Phase matrix characterization of long-range transported Saharan dust using multiwavelength polarized polar imaging nephelometry

Abstract

This work investigates the scattering matrix elements during different Saharan dust outbreaks over Granada (South-East Spain) in 2022 using the Polarized Imaging Nephelometer (PI-Neph PIN100, GRASP-Earth). The PI-Neph is a unique instrument capable of measuring continuously the phase function and polarized phase function (F₁₁ and -F₁₂/F₁₁) at three different wavelengths (405, 515 and 660 nm) with 1° resolution. Extreme dust events (PM₁₀ concentration above 1000 µgm⁻³) occurring in March 2022 are compared with more frequent and moderate events registered in summer 2022 (PM₁₀ concentration between 50 and 100 µgm⁻³). For F₁₁ there are no remarkable differences between extreme and moderate events. However, results of -F₁₂/F₁₁ show large differences between extreme and moderate events, especially for the 405 nm wavelength. These differences are also observed when studying the temporal evolutions during the extreme events and reveal that -F₁₂/F₁₁ patterns similar to laboratory measurements occurred during the more intense periods of dust influence. Other aerosol optical properties were derived from the PI-Neph, such as the asymmetry parameter (g), the fraction of backscattered light (Bs) and the lidar ratio (LR). In general, g and Bs show typical values (g > 0.65 and Bs ~ 0.1) for both extreme and moderate Saharan dust events. However, the LR shows more variable values for the different dust events, ranging from 20 to 60 sr⁻¹. The combination with additional in-situ instrumentation allowed to obtain scattering (SAE) and absorption (AAE) Ångström exponents and to conduct a typing classification that revealed extreme dust events as pure dust while moderate dust events were classified as a mixture of dust with urban background pollution. In addition, model simulations with the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) code reproduce well the PI-Neph measurements. Therefore, our results confirm that differences in the phase matrix elements of Saharan dust outbreaks of varying intensity can be explained by the mixing conditions of dust with the background particles, which varies from almost pure dust particles during extreme events, to a mixture of dust with local pollution during moderate events.