Optical measurements of atmospheric aerosols: Aeolian dust, secondary organic aerosols, and laser-induced incandescence of soot

Abstract

Aerosols suspended in earth’s atmosphere absorb and scatter sunlight. These optical effects can alter photochemistry or warm or cool the atmosphere and affect climate. This dissertation research contributes to a broader understanding of how aerosols affect our climate system by presenting advances in measurement devices used for optical measurements of aerosols. In addition, the dissertation provides salient optical property data for several aerosol types including Aeolian dusts, ammonium sulfate, secondary organic aerosols formed from pinene and toluene, biomass burning aerosols, black carbon (soot), and ambient aerosols collected at Lubbock, TX.
Chapter 2 outlines the optical properties of Amarillo and Pullman soil dusts that are widely dispersed in the Texas Panhandle. After collection of the soils, they were dispersed in air, and light absorption and extinction coefficients were measured at several visible wavelengths. Size distributions, mass concentration (µg particles / m3) and soil texture / color were also determined. Amarillo and Pullman soils had similar size distributions with generated particles having effective diameters in the 1-6 µm range. Both of the samples exhibited a wavelength dependent extinction and absorption. Extinction increased at larger wavelengths slightly with Angstrom extinction exponents of -0.11 and -0.17 for Pullman and Amarillo dust, respectively. Conversely, light absorption increased at shorter wavelengths with Angstrom absorption exponents of 1.73 and 2.17 for the Pullman and Amarillo dusts. Values for the mass-extinction coefficient ranged between 1.7 m2/g and 3.0 m2/g at 522 nm and single-scatter albedo (SSA) for both soil types ranged from 0.947 – 0.980 at visible wavelengths with SSA increasing at longer wavelengths. Chapter 3 outlines the development of a measurement method that combines the advantages of cavity ring-down spectrometry (CRDS) and integrating sphere nephelometry to measure both extinction and scattering of dispersed aerosols at 355 nm simultaneously. Aerosols including ammonium sulfate, secondary organic aerosols (SOA) resulting from the ozonlysis of α-pinene and photo-oxidation of toluene, Aeolian dusts (Amarillo and Pullman), biomass burning aerosols, and ambient aerosols were monitored though this approach in the near ultraviolet range. Scattering and extinction cross-sections for size-selected (diameter = 300 nm) particles were between 1.65–2.60 × 10–9 cm2 with the largest value corresponding to ammonium sulfate and the lowest value for pinene SOA. Neither ammonium sulfate, nor α-pinene SOA absorbed light at 355 nm. Toluene photo-oxidation SOA showed slight absorption. Aeolian dust samples and biomass burning aerosols absorbed light significantly at 355 nm with single scatter albedo (SSA) between 0.74 and 0.84. The near UV optical measurements this system can provide are useful for assessing the impact of aerosols on tropospheric photochemistry. The albedometer instrument described in chapter 3 was then modified to perform laser induced-incandescence (LII) of soot. The results of this study are presented in chapter 4. Instead of the 355 nm light beam, a co-linear beam containing both 532 and 1064 nm was applied to the aerosol. The 532 nm beam was meant to probe the particles scattering and extinction properties, while the 1064 nm beam heated the strongly absorbing soot resulting in broadband incandescence. The LII signal at 590 nm for fresh soot generated by a kerosene lamp was found to produce a linear response with soot mass concentration from the limit of detection of 0.5 to at least 100 µg / m3. Transmission electron microscopy (TEM) analysis and scanning mobility particle sizing (SMPS) data showed significant morphology changes and new particle production for lased soot samples compared with non-lased soot. These results suggest the LII process always irreversibly alters soot microphysical properties. Since these morphological changes were accompanied by a dramatic and significant drop in SSA after lasing with 1064 nm, we conclude that it is necessary for observations of aerosol optical properties and LII measurements to be made sequentially while allowing sufficient time for the measurement cell to be flushed with fresh aerosol sample.

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Keywords

Aerosols, Secondary organic aerosols, Soil dusts, Optical properties, Single scatter albedo, Scattering, Extinction, Cavity ring-down spectrometry, Integrating sphere nephelometry, Albedometer, Laser induced-incandescence

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