Radiative Transfer and Aerosol Scattering

One of the most fundamental energy flows in the climate system is constituted by the radiative input of energy from the sun and the outgoing energy flow from thermal infrared radiation. Aerosols in the atmosphere provide a significant modulation of these energy flows. Depending on the location of the aerosol, its composition, and its physical details (particularly its size), the aerosol may introduce a positive or negative net perturbation to the balance between these incoming and ougoing radiative flows. This radiative impact of aerosols has been observed to produce a subsantial short-term impact on climate in the case of major volcanic eruptions, which have resulted for some well-known instances in a significant global temperature anomaly persisting for more than a year. A quantitative understanding of aerosol perturbations to the climate radiative balance requires a detailed understanding of the processes that govern radiative transfer in the atmosphere. These processes rest on fundamental optical properties, such as the complex refractive index, of the condensed phase materials that constitute aerosols. While laboratory measurements to quantify complex refractive index are routine, the samples used for laboratory measurements may not be representative of atmospheric particulates in important ways. These differences can lead to substantially different quantitative assessments of the radiative perturbations caused by aerosol scattering. This is particularly the case for studies investigating the risks and efficacy of albedo modification by deliberate introduction of aerosols into the atmosphere as a form of climate intervention. This talk will survey relevant aspects of radiative transfer and aerosol scattering and examine their implications in recent research studying hypothetical scenarios of albedo modification.