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


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Grose, Michael Richard 2008 , 'The ocean-atmosphere cycles of methyl halide trace gases in the Southern Ocean and Tasmanian coastal waters', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Grose, Michael Richard
Keywords: Cape Grim Baseline Air Pollution Station, Ocean-atmosphere interaction, Ocean-atmosphere interaction, Chemical oceanography, Carbon cycle (Biogeochemistry), Atmospheric carbon dioxide
Copyright Holders: The Author
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Copyright 2008 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2008. Includes bibliographical references. Ch. 1. Introduction and literature review -- Ch. 2. Analysis of methyl halide and particle concentrations measured at the Cape Grim station -- Ch. 3. Production of methyl halide in the open Southern Ocean west of Tasmania -- Ch. 4. Production of methyl halides in the coastal waters of northwest Tasmania -- Ch. 5. Production of methyl halides in inshore waters less than 5km offshore at Cape Grim -- Ch. 6. Links between emission of methyl iodide, iodine and other gases from kelp beds and new particles detected at the Cape Grim station -- Ch. 7. Summary and conclusions

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