Tropospheric Age‐of‐Air: Influence of SF<sub>6</sub> Emissions on Recent Surface Trends and Model Biases
Clara Orbe, Darryn W. Waugh, S. A. Montzka, E. J. Dlugokencky, S. E. Strahan, Stephen D. Steenrod, Sarah A. Strode, James W. Elkins, B. D. Hall, Colm Sweeney, Eric J. Hintsa, F. L. Moore, Emma Penafiel
Abstract
Abstract 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 age” ( ), derived from surface and aircraft measurements of that are broader in spatial scope and cover a longer time period (1997–2018) than considered previously. At the surface, increases from near‐zero values north of N to 1.5 years over the Southern Hemisphere (SH) extratropics, with the largest meridional gradients occurring in the tropics. By comparison, vertical gradients in 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 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 age at sites in the SH has been decreasing by 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 age trends, show no decreases in an age‐of‐air tracer, reinforcing the fact that represents only an approximation to the mean age. Finally, the modeled ages are older than observed, by 0.3–0.4 years throughout the southern extratropics. We show that this bias is partly related to an overestimation in simulated near emissions regions, likely reflecting a combination of uncertainties in emissions and model transport.