Tropospheric ¹⁸O¹⁸O is an emerging proxy for past tropospheric ozone and free-tropospheric temperatures. The basis of these applications is the idea that isotope-exchange reactions in the atmosphere drive ¹⁸O¹⁸O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the ¹⁸O¹⁸O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the ¹⁸O¹⁸O proxy, we developed a scheme to simulate atmospheric ¹⁸O¹⁸O abundances (quantified as ∆₃₆ values) online within the GEOS-Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O₂ isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆₃₆ values are too high elsewhere. Investigations using a comprehensive model of the O-O₂-O₃ isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆₃₆ values: the anomalously efficient titration of ¹⁸O¹⁸O to form ozone. Incorporating these effects into the online ∆₃₆ calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of ¹⁸O¹⁸O in O₂, the modeled change in the mean tropospheric ∆₃₆ value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice-core ∆₃₆ record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.

中文翻译：

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臭氧同位素体的形成对大气O2聚集同位素组成的影响

对流层¹⁸ O ¹⁸ O 是过去对流层臭氧和自由对流层温度的新兴代表。这些应用的基础是大气中的同位素交换反应将¹⁸ O ¹⁸ O 丰度推向同位素平衡的想法。然而，之前的工作使用离线盒模型框架来解释¹⁸ O ¹⁸ O 预算，近似大气化学和传输的相互作用。这种方法虽然方便，但不确定性的特征很差。为了研究这些不确定性，并扩大¹⁸ O ¹⁸ O 代理的适用性，我们开发了一种模拟大气¹⁸O ¹⁸ O 丰度（量化为 ∆ ₃₆值）在 GEOS-Chem 化学传输模型中在线。这些结果与新的和以前发表的从地表到 33 公里的大气观测结果进行了比较。在 GEOS-Chem 中使用简化的 O ₂同位素平衡方案进行的模拟显示，与仅在平流层中部的测量结果一致；模拟的 ∆ ₃₆值在其他地方太高了。使用 OO ₂ -O ₃同位素光化学系统的综合模型和原理验证实验进行的研究表明，简单的平衡方案忽略了对 Δ ₃₆的重要压力依赖性值：异常有效的¹⁸ O ¹⁸ O滴定以形成臭氧。将这些影响纳入GEOS-Chem 中的在线 Δ ₃₆计算方案，可以为所有可用的观察结果产生定量一致性。虽然这种先前未识别的偏差影响了O ₂中¹⁸ O ¹⁸ O的大气收支，但自 1850 CE 以来平均对流层 Δ ₃₆值的模拟变化仅略有改变；它仍然是与冰芯Δ定量一致₃₆的记录，这意味着对流层臭氧负担在二十世纪增加了不到40％。