This project was aimed at understanding reactions in cloud and fog droplets. In the atmosphere, cloud and fog droplets act as tiny active chemical reactors, able to facilitate different chemical reactions from what happens in the air itself. Cloud and fog drops initially grow on aerosol particles, and the aerosol particle largely dissolves into the cloud drop. They also take up gasses from the air. The chemistry in the drops, mostly oxidation reactions, has two important impacts: it forms more acids, which contribute to acid deposition, and it increases the amount of material that stays in the particle phase after the droplet re-evaporates. An airmass that goes through a cloud can come out with larger aerosol particles—and more PM2.5 mass--as a result of these reactions. PM2.5 mass is strongly associated with air pollution-related health impacts. We collected aerosol particles in urban areas, from a ship burning conventional fuel and biodiesel, and in an area with substantial residential wood burning. The particles differed in their abilities to promote oxidation chemistry, with fresh engine emissions the least active, and biomass burning particles, collected after ‘aging’ by travelling through the air for at least a few hours, the most by far. The reactivity of engine emission particles increased after they were aged. Average urban emissions were more reactive than aged engine emissions, but much less reactive than aged biomass burning particles.

The chemistry leading to the observed reactivity was not known. Our project led to the discovery of novel chemistry that provides an additional source of the most reactive species in cloud and fog droplets, able to enhance the speed at which molecules are oxidized in clouds.  This chemistry involves soluble transition metals and organic compounds. Transition metals take many different forms in droplets, such as free metal ions, metals in inorganic and organic complexes, each with very different reactivities. Further probing the underlying chemistry, including the form of iron involved is the subject of future work.

The work is the product of a team of graduate students (4), undergraduates (10) a high school student, collaborating or visiting scientists (4), and the principle investigator. The students especially solved problems, overcame challenges, made discoveries, learned about themselves, plotted their career paths, and later got jobs they wanted in a variety of fields.

Last Modified: 02/28/2018
Modified by: Suzanne E Paulson