Optical properties of marine aerosol: modelling the transition from dry, irregularly shaped crystals to brine-coated, dissolving salt particles

Kahnert, Michael; Kanngießer, Franz

doi:10.1016/j.jqsrt.2022.108408

Cited by 6 publications

(13 citation statements)

References 57 publications

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“…The aerosol extinction and optical thickness corresponding to these non-spherical sea salt particles were low; aerosol extinction values were around 20 Mm −1 at 532 nm and the optical depth contributed by these non-spherical particles was about 0.03-0.04 at 532 nm which represented on average about 30%-40% of the total column AOD. These HSRL-2 multiwavelength measurements of aerosol depolarization and lidar ratio are examples of the type of field measurements that can be used to help refine models of the optical properties of marine aerosols under different meteorological conditions (Kahnert and Kanngießer, 2023).…”

Section: Discussionmentioning

confidence: 99%

Airborne HSRL-2 measurements of elevated aerosol depolarization associated with non-spherical sea salt

Ferrare

Hair

Hostetler

et al. 2023

Front. Remote Sens.

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Airborne NASA Langley Research Center (LaRC) High Spectral Resolution Lidar-2 (HSRL-2) measurements acquired during the recent NASA Earth Venture Suborbital-3 (EVS-3) Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment (ACTIVATE) revealed elevated particulate linear depolarization associated with aerosols within the marine boundary layer. These observations were acquired off the east coast of the United States during both winter and summer 2020 and 2021 when the HSRL-2 was deployed on the NASA LaRC King Air aircraft. During 20 of 63 total flight days, particularly on days with cold air outbreaks, linear particulate depolarization at 532 nm exceeded 0.15–0.20 within the lowest several hundred meters of the atmosphere, indicating that these particles were non-spherical. Higher values of linear depolarization typically were measured at 355 nm and lower values were measured at 1,064 nm. Several lines of evidence suggest that these non-spherical particles were sea salt including aerosol extinction/backscatter ratio (“lidar ratio”) values of 20–25 sr measured at both 355 and 532 nm by the HSRL-2, higher values of particulate depolarization measured at low (< 60%) relative humidity, coincident airborne in situ size and composition measurements, and aerosol transport simulations. The elevated aerosol depolarization values were not correlated with wind speed but were correlated with salt mass fraction and effective radius of the aerosol when the relative humidity was below 60%. HSRL-2 measured median particulate extinction values of about 20 Mm−1 at 532 nm associated with these non-spherical sea salt particles and found that the aerosol optical depth (AOD) contributed by these particles remained small (0.03–0.04) but represented on average about 30%–40% of the total column AOD. Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar aerosol measurements during several cold air outbreaks and CALIOP retrievals of column aerosol lidar ratio using column AOD constraints suggest that CALIOP operational aerosol algorithms can misclassify these aerosols as dusty marine rather than marine aerosols. Such misclassification leads to ∼40–50% overestimates in the assumed lidar ratio and in subsequent retrievals of aerosol optical depth and aerosol extinction.

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Section: Discussionmentioning

confidence: 99%

Airborne HSRL-2 measurements of elevated aerosol depolarization associated with non-spherical sea salt

Ferrare

Hair

Hostetler

et al. 2023

Front. Remote Sens.

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“…As discussed in Kahnert and Kanngießer (2023a), and as illustrated in Figure 1 (bottom), this braces the range within which the transition from dry nonspherical to wet spherical marine particles occurs. Although it is not straightforward to relate these mass fractions to relative humidity, it is estimated that, for pure sodium chloride crystals, this range would roughly correspond to a relative humidity of 43%-48% for efflorescence, and 60%-72% for deliquescence (Kahnert & Kanngießer, 2023a). However, field and satellite observations of lidar depolarization ratios of marine aerosols as a function of relative humidity suggest that in nature the transition from dry depolarizing to wet non-depolarizing particles is rather smooth and extends over a broad range of the relative humidity between 40% and 90% (Haarig et al, 2017;Thomas et al, 2022).…”

Section: Particle Modelmentioning

confidence: 82%

“…This is because within the limited range of salt‐mass fraction considered here, the wet radius is not dramatically larger than the corresponding dry radius. (As an example, a particle with dry radius 1.5 μm and a salt‐mass fraction of 0.91 would have a wet radius of 1.6 μm—see also Figure 11 by Kahnert and Kanngießer (2023a). ) Further, the corresponding color ratio (top right) is also insensitive to a variation in f m .…”

Section: Resultsmentioning

confidence: 99%

“…At a wavelength of λ = 532 nm, the complex refractive indices of sea salt and brine are assumed to be m = 1.5484 + 0i (Eldridge & Palik, 1997) and 1.3844 + 2.5330 ⋅ 10 −8 i, respectively. The latter is an updated value different from the one in Kahnert and Kanngießer (2023a), which accounts for the corrigendum in X. Li et al. (2018) to the parameterization given in X. Li et al.…”

Section: Methodsmentioning

confidence: 99%

“…The model accounts for this effect in calculating the wet radius as a function of the dry radius and the salt‐mass fraction. A detailed model description and the transformation from salt‐volume fraction to salt‐mass fractions is given in Kahnert and Kanngießer (2023a). Tests at a wavelength of 532 nm showed that the range of lidar ratios and linear depolarization ratios obtained with the model is consistent with existing field and satellite observations (Kahnert & Kanngießer, 2023a).…”

Section: Methodsmentioning

confidence: 99%

See 2 more Smart Citations

Optical Characterization of Marine Aerosols Using a Morphologically Realistic Model With Varying Water Content: Implications for Lidar Applications and Passive Polarimetric Remote Sensing

Kahnert,

Kanngießer

2024

Geophysical Research Letters

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Retrieving the physical properties and water content of marine aerosols requires understanding the links between the particles' optical and microphysical properties. By using a morphologically realistic model with varying salt mass fractions fm, describing the transition from irregularly shaped, dry salt crystals to brine‐coated geometries, optical properties relevant to polarimetric remote sensing are computed at wavelengths of 532 and 1,064 nm. The extinction cross section and its color ratio depend on particle size, but are insensitive to changes in fm; thus, measured extinction coefficients at two wavelengths contain information on both particle number and size. The lidar ratio's dependence on both size and wavelength has implications for inverting the lidar equation. The results suggest that active observations of the backscattering cross section's color ratio and the depolarization ratio, as well as, passive observations of the degree of linear polarization offer avenues to obtain the water content of marine aerosols.

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Optical Modeling of Sea Salt Aerosols Using in situ Measured Size Distributions and the Impact of Larger Size Particles

Lin,

2024

Adv. Atmos. Sci.

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Optical properties of marine aerosol: modelling the transition from dry, irregularly shaped crystals to brine-coated, dissolving salt particles

Cited by 6 publications

References 57 publications

Airborne HSRL-2 measurements of elevated aerosol depolarization associated with non-spherical sea salt

Airborne HSRL-2 measurements of elevated aerosol depolarization associated with non-spherical sea salt

Optical Characterization of Marine Aerosols Using a Morphologically Realistic Model With Varying Water Content: Implications for Lidar Applications and Passive Polarimetric Remote Sensing

Optical Modeling of Sea Salt Aerosols Using in situ Measured Size Distributions and the Impact of Larger Size Particles

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