Research Highlights
Optical properties of dust aerosols
Most global aerosol models approximate dust as spherical particles, whereas most remote sensing retrieval algorithms approximate dust as spheroidal particles with a shape distribution that conflicts with measurements. These inconsistent and inaccurate shape assumptions generate biases in dust single-scattering properties. Here, we obtain dust single-scattering properties by approximating dust as triaxial ellipsoidal particles with observationally constrained shape distributions. We find that, relative to the ellipsoidal dust optics obtained here, the spherical dust optics used in most aerosol models underestimate dust single-scattering albedo, mass extinction efficiency, and asymmetry parameter for almost all dust sizes in both the shortwave and longwave spectra. We further find that the ellipsoidal dust optics are in substantially better agreement with observations of the scattering matrix and linear depolarization ratio than the spheroidal dust optics used in most retrieval algorithms. However, relative to observations, the ellipsoidal dust optics overestimate the lidar ratio by underestimating the backscattering intensity by a factor of ∼2. This occurs largely because the computational method used to simulate ellipsoidal dust optics (i.e., the improved geometric optics method) underestimates the backscattering intensity by a factor of ∼2 relative to other computational methods (e.g., the physical geometric optics method). We conclude that the ellipsoidal dust optics with observationally constrained shape distributions can help improve global aerosol models and possibly remote sensing retrieval algorithms that do not use the backscattering signal.
These newly-developed dust optical properties are being implemented into several global aerosol models, including MONARCH (Klose et al., 2021), IMPACT (Ito et al., 2021), NCAR CESM (Meng et al., 2022; Li et al., 2022), and NASA GISS ModelE.
References:
Huang et al. (2023), Atmospheric Chemistry and Physics (paper link).
Li et al. (2022), Geoscientific Model Development (paper link).
Meng et al. (2022), Geophysical Research Letters (paper link).
Klose et al. (2021), Geoscientific Model Development (paper link)
Ito et al. (2021), Atmospheric Chemistry and Physics (paper link)
Dust diameter standardization
Global aerosol models use different types of diameters from measurements, and therefore diameter standardization is crucial. Photo after Fig. 1 of Huang et al. (2021).
References:
Huang et al. (2021), Geophysical Research Letters (paper link)
Formenti et al. (2021), Atmospheric Measurement Techniques (paper link)
Measurements of dust aerosol size usually obtain the optical or projected area-equivalent diameters, whereas model calculations of dust impacts use the geometric or aerodynamic diameters. Accurate conversions between the four diameter types are thus critical. However, most current conversions assume dust is spherical, even though numerous studies show that dust is highly aspherical. Here, we obtain conversions between different diameter types that account for dust asphericity. Our conversions indicate that optical particle counters have underestimated dust geometric diameter (Dgeo) at coarse sizes. We further use the diameter conversions to obtain a consistent observational constraint on the size distribution of emitted dust. This observational constraint is coarser than parameterizations used in global aerosol models, which underestimate the mass of emitted dust within 10 ≤ Dgeo ≤ 20 μm by a factor of ∼2 and usually do not account for the substantial dust emissions with Dgeo ≥ 20 μm. Our findings suggest that models substantially underestimate coarse dust emission.
Realistic shape of dust aerosols
Global aerosol models and remote sensing products (AERONET, MODIS, and MISR) substantially underestimate dust asphericity. Photo after Fig. 2 of Huang et al. (2020).
Reference:
Huang et al. (2020), Geophysical Research Letters (paper link)
Climate models and remote sensing retrievals generally assume that dust aerosols are spherical or spheroidal. However, measurements show that dust aerosols deviate substantially from spherical and spheroidal shapes, as ratios of particle length to width (the aspect ratio) and height to width (the height‐to‐width ratio) deviate substantially from unity. Here, we quantify dust asphericity by compiling dozens of measurements of aspect ratio and height‐to‐width ratio across the globe. We find that the length is on average 5 times larger than the height and that climate models and remote sensing retrievals underestimate this asphericity by a factor of ~3-5. Compiled measurements further suggest that North African dust becomes more aspherical during transport, whereas Asian dust might become less aspherical. We obtain globally-averaged shape distributions, from which we find that accounting for dust asphericity increases gravitational settling lifetime by ~20%. This increased lifetime helps explain the underestimation of coarse dust transport by models.
Dust emission from active sands
Field campaign at Oceano Dunes, California.
Photo after Fig. 1 of Huang et al. (2019).
Sand dunes and other active sands generally have a low content of fine grains and, therefore, are not considered to be major dust sources in current climate models. However, recent remote sensing studies have indicated that a surprisingly large fraction of dust storms are generated from regions covered by sand dunes, leading these studies to propose that sand dunes might be globally relevant sources of dust. To help understand dust emissions from sand dunes and other active sands, we present in situ field measurements of dust emission under natural saltation from a coastal sand sheet at Oceano Dunes in California. We find that saltation drives dust emissions from this setting that are on the low end of the range in emissions produced by non-sandy soils for similar wind speed. Laboratory analyses of sand samples suggest that these emissions are produced by aeolian abrasion of feldspars and removal of clay-mineral coatings on sand grain surfaces. We further find that this emitted dust is substantially finer than dust emitted from non-sandy soils, which could enhance its downwind impacts on human health, the hydrological cycle, and climate.
References:
Huang et al. (2019), Atmospheric Chemistry and Physics (paper link)
Swet et al. (2020), Journal of Geophysical Research (paper link)