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39577-AC6
Atmospheric Chemistry of Hydrocarbon Soot
In the final year of the grant, a number of projects were completed. In particular, the focus of the research is upon the reactive and hygroscopic properties of fresh hydrocarbon soot. We are interested in the degree to which such surfaces can be chemically modified by gas-surface interactions. In the first case, we completed the study of the interaction of NO2 with a variety of hydrocarbon soot surfaces. The focus of this work was on the formation of HONO in the gas phase, the surface modification that results, and the mechanism. We believe this is one of the most thorough studies of such a system and it is now published in the Journal of Physical Chemistry A (Aubin and Abbatt, 2007). The student working on this project, Dr. D. Aubin, has now graduated with a PhD degree.
With respect to the interactions of ozone with PAHs that are known to comprise a significant component of the surface of hydrocarbon soot, we also completed a study involving the factors that affect the reactivity of a specific PAH, anthracene. A particular finding is that the surface phase reaction rate constant between adsorbed anthracene and ozone is not nearly so sensitive to the nature of the surface as is the binding efficiency of ozone to such surfaces. We showed this by comparing the reactivity between different organic substrates, one more oxidized similar to an SOA particle and one more reduced and hydrophobic similar to the surface of a soot particle. We believe this may be a general finding, as described in the summary of this project which is currently in press with Journal of Physical Chemistry A (Kwamena, George, Staikova, Donaldson and Abbatt). The atmospheric implications of this work are that PAHs sitting on the surface of combustion soot particles are expected to be oxidized with ozone extremely rapidly under atmospheric conditions of ozone in the gas phase. The student working on this project, Ms. N. Kwamena, will defend her PhD in November, 2007.
As a follow-on from our work with NO2 oxidation of soot surfaces, a third project that was addressed in the final year of funding was an investigation of the degree to which n-hexane soot surfaces are oxidized by the gas-phase OH radical. (The project is being completed with funding from another source.) In the case of OH, we observed that small volatile organic compounds are formed from such heterogeneous chemistry, such as formic acid. We quantified their yield using chemical-ionization mass spectrometry. The yields, however, are quite small suggesting that little product volatilization occurs. Nevertheless, we do expect that the surfaces will be come significantly oxidized through such interactions.
