[A-AS12] 大気化学

[AAS12-08] スバルバール諸島ニーオルスンにおける大気主成分濃度・同位体比の観測から示唆される下層大気の分子拡散分離

*石戸谷 重之¹、後藤 大輔²、菅原 敏³、森本 真司⁴、青木 周司⁴、村山 昌平¹、中澤 高清⁴ (1.産業技術総合研究所、2.国立極地研究所、3.宮城教育大学、4.東北大学)

キーワード：大気成分拡散分離、Ar/N2比、主成分同位体比、気温逆転層

Recent technical advances has made it possible to observe a molecular diffusive separation of the atmosphere based on high precision measurements of the composition of atmospheric major components. In the middle to lower stratosphere, Ishidoya et al. (2013) reported the existence of observable gravitational separation based on the measurements of stratospheric air samples collected using a balloon-borne cryogenic air sampler. In the lower atmosphere, Adachi et al. (2006) reported the diffusive separation of Ar and N₂, mainly due to thermal diffusion, in the center of a wide desert during the nighttime when vertical temperature inversions are generated. To examine whether the diffusive separation of the atmosphere is also detectable near the surface in polar region, air samples collected at Ny-Ålesund, Svalbard (79°N, 12°E) have been analyzed for delta(Ar/N₂)_, delta(O₂/N₂), delta¹⁵N of N₂, delta¹⁸O of O₂ and delta⁴⁰Ar by using a mass spectrometer (Ishidoya and Murayama, 2014) since January 2013. It was found that delta¹⁵N and delta¹⁸O show small but significant seasonal cycles, with the seasonal maxima and minima in winter and summer, respectively. The peak-to-peak amplitudes of the respective seasonal cycles of delta¹⁵N and delta¹⁸O were about 2 and 4 per meg. On the other hand, no significant seasonal cycle was seen in delta(Ar/N₂). If we assume the seasonal cycles of delta¹⁵N and delta¹⁸O are attributed mainly to gravitational separation in a temperature inversion layer during polar night in winter and corrected the delta(Ar/N₂) for the separation by subtracting 12 x (delta¹⁵N + delta¹⁸O/2)/2 (delta(Ar/N₂)_cor), then the delta(Ar/N₂)_cor show clear seasonal cycle with a maximum in August. The peak-to-peak amplitude of the seasonal delta(Ar/N₂)_cor cycle is about 25 per meg, and the appearance time of seasonal maximum agrees with that of the sea surface temperature around Ny-Ålesund. These results suggest that gravitational separation is observable near the surface at Ny-Ålesund. Our suggestion would be supported by Keeling et al. (2004) who reported the delta(Ar/N₂) observed in the polar region may be detectably enriched near the ground by gravitational separation or thermal diffusion under condition of strong surface inversions.

References
Adachi, Y. et al. (2006) Diffusive separation of the lower atmosphere, Science, 311, 1429.
Ishidoya, S. et al. (2013) Gravitational separation in the stratosphere – a new indicator of atmospheric circulation. Atmos. Chem. Phys., 13, 8787–8796, www.atmos-chem-phys.net/13/8787/2013/, doi:10.5194/acp-13-8787-2013.
Ishidoya, S. & Murayama, S. (2014) Development of high precision continuous measuring system of the atmospheric O₂/N₂ and Ar/N₂ ratios and its application to the observation in Tsukuba, Japan. Tellus B, 66, 22574, http://dx.doi.org/ 10.3402/tellusb.v66.22574.
Keeling, R. F. et al. (2004) Measurement of changes in atmospheric Ar/N2 ratio using a rapid-switching, single-capillary mass spectrometer system, Tellus B, 56, 322–338.