New Measurements of Size-Dependent Aerosol Chemistry in the Lower Stratosphere

Airborne in-situ aerosol composition measurements show that the lower stratospheric aerosol is a combination of relatively pure sulfuric acid particles that originated in the stratosphere alongside mixed organic-sulfate particles that originated in the troposphere. New data from the Atmospheric Tomography (ATom) mission provide some of the first observations of aerosol composition in the Southern Hemisphere. The 2015 eruption of Calbuco was still affecting the Southern Hemisphere lower stratosphere in August 2016. These data show that in the lowermost stratosphere in both hemispheres the pure sulfuric acid particles are larger than the mixed organic-sulfate particles. Stratospheric mixed particles with significant organic content are almost identical to those in the upper troposphere, confirming their tropospheric origin. In contrast, the sulfuric acid particles are completely distinct from tropospheric particles. About a third of the total aerosol mass in the lower stratosphere is organic. The radiative forcing of stratospheric aerosol depends on infrared absorption more than is the case in the lower troposphere. This means that the organic content can affect the radiative forcing.