Characterization of methyl bromide and methyl chloride fluxes at temperate freshwater wetlands

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are important natural sources of halogens to the atmosphere. A total of 568 CH3Br and 418 CH3Cl net flux measurements were made for up to 2 years at the same locations within four different wetlands in Scotland. Mean (+/- 1 standard deviation (SD)) CH3Br and CH3Cl net fluxes across all measurements at each wetland were: Auchencorth Moss, 8 (+/- 7) and 3560 (+/- 1260) ngm(-2) h(-1); Old Castle Farm, 420 (+/- 70) and 500 (+/- 260) ngm(-2) h(-1); Red Moss of Balerno, 500 (+/- 90) and 140,000 (+/- 36,000) ngm(-2) h(-1); and St Margaret's Marsh, 3600 (+/- 600) and - 270 (+/- 450) ngm(-2) h(-1). None of the wetlands was a large net sink. Where substantial emissions were observed, these followed seasonal trends, increasing early in the growing season and declining in early autumn. Some diurnal cycles were observed, with emissions greatest during the day, although lower emissions were present at night. None of the measured environmental parameters was a strong universal driver for fluxes, which were heterogeneous within and between the wetlands, and larger on average than reported to date; plant species appeared to be the dominant factor, the latter confirmed by vegetation removal experiments. Calluna vulgaris and Phragmites australis emitted particularly large amounts of CH3Br, the former also emitting substantial CH3Cl. While acknowledging the substantial uncertainties in extrapolating globally, observations from this work suggest that wetlands contribute more CH3Br and CH3Cl to the atmosphere than current World Meteorological Organization estimates.