Determination of the atmospheric lifetime and global warming potential of sulfur hexafluoride using a three-dimensional model

Abstract

Abstract. We have used the Whole Atmosphere Community Climate Model (WACCM), with an updated treatment of loss processes, to determine the atmospheric lifetime of sulfur hexafluoride (SF 6 ). The model includes the following SF 6 removal processes: photolysis, electron attachment and reaction with mesospheric metal atoms. The Sodankylä Ion Chemistry (SIC) model is incorporated into the standard version of WACCM to produce a new version with a detailed D region ion chemistry with cluster ions and negative ions. This is used to determine a latitude-and altitudedependent scaling factor for the electron density in the standard WACCM in order to carry out multi-year SF 6 simulations. The model gives a mean SF 6 lifetime over an 11-year solar cycle (τ ) of 1278 years (with a range from 1120 to 1475 years), which is much shorter than the currently widely used value of 3200 years, due to the larger contribution (97.4 %) of the modelled electron density to the total atmospheric loss. The loss of SF 6 by reaction with mesospheric metal atoms (Na and K) is far too slow to affect the lifetime. We investigate how this shorter atmospheric lifetime impacts the use of SF 6 to derive stratospheric age of air. The age of air derived from this shorter lifetime SF 6 tracer is longer by 9 % in polar latitudes at 20 km compared to a passive SF 6 tracer. We also present laboratory measurements of the infrared spectrum of SF 6 and find good agreement with previous studies. We calculate the resulting radiative forcings and efficiencies to be, on average, very similar to those reported previously. Our values for the 20-, 100-and 500-year global warming potentials are 18 000, 23 800 and 31 300, respectively.

Quantifying the effect of mixing on the mean age of air in CCMVal-2 and CCMI-1 models

“…Except for UMUKCA-METO, all models show younger air than observed, particularly at high latitudes. However, especially at high latitudes AoA derived from observed SF 6 is overestimated because of the mesospheric sinks of SF 6 (see Haenel et al, 2015;Ray et al, 2017;Kovács et al, 2017). Overall, we can say that, compared to observations, AoA is too low in most of the models analyzed in our study.…”

Ground-based FTIR retrievals of SF<sub>6</sub> on Reunion Island

“…The profile of SF 6 trend shows a peak at altitude of 11-13 km from the MIPAS measurements, and a peak at 11.5-16.5 km from the ACE-FTS measurements. As the SF 6 emissions are all at the Earth's sur- face and there is almost no removal mechanism in the troposphere and stratosphere (Kovács et al, 2017), the SF 6 concentration should be well mixed in the troposphere (the tropopause height above Reunion Island is about 16.5 km) and decreases above the tropopause, which was confirmed by the airborne in situ measurements (Patra et al, 1997). Figure 7 shows the SF 6 monthly means and the number of measurements in each month from MIPAS and ACE-FTS.…”

“…It has an extremely long lifetime of 850 years (Ray et al, 2017) with global warming potential for a 100-year time horizon of 23 700 (relative to CO 2 ) (Kovács et al, 2017). Since SF 6 is a very stable trace gas in the atmosphere and its annual growth rate seems relatively constant during the last two decades (Hall et al, 2011), it is usually applied to calculate the age of air (Patra et al, 1997;Engel, 2002;Patra et al, 2009;Stiller et al, 2012;Haenel et al, 2015).…”

Evaluation of stratospheric age-of-air from CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>8</sub>, CHF<sub>3</sub>, HFC-125, HFC-227ea and SF<sub>6</sub>; implications for the calculations of halocarbon lifetimes, fractional release factors and ozone depletion potentials

“…For SF6, recent research suggests its lifetime has likely been overestimated, thus it may be giving high-biased mean ages. The 20 reduction in SF6 lifetime comes from both modelling and measurement studies, which have evaluated its stratospheric loss mechanisms via electron attachment (Kovács et al, 2017) and in the polar vortex (Andrews et al, 2001;Ray et al, 2017). The most recent (at time of writing) evaluation gives a revised lifetime of 850 (580-1400) years (Ray et al, 2017).…”

“…This is considerably lower than the 3200 year lifetime used in the most recent assessments of the Intergovernmental Panel on Climate Change (IPCC) and the World Meteorological Organization (WMO) (IPCC, 25 2013;WMO, 2014). A revised lifetime will impact the estimated global warming potential of SF6 (Kovács et al, 2017). These limitations do not preclude the use of CO2 and SF6 as age tracers, but may require complex corrections or limit the suitability of these gases to act as tracer in certain regions (Andrews et al, 2001;Bönisch et al, 2009).…”