Composition Dependence of Stratospheric Aerosol Shortwave Radiative Forcing in Northern Midlatitudes

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.