The NPOESS Aerosol Polarimetry Sensor

Two of the largest potential sources of error in climate models are uncertainties in the direct radiative forcing by atmospheric aerosols and in their indirect effects through their impacts on the cloud field. The Aerosol Polarimetry Sensor (APS) instrument is being developed for the National Polar-orbiting Operational Environmental Satellite System (NPOESS) to measure the properties of the atmospheric aerosols in sufficient detail and accuracy to help reduce these uncertainties. The APS will be a high-precision multi-spectral photopolarimeter which will scan along the sub-orbital track as the satellite passes overhead providing multiple views of the each point along the ground track. The measurements will be made at 8 different wavelengths between 400 and 2400 nm, through pairs of Wollaston prism analyzers, which allow the simultaneous determination of the Stokes polarization parameters I, Q, and U. These measurements will be used to determine the aerosol optical thickness, the effective radius, and the effective variance for both the accumulation and coarse particle modes of a bimodal aerosol size distribution. The real part of the aerosol refractive index and the aerosol single-scattering albedo will also be produced along with a determination of whether or not the aerosol particles are spherical. NASA is planning an early flight for an APS instrument as part of the Glory Mission. The APS instrument is scheduled to fly on the first NPOESS platform, which will be in a 2130 ascending node polar orbit. The planned APS measurements and the resulting Environmental Data Records (EDRs) will be described.