Objective

The main objective was to investigate the distribution and production of volatile halogenated organic compounds (halocarbons) in different habitats in the Arctic during polar sunrise. Special emphasis was put on the contribution of halocarbons to the atmosphere produced by organisms in snow and ice compared to the contribution from pelagic organisms.

Halocarbons are ubiquitous trace constituents of the oceans and the atmosphere. Their role in the global circulation of halogens and in atmospheric chemical reactions has been discussed extensively within the last few years in connection with their ability to affect the atmospheric ozone budget. Unlike the chlorofluorohydrocarbons (CFCs), which are ozone-depleting compounds of anthropogenic origin, other chlorinated, brominated and iodinated compounds are involved in a number of chemical and biological processes. It is known that a number of brominated and chlorinated compounds deliver chlorine and bromine to the stratosphere, and it has been shown that in the stratosphere bromine is about 50 times more efficient in depleting ozone than is chlorine. It has also been shown that the synergistic effect of chlorine and bromine species accounts for approximately 20% of the polar stratospheric ozone depletion. The iodinated substances have relatively short lifetimes in the atmosphere and are therefore only involved in reactions in the lower troposphere.

The formation of halocarbons is presumed to be closely connected to the formation of hydrogen peroxide. We have suggested that during the reduction of hydrogen peroxide by haloperoxidases halide ions are oxidised, thus forming hypochlorite and hypobromite, which will react with the dissolved organic matter, thus forming a number of halogen containing organic compounds.

In accordance with the objective the following studies have been performed during Arctic Ocean 2002:

• Studies of the production of halocarbons by pelagic, ice-living and snow micro-organisms
• Studies of the surface water concentrations of halocarbons during transects
• Studies of the diurnal production of halocarbons and the subsequent flux of halocarbons from the sea surface to the atmosphere
  

  Photo: Pauline Snoeijs Leijonmalm

Sampling strategy and analytical procedures

Seawater samples were collected from the rosette sampler along seven transects (45 stations) for the determinations of halocarbons (figure 1).

Eight ice stations were occupied during the cruise. Six of the stations had a duration of six hours and two of 36 hours. During the shorter stations, brine was collected from several holes drilled in the ice. Water from underneath the ice was also collected. During the 36-hour stations samples of brine were collected every one and a half hours, except for the station in Storfjorden where bad weather prohibited work on the ice. The samples collected were used for incubation experiments onboard the ship. Snow samples were also collected.

The experiments were performed in 60 ml glass bottles and incubated at +1°C and 150 μE. The formation of halocarbons was determined in brine of different salinities and filters were stored in liquid nitrogen to determine pigment composition, in order to relate the production rates to classes of organisms. After approximately 24 hours the amounts of halocarbons were determined.

During the entire cruise air was sampled through a Teflon tube. The air was drawn from the front of the ship at a height of approximately 12 m above the sea surface.

The halocarbons were pre-concentrated with a purge-and-trap technique and then determined by capillary gas chromatography with either electron capture detection or mass spectrometric detection. The compounds measured were: iodomethane, iodoethane, 1-iodopropane, 2-iodopropane, 1-iodobuthane, 2-iodobuthane, diiodomethane, dibromomethane, tribromomethane, dibromochloromethane, bromodichloromethane, bromochloromethane, chloroiodomethane, dichloromethane, trichloromethane, trichloroethene and tetrachloroethene.

Preliminary results

Several factors are uncertain in the estimate of the emissions of halocarbons from the oceans to the atmosphere. It has been shown earlier that there are seasonal as well as geographical and diurnal differences in the production of halocarbons by marine algae. In addition, earlier studies have shown that there is a large release of halocarbons during spring in the Arctic, and it has been questioned whether this “burst” is due to abiotic or biotic factors.

In order to establish the importance of the halocarbon production in sea ice, several incubation experiments were performed with brine. As can be seen in figure 2, the increase in concentration of halocarbons was related to the salinity of the brine.

The data presented in figure 2 were used to calculate the corresponding production rates, and the rate of production was observed to increase with salinity (figure 3 to the left). This is probably due to more efficient enzymatic systems in organisms occupying the different habitats. This was also demonstrated in our experiments with differently sized organisms, where the smallest-sized organisms had the highest production rates (figure 3 to the right ). The composition of micro-algae will be determined by identification of pigments present in the samples by high performance liquid chromatography.

Halocarbon concentrations were monitored every one and a half hours during the 36-hour stations. The measurementswere performed in seawater, brine and snow. Preliminary estimates of the production and release of halocarbons are presented in table 1. Interestingly, the production and release rates in snow were high. Some samples were collected in order to establish if a photochemicalproduction of hypobromite (HOBr) could be an explanation for the relatively high formation rates, since the biological activity seemed to be low. No formation of HOBr could be established. Therefore we presently believe that bacterial activity could account for the production of halocarbons in snow.

The distribution of halocarbons was studied during the seven transects. The concentrations of brominated and iodinated compounds along the transects show rather low values in deep waters, while the highest values were found in the upper layers. Maximum concentrations were found on the Icelandic shelf and relatively high values could be measured over the Greenland continental shelf. Two examples of the distribution of brominated and iodinated compounds are shown in figure 4.

To conclude, the results from the the expedition have increased our insights regarding the impact of halocarbons on the lower atmosphere of the Arctic.