Brimstone in the air
Posted by sogasex on March 13, 2008
By Byron Blomquist, University of Hawaii
Sulfur in the air is not something a typical person spends much time thinking about. You may have heard of sulfur dioxide from power plants causing acid rain in Asia, Europe and North America. You may have heard that volcanos also emit large quantities of sulfur dioxide. And you may be familiar with hydrogen sulfide emitted at hot springs and other geothermal vents. Sulfur in this form gives some hot springs a distinctive, and most would say unpleasant odor. The oceans cover two-thirds of our planet and, except for occasional ship exhaust, the amount of sulfur in the marine atmosphere is very low. Yet in places like the Southern Ocean, sulfur gases participate in fascinating and important climate-related processes.
As Steve Archer mentioned in his blog on March 3, one object of our attention on this cruise is dimethylsulfide, or DMS, and it begins it’s story in the individual cells of minute marine plants called phytoplankton. A sulfur containing chemical released from plankton cells is converted to DMS in seawater, and most of this is subsequently consumed by bacteria and other organisms. However, a small fraction of the DMS escapes to the atmosphere. So, you may ask, how much DMS is in the air down here? Well, today I’m measuring a concentration of about 250 parts-per-trillion (ppt). How much is that?? Let’s say you had 1 liter of ink and you mixed it into a volume of water equal to 1,600 Olympic-sized swimming pools. You would have a 250 ppt solution of ink. A very small concentration indeed, and in the case of DMS in the atmosphere, a concentration you would never smell or otherwise detect without very sophisticated equipment. And 250 ppt is more than the usual amount of DMS for the marine atmosphere. Less than 100 ppt is more typical.
Yet the consequences of this minute amount of sulfur are potentially profound. In the atmosphere, a chain of reactions initiated by sunlight converts DMS into very small sulfate-containing particles. Sulfate strongly attracts water and either acts as a “seed” or nucleus for the formation of cloud droplets, or it enhances the ability of other existing particles to attract water. Regions rich in natural and human-produced particles (from dust, volcanic emissions, forests and urban/industrial activity, for example) have a surplus of potential cloud nuclei. But the remote marine atmosphere is far from urban and terrestrial particle sources, and thus contains fewer cloud nuclei. This is especially the case for the Southern Ocean. In this region sulfate particles from DMS are probably the principle source of cloud nuclei.
One of the most challenging aspects of climate research involves predicting how and when clouds will form, and how changes in global temperature and ocean currents will effect clouds and cloud formation into the future. Part of this problem lies in predicting where and when DMS occurs in the ocean, how much DMS escapes from the ocean, how DMS is converted into cloud nuclei, and how all of these things may change in the future. Our focus on this project is measuring the rate of DMS emission from the the ocean’s surface and studying how the rate of DMS emission depends on wind, waves and other phenomena. If we’re successful, our ability to predict DMS emission from the ocean will improve, refining one small piece of the climate puzzle. And perhaps a better understanding of how DMS escapes the ocean can improve our understanding of air/sea exchange in general. More on this in a future post…
Dark clouds and drizzle over the Southern Ocean is the backdrop for the ship’s bow mast, where many of the DMS, CO2, and wind sensors are mounted.