Hygroscopic Phase Transitions of Atmospheric Particles

Harvard University, Cambridge MA

Investigators

Abstract

The objective of the work is to understand the connections between chemistry and hygroscopic phase transitions for complex organic materials mixed with sulfate. The complex organic materials will include both those produced by different types of secondary organic aerosol (SOA) in the laboratory and those naturally present in the Southern Great Plains, Oklahoma (i.e., field campaign). The first goal is to measure the values of the deliquescence relative humidity (DRH) and crystallization relative humidity (CRH) of mixed SOA organic-sulfate particles. The second goal is to connect the variability of the organic composition to the variability in the phase-transition behavior of the mixed particles. Hygroscopic phase transitions switch aerosol particles between states of lower and higher heterogeneous chemical reactivity and between states of lower and higher light scattering. These effects are becoming increasingly quantified as a result of a combination of recent ambient measurements, modeling, and laboratory studies. What emerges is that the sulfate content of atmospheric particles is the anchor point for understanding and predicting the hygroscopic phase transitions. Other chemical components such as acidity, nitrate, or organic material have a modulating influence. A key challenge for further progress is a better understanding of the effects of organic material on sulfate phase transitions; previous laboratory work has largely focused on sulfate mixed with a single up to several types of organic molecules. Atmospheric particles, however, consist of a mixture of sulfate with a multitude of different types of organic molecules. Such mixtures are expected to behave quite differently than simpler compositions, especially in regard to phase transitions, because the entropy of the multitude implies that crystal-aqueous phase transitions, when present, will be those solely of sulfate. A summer undergraduate project will broaden the participation of underrepresented groups in the sciences. A program is in place for recruitment from Historically Black Colleges and Universities and the University of Puerto Rico. He has also developed an AerosolCalculator website, and in this project the principal investigator will continue with its development. The motivation is to create a site useful for new students (as well as professionals) to develop a careful and thorough understanding of the complexities of aerosol science and technology. The website provides a ready interface (i.e., a web browser such as Internet Explorer) for the learning and exploration of the properties of aerosols. The use of a website and a web browser makes the site accessible worldwide, allowing for broad dissemination of this educational tool.