Development of a Cold Stage to Measure Ice Nucleating Activity Produced by Biomass Burning: Ash, Aerosol, and Aging
Creators: name: Polen, Michael
Publisher: name: Carnegie Mellon University
Year: 2019
Subjects: subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences

description: 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 the<br>properties 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 examined<br>the 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 the<br>ice 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.<br>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 never<br>examined 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 amorphous<br>material. 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>