In this project, field measurements and controlled experiments were used to investigate the magnitude of, and controls on, some terrestrial sources of CH<sub>3</sub>Br to help in assessing global fluxes. The influence of factors such as temperature, soil properties, sunshine level, plant species and water table, and correlation to methane (CH<sub>4</sub>) fluxes, were investigated. Field measurements were carried out using eight static flux chapters installed in pairs in the lower and upper areas of a salt marsh in East Lothian (Scotland) and four at a woodland site in Edinburgh. Mean annual measured net CH<sub>3</sub>Br emissions from the salt marsh were 350 ng m<sup>-2</sup> h<sup>-1</sup>, with a large variability in fluxes from zero to ~4000 ng m<sup>-2</sup> h<sup>-1</sup>, which increased seasonally. Summer emissions were on average about three times higher than winter emissions. Variation in CH<sub>3</sub>Br fluxes was larger between individual chambers indifferent pairs at the same salt marsh area (upper or lower) than between the two areas. There was also a strong association between sunny conditions and the timings of the frequent peaks in CH<sub>3</sub>Br emissions from the high emitting chambers above the general annual trend. There was a distinct diurnal variation in CH<sub>3</sub>Br fluxes from the salt marsh chambers throughout the year, also strongly correlated to the solar flux, showing highest emissions at mid-day and lowest at night. Salt marsh CH<sub>3</sub>Br fluxes were highly spatially variable, with a small proportion of “hot spots” accounting for the bulk of the spatially-integrated net emissions. There was no obvious explanation for the spatial heterogeneity of CH<sub>3</sub>Br emissions. Net CH<sub>3</sub>Br fluxes at the woodland site were an order of magnitude lower than at the salt marsh site, fluctuating between net uptake and net emission and with a mean in annual measured emissions (minimum and maximum in parentheses) of 27 (-73-279) ng m<sup>2</sup> h<sup>-1</sup>. Net fluxes from the woodland site showed no seasonal trend and only fairly small diurnal variation. There were no or only modest correlations of net CH<sub>3</sub>Br fluxes with air and soil temperature, CH<sub>4</sub> flux and water table depth. Scaling-up the salt marsh CH<sub>3</sub>Br fluxes from this work globally gave estimated annual emissions of 1 (0.5-3) Gg y<sup>-1</sup>, which was only ~10% of the global salt marsh emission estimate regularly quoted in the methyl bromide literature. In contrast, the emissions from the temperate woodland gave a global average of 2.7 (1.3-5) Gy y<sup>-1</sup> which contradicted the common assumption that woodland soils are only sinks for CH<sub>3</sub>Br. Global annual net CH<sub>3</sub>Br fluxes from leaf litter were calculated to be 0.9 (-1.3-4) Gg y<sup>-1</sup> which was within the range quoted in the literature, and from conifer needles to 5.3 (1.5-11.6) Gy y<sup>-1</sup>, slightly higher than previously reported.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:649723 |
| Date | January 2007 |
| Creators | Drewer, Julia |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13722 |
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