Retrieving volcanic, desert dust, and sea-salt particle properties from two/three-component particle mixtures after long-range transport using UV-VIS polarization Lidar and T-matrix

During transport by advection, atmospheric nonspherical particles, such as volcanic, desert dust or sea-salt particles experience several chemical and physical processes, leading to a complex vertical atmospheric layering at remote sites where intrusion episodes occur. In this contribution, a new methodology is proposed to analyze this complex vertical layering in the case of a two/three-component particle external mixtures after long-range transport. This methodology relies on a precise analysis of the spectral and polarization properties of the light backscattered by atmospheric particles. It is based on combining a sensitive and accurate UV-VIS polarization Lidar experiment with accurate T-matrix numerical simulations and air mass back-trajectories. The Lyon UV-VIS polarization Lidar is used to efficiently partition the particle mixture into its nonspherical components, while the T-matrix algorithm is used for computing backscattering and depolarization properties specific to nonspherical volcanic, desert dust and sea-salt particles, the latter being described in the cubic shape approximation. It is shown that, after long-range transport, the particle mixtures' depolarization ratio δp differs from the nonspherical particles' depolarization ratio δns due to the presence of spherical particles in the mixture. Hence, after identifying a tracer for nonspherical particles, particle backscattering coefficients specific to each nonspherical component can be retrieved in a two component external mixture. For three-component mixtures, the spectral properties of light must in addition be addressed by using a dual-wavelength polarization Lidar. Hence, for the first time, in a three-component external mixture, the nonsphericity of each particle is taken into account in a so-called 2β + 2δ formalism. Applications of this new methodology are then demonstrated in two case studies carried out in Lyon, France, related to the mixing of Eyjafjallajökull volcanic ash with sulphate particles (case of a two-component mixture) and to the mixing of dust with sea-salt and water-soluble particles (case of a three-component mixture). This new methodology, which is able to provide separate vertical profiles of mixed atmospheric dust, sea-salt and water-soluble particles, may be useful for accurate radiative forcing assessments.