Temporal and spatial variation in methyl bromide flux from a salt marsh

Abstract

[1] Methyl bromide (CH 3 Br) is a trace gas involved in stratospheric ozone depletion with both anthropogenic and natural sources. Estimates of natural source strengths are highly uncertain. In this study, >320 highly temporally and spatially resolved measurements of CH 3 Br emissions from a salt marsh in Scotland (56°00 0 N, 2°35 0 W) were made during one year using eight static enclosures. Net emissions showed both strong seasonal and diurnal cycles. Day-today maxima in emissions were associated with sunny days. Emissions dropped to zero when vegetation was removed. Mean measured CH 3 Br emission was 350 ng m À2 h À1 , but a few ''hot spots'' (measured maximum 4000 ng m À2 h À1 ) dominated integrated emissions. A crude scale-up of the annual mean emission yields an estimate for global CH 3 Br emission of $1 (0.5-3) Gg y À1 (range uses annual mean from lowest and highest emitting enclosures), $10% the global salt marsh emission regularly quoted in the literature.

Long-term CH<sub>3</sub>Br and CH<sub>3</sub>Cl flux measurements in temperate salt marshes

“…However, there was no significant influence of PAR on net methyl halide fluxes from either salt marsh, during the growing season. This lack of correlation is surprising since it has been suggested that sunlight levels are important drivers for methyl halide production by plants (Dimmer et al, 2001;Manley et al, 2006;Wang et al, 2006;Drewer et al, 2006). However, separating individual effects of PAR and temperature is difficult because of their intrinsic tendency to correlate.…”

“…Thus gaps remain in both quantification and characterisation which must be filled in order to gain a clearer picture of the current and future impact of salt marshes on atmospheric chemistry. The aim of this study was to extend considerably a previous monitoring project for CH 3 Br fluxes at Heckie's Hole salt marsh on the east coast of Scotland (Drewer et al, 2006) by including a second site at Hollands Farm salt marsh on the west coast and measurement of CH 3 Cl. Measurements were conducted for more than 2 yr at the two salt marshes, comprising more than 450 individual daytime measurements in all seasons and more than 100 diurnal measurements.…”

“…The different groups found either sunlight, temperature, season or different plant growth stages as the most likely drivers of temporal methyl halide flux variations, with Rhew et al (2000), Manley et al (2006) and Rhew and Mazéas (2010) identifying plant species as the cause of spatial flux variation. Of the other studies, only Drewer et al (2006) attempted to identify factors underlying the spatial variation and suggested sediment bromide content as a possible contributor. In general, fluxes of CH 3 Br and CH 3 Cl were found to be positively correlated with each other.…”

“…Whilst extrapolation of flux measurements conducted in southern California (Rhew et al, 2000(Rhew et al, , 2002Manley et al, 2006) suggested that salt marshes could be responsible for 7-12% and 4-5% of global CH 3 Br and CH 3 Cl production, respectively, extrapolation of measurements in Tasmania by Cox et al (2004), in Ireland by Dimmer et al (2001) and in Scotland by Drewer et al (2006) suggested that salt marshes were likely to account for less than 2% of global CH 3 Br and no more than 0.03% of global CH 3 Cl production (all estimates based on a global salt marsh area of 0.38 × 10 12 m 2 , Woodwell et al, 1973). Recent measurements by Rhew and Mazéas (2010) at northern California salt marshes show notably smaller methyl halide net Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Growing season methyl bromide and methyl chloride fluxes at a sub‐arctic wetland in Sweden

“…[5] Seasonal and diurnal trends in CH 3 Br and CH 3 Cl fluxes from non-polar ecosystems have been reported in a number of studies [e.g., Redeker et al, 2000;Rhew et al, 2002;White et al, 2005;Drewer et al, 2006;Manley et al, 2006], but there has been no consistent evidence for external parameters driving CH 3 Br or CH 3 Cl fluxes across all ecosystem types; individual studies have suggested that light [Drewer et al, 2006], temperature [Rhew et al, 2000[Rhew et al, , 2002Redeker and Cicerone, 2004], or soil pore-water saturation [Redeker and Cicerone, 2004] may affect emissions. In arctic Alaskan tundra it was found that drained sites had greater rates of CH 3 Br and CH 3 Cl uptake than flooded sites in both coastal and inland areas [Rhew et al, 2007;Teh et al, 2009], with water table depth correlating most strongly with CH 3 Cl and CH 3 Br net uptake [Teh et al, 2009].…”