Abstract
Simultaneous observations of atmospheric potential oxygen (APO=O2+1.1×CO2) and air–sea O2 flux, derived from dissolved oxygen in surface seawater, were carried out onboard the research vessel MIRAI in the northern North Pacific and the Arctic Ocean in the autumns of 2012–2014. A simulation of the APO was also carried out using a three-dimensional atmospheric transport model that incorporated a monthly air–sea O2 flux climatology. By comparing the observed and simulated APO, as well as the observed and climatological air–sea O2 fluxes, it was found that the large day-to-day variation in the observed APO can be attributed to the day-to-day variation in the local air–sea O2 fluxes around the observation sites. It was also found that the average value of the observed air–sea O2 fluxes was systematically higher than that of the climatological O2 flux. This could explain the discrepancy between the observed and simulated seasonal APO cycles widely seen at various northern hemispheric observational sites in the fall season.
Highlights
High precision measurements of the atmospheric O2/N2 ratio have been carried out since the early 1990s to estimate the global terrestrial biospheric and oceanic CO2 uptake (e.g. Keeling et al, 1996; Bender et al, 2005; Manning and Keeling, 2006; Tohjima et al, 2008; Ishidoya et al, 2012a, 2012b)
Based on the evidence given in the previous paragraph, we conclude that the observed change rate of d(O2/N2)/CO2 can be attributed to the airÁsea exchange of O2 and N2 rather than to fossil fuel combustion
Atmospheric d(O2/N2), CO2 concentration and d13C of CO2 were observed onboard R/V MIRAI in the northern North Pacific and the Arctic Ocean during the autumn seasons of 2012Á2014
Summary
High precision measurements of the atmospheric O2/N2 ratio have been carried out since the early 1990s to estimate the global terrestrial biospheric and oceanic CO2 uptake (e.g. Keeling et al, 1996; Bender et al, 2005; Manning and Keeling, 2006; Tohjima et al, 2008; Ishidoya et al, 2012a, 2012b). On the other hand, Tohjima et al (2005, 2012, 2015) have carried out long-term observations of APO in the Pacific region since the early 2000, elucidating in much more detail the salient features of the seasonal cycle, latitudinal gradient and secular trend in APO In one of their studies (Tohjima et al, 2012), the observed seasonal APO cycles were reproduced relatively well by an atmospheric transport model (Maksyutov and Inoue, 2000) driven by an airÁsea climatology of O2 and N2 fluxes (Garcia and Keeling, 2001). Garcia and Keeling, 2001), simultaneous observations of APO and the airÁsea O2 flux are needed To achieve this objective, we collected flask air samples onboard the research vessel MIRAI during its cruises in the northern North Pacific and the Arctic Ocean during the fall seasons of 2012Á2014, to analyse for the atmospheric O2/N2 ratio, CO2 concentration and stable isotopic ratio (d13C) of CO2, along with continuous measurements of dissolved oxygen in the surface seawater. We compare the observed APO with simulated APO using an atmospheric transport model (Taguchi et al, 2002) driven by climatological airÁsea O2 and N2 fluxes (Garcia and Keeling, 2001), to evaluate differences between the observed and climatological O2 fluxes during the fall season in the northern North Pacific and the Arctic Ocean
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