Long-term observation of mass-independent oxygen isotope anomaly in stratospheric CO2

[摘要] Stratospheric and upper tropospheric air samples were collected during1994–2004 over Sanriku, Japan and in 1997 over Kiruna, Sweden. Using thesearchived air samples, we determined the triple oxygen-isotope composition ofstratospheric CO₂ and the N₂O mixing ratio. The maximum Δ¹⁷O_CO2value of +12.2‰, resembling that observed previously inthe mesosphere at 60 km height, was found in the middle stratosphere overKiruna at 25.6 km height, suggesting that upper stratospheric andmesospheric air descended to the middle stratosphere through strong downwardadvection. A least-squares regression analysis of our observations on aδ¹⁸O_CO2−δ¹⁷O_CO2 plot (r²>0.95)shows a slope of 1.63±pm0.10, which is similar to the reported value of1.71±0.06, thereby confirming the linearity of three isotopecorrelation with the slope of 1.6–1.7 in the mid-latitude lower and middlestratosphere. The slope decrease with increasing altitude and a curvy trendin three-isotope correlation reported from previous studies were notstatistically significant. Using negative linear correlations of Δ¹⁷O_CO2and δ¹⁸O_CO2 with the N₂O mixingratio, we quantified triple oxygen-isotope fluxes of CO₂ to thetroposphere as +48‰ GtC/yr (Δ¹⁷O_CO2) and +38‰ GtC/yr(δ¹⁸O_CO2) with ~30% uncertainty. Comparing recentmodel results and observations, underestimation of the three isotope slopeand the maximum Δ¹⁷O_CO2 value in the model were clarified,suggesting a smaller O₂ photolysis contribution than that of the model.Simultaneous observations of δ¹⁸O_CO2, δ¹⁷O_CO2, and N₂O mixing ratios can elucidate triple oxygenisotopes in CO₂ and clarify complex interactions among physical,chemical, and photochemical processes occurring in the middle atmosphere.

[学科分类] 大气科学