Science at the CASLab
Primary goals for the laboratory include:
- Characterisation of coastal Antarctic atmospheric chemistry
- Studies of snow photochemistry and air/snow exchange
- Studies of atmospheric chemistry associated with the sea ice zone e.g. halogens, surface ozone and mercury depletion
- Studies of Southern Ocean air/sea exchange signals and processes
- Monitoring for regional and global trends (given the clean and uncomplicated background system)
- Deriving baseline signals to understand ice core data (because understanding today’s system gives a chance to model the past)
Snow photochemistry and air/snow exchange
Rather than being an inert medium, the polar snowpack is highly chemically active and, consequently, is a major source of reactive trace gases to the overlying boundary layer. For NOx, the major precursor is nitrate impurities from which NO2 (as the major product) and NO (as the minor) are generated via photolysis. There is evidence for the photochemical production of many other reactive trace gases from snowpack impurities, including HCHO, CO, C2H2 and HONO. There is also extensive physical exchange of species such as HCHO and H2O2. Critically, many of these emissions are either a direct or an indirect source of OH, so the snowpack effectively controls the oxidising capacity of the atmosphere.
While considerable progress in understanding and quantifying snow photochemistry and air/snow exchange has been made in recent years, there is still some way to go before reliable assessments of the wider implications can be made.
- Jones, A.E., Weller, R., Wolff, E.W., Jacobi, H-W., Speciation and Rate of Photochemical NO and NO2 Production in Antarctic Snow, Geophys. Res. Lett., 27, 3, 345–348, 2000.
Atmospheric chemistry associated with the sea ice zone e.g. halogens, surface ozone and mercury depletion
Tropospheric ozone depletion events (ODEs) are well known phenomena that are observed in both polar regions during spring. Surface ozone drops from background concentrations to below instrumental detection limits (of a few parts per billion by volume). At coastal stations, concentrations can remain suppressed on the order of hours to days; observations over the frozen Arctic Ocean have found surface ozone to be depleted for several weeks at a time. No equivalent observations from the frozen Southern Ocean exist. Vertical profiling measurements have shown that the depletion can extend from the ground up to ∼2 km altitude.
The ozone depletion is driven by halogens, with bromine compounds (Br/BrO) appearing to play an especially important role. The source of Br/BrO appears tightly linked with the sea ice zone, although the precise mechanism(s) for delivering halogens to the atmosphere are still under debate.
Enhanced reactive bromine emanating from the sea ice zone also causes major perturbations to the mercury cycle. Atmospheric gaseous elemental mercury is converted to more reactive forms which can then be deposition to the surrounding ecosystems. While the biogeochemistry of mercury has predominantly been studied in the Arctic, such processes have also been reported from the Antarctic.
Studies of Southern Ocean air/sea exchange signals and processes
The Southern Ocean is an important CO2 sink, estimated to presently take up ∼7% of global CO2 emissions every year. It has been suggested that CO2 uptake by the Southern Ocean has reduced in efficiency over the past few decades, as increased winds over the ocean have enhanced the natural carbon outgassing rate over that of anthropogenic CO2 uptake. This work is controversial, and a primary constraint on more definitive conclusions is the dearth of atmospheric observations in the Southern Ocean.
A recent biogeochemical modeling study has suggested that Antarctic continental shelves are major, but as yet unaccounted for, sinks of atmospheric CO2. They calculated that the CO2 sink of the Ross Sea, a major embayment within the Pacific Ocean sector, is equivalent to 27% of the entire Southern Ocean. Halley lies at the edge of the Weddell Sea, the other major embayment.
Air arrives at Halley with a variety of histories, including passage over the open ocean and the nearby Weddell Sea. Access to such air masses, together with the longevity of the station, makes Halley an ideal location from which to study Southern Ocean air/sea exchange.
Monitoring for regional and global trends
Since 1983, Halley station has contributed to the carbon cycle measurement program of the NOAA collaborative sampling network. Whole air samples are collected approximately weekly in glass flasks and returned to NOAA’s Global Monitoring Division in the US for analysis of various gases including CO2, CH4, CO, H2, SF6, N2O, 13C/12C in CO2, 18O/16O in CO2.
Example time series for CO2 (Conway et al., 2011) and CH4 (Dlugokencky et al., 2011) are shown here. Data shown in orange are preliminary; all other data have undergone rigorous quality assurance.
Deriving baseline signals to understand ice core data
A key to understanding the chemical development of our atmosphere is through study of changing chemical signatures in ice cores. They hold information on the state and composition of the atmosphere in the past, and hence how and why it has evolved. The way to access this information is to find constituents that might be used as proxies for environmental changes in the past, but in order to do so it is necessary to understand the processes that created the ice core record. Such factors might include the nature of the sources, subsequent chemical reaction, transport to the deposition site, a range of deposition processes (in-cloud, below cloud, dry deposition), and post-depositional losses and movement. A proper understanding of the relationship between the chemical climatology of the atmospheric boundary layer and the chemical constituents in firn and ice is crucial for the correct interpretation of the cores that are used in palaeo-atmospheric reconstruction. In essence, we cannot hope to correctly reconstruct a past atmosphere if we do not properly understand that of the present day.