Polarimetric remote sensing of aerosols over land

We present a new approach to retrieve the aerosol properties over land that uses accurate polarization measurements over a broad spectral (410-2250 nm) and angular (+/- 60 degrees from nadir) ranges. The approach uses longer wavelength observations to accurately estimate the surface effects, and it is incorporated into an optimal estimation framework for retrieving the particle number density and a detailed aerosol microphysical model: effective radius, variance, and complex refractive index. A sensitivity analysis shows that the uncertainties in aerosol optical thickness (AOT) increase with AOT while the uncertainties in the microphysical model decrease. The uncertainty in the single scattering albedo (SSA) is notably less than 0.05 by the time the AOT is greater than 0.2. We find that calibration is the major source of uncertainty and that perfect angular and spectral correlation of calibration errors reduces the uncertainties in retrieved quantities. Finally, we observe that shorter wavelength (< 500 nm) observations are crucial for determining the aerosols vertical extent and imaginary refractive index from polarization measurements. The retrieval approach is tested under pristine and polluted conditions using observations made by the Research Scanning Polarimeter during the Aerosol Lidar Validation experiment and over California Southern wild fires. In both cases we find that the retrievals are within the combined uncertainties of the retrieval and the Aerosol Robotic Network Cimel products and Total Ozone Mapping Spectrometer Aerosol Index. This demonstrates the unique capability of polarization measurements to accurately retrieve AOTs under pristine conditions and provide estimation of the SSA at higher AOTs.