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. Accurate simulation of atmospheric circulation,particularly in the lower stratosphere, is challenging due to unresolvedwave–mean flow interactions and limited high-resolution observations for validation. Gravity-induced pressure gradients lead to a small butmeasurable separation of heavy and light gases by molecular diffusion in thestratosphere. Because the relative abundance of Ar to N2 is exclusivelycontrolled by physical transport, the argon-to-nitrogen ratio (Ar∕N2)provides an additional constraint on circulation and the age of air (AoA),i.e., the time elapsed since entry of an air parcel into the stratosphere. Here we use airborne measurements of N2O and Ar∕N2 from ninecampaigns with global coverage spanning 2008–2018 to calculate AoA and toquantify gravitational separation in the lowermost stratosphere. To thisend, we develop a new N2O–AoA relationship using a Markov chain Monte Carlo algorithm. We observe that gravitational separation increases systematically with increasing AoA for samples with AoA between 0 and 3 years. These observations are compared to a simulation of the TOMCAT/SLIMCAT 3-D chemical transport model, which has been updated to includegravitational fractionation of gases. We demonstrate that although AoA atold ages is slightly underestimated in the model, the relationship betweenAr∕N2 and AoA is robust and agrees with the observations. Thishighlights the potential of Ar∕N2 to become a new AoA tracer that is subject only to physical transport phenomena and can supplement the suite of available AoA indicators.

atmospheric chemistry and physics

Gravitational separation of Ar∕N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and age of air in the lowermost stratosphere in airborne observations and a chemical transport model