Collaborative Research: Impact of Surface Chemistry for Black Carbon Cloud Droplet Formation

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This award is funded by the Environmental Chemical Sciences Program in the Division of Chemistry. Professors Howard Fairbrother of Johns Hopkins University and Akua A. Asa-Awuku of the University of Maryland College Park are supported to develop a molecular level understanding of how the chemical composition of the surface of soot (black carbon, BC) influences its ability to act as Cloud Condensation Nuclei (CCN) and form rain droplets. Black carbon particles are released into the Earth's atmosphere from forest fires, biomass burning, and coal and diesel engines. Experimental evidence provides strong support for the idea that the ability of black carbon particles to uptake water depends on the presence of water-attracting functional groups on the black carbon particle surface. However, atmospheric black carbon is considered water-repellant in scientific models and thus not aptly addressed in aerosol-cloud climate interactions, contributing to the largest source of uncertainty in climate predictions. The surface chemistry of black carbon particles is complex and varies considerably due to differences in the carbon source, combustion conditions and subsequent oxidation (aging) in air. This project seeks a better understanding of how surface reactivity influences black carbon's ability to act as cloud condensation nuclei. Establishing relationships between the surface chemistry and CCN properties of BC particles in the atmosphere improves the predictive capabilities of global climate models and enhances our scientific understanding of air quality and climate, two important societal concerns. While conducting the project, the students benefit from acquiring skills in surface chemistry and atmospheric science. A plan that targets the participation of female undergraduate students in science majors at the Notre Dame University of Maryland, a local, primarily teaching, institution is initiated. The project aims to develop a molecular level understanding of how the chemical composition of the surface of black carbon influences its ability to act as Cloud Condensation Nuclei (CCN). The goal is accomplished by measuring and analyzing the CCN properties of a suite of black carbon particles whose surface chemistry has been systematically varied via exposure to different oxidizing conditions. The specific tasks are to develop experimental methods to modify and characterize the chemistry and surface properties of atmospherically-relevant aerosol, and to address unknown contributions of BC to atmospheric CCN and the subsequent aerosol-indirect effect. Detailed information on the surface composition and distribution of surface oxides is obtained with various analytical techniques, including X-ray Photoelectron Spectroscopy, augmented by chemical derivatization and infrared spectroscopies. The effect of other heteroatoms on CCN properties is also explored as well as the relationship between CCN activation and more fundamental BC surface properties, such as water adsorption energies.