We have new measurements of the size and composition of particles in the lower stratosphere. The lower stratosphere has had frequent perturbations not only from moderate volcanic eruptions but also other sources. These measurements have implications for the climate forcing and chemical impacts of aerosol particles in the lower stratosphere in the present-day stratosphere as well as the implications for volcanic or intentionally added material.
Some of the impacts of particles in the lower stratosphere are climate forcing, surface area for heterogeneous chemistry, infrared heating, production of diffuse light, and possible effects on photolysis. Sulfuric acid particles in the background stratosphere are near the optimal diameter for climate forcing. Mixed tropospheric particles that make their way into the stratosphere are less efficient at climate forcing but have significant surface area for possible heterogeneous chemistry that can affect ozone. Sulfuric acid particles after the Mount Pinatubo eruption had relatively little surface area compared to their climate impact. Added material could therefore easily have more impact on heterogeneous chemistry than an analogy to volcanic eruptions would suggest. Infrared heating is more important for aerosol in the lower stratosphere than it is in the lower troposphere. There is no optimum size for added material that simultaneously minimizes all potential side effects such as heterogeneous chemistry and reduction of direct sunlight.