The ocean-atmosphere cycles of methyl halide trace gases in the Southern Ocean and Tasmanian coastal waters

The trace gases methyl bromide and methyl iodide are important vectors for the transport of halogens from the ocean to the atmosphere. The halogens play roles in atmospheric reactions including catalytic ozone destruction and iodine has a role in the formation of new particles. The oceans are a large source and sink of the methyl halide gases and there are known biological and photochemical sources in the surface ocean. However the ocean-atmosphere cycle and the variables controlling production are still quite poorly understood. This study investigates the marine sources and sinks of marine methyl halides, particularly the biogenic source in the open ocean, coastal and inshore waters, in the region of the Cape Grim Baseline Air Pollution Station, Tasmania, in the period 2003 to 2007. Methyl halide concentrations measured at Cape Grim under the Advanced Global Atmospheric Gas Experiment are made under all conditions, but this study focuses on the measurements made in baseline air that originates over the Southern Ocean and has no contact with land for some time prior to reaching Cape Grim. Methyl bromide in baseline air in 2004-2005 showed a mean concentration of 7.27 pptv, with very minor seasonal changes with no evidence of a strong daily cycle. In contrast, methyl iodide concentration in baseline air in 2004-2006 showed a seasonal cycle with a maximum of 1.62 pptv in late summer to autumn and a minimum of 0.87 pptv in late winter to spring, and was correlated with sea surface temperature and to primary production in the adjacent ocean, with a time offset. This was consistent with a source from both photochemical production and biological production, together with temperature-dependent chemical loss and flux processes. The methyl iodide record also showed increased frequency and magnitude of short-term peaks over summer, consistent with local sources from coastal waters. There was also a distinct daily cycle consistent with the atmospheric photochemical sink. The concentrations of methyl halides in seawater and air were measured in the waters off Cape Grim using gas chromatography with electron capture detection. Seawater concentration and sea-air saturation of methyl halides were typical of temperate waters, with a pattern of higher magnitude and variability closer to the shore. An exponential decrease in concentration with distance from the coast out to 5 km was observed, then a further decrease out across the continental shelf and into the open ocean. Mean methyl bromide seawater concentration from all measurements was 30 pM inshore, 7 pM at 5 km, 4 pM in shelf waters and 1.5 pM in open ocean. Mean methyl iodide seawater concentration was 44 pM at the shore, 11 pM at 5 km, 2 pM in shelf waters and 0.7 pM in open ocean. This created a strong source region in coastal waters with consistent positive saturations, falling to a minor source or sink in the open ocean. Open ocean methyl iodide concentration showed a positive relationship with inorganic iodide, suggesting a positive or synergistic relationship. Production of methyl halides in coastal waters showed some correlations to phytoplankton numbers during high biomass events, including a bloom of diatoms of the genus Chaetoceros. Methyl halide concentrations were higher and more variable in inshore waters near beds of bull kelp (Durvillaea potatorum), and one period of high methyl iodide saturation at the coast in summer was followed by detection of a peak in ultra-fine condensation nuclei at the station. It is unlikely the methyl iodide acted as a precursor to new particle formation in this instance, but this observation suggests that the peak in methyl iodide was concurrent with an emission of other precursor gases or particles directly from kelp. Bull kelp emits moderate amounts of iodine gas (18.1 pmol g\(^{-1}\) min\(^{-1}\), using fresh weight of kelp) when exposed to the air and under photo-oxidative stress, and this is also unlikely to act as a precursor to new particle formation. Chamber experiments showed that particles formed above kelp under photo-oxidative stress consisted of aromatic organic compounds. It appears likely that the role of iodine in particle formation processes is not as significant as at other locations such as Mace Head, Ireland, but emissions of an unknown and possibly organic precursor gas from kelp is likely to contribute new particles at Cape Grim