Utilising radon measurements at Macquarie Island to estimate CO2 and CH4 transport across the Southern Ocean from Australia and New Zealand
Six years of Radon-222 (radon), carbon dioxide (CO2), and methane (CH4) observations at Macquarie Island were used to investigate the strength and seasonality of air mass transport from Australia and New Zealand into the mid-Southern Ocean, which remains poorly characterised. The unique physical cha...
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School of Earth, Atmospheric and Life Sciences
2021
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| Online Access: | https://ro.uow.edu.au/thsci/184 https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1207&context=thsci |
| Summary: | Six years of Radon-222 (radon), carbon dioxide (CO2), and methane (CH4) observations at Macquarie Island were used to investigate the strength and seasonality of air mass transport from Australia and New Zealand into the mid-Southern Ocean, which remains poorly characterised. The unique physical characteristics of radon allow it to be used to identify both air masses characteristic of hemispheric mean ‘baseline’ conditions, as well as air masses modified by recent terrestrial influence. A strong seasonality in the number and duration of recently terrestrially influenced air masses was observed, characterised by a December-February minimum (with an average of 16 events with a mean length of 8.37 hours) and a June-September maximum (with an average of 35 events with a mean length of 15.97 hours). The terrestrial fetch regions for these events changed from north of Tasmania in summer and around Sydney in winter. Once long-term accumulation trends were characterised and removed, a pronounced seasonality was observed in ‘baseline’ CO2 and CH4 concentrations, characterised by a February-March minimum and September maximum. Relative to baseline concentrations, a relatively consistent seasonality was observed in the deviations of CO2 (amplitude ~4 ppm) and CH4 (amplitude ~20 ppb) in terrestrial events that was broadly out of phase with the baseline seasonality of these trace gases. This seasonality in terrestrial transport characteristics into the mid-Southern Ocean was found to be through a combination of two main influences: i) seasonally changing fetch regions, and ii) seasonally changing emission strengths from these fetch regions derived using the radon calibrated flux technique (CO2: -2.5 x106 ± 2.3 x106 kg CO2 km-2 y-1 to 3.6 x106 ± 2.5 x106 kg CO2 km-2 y-1; CH4: 2.1 x103 ± 5.4 x103 kg CH4 km-2 y-1 to 9.4 x103 ± 4.5 x103 kg CH4 km-2 y-1). Significant and unexpected CH4 depletion events were identified as part of this study, for which a chlorine (Cl) sink mechanism is proposed but will require further research. |
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