Data reported in: Effects of ozone isotopologue formation on the clumped-isotope composition of atmospheric O2

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 C.E. 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.

对流层¹⁸O¹⁸O是一种新兴的古对流层臭氧与自由对流层温度同位素代用指标（proxy）。这类应用的核心理论基础是，大气中的同位素交换反应会驱使¹⁸O¹⁸O的丰度向同位素平衡态趋近。然而，此前的研究采用离线箱式模型（box-model）框架阐释¹⁸O¹⁸O的大气收支，通过近似方式模拟大气化学与大气输送的相互作用过程。该方法虽简便易行，但不确定性的特征尚未得到充分表征。
为探究这些不确定性并拓展¹⁸O¹⁸O代用指标的适用范围，我们开发了一套可在GEOS-Chem化学输送模型（chemical transport model）中在线模拟大气¹⁸O¹⁸O丰度（以Δ36值量化）的方案。我们将该模拟结果与新获取及已发表的、从地表至33 km高度的大气观测数据进行了对比。在GEOS-Chem中采用简化的O₂同位素平衡方案开展的模拟结果显示，仅在平流层中部与观测数据达成定量一致；其余区域的模拟Δ36值均偏高。
通过采用完整的O-O₂-O₃同位素光化学系统模型以及原理验证实验（proof-of-principle experiments），我们发现该简化平衡方案忽略了Δ36值对气压的重要依赖关系：即¹⁸O¹⁸O通过反常高效的滴定反应生成臭氧的过程。将这些效应纳入GEOS-Chem中的在线Δ36计算方案后，模拟结果与所有可用观测数据均实现了定量一致。尽管这一此前未被识别的偏差会影响O₂中¹⁸O¹⁸O的大气收支，但自公元1850年以来，模拟得到的对流层平均Δ36值仅发生了小幅变化；其仍与冰芯Δ36记录定量吻合，这意味着20世纪全球对流层臭氧负荷的增幅不足40%。