Measurements of ^{17}O^{18}O and ^{18}O^{18}O in atmospheric air O_{2} and comparison with model simulations

The conventional isotopic composition of atmospheric oxygen in governed by photosynthesis and hydrological process in the troposphere. However photochemical reactions also affect the isotopic compositions in the tropopause (TP) and stratospheric regions. In this study, we present measurements of doubly substituted 17O18O and 18O18O (known as clumped isotopes), denoted as Δ35 and Δ36respectively, in atmospheric air samples collected from the surface to the lowermost stratospheric regions (LMS) of up to 20km. Isotope exchange between O2 and O(3P), a product of photolytic dissociation of O3, resets the isotopic signatures of oxygen. The clumped isotope signatures depend on the resetting ambient temperature as well as on the O(3P) concentrations. The Δ36 and Δ35 values are higher for air sampled at colder temperatures in the TP and LMS regions (2.9‰ to 3.1‰ for Δ36 ; 1.4‰ to 1.6‰ for Δ35 ), whereas lower values (2.2‰ to 2.9 ‰ for Δ36 ;1.1‰ to 1.9‰ for Δ35) are observed in the tropospheric regions. As a result of the high abundance of O3 in the TP and LMS, the clumped isotope values are close to the isotopic equilibrium values there. In contrast, O2 in the troposphere is a mixture of that has been isotopically exchanged at relatively warmer temperature in the troposphere, and the O2 that has retained higher clumped isotopes values from the time that it was isotopically equilibrated in the cold stratosphere. Full isotopic equilibration in the troposphere does not occur due to the low O(3P) concentrations there. Thus, Δ36 values are influenced by atmospheric transport and circulation. To investigate this in more detail, the clumped isotopic composition of O2 was modeled with the European Centre for Medium-Range Weather Forecasts - Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model. The model resets the clumped isotopic composition of O2 to the thermodynamically expected values depending on temperature during isotope exchange reactions, and it explicitly takes into account the transport. The model reproduces the mean measured tropospheric clumped isotope within the analytical uncertainty. The results obtained from the measurements and model will be discussed.