Development of a Cold Stage to Measure Ice Nucleating Activity Produced by Biomass Burning: Ash, Aerosol, and Aging

Ice nucleation is a fundamental process in the atmosphere that is difficult to study in a controlled manner. In this dissertation I developed a cold stage instrument to study theproperties of ice nucleating particles found in the atmosphere. The CMU-Cold Stage has been used to probe many different particle types to determine the impact they have on cloud glaciation. To properly assess the capabilities of the CMU-CS, I performed numerous tests of the background pure water signal and impacts of water source and surface interactions. To compare the abilities this system against other ice nucleation instruments I examinedthe ice nucleating ability of a biological particle surrogate, Snomax. However, atmospheric ice nucleating particles are much more complex than simple ice nucleating standards, so I studied different ice nucleating particles produced by biomass combustion. In particular, I examine the mechanisms and ice activity of the ash generated by biomass burning and the emissions given off by the combustion process. This work for the first time explores theice nucleating activity of different products of biomass burning and their impacts on cloud glaciation. I designed a substrate-based droplet freezing instrument to explore immersion mode freezing of microliter volume droplets. For any substrate-based freezing experiments, I identified important considerations that should be tested by all researchers. I examined the effect of different surfaces on pure water nucleation as well as water generation and source. I established a list of recommendations for all researchers to follow to improve the uniformity of ice nucleation methods. I compared the CMU-CS against other methods by using the biological particle surrogate Snomax. Snomax has previously been shown to be a good proxy for biological ice nucleating particles. However, I found that the product does not behave the consistently when stored for long periods of time. Its IN ability decreases to the level of weaker monomers and dimers of the ice-active protein as opposed to the high temperatures of the aggregates. This effect was also observed for repeated freezing and thawing of the droplets containing Snomax. I saw similar activity to other methods with the caveats of that activity not being consistent over time. For the first time I showed that Snomax should be cautiously used as a surrogate for biological particles and never over long periods of time.I then used the CMU-CS to study the products of biomass burning ash. Ash from combustion was recently found to contain ice nucleating particles. However, this was neverexamined for grasses, which are the most susceptible to wildfires. I found that grass ash, unlike wood ash, contains high amounts of mineral phases compared to amorphousmaterial. This is correlated to the amount of ice nucleating potential of the ash. Grasses burn with much higher temperature, which is also correlated to the amount of mineral phases present in the ash produced. For the first time I linked biomass burning efficiency to the ice nucleating particles present in the ash generated. I also performed experiments examining the ice nucleating particles emitted from biomass burning. These particles are known to be inconsistently produced by combustion of biomass and the mechanisms that cause them to be active are still unknown. I also examined how this ice nucleating ability was altered by atmospheric chemical aging by various oxidation mechanisms. I found that many tall grasses produce ice nucleating particles which are enhanced by oxidation. However, one instance where elevated organic aerosol was present I saw a decrease in ice nucleating ability upon aging. I saw that emission from wood burning contain almost no ice nucleating particles unlike grasses, a similar result to the ash findings. I also began testing a microfluidic device to lower the variable background of the CMU-CS traditional method, which helps to detect low activity particles from biomass burning aerosol. These experiments improve our current understanding of the ice nucleating properties of particles in the atmosphere and how we measure them. While the method itself is not new, the methods I used to approach these measurements were novel and important to developing our understanding of ice nucleation. This work will lead to better understanding of atmospheric ice nucleation and its effects on clouds, weather, and climate.<br

A New Multicomponent Heterogeneous Ice Nucleation Model and its Application to Snomax Bacterial Particles and a Snomax–illite Mineral Particle Mixture

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 15.0px; font: 13.0px Calibri; color: #000000; -webkit-text-stroke: #575757; background-color: #ffffff} p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 15.0px; font: 13.0px Calibri; color: #000000; -webkit-text-stroke: #575757; background-color: #ffffff; min-height: 15.0px} span.s1 {font-kerning: none} Some biological particles, such as Snomax, are very active ice nucleating particles, inducing heterogeneous freezing in supercooled water at temperatures above −15 and up to −2°C. Despite their exceptional freezing abilities, large uncertainties remain regarding the atmospheric abundance of biological ice nucleating particles, and their contribution to atmospheric ice nucleation. It has been suggested that small biological ice nucleating macromolecules or fragments can be carried on the surfaces of dust and other atmospheric particles. This could combine the atmospheric abundance of dust particles with the ice nucleating strength of biological material to create strongly enhanced and abundant ice nucleating surfaces in the atmosphere, with significant implications for the budget and distribution of atmospheric ice nucleating particles, and their consequent effects on cloud microphysics and mixed-phase clouds. The new critical surface area “g” framework that was developed by Beydoun et al. (2016) is extended to produce a heterogeneous ice nucleation mixing model that can predict the freezing behavior of multicomponent particle surfaces immersed in droplets. The model successfully predicts the immersion freezing properties of droplets containing Snomax bacterial particles across a mass concentration range of 7 orders of magnitude, by treating Snomax as comprised of two distinct distributions of heterogeneous ice nucleating activity. Furthermore, the model successfully predicts the immersion freezing behavior of a low-concentration mixture of Snomax and illite mineral particles, a proxy for the biological material–dust (bio-dust) mixtures observed in atmospheric aerosols. It is shown that even at very low Snomax concentrations in the mixture, droplet freezing at higher temperatures is still determined solely by the second less active and more abundant distribution of heterogeneous ice nucleating activity of Snomax, while freezing at lower temperatures is determined solely by the heterogeneous ice nucleating activity of pure illite. This demonstrates that in this proxy system, biological ice nucleating particles do not compromise their ice nucleating activity upon mixing with dust and no new range of intermediary freezing temperatures associated with the mixture of ice nucleating particles of differing activities is produced. The study is the first to directly examine the freezing behavior of a mixture of Snomax and illite and presents the first multicomponent ice nucleation model experimentally evaluated using a wide range of ice nucleating particle concentration mixtures in droplets.</p

Cleaning up our water: reducing interferences from nonhomogeneous freezing of “pure” water in droplet freezing assays of ice-nucleating particles

Droplet freezing techniques (DFTs) have been used for half a century to measure the concentration of ice-nucleating particles (INPs) in the atmosphere and determine their freezing properties to understand the effects of INPs on mixed-phase clouds. The ice nucleation community has recently adopted droplet freezing assays as a commonplace experimental approach. These droplet freezing experiments are often limited by contamination that causes nonhomogeneous freezing of the pure water used to generate the droplets in the heterogeneous freezing temperature regime that is being measured. Interference from the early freezing of water is often overlooked and not fully reported, or measurements are restricted to analyzing the more ice-active INPs that freeze well above the temperature of the background water. However, this avoidance is not viable for analyzing the freezing behavior of less active INPs in the atmosphere that still have potentially important effects on cold-cloud microphysics. In this work we review a number of recent droplet freezing techniques that show great promise in reducing these interferences, and we report our own extensive series of measurements using similar methodologies. By characterizing the performance of different substrates on which the droplets are placed and of different pure water generation techniques, we recommend best practices to reduce these interferences. We tested different substrates, water sources, droplet matrixes, and droplet sizes to provide deeper insight into what methodologies are best suited for DFTs. Approaches for analyzing droplet freezing temperature spectra and accounting and correcting for the background pure water control spectrum are also presented. Finally, we propose experimental and data analysis procedures for future homogeneous and heterogeneous ice nucleation studies to promote a more uniform and reliable methodology that facilitates the ready intercomparison of ice-nucleating particles measured by DFTs.</p