The Center for Atmospheric Sciences | » Characterizing aerosol properties in the upper troposphere and stratosphere from limb scatter radiance

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  • Characterizing aerosol properties in the upper troposphere and stratosphere from limb scatter radiance

    The most important stratospheric aerosol property for most applications is the aerosol extinction coefficient ka, which represents the rate at which energy is removed by aerosols from a beam per unit of path length traversed, and varies with both altitude z and wavelength λ. The aerosol single-scattering albedo (SSA) indicates the fraction of aerosol extinction that is caused by scattering (rather than absorption). The pattern of radiation scattered by aerosol varies with single-scattering angle Θ as characterized by the aerosol scattering phase function Pa(Θ). The optical properties ka, Pa(Θ) and SSA are determined by the aerosol composition (through the complex aerosol refractive index n), aerosol shape, and aerosol size distribution.

    During volcanically active periods, the stratospheric aerosol distribution is highly variable and particles are either represented by a uni-modal lognormal particle size distribution or a bimodal lognormal particle size distribution. By viewing these particles from different geometries at the same time and location, measurements should be independent of scattering angle if the knowledge of the particle size distribution and hence the phase function is accurate. Consequently, if the particle size distribution is wrongly assumed, the phase function will be inaccurate and the retrievals would be skewed towards compensating for the varying amounts of aerosols with respect to the different scattering angles.