The Environmental Chemical Sciences (ECS) program of the Division of Chemistry (CHE) and the Atmospheric Chemistry program of the Division of Atmospheric and Geospace Sciences (AGS) will support the research project of Prof. Jesse Kroll of Massachusetts Institute of Technology (MIT). Prof. Kroll and his students will study the mechanisms of oxidation of organic species in the environment using an environmental chamber at MIT and a flow reactor at the Advanced Light Source at Lawrence Berkeley National Laboratory (LBNL). A highly innovative feature of the study is the direct and controlled formation of alkylperoxy and alkoxy radicals by the photolysis of radical precursors like alkyl iodides and alkyl nitrites . Alkylperoxy and alkoxy radicals are key intermediates that are directly relevant to the formation and evolution of atmospheric organic aerosol, processes of central importance to global climate and human health.
The project will greatly improve our understanding of the oxidation mechanisms central to a number of key environmental issues, such as the degradation of pollutants and the production of secondary species, leading to the improved prediction of their rates and environmental impacts. In particular, the insights into the chemistry underlying the formation and evolution of atmospheric organic aerosol will inform policy on aerosol-climate and aerosol-health interactions, two of the major environmental issues confronting policymakers around the world. The study will provide excellent opportunities to graduate and undergraduate students at MIT to work on a cutting edge research project in environmental chemical sciences.
Organic compounds make up a large fraction of atmospheric fine particulate matter, and therefore can have important impacts on global climate and human health. However, our ability to predict the amounts and chemical composition of particulate organic species is limited, due largely to their immense chemical complexity and reactivity. In particular, the atmospheric oxidation reactions that control the formation and evolution of organic particulate matter involve a large number of chemical pathways, radical intermediates, and oxidation products, as well as multiple reaction generations, limiting the level of mechanistic insight that can be gained from most laboratory studies. In this project we have developed a novel approach toward the laboratory study of such chemistry, by photolytically generating large organic radicals of known chemical structure and studying their particulate products. This allows for the isolation and examination of individual reactive pathways relevant to the formation and evolution of particulate matter in the atmosphere. In these experiments, key oxidation intermediates (alkylperoxy and alkoxy radicals) were generated from the UV photolysis of radical precursors (alkyl iodides and alkyl nitrites, respectively), and the particulate products were measured in real time using aerosol mass spectrometry. Most experiments focused on the formation of secondary organic aerosol (SOA), a key class of organic particulate matter, in which the reactions of gas-phase species lead to the formation of particle mass. Both alkylperoxy and alkoxy radicals (with 8 or more carbon atoms per molecule) were shown to generate SOA, and the amount and composition of the SOA formed were shown to be strongly dependent on chemical structure (the location of the radical center on the carbon skeleton, and the structure of the carbon skeleton itself). In other experiments, radicals were generated directly within the particles, in order to study the oxidation chemistry that can occur within particles over the course of their atmospheric lifetimes. In that case it was shown that the radicals may react not only by the reactions available to them in the gas phase, but also by reactions with other particle-phase organic species, complicating their chemistry substantially. As part of this project we also investigated novel ways to measure the chemical composition of particles in real time, by using infrared laser vaporization and/or vacuum ultraviolet ionization within the aerosol mass spectrometer, and examining the response of different chemical species to these detection schemes. Together, these studies improve our ability to chemically characterize atmospheric organic particles, and moreover improve our understanding of the chemical reactions that control the loadings and key properties of such particles.
