The overall goal of this project is to reduce uncertainties related to the formation, oxidative aging, and cloud condensation nucleus (CCN) activity of organic aerosol particles, thereby providing a more reliable base for modeling and predicting climate change. A unique laboratory setup to characterize the composition, size, morphology, and density of aerosol particles and to measure CCN activity of these particles will be used to characterize and understand secondary organic aerosol (SOA) formation/evolution mechanisms and yields from atmospherically relevant precursor gases oxidized by hydroxyl and hydroperoxyl radicals and ozone. Multi-step heterogeneous oxidation of atmospherically relevant organic compounds will also be studied. The results will provide information necessary to characterize chemical composition of SOA from given precursors and to accurately represent their CCN proclivity.
The principal investigators (PIs) will actively participate in an initiative funded by the Gelfand Family Charitable Trust to mentor science students in four Boston inner city high schools, a program guided by the TSIP concept (Teaching Science through the Inquiry Process). Students will be mentored in small groups and in one-on-one settings with the aim of helping them prepare science fair projects. Two of the schools have science fairs already (Monument High School and The Engineering School). In the other two, working with the teachers and students, the PIs will help organize such fairs. The vehicle for potential science discussions will be current environmental issues with a specific focus on the atmosphere.
This report summarizes the project outcomes during the three years (09/15/2009 to 08/31/2012) of a collaborative project, entitled "Collaborative Research: Laboratory Studies of Formation, Aging, and Physiochemical Properties of Atmospheric Aerosols", performed by researchers at Boston College, Aerodyne Research Inc., and Pennsylvania State University. This project consisted of laboratory and field studies focused on characterizing the chemical, physical, and optical properties of two important types of atmospheric particles: black carbon (soot) particles and secondary organic aerosol particles. The first component of this project involved laboratory and field studies of black carbon (soot) particles, which are a by-product of incomplete combustion. Soot particles are the only particle types to strongly absorb all colors of sunlight and thus are an important element in global climate change. We conducted laboratory studies characterizing and comparing the performance of state-of-the-art instruments designed to measure the optical, physical and chemical properties of black carbon particles. In addition, we participated in a field study using these instruments to directly correlate the chemical and optical properties of black carbon-containing particles. The results from the field study suggest that the currently modeled climate forcing effect of black carbon particles may be significantly overestimated. This work provides direction for future studies geared towards more accurate modeling of the global climate forcing effects of black carbon particles. The second component of this project focused on the use of laboratory techniques to generate secondary organic aerosol (SOA) particles, which are complex mixtures of hundreds/thousands of individual organic compounds formed from gas-to-particle conversion processes in the atmosphere. Organic material typically dominates the particulate mass loadings in the US and around the world, confirming that SOA is an extremely important component of atmospheric aerosol. Central to our work was the development and characterization of a new bench-top flow reactor, which, unlike established environmental chamber techniques, is capable of simulating the formation and oxidation of SOA particles over multiple equivalent days in the atmosphere in a few minutes in the laboratory. This technique was used in a range of studies designed to measure physical and chemical properties of SOA: (1) We simulated the formation and evolution of SOA particles over approximately 1 to 10 days of equivalent atmospheric processing. We found that as SOA particles became more heavily oxidized, they take up water vapor more efficiently and can more readily form cloud droplets. This result suggests an important link between the chemical properties and the indirect climate forcing potential of SOA. (2) We performed laboratory measurements to understand ambient observations of organic aerosol particles measured downwind of the June 2010 Gulf oil spill. Using laboratory surrogates for vapors of the spilled oil, we generated SOA particles with the same chemical composition as atmospheric aerosol particles measured near the spill. Our measurements support the hypothesis that these aerosol particles were formed by the atmospheric chemical processing of hydrocarbon vapors evaporating from the oil slick. As SOA particles are continually oxidized in the atmosphere, low-vapor-pressure compounds in the SOA are eventually converted to high-vapor-pressure gas-phase species. This may alter the role of SOA particles in global climate forcing. (3) We measured the phase state of SOA particles as a function of atmospheric processing. Under most conditions, SOA particles were characterized by a solid phase at low relative humidity. As the SOA was exposed to higher relative humidity, a solid-to-liquid phase transition was observed. The phase state of SOA particles influences the ability to take up water vapor and form clouds, thereby affecting their atmospheric lifetimes and climate forcing potential. In a related study, we characterized the ice-forming potential of SOA particles at lower temperatures relevant to cirrus cloud formation (-70 to -10 oC), and found that solid SOA particles formed ice crystals more efficiently than liquid SOA particles.
