Terrestrial sources and sinks of halomethanes near Cape Grim, Tasmania

The terrestrial sources and sinks of halomethanes, at and nearby the Cape Grim Baseline Air Pollution Station, Tasmania (41°S, 145°E), have been investigated over 12 months (July 2000–June 2001) using a flux chamber technique. The sites studied are representative of the soils and vegetation within 20 km of Cape Grim and include soil/plant litter under melaleuca scrub and eucalypt forest canopies, native tussock grass/soil, improved pasture grass/soil and coastal wetland. On average, the soil, tussock and pasture sites were found to be sinks for methyl chloride (CH3Cl), whereas the coastal wetland site was a source for CH3Cl, methyl bromide (CH3Br) and methyl iodide (CH3I). The grassland sites (pasture and tussock) were a small source for CH3Br and the soil sites (melaleuca and eucalypt) a sink. All sites were significant sources of chloroform (CHCl3). The interspecies ratios for significantly correlated halomethane net fluxes at some terrestrial sites were compared with the ratios found for AGAGE Cape Grim in situ data characterising north and west coast Tasmania. Flux ratios for CH3Br/CH3I at the coastal wetland and pasture sites were found to be similar to ratios found for the Cape Grim in situ data. Common production mechanisms were indicated for CH3Br and CH3Cl which were significantly correlated at the coastal wetland, eucalypt and melaleuca sites and for CH3Br and CH3I at the pasture and coastal wetland sites. A seasonal variation in the net flux was observed at the coastal wetland site for CH3Cl and CH3Br.