The Environmental Chemical Sciences (ECS) program of the Division of Chemistry and the Atmospheric Chemistry (ATC) program of the Division of Atmospheric and Geospace Sciences will support the collaborative research project of Prof. Neil Donahue and Prof. Allen Robinson of Carnegie Mellon University. Profs. Donahue and Robinson and their students will utilize advanced mass spectrometric techniques to elucidate mixing thermodynamics and kinetics of aerosol suspensions of varying size and composition. The collaborative team will also explore the role of water in the formation and dynamic nature of aerosols. The results will also provide vital input to a framework for organic-aerosol representation in atmospheric models.
The proposed research addresses the impact of aerosols on climate model predictions. This is one of the main uncertainties in current climate models. The study has the potential to transform our understanding of climate processes. It will provide excellent training opportunities to students who wish to train in a cutting edge research field of great societal importance.
Fine particles have extremely profound and uncertain environmental effects, and organic compounds are dominate both the effects and uncertainties. More than 50,000 people die every year in the US, and more than two million globally from inhalation of fine particles. Effects from particles constitute the largest uncertainty in climate forcing, depending in part on the number of particles larger than about 100 nanometers in diameter. Almost all of these grew up from much smiler sizes, either literally forming from vapors via nucleation as clusters smaller than 1 nm or being emitted by combustion between 10 and 30 nm or so. These particles must grow or die, because they tend to bump into larger particles and get swallowed up if they do not grow. The dynamics of organic aerosol (OA) condensation (vapors to particles) and evaporation (particles to vapors) has received considerable attention over the past few years, for good reasons. First, it is now clear that a very large fraction (80-90%) of the submicron particle mass arrives via condensation. Second, where in the size distribution the condensation occurs has huge implications for the growth rate and thus survival probability of ultrafine particles. Third, organic particles may sometimes exist in a highly viscous, glassy state; the resulting low diffusivity may then limit the rate of internal mixing of those particles enough to slow their approach to equilibrium. There is a very real possibility that the diffusivity of organics in organic aerosol spans a huge range, depending on the diffusing molecule, the composition of the matrix, and environmental conditions such as temperature and relative humidity. Research at Carnegie Mellon University (CMU) has shown that organic vapors from one particle type can indeed transfer to other particles of a different type, in most cases without evident limitations related to high viscosity (glassiness). This was demonstrated via novel experiments using mass spectrometers capable of quantitative measurements on individual particles, starting with two distinct particle populations labeled using isotopes. This enables experiments studying the process of organic condensation under conditions much closer to those found in the atmosphere than previously studied. The CMU research team also joined a consortium called CLOUD at the European Research Center CERN in Geneva, Switzerland to study new-particle formation, bringing expertise in the role of organic compounds in this process. Most particles in the atmosphere form by nucleation, but they are so small that most are lost to larger particles. CLOUD is a unique facility, both in terms of unprecedented cleanliness and exceptional instrumentation. Experiments have for the first time shown nucleation rates and particle growth rates in line with atmospheric observations, finding that particle formation can occur via a combination or sulfuric acid and organic compounds — either organic bases like dimethyl amine, or oxidized organics of the sort that form most organic aerosols. Both lines of research are adding significantly to our understanding of how particles form and grow in the atmosphere. These results are being incorporated into chemical transport models and climate models and will contribute to a significant reduction in our uncertainty regarding climate forcing.
