Case studies of aerosol remote sensing with the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI)

The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) is an 8-band (355, 380, 445, 470, 555, 660, 865, 935 nm) pushbroom camera, measuring polarization in the 470, 660, and 865 nm bands, mounted on a gimbal to acquire multiangular observations over a ±67° along-track range with 10-m spatial resolution across an 11-km wide swath. Among the instrument objectives are exploration of methodologies for combining multiangle, multispectral, polarimetric, and imaging observations to retrieve the optical depth and microphysical properties of tropospheric aerosols. AirMSPI was integrated on NASA's ER-2 high-altitude aircraft in 2010 and has successfully completed a number of flights over land and ocean targets in the Southern California vicinity. In this paper, we present case studies of AirMSPI imagery, interpreted using vector radiative transfer theory. AirMSPI observations over California's Central Valley are compared with model calculations using aerosol properties reported by the Fresno AERONET sunphotometer. Because determination of the radiative impact of different types of aerosols requires accurate attribution of the source of the reflected light along with characterization of the aerosol optical and microphysical properties, we explore the sensitivity of the Fresno measurements to variations in different aerosol properties, demonstrating the value of combining intensity and polarimetry at multiple view angles and spectral bands for constraining particle microphysical properties. Images over ocean to be presented include scenes over nearly cloud-free skies and scenes containing scattered clouds. It is well known that imperfect cloud screening confounds the determination of aerosol impact on radiation; it is perhaps less well appreciated that the effect of cloud reflections in the water can also be problematic. We calculate the magnitude of this effect in intensity and polarization and discuss its potential impact on aerosol retrievals, underscoring the value of spatially resolved imaging to differentiate the contributions of various scene elements to upwelling top-of-atmosphere radiation.