Optimization of the Polarized Imaging Nephelometer (PI-Neph) for continuous monitoring of multiwavelength aerosol phase functions in support of space polarimetry missions

Abstract

This work deals with the set up and optimization of a commercial multiwavelength polarized imaging nephelometer (PI-Neph, Airphoton PIN100) designed to measure F11 (phase function) and -F12/F11 (polarized phase function) from ambient aerosol samples. Such measurements are critical for the success of upcoming satellite missions based on polarimetry measurements because they will support and validate retrieval techniques. The main novelty of the instrument with respect to previous versions consists of using only one beam instead of a mirror system to fold it, avoiding internal reflections of light and loss of energy within the laser beam. The instrument also includes the automatization of the imaging technique that makes possible continuous measurements in near real time of the aerosol phase functions at different states of linear polarization directly from ambient air. Laser emits at three different wavelengths (405, 515 and 660 nm) and the beam goes through a wire grid polarizer for linearly polarizing the light, followed by two liquid crystal retarders that control the state of linear polarization. The detector is a CMOS camera perpendicular to the beam which allows to obtain final phase functions in the range 5° - 175° (losses of some angles by physical limitations and stray light). The calibration of the instrument is discussed and includes geometric calibration (to account for different light paths to the camera) plus absolute calibration (to obtain physical units). Temporal stability of calibrations is observed, and different data quality tests are presented in order to obtain reliable values of F11 and -F12/F11. For an automatic operation of the instrument that permits real time phase matrix measurements, a data quality check algorithm that avoids non-physical measurements and filters outliers has been developed. Validation of the scattering matrix elements based on monodisperse aerosol measurements is also done with polystyrene latex spheres (PSL), showing good agreement with Mie theory. Finally, first results of the instrument operating at the University of Granada (UGR) station from the Andalusian Global ObseRvatory of the Atmosphere (AGORA) since April 2022 are presented. Very good agreements of the integrated scattering coefficients with an integrating nephelometer (TSI, model 3563) were obtained with high correlation coefficients for the three wavelengths (R2 >0.81). The calculated root-mean-square error (RMSE) reveals that the performance of the PI-Neph in the measurement of the scattering coefficient is better in the 660 nm and 515 nm wavelengths than in the 405 nm one, although differences between datasets are still low. The observed differences in phase functions and especially polarized phase functions (both in shape and spectral differences) under changing atmospheric scenarios (Saharan dust, pollution and background aerosol conditions) indicated that these variables are sensitive and reliable to classify the type of ambient aerosol. Particularly, the combined measurements of F11 and -F12/F11 shows a unique potential of the PI-Neph for aerosol typology.