Observational Constraints on the Vertical and Interhemispheric Gradient of the Ar/N2 Ratio in the Troposphere

Measurements of the atmospheric Ar/N₂ ratio at surface stations show small but detectable variations due to the changing solubility of argon in surface waters and subsequent air-sea exchange. Such measurements are challenging to perform, since the potential for artificial fractionation of Ar relative to N₂ during sampling is very high. We present a synthesis of surface flask time series and airborne measurements from nine global campaigns to resolve the variability of δ(Ar/N₂) with latitude, altitude, and season. We discuss these results with special consideration for possible sampling artifacts. New observations from the Atmospheric Tomography (ATom) mission suggest that on an annual basis δ(Ar/N₂) should be constant within the detection limit from the lower to mid troposphere, in contrast to previous campaigns. After accounting for seasonality, we find that δ(Ar/N₂) is enriched in the Southern Hemisphere relative to the Northern, and discuss these results in the context of interhemispheric heat transport and other natural processes which can affect the vertical distribution of δ(Ar/N₂), such as stratosphere-troposphere exchange. We explore our results with the aid of several ocean and earth system models, through forward runs in a tracer transport model of simulated argon air-sea exchanges, derived from ocean heat fluxes.