Abstract
Abstract. The Southern Ocean (south of 30∘ S) is a key global-scale sink of carbon dioxide (CO2). However, the isolated and inhospitable nature of this environment has restricted the number of oceanic and atmospheric CO2 measurements in this region. This has limited the scientific community's ability to investigate trends and seasonal variability of the sink. Compared to regions further north, the near-absence of terrestrial CO2 exchange and strong large-scale zonal mixing demands unusual inter-site measurement precision to help distinguish the presence of midlatitude to high latitude ocean exchange from large CO2 fluxes transported southwards in the atmosphere. Here we describe a continuous, in situ, ultra-high-precision Southern Ocean region CO2 record, which ran at Macquarie Island (54∘37′ S, 158∘52′ E) from 2005 to 2016 using a LoFlo2 instrument, along with its calibration strategy, uncertainty analysis and baseline filtering procedures. Uncertainty estimates calculated for minute and hourly frequency data range from 0.01 to 0.05 µmol mol−1 depending on the averaging period and application. Higher precisions are applicable when comparing Macquarie Island LoFlo measurements to those of similar instruments on the same internal laboratory calibration scale and more uncertain values are applicable when comparing to other networks. Baseline selection is designed to remove measurements that are influenced by local Macquarie Island CO2 sources, with effective removal achieved using a within-minute CO2 standard deviation metric. Additionally, measurements that are influenced by CO2 fluxes from Australia or other Southern Hemisphere land masses are effectively removed using model-simulated radon concentration. A comparison with flask records of atmospheric CO2 at Macquarie Island highlights the limitation of the flask record (due to corrections for storage time and limited temporal coverage) when compared to the new high-precision, continuous record: the new record shows much less noisy seasonal variations than the flask record. As such, this new record is ideal for improving our understanding of the spatial and temporal variability of the Southern Ocean CO2 flux, particularly when combined with data from similar instruments at other Southern Hemispheric locations.
Highlights
Greenhouse gases, such as CO2 released by human activity, are primarily responsible for global warming over the last century
This paper focuses on the technical aspects of the Macquarie Island in situ CO2 measurement programme, including site details, instrumentation and calibration (Sects. 2 and 3), data characteristics and comparison with the flask record
Combining in quadrature the 2B propagation uncertainty with Type 1, 2, 3 and 4 uncertainties estimated based on the worst-case 2G uncertainties, the 2G World Meteorological Organisation (WMO) X2007 propagation error was estimated as 0.024 μmol mol−1
Summary
Greenhouse gases, such as CO2 released by human activity, are primarily responsible for global warming over the last century. The Southern Ocean and Antarctic regions, remote from significant industrial and terrestrial biosphere activity, are ideally located to measure global-scale changes and long-term trends in the concentrations of these gases. Most atmospheric measurements from this region are flask samples, and previous work (Law et al, 2008) has shown that the Southern Ocean flux trends calculated by inversions are sensitive to atmospheric CO2 data quality. The island is ideally situated in the middle of the Southern Ocean near the subantarctic front, the boundary between the subantarctic zone and polar frontal zone This is a highly active oceanic region, known to be a CO2 sink in the summer months due to biological production, and a CO2 source in some areas during winter as a result of deepwater mixing (Lenton et al, 2013).
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