Tropospheric Age‐of‐Air: Influence of SF6 Emissions on Recent Surface Trends and Model Biases. Issue 19 (29th September 2021)

• Record Type: Journal Article
• Authors: Orbe, Clara; Waugh, Darryn W.; Montzka, Stephen; Dlugokencky, Edward J.; Strahan, Susan; Steenrod, Stephen D.; Strode, Sarah; Elkins, James W.; Hall, Bradley; Sweeney, Colm; Hintsa, Eric J.; Moore, Fred L.; Penafiel, Emma
• Is Part Of: Journal of geophysical research. Volume 126:Issue 19(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-29
• Subjects: atmospheric transport -- interhemispheric exchange -- tracers -- SF6 emissions; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD035451
• Languages: English
• ISSNs: 2169-897X

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 " S F 6 age" ( Γ SF 6 ), derived from surface and aircraft measurements of S F 6 that are broader in spatial scope and cover a longer time period (1997–2018) than considered previously. At the surface, Γ SF 6 increases from near‐zero values north of 30 ° 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 Γ SF 6 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 Γ SF 6 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 S F 6 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 S F 6 age trends, show no decreases in an age‐of‐air tracer, reinforcing the fact that Γ SF 6 represents only an approximation to the mean age. Finally, the modeled S F 6 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 S F 6 near emissions regions, likely reflecting a combination of uncertainties in emissions and model transport. Plain Language Summary: The mean age since air was last at the Northern Hemisphere midlatitude surface is a fundamental timescale of tropospheric transport. The mean age is not directly observable, but can be estimated from measurements of S F 6 to derive an " S F 6 age" ( Γ SF 6 ), or the time lag since the S F 6 mixing ratio at a given location equaled the mixing ratio over a northern midlatitude source region. Here we use new surface and aircraft measurements of S F 6 to construct an estimate of the mean age that covers a longer period (1997–2018) and is more globally resolved, compared to previous estimates. The broader spatial coverage reveals strong variations in the seasonal cycle of Γ SF 6 within the (sub)tropics that are weaker over the Atlantic and Pacific oceans, compared to over the Indian Ocean. The longer temporal record also reveals that Γ SF 6 has been decreasing by ∼ 0.12 years/dec. Quite importantly, this decrease is not due to underlying changes in transport but, rather, is likely related to changes in S F 6 emissions, which increase globally while shifting from northern midlatitudes into the subtropics. We also show that the longstanding old bias in modeled Γ SF 6 is partly related to an overestimation in simulated S F 6 near emissions regions. Key Points: The mean age since air was last at the Northern Hemisphere (NH) midlatitude surface features large (small) meridional gradients in the tropics (extratropics) Recent mean age trends in the Southern Hemisphere (SH), estimated from measurements of SF6, likely reflect shifts in SF6 emissions, not transport changes Modeled SF6 ages in the SH are older than observed, partly due to overestimation in simulated SF6 mixing ratios near NH emissions regions