Jesse Kroll, PhD, Massachusetts Institute of Technology
Project Goals and Activities. This project is aimed at gaining an improved understanding of oxidation mechanisms of complex hydrocarbons. Such reactions play key roles in several chemical processes important to energy, industry, and the environment, including combustion of fossil fuels, weathering of organic materials such as oil and asphalt, environmental degradation of pollutants, and formation of secondary atmospheric species such as tropospheric ozone and secondary organic aerosol (SOA). The goals are to gain an improved understanding of the chemistry of key radical species –alkoxy radicals (RO) and alkylperoxy radicals (RO2) – by forming these intermediates not by reactions of organics with oxidants but rather by photolysis of radical precursors. Such an approach avoids much of the drawbacks associated with initiating reactions with gas-phase oxidants, the typical approach for studying hydrocarbon oxidation processes. These drawbacks include interferences from multiple reaction channels, multiple oxidation generations, and competing chemistry, all of which are minimized when the radicals are generated directly. Two approaches are used for generating radicals: the 254 nm photolysis of alkyl iodides (RI) in the presence of oxygen to form RO2 radicals, and the ~350 nm photolysis of alkyl nitrites (RONO) to form RO radicals. A key to this project is the use of aerosol mass spectrometry (AMS) for the real-time characterization of the mass and chemical composition of low-volatility organic compounds.
In Year 2 of the project we built on our results from Year 1, which had shown that the photolysis of simple iodide and nitrite species could indeed form lower-volatility species. Two major types of experiments were carried out this year: (1) the photolysis of gas-phase alkyl nitrites under a range of experimental conditions in an environmental chamber, in order to understand the gas-phase chemistry of large RO radicals, and (2) the photolysis of particle-phase alkyl iodides and nitrites, in order to study the chemistry of condensed-phase R, RO, and RO2 radicals.
Photolysis of gas-phase alkyl nitrites. A major research effort involved the study of gas-phase alkoxy radical chemistry within the large (7.5 m3) environmental chamber in our laboratory. We have synthesized a num of a number of isomeric alkyl nitrite species (for example, for a C7 alkane chain, heptyl-1-nitrite, heptyl-2-nitrite, heptyl-3-nitrite, and heptyl-4-nitrite), which are then introduced into the chamber, and photolyzed using blacklights (300-400 nm output). Ammonium sulfate seed particles are added in order to correct for losses of any particles to the walls, and in some cases, co-reagents (O3, NO, etc.) are added as well, in order to influence the subsequent chemistry. The gas-phase nitrite concentrations are then tracked, and the loading and composition of any SOA formed is measured. For some experiments, chemical ionization mass spectrometry (CIMS) is also used to characterize gas-phase reaction products.
The experiments led to the collection of a rich dataset of aerosol formation and chemistry as a function of carbon number of the precursor, radical position on the carbon skeleton, and NOx level. The chemistry data are still being interpreted, but several key results about overall aerosol formation have been obtained. First, primary radicals form much more aerosol than do secondary radicals, probably due to the lower vapor pressure of molecules with functional groups located at the terminal position of the carbon chain. Secondly, the addition of NO strongly inhibits aerosol growth, likely by suppressing the formation of secondary oxidants (O3 and NO3). Such oxidants react with photolysis products, and these reactions appear to be crucial in the formation of low-volatility species. Current work is focused on the analysis and interpretation of the gas- and condensed-phase chemistry data, in order to better understand the underlying chemistry of the alkoxy radicals. The nature of the secondary reactions leading to later-generation, low-volatility products is an important area of future research for our lab, and builds directly off this project.
Photolysis of condensed-phase alkyl nitrites and iodides. The other major research effort this year involved understanding the reactions of condensed-phase RO, and RO2 radicals, important intermediates in combustion, atmospheric chemistry, and organic weathering processes. We access this chemistry by the photolysis of submicron particles made up of low-volatility radical precursors, followed by the real-time measurement of the changing chemical composition and mass of the particles. To demonstrate this novel approach toward the study of condensed-phase radical chemistry, we examined the 254 nm photolysis C18H37I, which forms condensed-phase RO2 radicals when O2 is present, and the 300-400 nm photolysis of C20H41ONO, which forms condensed-phase RO radicals. These experiments were carried out at the Advanced Light Source at Lawrence Berkeley National Laboratory, so the particles could be measured using vacuum ultraviolet (VUV) photoionization aerosol mass spectrometry, a “soft” technique for the molecular-level identification of specific aerosol components. A number of experiments were carried out using these two precursors, in which the concentrations of radical precursor, NO levels, and mass spectrometry techniques were all varied.
Contrary to our expectation of the generation of only a few product species formed according to well-understood chemical mechanisms, we found that both radical precursors formed a large, highly complex array of products. The alkyl nitrite species shows clear signs of secondary chemistry – the major products are not from unimolecular transformations of the radical but rather from reactions of the radical with other organic molecules, most likely via hydrogen-atom abstraction (RO + RH -> ROH + R). This may be an important radical-propagating channel in the oxidation of condensed-phase hydrocarbons; such autoxidation processes are well known in combustion but have received less attention for lower-temperature processes (such as in atmospheric chemistry). Photolysis of the alkyl iodide leads to a dramatic change in the particles: particle mass decreases by ~90%, indicating the formation of predominantly high-volatility products (which is unexpected given what we know about R and RO2 chemistry). The remaining low-volatility products are high in mass, with more carbon atoms than the precursor, indicating the importance of radical-radical or radical-molecule addition reactions (possibly RO2+alkene). It is expected that this novel dataset with provide insight into the mechanisms by which hydrocarbons oxidize in the condensed phase.