Tropospheric Age-Of-Air: Influence Of Sf6 Emissions On Recent Surface Trends And Model Biases

The mean age since air was last at the Northern Hemisphere (NH) midlatitude surface is a fundamental property of tropospheric transport. Here we approximate the mean age in terms of an "SF6 age" (Gamma SF6), derived from surface and aircraft measurements of SF6 that are broader in spatial scope and cover a longer time period (1997-2018) than considered previously. At the surface, Gamma SF6 increases from near-zero values north of 30 degrees N to similar to 1.5 years over the Southern Hemisphere (SH) extratropics, with the largest meridional gradients occurring in the tropics. By comparison, vertical gradients in Gamma SF6 are weak throughout, with only slight increases/decreases with height in the NH/SH. The broader spatial coverage of the measurements reveals strong variations in the seasonal cycle of Gamma SF6 within the (sub)tropics that are weaker over the Atlantic and Pacific oceans, compared to over the Indian Ocean. Observations from 2000 to 2018 reveal that the SF6 age at sites in the SH has been decreasing by similar to 0.12 years/dec. However, this decrease is not due to changes in transport but, rather, is likely related to changes in emissions, which have increased globally and reportedly shifted from northern midlatitudes into the subtropics. Simulations, which reproduce the SF6 age trends, show no decreases in an age-of-air tracer, reinforcing the fact that Gamma SF6 represents only an approximation to the mean age. Finally, the modeled SF6 ages are older than observed, by similar to 0.3-0.4 years throughout the southern extratropics. We show that this bias is partly related to an overestimation in simulated SF6 near emissions regions, likely reflecting a combination of uncertainties in emissions and model transport.