The Role of Sulfur Dioxide in Stratospheric Aerosol Formation Evaluated Using In-Situ Measurements in the Tropical Lower Stratosphere

Abstract

Stratospheric aerosols (SAs) are a variable component of the Earth's albedo that may be intentionally enhanced in the future to offset greenhouse gases (geoengineering). The role of tropospheric-sourced sulfur dioxide (SO₂) in maintaining background SAs has been debated for decades without in-situ measurements of SO₂ at the tropical tropopause to inform this issue. Here we clarify the role of SO₂ in maintaining SAs by using new in-situ SO₂ measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO₂ mixing ratios to estimate the global flux of SO₂ across the tropical tropopause. These analyses show that the tropopause background SO₂ is about 5 times smaller than reported by the average satellite observations that have been used recently to test atmospheric models. This shifts the view of SO₂ as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget.

Figure 1

Flight tracks from the VIRGAS experiment during October 2015.

Figure 2

Mean temperature (left) and ozone (O₃, right) profiles for four latitude ranges sampled during VIRGAS. Similarities of temperature and O₃ from 10 – 25 °N suggest data up to 25 °N are representative of tropical air masses on these flights.

Figure 3

Measured and modeled SO₂ profiles in the tropical (10 – 25 °N) UT/LS. (a) Blue line and shaded region show the VIRGAS in-situ measurement median and interquartile range. WACCM and GEOS-5 have been adjusted upwards by 1 km to match the aircraft ozone and thermal tropopause level. Two profiles each are shown for WACCM and GEOS-5: one for the zonal mean for 2015 (dash lines), and another showing data sampled from the models along the flight track locations /times (solid lines). (b) ACE-FTS median and interquartile range (2004–2010). MIPAS median and interquartile range of monthly means (2002 – 2012). Data during periods affected by major volcanic events were omitted from the ACE-FTS and MIPAS data [Höpfner et al., 2013]. WACCM and GEOS-5 profiles are the same zonal mean profiles shown in panel (a). WACCM 02–12 profile (black) shows the mean profile obtained by sampling the WACCM run during the 2002 – 2012 MIPAS period [Mills et al., 2016], from the same times and locations as the MIPAS data that are averaged to derive the blue MIPAS profile.