Volume-to-extinction ratio: an important property of dust

The volume-to-extinction ratio (ζ) is an important aerosol property, allowing to relate gravimetric and optical quantifications, widely used in remote sensing and in climate models. The ζ ratio is affected by the microphysical properties of aerosol particles, including their size, shape and composition. This study presents a synergistic approach combining airborne in-situ observations and ground-based remote sensing to study this ratio during dust events originating in the Middle East and Saharan regions, and to examine its vertical variability and general estimation uncertainty. The data were collected during the 2021 Cyprus Fall Campaign and the 2022 ASKOS campaign in Cabo Verde. The combination of observations offered vertically-resolved information on the particle size-distribution and volume-to-extinction ratio. The findings of this study reveal pronounced differences in the ζ ratio and effective radius across events and regions, reflecting variations in the degree of mixing with fine particles, as well as some variability with altitude due to varying particle size and shape. During Middle East dust events in Cyprus in fall 2021 the observed average ζ was the lowest with ζ=0.53±0.24 µm, whilst for a Saharan dust case in Cabo Verde in summer 2022 observations showed the highest values with ζ=1.14±1.01 µm, both values obtained at the dust layer altitude in some of the reported cases. The analysis highlights large discrepancies compared to AERONET-derived values and previous literature, especially in the presence of super-coarse and giant particles. Scattering computations allowed to evaluate the experimental results and provide insights into the role of particle asphericity. Atmospheric model simulations also showed discrepancies, mainly due to assumptions that neglect larger particles. These findings suggest that improved dust representation in models is essential for accurate climate assessment.