[A-AS11] 大気化学

[AAS11-P03] 10-years observation of atmospheric Ar/N₂ ratio and its comparison with some indexes associated with climate change

*石戸谷 重之¹、遠嶋 康徳²、菅原 敏³、後藤 大輔⁴、坪井 一寛⁵、村山 昌平¹、石島 健太郎⁵、丹羽 洋介²、青木 伸行¹ (1.産業技術総合研究所、2.国立環境研究所、3.宮城教育大学、4.国立極地研究所、5.気象研究所)

キーワード：大気中アルゴン濃度、大気重力分離、海洋貯熱量、全球表面温度、太平洋十年規模振動

Variations in the tropospheric and the stratospheric Ar/N₂ ratio are driven mainly by air-sea Ar and N₂ flux due to changes in solubility and gravitational separation of Ar and N₂, respectively (e.g. Keeling et al., 2004; Ishidoya et al., 2013). Recently, Ishidoya et al. (2021) reported temporal variations of gravitational separation in the middle atmosphere could also modify the long-term trend of the tropospheric Ar/N₂ ratio. Therefore, Ar/N₂ ratio has been applied not only to evaluating seasonal air-sea heat flux (Morgan et al., 2021) and atmospheric circulation in the stratosphere (e.g. Sugawara et al, 2018), but also to estimate changes in the ocean heat content (OHC) by simultaneous analyses of the surface and the stratospheric long-term trends (Ishidoya et al., 2021). We have continued observations of the Ar/N₂ ratio by using a mass spectrometer at Tsukuba (36°N, 140°E) and Hateruma Island (24°N, 124°E) for 10 years since 2012, and started observations at Cape Ochiishi (43°N, 146°E), Ryori (39°N, 142°E), Takayama (36°N, 137°E), and Minamitorishima (24°N, 154°E), Japan and Syowa station (69°S, 40°E), Antarctica since 2013 – 2020. In this study, we compared the long-term Ar/N₂ ratio data with some indexes associated with climate change; global OHC, global surface temperature, ENSO and Pacific Decadal Oscillation (PDO). The observed Ar/N₂ ratio showed secular increases before 2018 and subsequent decreases, although the global OHC showed secular increase throughout the period (updated from Levitus et al., 2012, https://www.ncei.noaa.gov/access/global-ocean-heat-content/). It was also found that the Ar/N₂ ratio at Tsukuba and Hateruma showed interannual variations in phase with the global OHC, and the minima of the rate of change of Ar/N₂ratio and the global OHC appeared at the beginning of 2016 and 2020 when maxima in the surface temperature anomaly appeared (JMA, https://www.data.jma.go.jp/cpdinfo/temp/nov_wld.html). Secular increases and subsequent decreases of Ar/N₂ ratio also seems to be corresponding to the variations in PDO (updated from Mantua, 1999, https://www.ncdc.noaa.gov/teleconnections/pdo/), of which links to variability in Earth’s energy imbalance have been suggested by past studies (Loeb et al., 2021). It seems that we have detected long-term variations in Ar/N₂ ratio corresponding to climate change, however, further studies are indispensable for quantitative interpretation.

Acknowledgements
We thank staff of Global Environmental Forum (GEF), the Science Program of the Japan Antarctic Research Expedition (JARE) and Japan Meteorological Agency (JMA) for their works to collect the air samples at Hateruma and Ochiishi stations (GEF), Syowa station (JARE), Ryori and Minamitorishima (JMA), respectively. This study was partly supported by the JSPS KAKENHI (grant nos. 15H02814, 19H01975 and 19K03963) and the Global Environment Research Coordination System from the Ministry of the Environment, Japan (grant nos. METI1454 and METI1953).

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