Relationship between particle size and imaginary refractive index in atmospheric dust

Lindberg, James D.; Gillespie, James B.

doi:10.1364/ao.16.002628

Cited by 40 publications

(14 citation statements)

References 6 publications

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“…The imaginary component estimates determined from the data (average of 0.004) are in agreement with values obtained for the 500-700 nm wavelength range from a number of other desert aerosol experiments ( e g , De Luisi et al, 1970;Grams et al, 1974;Lindberg and Laude, 1974;Spinhirne et al, 1980). Although imaginary indices in the range of 0.001-0.01 do not correspond to any specific substance commonly associated with atmospheric aerosols, the occurrence of values in t h s range is possibly a result of small amounts of carbon mixing with otherwise very weakly absorbing particles (Lindberg and Gillespie, 1977;Ackerman and Toon, 1981).…”

Section: Electron Microscope Analysissupporting

confidence: 90%

Lidar and Balloon-Borne Cascade Impactor Measurements of Aerosols: A Case Study

Reagan

Apte

Bruhns

et al. 1984

Aerosol Science and Technology

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Aerosol size distributions, elemental components, complex refractive indices, extinction profiles and extinctionto-backscatter ratios have been measured and inferred from balloon-borne cascade impactor and lidar observations made during a cooperative joint experiment conducted during the period 4-10 April, 1980 in Tucson, AZ. Size distributions obtained from quartz crystal microbalance (QCM) cascade impactor measurements at different heights (1 to 1000 m) and times over a period of several days were fairly similar in form, being clearly bimodal in their mass distributions with the coarse particle mode being dominant. Electron microscope and energy dispersive X-ray analyses of particles deposited on the QCM stages over the particle radii range -0.5-4.0 ym revealed that the particle samples were elementally dominated by both sulfur and crustal type (Al, Ca, Mg and Si) elements. Complex refractive index estimates for a wavelength of 649 nm were obtained by comparing the lidar inferred aerosol extinction-to-backscatter ratios with theoretically computed values calculated for the impactor-derived size distributions. The real part of the index was estimated to be 1.45 for most cases, while the estimates for the imaginary part ranged between 0.000 and 0.01. Aerosol extinction coefficients calculated for the impactor-derived size distributions were found to be somewhat smaller but in fair agreement with the extinction coefficients retrieved from the lidar measurements.

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Section: Electron Microscope Analysissupporting

confidence: 90%

Lidar and Balloon-Borne Cascade Impactor Measurements of Aerosols: A Case Study

Reagan

Apte

Bruhns

et al. 1984

Aerosol Science and Technology

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“…The resulting curves are shown in Fig. 6 and demonstrate significant variability of the optical constants over the particle size and wavelength ranges (Lindberg and Gillespie, 1977; Volz, 1983). Note that the smallest particles, less important for optical effects compared to larger ones, exhibit low imaginary parts in the short‐wave, too.…”

Section: Samum Measurementsmentioning

confidence: 99%

Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles

Otto¹,

Bierwirth²,

Weinzierl³

et al. 2009

Tellus B: Chemical and Physical Meteorology

124

150

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Citing articles: 6 View citing articles Tellus (2009), 61B, 270-296 C were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51-1.55 and imaginary parts of 0.0008-0.006 at 550 nm wavelength. At this spectral range a single scattering albedo ω o and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: ω o is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert.

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“…Analyses of particle samples collected near Tucson by both aircraft (Reagan et al, 1977) and a balloon-borne cascade impactor (Reagan et al, 1984) have revealed ME abundance of sulfur for partides ' iyTith radii as large as -1.0 pm. Larger imaginary refractive index values in the 0.005 to 0.01 range do not correspond to any spec~fic substance commonly associated with atmospheric aerosols, but such values may possibly result from small amounts of absorptive material such as carbon mixing with otherwise very weakly absorbing particles (e.g., Lindberg and Gillespie, 1977;Ackerman and Toon, 1981).…”

Section: Rrdius [Microns)mentioning

confidence: 99%

“…As noted earlier, small amounts of absorptive material, such as carbon, mixed with otherwise very weakly absorbing particles may be the cause of average or effective imaginary refractive index values in the 0.001 to 0.01 range sometimes inferred for aerosol particles; such values are not representative of any specific substance commonly associated with atmospheric aerosols. The carbon was assumed to be restricted to the accumulation mode because it is typically only detected in significant amounts in the very small particle range (Lindberg and Gillespie, 1977;Sloane, 1983;Heintzenberg and Covert, 1984). Table 4 lists Sa values computed for the mean balloon impactor distribution with different amounts of carbon particles (refractive index for carbon assumed to be m = 1.8-0.5i) in the accumulation mode and all remaining particles with refractive index values of m = 1.4-0.0005i or 1.5-0.000i.…”

Section: Radius (Pm)mentioning

confidence: 99%