Recent field measurements have shown that there are large, sustained, unidentified sources of secondary organic aerosol (SOA). The alpha-dicarbonyl compounds glyoxal and methylglyoxal are receiving increasing scrutiny as potential SOA sources because of their anthropogenic and biogenic precursors, their efficient uptake by atmospheric water droplets and liquid aerosol particles, and their oligomer-forming potential. Recent work shows that the key to aqueous phase alpha-dicarbonyl reactivity is dehydration followed by nucleophilic attack. The nucleophiles methylamine and several amino acids have been identified in cloud- and fog-water droplets in the atmosphere in micromolar concentrations similar to the alpha-dicarbonyl compounds. These nucleophiles have been identified in preliminary experiments as efficient reaction partners with glyoxal and methylglyoxal, forming products upon drying that irreversibly alter gas-particle partitioning of the reactants, even when re-dissolved. The goal of this research is to characterize these reactions and determine their reaction products, measure the reaction kinetics under realistic conditions, and thereby determine the atmospheric significance of these reactions in forming SOA and oligomeric material via cloud and aerosol processing. The reactions will be studied by several methods. First, scanning mobility particle sizing studies of drying monodispersed cloud droplets will determine the extent of reaction under a range of atmospheric conditions and the effect of reactions on particle volatility, off-gassing, and subsequent cloud processing cycles. Second, bulk ESI-MS and NMR studies on aqueous solutions that have been dried and re-dissolved to simulate cloud processing will be used to chemically characterize reaction products and determine reaction pathways. Third, aerosol mass spectrometric (AMS) analysis of particles produced by simulated cloud processing and by direct aerosol phase reactions will determine if bulk phase reaction products and pathways are applicable under atmospheric conditions. Finally, temperature dependent reaction kinetics will be measured by NMR. Information gained by these experiments will determine the atmospheric significance of alpha-dicarbonyl + amine reactions.
Airborne particles influence climate, degrade visibility, and impact human health. The results will help improve atmospheric models used to understand the origin of organic aerosols. These models are widely used to assess the potential effects of aerosols on global climate, regional visibility, and air quality, which are all important societal problems. The project will also support the education of a number of undergraduate students.
In this project, our research group studied reactions between chemicals that are commonly found in clouds and aerosol particles to identify what chemical transformations might take place. Focusing on chemicals like those shown in reaction 1 on the right (dicarbonyls such as glyoxal, and amines), we found that nitrogen-containing rings were the major products, along with nitrogen-containing polymers and brown chemicals. The formation of brown chemicals is especially important, since they would absorb sunlight and contribute to global warming (like soot). These products have all been seen in atmospheric aerosol particles, but no one knew where they were coming from, nor how they were being produced. Our reaction rate measurements showed that these reactions proceed at a leisurely pace in water, but become very quick as the water evaporates. The result is the production of brown aerosol particles right above clouds (or as morning fog burns off), as the evaporation of cloud or fog drops triggers these reactions to take place. These reactions therefore contribute to the formation of particulate air pollution, which is a significant health hazard in many parts of the country. Our work suggests that reducing emissions of ammonia and amine compounds could help reduce particulate air pollution. We also identified a reaction that can take place in high-altitude, icy clouds. (The illustration at right, taken from our paper (Journal of Physical Chemistry A 116 (24) 6180 – 6187 (2012)), also shows which emitted gases react into glyoxal once in the atmosphere). The reaction of glyoxal on ice surfaces in the presence of a tiny amount of nitric acid (reaction 2) produces a carbon-based acid that stays on the ice and a nitrogen oxide compound that evaporates. We estimate that this reaction significantly increases the amount of aerosol particle material high in the atmosphere. As a part of this project, one high school student and 15 undergraduate students were mentored and trained in research. One postdoctoral researcher was also trained. Of these 17 people involved with the project, 12 are female and 2 are African-American. Ten undergraduate students (so far) have co-authored journal articles describing their results, and four undergraduates have presented their work at conferences. The postdoctoral researcher now holds a tenure-track position in the Chemistry Department at Harvey Mudd College in Claremont, CA.
