[A-AS07] Atmospheric Chemistry

[AAS07-P01] Interannual variations in delta(APO) and delta(Ar/N₂) observed at four Japanese stations for the period 2012-2022

*Shigeyuki Ishidoya¹, Yasunori Tohjima², Shohei Murayama¹, Yosuke Niwa², Kentaro Ishijima³, Satoshi Sugawara⁴, Tsuboi Kazuhiro³, Nobuyuki Aoki¹ (1.National Institute of Advanced Industrial Science and Technology (AIST), 2.National Institute for Environmental Studies, 3.Meteorological Research Institute, 4.Miyagi University of Education)

Keywords:Atmospheric Potential Oxygen (APO), Atmospheric Ar/N2 ratio, Interannual variations

Atmospheric Potential Oxygen (APO=O₂+1.1xCO₂) (delta(APO)) varies due to air-sea O₂, N₂ and CO₂ fluxes and fossil fuel O₂ and CO₂ fluxes, while atmospheric Ar/N₂ ratio (delta(Ar/N₂)) varies due only to air-sea Ar and N₂ fluxes. It has been reported by past studies that seasonal and interannual variations in delta(APO) are driven mainly by the air-sea O₂ and N₂fluxes, although the air-sea CO₂ and fossil fuel fluxes cause a secular delta(APO) trend (e.g. Tohjima et al., 2019; Ishidoya et al., 2021). As to the air-sea exchange, Ar and N₂ fluxes are driven by solubility change, and O₂ flux is driven by both solubility and biospheric changes. Therefore, it is expected that we can separate an interannual variation in delta(APO) due to the solubility change (delta(APO)_therm) from that to the net marine biological activities (delta(APO)_netbio) by a combined analysis of delta(APO) and delta(Ar/N₂). We have conducted simultaneous observations of delta(APO) and delta(Ar/N₂) at various observational sites, and the data longer periods than 10 years have been obtained at Tsukuba (36°N, 140°E), Hateruma Island (24°N, 124°E), Cape Ochiishi (43°N, 146°E), and Takayama (36°N, 137°E), Japan (updated from Ishidoya et al., 2021). The annual change rate of the average delta(APO)_therm at the four sites, obtained by multiplying a coefficient of 0.9 derived from differences in the solubility in O₂ and Ar (Weiss, 1970), was found to vary in phase with the Southern Oscillation Index (SOI) and the annual change rate of the global ocean heat content. On the other hand, the corresponding annual change rate of the average delta(APO)_netbio, obtained by subtracting the rate of delta(APO)_therm from delta(APO) (small contributions of the air-sea CO₂ and fossil fuel fluxes are also subtracted based on a simulation using an atmospheric transport model), varied in opposite phase with SOI. These responses of delta(APO)_therm and delta(APO)_netbio to El Niño / La Niña events are qualitatively consistent with those expected from the simulations based on a community earth system model by Eddebbar et al. (2017).

Acknowledgements
We thank staff of Global Environmental Forum (GEF) for their works to collect the air samples at Hateruma and Ochiishi stations. This study was partly supported by the JSPS KAKENHI (grant nos. 22H05006, 19H01975 and 15H02814) and the Global Environment Research Coordination System from the Ministry of the Environment, Japan (grant nos. METI1454 and METI1953).

References
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