Observations of the Age of Air from the Northern Hemisphere Midlatitude Surface: New Estimates from the NASA Atmospheric Tomography Mission (ATom)

Understanding and modeling the interhemispheric transport (IHT) of trace gases and aerosols is important for interpreting past (and projecting future) changes in atmospheric composition. One measure of interhemispheric transport -- the mean age since air was last at the Northern Hemisphere (NH) midlatitude surface – represents a fundamental property of tropospheric transport that can be estimated from measurements of sulfur hexafluoride (SF₆). Furthermore, unlike hemispherically integrated measures likes the interhemispheric exchange time, the mean age provides a much richer description of IHT as it is calculated for locations throughout the troposphere. Here we use SF₆ measurements obtained from the recent NASA Atmospheric Tomography Mission (ATom) to construct observational estimates of the mean age throughout the troposphere. The use of the ATom data, which spans all four seasons over the period 2016-2018 and covers both the Pacific and Atlantic Oceans, affords a much more extensive window into the seasonal and longitudinal variations of the mean age as compared to previous observational estimates that were based solely on surface measurements and/or presented vertical profiles limited to only one season and ocean basin. In order to interpret the observed variations in the mean age and their relationship with the large-scale flow we use simulations of the Goddard Earth Observing System (GEOS) general circulation model, constrained with meteorological fields derived from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), performed over the ATom period. Simulations are also extended to the entire MERRA-2 period (1980-2016) in order to place the ATom-based results in the context of the longer observational record.