Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Margaret A. Tolbert

Second Advisor

Veronica Bierbaum

Third Advisor

Bob Sievers

Fourth Advisor

Robert Parsons

Fifth Advisor

Brian Toon

Abstract

The direct interaction of ambient atmospheric particles with solar radiation affects Earth's climate and local visibility. Light extinction by particles in the atmosphere is strongly dependent on particle size, chemical composition, hygroscopic growth properties and particle mixing state. Changes in relative humidity can influence particle extinction because hygroscopic growth causes the particles to increase in size. Laboratory quantification of the optical growth of various particle compositions is needed for inclusion in radiative transfer models. While the optical growth of pure particles is well understood, less is known about mixed particles. Atmospheric aerosols are typically 50-80% organic, but also contain sulfates. This thesis describes laboratory investigations into the role of organics on the optical hygroscopic growth of ammonium sulfate (AS). The influence of an organic 1,2,6-hexanetriol coating on an ammonium sulfate particle was first studied. The particle optical growth factor, fRHext, was measured using cavity ring-down aerosol extinction spectroscopy at 532 nm. The particles were composed of pure ammonium sulfate, pure 1,2,6-hexanetriol, and mixed particles containing a wet or dry ammonium sulfate core and 1,2,6-hexanetriol coating. Dry, coated particles were generated by atomization followed by drying. Wet, coated particles were formed via liquid-liquid phase separation (LLPS). LLPS was achieved by deliquescing the mixed particles, and then the particles were dried to a relative humidity (RH) between the phase separation RH and the efflorescence RH. For the LLPS particles, the fRHext at each RH was found to be between the fRHext of ammonium sulfate and 1,2,6-hexanetriol. In contrast, for the mixed dry, coated particles, the fRHext was the same as 1,2,6-hexanetriol particles. This work shows that at room temperature, the water uptake properties of AS coated with 1,2,6-hexanetriol are largely dictated by the phase of the AS. Thus, the total water uptake depends on the RH history of the particle and the resulting phase of AS. The influence of adding semi-solid, glassy particles to AS particles on the particle optical growth was then studied. Particles were composed of ammonium sulfate (AS), 1,2,6-hexanetriol, sucrose, raffinose, and mixed particles containing AS and either sucrose or raffinose. Both sucrose and raffinose were used because they can be semi-solid or glassy at room temperature. For the pure 1,2,6-hexanetriol, sucrose, and raffinose, the particles optical growth begins to occur around an RH between 34 and 40%. The liquid 1,2,6-hexanetriol exhibits similar water uptake behavior as the glassy sucrose and raffinose. Therefore, pure glassy aerosols may be treated similar to liquids is climate change models. For the mixed AS and sucrose or raffinose particles, the component weight % significantly affects the RH where water uptake is observed. When particles contain more AS than organic, it acts similar to a pure AS particle. The AS in the mixture is crystalline, and the particle doesn't take up much water until it deliquesces. When the particle contains more organic, it acts like a liquid particle and has slow continuous water uptake. This significantly simplifies the addition of mixed glassy aerosols in climate change models.

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