NASA - National Aeronautics and Space Administration

Both observations and recent model results reveal a significant organic component in stratospheric aerosols. However, intrinsic optical properties (i.e., complex refractive index) of this organic component and the mixing state of organic and ubiquitous sulfuric acid components are quite uncertain. We examine the effects of different complex refractive indices and particle mixing states on shortwave radiative forcing (RF) of stratospheric aerosols in northern midlatitudes. In the absence of large volcanic sulfur emissions, our calculations show that organic components may have substantial impacts on stratospheric aerosol optical depth (AOD). Compared to pure sulfuric acid/water aerosol, organic-containing aerosols could cause ±100% change in shortwave RF (for low AOD conditions) depending on the refractive index and mixing state. The range found here of shortwave RF results, for different scenarios of organic complex refractive index and mixing state, call for better understandings of chemical and transport processes determining aerosol optical properties in the stratosphere. Plain Language Summary While it is generally assumed that stratospheric aerosol is dominated by pure sulfuric acid plus water aerosols, recent modeling studies and in situ measurements suggest that organic matter makes up a significant fraction of lower stratospheric aerosol. The implications of this organic component are uncertain but may require significant revision of our understanding of the stratosphere's climate influence. Here, we perform a sensitivity study of shortwave RF of stratospheric aerosols in northern midlatitudes, examining the influence of different plausible values of complex refractive index and particle morphologies. A radiative transfer model indicates that both complex refractive index of organics, and particle morphologies, could have a consequential influence on the shortwave RF of stratospheric aerosols. Currently, however, there is very little data to constrain the mixing state and refractive index of these organic-containing aerosols. Reducing uncertainty in details of these aerosols requires a concerted effort to bring models, observations, and laboratory studies together in a unified framework.