Atmospheric bromoform at Mace Head, Ireland: seasonality and evidence for a peatland source

[摘要] In situ atmospheric observations of bromoform (CHBr₃) made over a 2.5year period at Mace Head, Ireland from May 2001- Dec 2003, including duringthe NAMBLEX (North Atlantic Marine Boundary Layer Experiment) campaign, showbroad maxima from spring until autumn and winter minima, with mixing ratiosof 5.3+1.0 pptv (mid March - mid October) and 1.8+0.8 pptv(December-February). This indicates that, unlike CHCl₃, which has asummer minimum and winter maximum at Mace Head, local biological sources ofCHBr₃ have a greater influence on the atmospheric data thanphotochemical decay during long-range transport. The emission sources arepredominantly macroalgal, but we find evidence for a small terrestrial fluxfrom peatland ecosystems, which so far has not been accounted for in theCHBr₃ budget. Sharp increases in CHCl₃ and CHBr₃concentrations and decreases in O₃ concentrations occurred at nightwhen the wind direction switched from an ocean- to a land-based sector (landbreeze) and the wind speed dropped to below 5 ms^-1. These observationsinfer a shallow atmospheric boundary layer with increased O₃ depositionand concentration of local emissions of both CHCl₃ and CHBr₃. Theratio of ΔCHCl₃/ΔCHBr₃ varied strongly accordingto the prevailing wind direction; from 0.60+0.15 in south-easterly(100-170° and northerly (340-20°) air to 2.5+0.4 in north-easterly(40-70°) air. Of these land-sectors, the south-easterly air massesare likely to be strongly influenced by macroalgal beds along the coast andthe emission ratios probably reflect those from seaweeds in addition to landsources. The north-easterly airmasses however had an immediate fetch inland,which locally is comprised of coastal peatland ecosystems (peat bogs andcoastal conifer plantations), previously identified as being strong sourcesof atmospheric CHCl₃ under these conditions. Although we cannotentirely rule out other local land or coastal sources, our observations alsosuggest peatland ecosystem emissions of CHBr₃. We use correlationsbetween CHCl₃ and CHBr₃ during the north-easterly land breezeevents in conjunction with previous estimates of local wetland CHCl₃release to tentatively deduce a global wetland CHBr₃ source of 20.4(0.4-948)Gg yr^-1, which is approximately 7% of the total globalsource.

[学科分类] 大气科学