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

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 (SO) in maintaining background SAs has been debated for decades without in-situ measurements of SO at the tropical tropopause to inform this issue. Here we clarify the role of SO in maintaining SAs by using new in-situ SO measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO mixing ratios to estimate the global flux of SO across the tropical tropopause. These analyses show that the tropopause background SO 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 SO as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget.