15 June 2021
A new study by scientists at CSL and CIRES, led by CSL's Daniel Murphy, examines the size and chemical composition of aerosol particles in the global lower stratosphere from observations obtained during the recent Atmospheric Tomography (ATom) airborne field campaign.
The authors observed two distinct size modes of particles: the larger size mode consisted of particles produced in the stratosphere, whereas the smaller size mode consisted of particles transported up from the troposphere. Because of their size and composition, these two modes have very different roles in radiative climate forcing and in surface-based chemical reactions (such as, e.g., chlorine activation). The larger mode particles are more efficient at scattering visible light, whereas the smaller particles scatter UV light, and infrared heating (warming) is relatively independent of particle size. This complex relationship means that sufficiently large or small particles will both cause net heating of the Earth, and only mid-size particles (~0.5 µm diameter) will have a net cooling effect. This has important implications for the feasibility of climate intervention through stratospheric aerosol injection and the challenges that would be associated with generating aerosols of a particular size and composition to have the intended cooling effect.
Murphy, D.M., K.D. Froyd, I. Bourgeois, C.A. Brock, A. Kupc, J. Peischl, G.P. Schill, C.R. Thompson, C.J. Williamson, and P. Yu, Radiative and chemical implications of the size and composition of aerosol particles in the existing or modified global stratosphere, Atmospheric Chemistry and Physics, doi:10.5194/acp-21-8915-2021, 2021.
The size of aerosol particles has fundamental effects on their chemistry and radiative effects. We explore those effects using aerosol size and composition data in the lowermost stratosphere along with calculations of light scattering. In the size range between about 0.1 and 1.0 µm diameter (accumulation mode), there are at least two modes of particles in the lowermost stratosphere. The larger mode consists mostly of particles produced in the stratosphere, and the smaller mode consists mostly of particles transported from the troposphere. The stratospheric mode is similar in the Northern and Southern Hemisphere, whereas the tropospheric mode is much more abundant in the Northern Hemisphere. The purity of sulfuric acid particles in the stratospheric mode shows that there is limited production of secondary organic aerosol in the stratosphere, especially in the Southern Hemisphere. Out of eight sets of flights sampling the lowermost stratosphere (four seasons and two hemispheres) there were three with large injections of specific materials: volcanic, biomass burning, or dust. The stratospheric and tropospheric modes have very different roles for radiative effects on climate and for heterogeneous chemistry. Because the larger particles are more efficient at scattering light, most of the radiative effect in the lowermost stratosphere is due to stratospheric particles. In contrast, the tropospheric particles can have more surface area, at least in the Northern Hemisphere. The surface area of tropospheric particles could have significant implications for heterogeneous chemistry because these particles, which are partially neutralized and contain organics, do not correspond to the substances used for laboratory studies of stratospheric heterogeneous chemistry. We then extend the analysis of size-dependent properties to particles injected into the stratosphere, either intentionally or from volcanoes. There is no single size that will simultaneously maximize the climate impact relative to the injected mass, infrared heating, potential for heterogeneous chemistry, and undesired changes in direct sunlight. In addition, light absorption in the far ultraviolet is identified as an issue requiring more study for both the existing and potentially modified stratosphere.