Identification of regional sources of methyl bromide and methyl iodide from AGAGE observations at Cape Grim, Tasmania

There are large uncertainties in identifying and quantifying the globally significant sources and sinks of methyl bromide (CH3Br) and methyl iodide (CH3I). Long-term, quasi-continuous observations can provide valuable information about their regional sources, which may be significant in the global context. We report 3 years of in situ observations of these trace gases from the AGAGE ( Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41 degrees S, 145 degrees E). The average background levels of CH3Br and CH3I during March 1998 - March 2001 were 8.05 and 1.39 ppt ( dry air mole fractions expressed in parts per 10(12)), respectively. The CH3Br background data showed little seasonal variability. Trajectory analyses reveal that air masses showing elevated CH3Br levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater and/or urban source regions. The CH3I background data showed a seasonal cycle with a 3-year average amplitude of 0.47 ppt and maximum concentrations in summer, suggesting that the Southern Ocean is a significant source. Trajectory analyses reveal that air masses showing highly elevated CH3I levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater regions and/or the open-ocean regions of Bass Strait and the Tasman Sea.