158 Polarization helps characterize aerosol mixtures, but small particles are hard to see: measure volume-absorption and in the UV

Monday, 7 July 2014
William GK Martin, NASA/GISS and Columbia University, New York, NY; and B. Cairns

This talk focuses on the sensitivity of single-scattering measurements to aerosol microphysical properties, like size distribution and complex refractive index. We present a theoretical study which analyzes several hypothetical sets of single-scattering measurements at three visible wavelengths (405, 532, and 780nm). Full data includes all angularly dependent scattering-matrix elements and both the volume scattering and absorption coefficients. Information content and microphysical parameter resolution is computed for full data and for several incomplete data subsets. Comparing resolution for these measurement sets allows us to quantify, for instance, the importance of polarization for resolving the size-distribution and complex refractive index.

Following a brief review of single-scattering by airborne particles, theoretical results about the sensitivity of single-scattering measurements to aerosol properties will be discussed: (1) While the phase-function alone offers reasonable constraints on the size distribution for larger particles, all single-scattering measurements fail to constrain the size distribution for very-small particles. (2) Uncertainty estimates for particle number concentration are much worse than for other loading parameters, like particle surface-area density and particle volume density. (3) Polarimetric measurements improve retrieval uncertainty estimates in the size distribution and complex refractive index for larger particles. (4) Measuring the absorption coefficient becomes increasingly important for determining the complex refractive index of smaller particles as their scattering behavior approaches the Rayleigh limit.

While these results apply to single-scattering measurements only, they may provide useful intuition about satellite and aircraft retrievals of aerosols in the atmosphere.

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