The Aerosol Limb Imager: multi-spectral polarimetric observations of stratospheric aerosol

Abstract. The Aerosol Limb Imager (ALI) is designed to measure stratospheric aerosol by imaging limb-scattered sunlight. Each image taken by ALI is spectrally filtered at a tunable wavelength and refined to consist of either horizontally or vertically polarized light. Novel to limb imaging, these polarized observations of ALI provide a means to isolate tangent altitudes which have signal contaminated by clouds as identified by the depolarization of the scattered radiance. This avoids the ambiguity caused by clouds to be interpreted as aerosol in a retrieval. We present a polarized aerosol retrieval methodology which retrieves vertically resolved aerosol number density and median radius of a unimodal log-normal distribution, in addition to a scalar (uniform in altitude) width of the log-normal distribution. We explore the cloud discrimination and aerosol retrieval of ALI in simulation as validation of the efficacy and the limits to the technique. We then apply the retrieval to three example sets of observations taken from the most recent high-altitude balloon flight of ALI. One set provides a nominal exemplar, while the other two represent more difficult retrieval conditions of an increasingly polarized atmosphere. We compare the aerosol extinction of ALI in all three exemplar cases to the best coincident extinctions of three space based instruments: the Stratospheric Aerosol and Gas Experiment (SAGE III/ISS), the Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP), and the Optical Spectrograph and InfraRed Imaging System (OSIRIS). We provide discussion on the agreement of all three cases against the comparison instruments with respect to the efficacy of our approach. However, we find the retrieved aerosol extinction of ALI in the nominal case (considering the limitations of ballooning) is in good agreement (a median absolute percent difference < 29 %) to the extinction reported by SAGE III/ISS, OMPS-LP, and OSIRIS while also yielding aerosol particle size information. In the case of SAGE III/ISS, the comparison with ALI results is extended into the aerosol size parameters. In a nominal case SAGE III/ISS and ALI aerosol effective radius agree within uncertainty for the majority of altitudes.