SENTINEL
Researchers aboard the Canadian Coast Guard icebreaker Louis S. St-Laurent set this buoy into Arctic Ocean ice in August.
CONNECTING ICE AND AIR Scientists probe the role of sea ice in
ATMOSPHERIC CHEMISTRY as the Arctic warms JYLLIAN KEMSLEY, C&EN WEST COAST NEWS BUREAU
AT THE END OF SUMMER, sea ice covered 4.6 million km2 of the Arctic, according to satellite data analyzed by the National Snow & Ice Data Center. That was the second-lowest amount of ice observed since 1979, when satellites started keeping track. From 1979 to 1983, Arctic ice covered about 7.5 million km2. The lowest coverage was 4.3 million km2, in 2007. Caused by rising global temperatures, shrinking sea ice in the Arctic poses significant concerns: Because sea ice reflects sunlight, having less of it could accelerate global warming and its attendant effects on weather patterns and ecosystems worldwide. The loss of Arctic sea ice may also change atmospheric chemistry, because reactions on the ice surface play a role in the chemistry of the air above. Researchers are still in the early stages of understanding the interplay between ice and air in a remote region where concentrations of key compounds may be, at most, parts per trillion. Understanding these effects may provide clues to how rising temperatures in the Arctic will affect the globe. Arctic atmospheric chemistry centers on halogens. Seawater contains chloride,
bromide, and small amounts of iodide. As seawater freezes, those ions and compounds other than water get excluded from the bulk ice. The result is that sea ice somewhat resembles Swiss cheese, with brine contained in pockets and channels between ice crystal grain boundaries, says University of Alaska, Fairbanks, chemistry professor William R. Simpson. The brine also winds up on the ice surface, where its contents can react with the air above. Undetermined chemicals in the atmosphere oxidize Br– and Cl– on the ice to form Br2 and Cl2, which enter the atmosphere. Sunlight then photolyzes Br2 and Cl2 to extremely reactive Br• and Cl•. The halogen radicals cleanse the atmosphere of ground-level pollutants by reacting with ozone and a variety of organic and inorganic compounds, including SO2 and mercury. The same can happen with iodide, although iodine chemistry appears to be more prevalent in the Antarctic, for reasons scientists don’t understand. The polar halogen chemistry is unlike the atmospheric chemistry of the rest of MORE ONLINE
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the planet, which is driven primarily by ozone and HO•, says Paul B. Shepson, a chemistry professor at Purdue University. “As far as we know it’s a natural effect, although we don’t have a lot of evidence for what it looked like before there was human activity,” he adds. And still a lot remains unknown, such as the identities of the initial atmospheric oxidants and the mechanisms of the reactions. Scientists are also probing the roles newly frozen, “first year” ice plays compared with “multiyear” ice that has survived at least one summer and is less salty. As ice ages over the summer, the salty parts melt first and flow to the ocean. Consequently, any ice that remains by fall and lasts through the winter contains less salt. For this reason, scientists expect first-year ice to promote more halogen chemistry than does multiyear ice. Simpson and colleagues evaluated that hypothesis in a field campaign in Barrow, Alaska, in 2005. The group used differential optical absorption spectroscopy (DOAS) to measure concentrations of BrO, which is formed from the reaction of Br• and O3 and is an indicator of reactive bromine in the air. They found higher BrO concentrations in air that had contact with first-year ice rather than with either frost flowers—icy structures that form on top of the ice—or multiyear ice (Atmos. Chem. Phys., DOI: 10.5194/acp-7-621-2007). What this finding means for atmospheric chemistry overall in a warming Arctic, in which the ratio of first-year to multiyear ice is increasing, is an open question. IN ADDITION TO ground-based mea-
surements, satellites also measure BrO in the air. Satellites have the advantage of constant monitoring as well as a geographically comprehensive view of the Arctic atmosphere. They also show a total column concentration that includes what’s in both the troposphere and stratosphere. A comparison of ground-based data and satellite data indicated that some of the BrO must be in the stratosphere, contrary to researchers’ previous assumptions that satellite-observed BrO “hot spots” were all close to the ground (Geophys. Res. Lett., DOI: 10.1029/2010GL043798). Localizing BrO to the stratosphere is “a really key finding because it tells us that we have to do a little more work to interpret that satel-
See a buoy’s view of the Arctic seascape over time at cenm.ag/buoy. DECEMBER 5, 2011
Donaldson’s group has used glancing-angle Raman and fluorescence techniques to look at the reaction between ozone and bromide or iodide at the surface Asia of frozen salt solutions. The exBering periments demonstrated that Sea such reactions could form Br2 or I2, indicating that ozone may be a Arctic possible route to those gases in the Barrow, Ocean environment (J. Geophys. Res., DOI: Alaska 10.1029/2010JD013929). Donaldson and colleagues are now trying to determine the pH of the brine layer and how it might change under different condiNorth America tions. Seawater normally has a pH of about 8, but its buffering caEurope pacity might change as it freezes because carbonates precipitate at Greenland about –2 ºC. Because the halogen chemistry likely depends on pH, A FIELD CAMPAIGN in spring determining the pH of the brine Atlantic 2012 should reveal further details layer is important, Donaldson Ocean of Arctic atmospheric chemistry. notes. Researchers will use DOAS meaComputational modeling can ◼ Arctic sea ice in September 2011 surements from an airplane to also help unravel some of the – Median sea ice extent in look for free-radical intermediates brine-ice interactions. Molecular September from 1979 to 2000 • produced when Br reacts with O3 dynamics simulations by Shepto form BrO. They will also excise son and coworkers, for example, ice samples and pass oxidants show that the large polarizability over the surfaces to try to determine the for BrO. They also house meteorological of iodide and bromide ions drives them to airborne species that convert Cl– to Cl2 or sensors and a satellite communications the air-water interface of a salt solution (J. Br– to Br2, Shepson says. system (Atmos. Meas. Tech., DOI: 10.5194/ Phys. Chem. A, DOI: 10.1021/jp110208a). Conducting field campaigns in the Arcamt-3-249-2010). The enhanced surface concentrations of tic isn’t easy. Locations with the necessary The project has placed five buoys on the those ions may explain why air contains a power and other supplies are few. And ice since 2009. A buoy gets deployed in the higher ratio of iodine or bromine to chlothose sites are on coasts rather than on spring or summer; it collects data until the rine species compared with the concentrathe ice itself, leaving scientists to “hope ice beneath it melts, leaving the buoy in tion ratios found in seawater, Shepson says. that nature brings air over to your site so open water and vulnerable to toppling by Computational work can also help reyou can study it,” Shepson says. During ice chunks floating nearby. O-Buoy then veal details of brine formation. Columbia the occasional opportunities to go on ice, tries to recover the buoys and redeploy University chemical engineering profeshe adds, “there’s Murphy’s Law to deal them. The team is still analyzing data from sor V. Faye McNeill, for example, has used with: If you put your instruments in a little the first deployments, but “I think the modeling to determine that the thickness shed on the ice and have a little generator promise of the project is that we’ll really be of the brine layer depends on temperature to provide power and heat, God will wait able to step away from the coastal sites and as well as the identity and quantity of brine until you have everything up and running understand what’s going on in the middle solutes—those that come from the ice as and then the ice will crack and dump everyof the Arctic Ocean,” Simpson says. well as some that might be deposited from thing into the water.” Away from the Arctic, laboratory studies air, such as nitric acid (Atmos. Chem. Phys., To get around those problems, at least of ice chemistry present their own chalDOI: 10.5194/acp-11-9971-2011). in part, Shepson and others—Simpson, lenges: In particular, the high vapor presAs researchers unravel the pieces of sea research scientist Paty Matrai and colsure of water means that standard surface ice-air interactions, it’s still unclear what leagues from the Bigelow Laboratory for science techniques, which are done under changes are in store for the Arctic region Ocean Sciences, and collaborators at other vacuum, can’t be used to study ice, says as it warms. That is both the difficulty and institutions—are working on a field projJames Donaldson, a chemistry professor the urgency of the work, Shepson says. ect called O-Buoy that places battery- and at the University of Toronto. Research “Climate change is a reality,” he says, “and solar-powered buoys on the ice. The buoys has also shown that results obtained from that reality is very dramatically apparent in are loaded with an ultraviolet absorption doing reactions with ice and then melting the Arctic.” Understanding the chemistry sensor to measure ozone, an infrared specit for analysis are different from those obthat goes along with it matters deeply for trometer for CO2, and a DOAS instrument tained when studying the ice directly. understanding the planet. ◾ NATIONAL SNOW & ICE DATA CENTER
lite data,” Simpson says. While it’s not yet possible to extract BrO altitude information from the satellite data, it may someday be possible by correlating with other observations, he adds. In 2009, Arctic atmosphere researchers were able to augment their understanding by using mass spectrometry for the first time to look for chlorine as well as bromine species. Georgia Institute of Technology atmospheric chemistry professor L. Gregory Huey and colleagues found Cl2 that was “clearly being produced on snow and ice surfaces in the area around Barrow,” Huey says. The group found that, in turn, Cl• was oxidizing most of the methane in the air.
SHRINKING FOOTPRINT A map of the Arctic region shows the extent of sea ice in September compared with the median observed for 1979–2000.
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