Size-dependent aerosol radiative impacts in the lower stratosphere

Abstract:

Particles in the lower stratosphere consist of at least two modes, a larger mode that is almost pure sulfuric acid and a smaller mode of particles from the troposphere with significant organic content. Because the larger particles are more efficient at scattering light, most of the radiative effect in the lowermost stratosphere is due to the sulfuric acid particles. In contrast, the mixed organic-sulfate particles can have more surface area, at least in the Northern Hemisphere. This could have significant implications for heterogeneous chemistry because these particles are partially neutralized and contain organics and therefore do not correspond to the surfaces used for laboratory studies of stratospheric heterogeneous chemistry.

We then extend the analysis of size-dependent properties to particles considered for intentional climate modification. There is no single size that will simultaneously optimize the climate impact compared to the amount of mass needed or side effects such as infrared heating, potential for heterogeneous chemistry, and undesired changes in direct sunlight. The sulfuric acid particles after the Mount Pinatubo eruption were of a size with little surface area compared to their climate impact. They therefore may not be a reliable analog for heterogeneous chemistry on added particles. In addition, light absorption in the far ultraviolet is identified as an issue requiring more study for both the existing and potentially modified stratosphere.