Chen, Yalei, author2022-05-052022-05-051999-10https://hdl.handle.net/10217/234929October 1999.Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1999.Studies were conducted to investigate ice formation by aerosol particles at upper tropospheric conditions. The continuous flow thermal diffusion chamber ( CFD) was used in both field and lab experiments to determine the conditions for the onset of ice formation in ambient and lab aerosols. After confirming the capability of the CFD to separate ice nucleating particles (IN) from non-IN, we collected samples of both IN and non-IN from the upper troposphere (UT) and the lower stratosphere during the 1996 NASA SUCCESS airborne field campaign. The chemical compositions of these particles were measured by electron microscopy, an individual particle analysis technique. From these results, as well as previous studies, ammoniated sulfates, sulfuric acid, and soot particles with different coatings were selected as representative UT aerosols for further laboratory investigation of their ice formation behaviors. In controlled laboratory studies, the phase states of (NH4)2SO4 and NH4HSO4 particles were found to have important impacts on their ice formation capabilities. Dry (NH4)2SO4 particles nucleated ice only at high relative humidity with respect to water at temperatures between -40°C and -60°C. Ammonium sulfate particles that entered the diffusion chamber in a liquid state froze by homogenous freezing at relative humidities that were 10% lower than where ice nucleated on dry particles. Likewise, crystalline or partially crystallized (as letovicite) NH4HSO 4 particles required higher relative humidities for ice nucleation than did initially liquid bisulfate particles . Based on our observations, 0.2 μm particles composed of either ammonium sulfate or bisulfate in a liquid solution freeze at lower RH at UT temperatures than do 0.05 μm sulfuric acid aerosol particles. The results indicate that, for liquid droplets, the size effect may be more important than the degree of ammoniation of the sulfate compound. Soot particles with different coating treatments were investigated in a similar manner as the sulfates. Submicron particles of commercial soot were coated with H2S04 in amounts of zero to several percent by weight. Untreated soot particles showed activity as deposition/sorption ice nuclei; and soot particles with approximately one monolayer equivalent coating of sulfuric acid froze at humidities slightly higher than those of untreated soot. These observations suggest that dilution of sulfuric acid was required before homogeneous freezing. Heterogeneous freezing was observed on particles with multilayer coverage at cold temperatures (< -53°C). We also generated soot particles by combustion of jet fuels and examined the freezing behavior of 0.05 μm monodisperse particles. The jet fuel soot particles contain about 10% by weight soluble matter, and did not freeze below 95% RH. Similarities existed in the ice formation behaviors of these jet fuel soot particles and 0.016 μm pure sulfuric acid particles. These laboratory results suggest that small solution droplets, with diameters of several hundredths of microns, require relatively high humidity to freeze homogeneously, and are therefore unlikely to be the particles that form cirrus clouds with continental origins. Dry crystals of sulfate form ice near water saturation. Sulfuric acid coating changes the ice formation properties of soot particles dramatically. With multilayer sulfuric acid coating, soot particles initiate homogeneous freezing at cirrus cloud conditions at cold temperatures. This suggests a heterogeneous nucleation pathway for continental cirrus formation. Pure soot particles and those with a small amount of sulfuric acid coating, like fresh aircraft exhaust, only form contrails near water saturation.reportsengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.IceAerosols -- CompositionAerosols -- Environmental aspectsTroposphereText