2:00 PM - 2:30 PM
[ACG50-02] Research on the atmosphere-ocean gas exchange based on precise measurements of the atmospheric O2 and CO2 concentrations
★Invited Papers
Keywords:atmospheric oxygen, atmospheric CO2, atmosphere-ocean gas exchange, atmospheric potential oxygen
Atmospheric carbon dioxide (CO2) concentration shows a seasonal variation with maximum in winter and minimum in summer due mainly to seasonality in the land biotic CO2 flux. This is because the land biosphere takes up CO2 in spring and summer when the photosynthesis exceeds the respiration, and it emits CO2 in autumn and winter vice versa. In contrast, since the land biotic CO2 and oxygen (O2) fluxes are tightly coupled via a constant exchange rate (−O2:CO2≈1.1) for the respiration and photosynthesis, the atmospheric O2 is expected to show the opposite seasonal variation of CO2. Although the observed atmospheric O2 shows an expected seasonal variation, the seasonal amplitude is significantly larger than that of CO2. This larger seasonal amplitude of O2 is explained by the fact that the seasonality of the atmospheric O2 is attributed to the seasonality in not only the land biotic O2 flux but also oceanic O2 flux. The reason why the atmospheric CO2 seasonal cycle seldom includes the oceanic component is because the atmosphere-ocean CO2 exchange is considerably suppressed by chemical equilibrium between dissolved CO2 gas and dissolved inorganic carbon including bicarbonate and carbonate ions. The atmospheric O2 seasonality also reflects the seasonality in the oceanic O2 flux due to the rather faster atmosphere-ocean exchange rate of O2 in comparison with that of CO2. Focusing on these characteristics, Stephens et al. (1998) defined a tracer of atmospheric potential oxygen (APO) as a sum of the CO2 concentration weighted by 1.1 and the O2 concentration (APO=O2+1.1·CO2). From the definition, APO is invariant with respect to land biotic exchanges and only slightly depend on the fossil fuel combustion, which causes the observed long-term APO decrease because of the slightly larger −O2:CO2 exchange ratio for the average fossil fuel combustion (~1.4) than that for the land biotic exchange (1.1). APO mainly reflects the atmosphere-ocean gas (O2 and CO2) exchange, which is tightly related to the ocean biological activity, ocean circulation, and atmosphere-ocean heat exchange. For example, the amplitude of the seasonal variation of the APO reflects the strength of biological production in the ocean and vertical mixing in the ocean in winter, while the annual mean value of the APO reflects the distribution of the large-scale net gas exchange between the atmosphere and ocean. Therefore, spatio-temporal variation in APO based on the precise measurements of the atmospheric O2 and CO2 can be used to constrain such ocean processes.
The National Institute for Environmental Studies has conducted long-term and wide-area observations of the atmospheric O2 and CO2 concentrations by collecting flask air samples from remote sites in Japan and from cargo ships sailing in the northern and western Pacific. These observations have revealed the details of the spatio-temporal distribution of APO. In this presentation, we will present research findings on the atmosphere-ocean gas exchange in the Pacific area based on the APO data.
The National Institute for Environmental Studies has conducted long-term and wide-area observations of the atmospheric O2 and CO2 concentrations by collecting flask air samples from remote sites in Japan and from cargo ships sailing in the northern and western Pacific. These observations have revealed the details of the spatio-temporal distribution of APO. In this presentation, we will present research findings on the atmosphere-ocean gas exchange in the Pacific area based on the APO data.