Transported African Dust in the Lower Marine Atmospheric Boundary Layer is Internally Mixed with Sea Salt Contributing to Increased Hygroscopicity and a Lower Lidar Depolarization Ratio

Abstract. Saharan dust is transported across the Atlantic, yet the chemical, physical, and morphological transformations dust undergoes within the marine atmospheric boundary layer (MABL) remain poorly understood. These transformations are critical for understanding dust's radiative and geochemical impacts, representation in atmospheric models, and detection via lidar remote sensing. Here, we present coordinated observations from the Office of Naval Research's Moisture and Aerosol Gradients/Physics of Inversion Evolution (MAGPIE) August 2023 campaign at Ragged Point, Barbados. These include vertically resolved single-particle analyses, mass concentrations of dust and sea spray, and High Spectral Resolution Lidar (HSRL) retrievals. Single-particle data show that dust within the Saharan Air Layer (SAL) remains externally mixed, with a corresponding high HSRL-derived linear depolarization ratio (LDR) of ~0.3. However, at lower altitudes, dust becomes internally mixed with sea spray, resulting in increased particle sphericity likely due to an increase in hygroscopicity, which suppresses the LDR signal to below 0.1 even in the presence of high dust loadings (e.g., ~120 µg/m3). The low depolarization in the presence of high dust in the MABL is likely due to a combination of the differences between the single scattering properties of dust and spherical particles, and the potential modification of the dust optical properties from an increased hygroscopicity of dust caused by the mixing with sea salt in the humid MABL. These results highlight the importance of the aerosol particle mixing state when interpreting LDR-derived dust retrievals and estimating surface dust concentrations in satellite products and atmospheric models.

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Published
2025-09-25
DOI
https://doi.org/10.5194/egusphere-2025-4584
Primary Topic
Atmospheric aerosols and clouds
Type
article
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article

Transported African Dust in the Lower Marine Atmospheric Boundary Layer is Internally Mixed with Sea Salt Contributing to Increased Hygroscopicity and a Lower Lidar Depolarization Ratio

Andrew P. Ault, Cassandra J. Gaston, Zezhen Cheng, Mira L. Pöhlker et al.
Atmospheric aerosols and clouds
article

Transported African Dust in the Lower Marine Atmospheric Boundary Layer is Internally Mixed with Sea Salt Contributing to Increased Hygroscopicity and a Lower Lidar Depolarization Ratio

Andrew P. Ault, Cassandra J. Gaston, Zezhen Cheng, Mira L. Pöhlker, Elena Lind, A. Ortiz, A. Joy Allen, Edwin Eloranta, Swarup China, Robert E. Holz, T. F. Eck, Nurun Nahar Lata, Hope E. Elliott, Zachary N. Buckholtz, Rupesh Agrawal, Jesus Ruiz‐Plancarte, Rebecca Chewitt-Lucas, Jeffrey S. Reid, Willem Marais, Anthony Bucholtz, I. A. Razenkov, Ryan Yamaguchi, Elizabeth A. Reid, Qing Wang, Sujan Shrestha, Edmund Blades, Devon Blades
article en

Abstract

Abstract. Saharan dust is transported across the Atlantic, yet the chemical, physical, and morphological transformations dust undergoes within the marine atmospheric boundary layer (MABL) remain poorly understood. These transformations are critical for understanding dust's radiative and geochemical impacts, representation in atmospheric models, and detection via lidar remote sensing. Here, we present coordinated observations from the Office of Naval Research's Moisture and Aerosol Gradients/Physics of Inversion Evolution (MAGPIE) August 2023 campaign at Ragged Point, Barbados. These include vertically resolved single-particle analyses, mass concentrations of dust and sea spray, and High Spectral Resolution Lidar (HSRL) retrievals. Single-particle data show that dust within the Saharan Air Layer (SAL) remains externally mixed, with a corresponding high HSRL-derived linear depolarization ratio (LDR) of ~0.3. However, at lower altitudes, dust becomes internally mixed with sea spray, resulting in increased particle sphericity likely due to an increase in hygroscopicity, which suppresses the LDR signal to below 0.1 even in the presence of high dust loadings (e.g., ~120 µg/m3). The low depolarization in the presence of high dust in the MABL is likely due to a combination of the differences between the single scattering properties of dust and spherical particles, and the potential modification of the dust optical properties from an increased hygroscopicity of dust caused by the mixing with sea salt in the humid MABL. These results highlight the importance of the aerosol particle mixing state when interpreting LDR-derived dust retrievals and estimating surface dust concentrations in satellite products and atmospheric models.

United States Naval Research Laboratory (US), Goddard Space Flight Center (US), University of Wisconsin–Madison (US), Pacific Northwest National Laboratory (US), University of Miami (US), University of Michigan (US), Environmental Molecular Sciences Laboratory (US), Naval Postgraduate School (US), Leibniz Institute for Tropospheric Research (DE), Caribbean Disaster Emergency Management Agency (BB)
National Science Foundation, U.S. Department of Energy, National Aeronautics and Space Administration, Office of Science, Office of Naval Research, Biological and Environmental Research
Openalex Percentile: Top 75%
Atmospheric aerosols and clouds
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