H2SO4, which "either condenses on existing particles or nucleates new particles," as described by Kulmala et al. (2007). These sulfur components then increase the hydroscopicity of the particles on which they condense and enhance their cloud condensation nucleating activity, as per Peters and Kreidenweis (2007). In addition, they state that "DMS oxidation products formed further over the ice pack condense on the locally-produced soluble and insoluble organic particles formed from bubble bursting and from other oceans," finally indicating that these steps complete the well-known CLAW hypothesis of Charlson et al. (1987), which posits that "new particles formed from DMS oxidation can impact climate by enhancing cloud reflectance," which ultimately exerts a cooling influence on the locally-warming region.

What was done
Sharma et al., as they describe it, used "29 years of atmospheric MSA measurements at Alert [Nunavut, Canada], 11 years at Barrow (Alaska, USA) and 14 years at Ny-Alesund (Svalbard, Norway) to document changes in MSA concentrations and look for "connections of MSA to sea ice extent, atmospheric transport patterns, and changes in source regions," which they did "for the spring and summer periods, when phytoplankton productivity in the source regions and MSA concentrations at the measurement sites are the highest."

What was learned
The fourteen researchers report that "since 2000, a late spring increase in atmospheric MSA at the three sites coincides with the northward migration of the marginal ice edge zone where high DMS emissions from ocean to atmosphere have previously been reported." And they write that "these results suggest that a decrease in seasonal ice cover influencing other mechanisms of DMS production could lead to higher atmospheric MSA concentrations," which indeed appears to be what is happening as Arctic ice cover declines.

What it means
If the CLAW hypothesis of Charlson et al. (1987) is indeed correct - and a wealth of studies suggest that it is (see materials archived under Feedback Factors (Biophysical - Dimethyl Sulfide) in our Subject Index) - this self-regulating aspect of earth's climate system may well be helping to reduce the rate at which global warming had been proceeding prior to the advent of the 21st century and the enhanced northward migration of the Arctic Ocean's marginal ice edge zone.

References
Charlson, R.J., Lovelock, J.E., Andrea, M.O. and Warren, S.G. 1987. Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326: 655-661.

Kulmala, M., Riipinen, I., Sipila, M., Manninen, H.E., Petaja, T., Junninen, H., Maso, M.D., Mordas, G., Mirme, A., Vana, M., Hirsikko, A., Laakso, L., Harrison, R.M., Hanson, I., Leung, C., Lehtinen, K.E.J. and Kerminen, V.-M. 2007. Toward direct measurement of atmospheric nucleation. Science 318: 89-92.

Petters, M.D. and Kreidenweis, S.M. 2007. A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmospheric Chemistry and Physics 7: 1961-1971.

Reviewed 14 November 2012