Concept, absolute calibration and validation of a new, bench-top laser imaging polar nephelometer

Abstract. Polar nephelometers provide in situ measurements of aerosol angular light scattering and play an essential role in validating numerically calculated phase functions or inversion algorithms used in space-borne and land-based aerosol remote sensing. In this study, we present a prototype of a new polar nephelometer called uNeph. The instrument is designed to measure the phase function, F₁₁, and polarized phase function, –F₁₂/F₁₁ over the scattering range of around 5° to 175° with an angular resolution of 1° at a wavelength of 532 nm. In this work, we present details of the data processing procedures and instrument calibration approaches. The uNeph was validated in a laboratory setting using mono-disperse polystyrene latex (PSL) and Di-Ethyl-Hexyl-Sebacate (DEHS) aerosol particles over a variety of sizes, ranging from 200 nm to 800 nm. An error model was developed and the level of agreement between uNeph measurements and Mie theory was found to be consistent within the uncertainties of the measurements and the uncertainties of the input parameters for the theoretical calculations. The estimated measurement errors were between 5 % to 10 % (relative) for F₁₁ and smaller than ~0.1 (absolute) for –F₁₂/F₁₁. Additionally, by applying the Generalized Retrieval of Aerosol and Surface Properties (GRASP) inversion algorithm to the measurements conducted with broad unimodal DEHS aerosol particles, the volume concentration, size distribution and refractive index of the ensemble of aerosol particles were accurately retrieved. This paper demonstrates that the uNeph prototype can be used to conduct accurate measurements of aerosol phase function and polarized phase function and to retrieve aerosol properties through inversion algorithms.