Oxidation of the PAH Coronene by Ozone and the Hydroxyl Radical

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic pollutants, consisting of two or more fused benzene rings, emitted directly into the atmosphere primarily through incomplete combustion processes and known to have allergenic, mutagenic, and carcinogenic effects. In the atmosphere, smaller PAHs are found primarily in gas-phase, whereas three or four- member ring compounds are partitioned between gas and particulate matter, and compounds with greater than five-member rings mostly reside in the particle phase. The atmospheric fate of these heavier PAHs is governed by heterogeneous reactions between the surface bound PAHs and gas-phase atmospheric oxidants such as ozone, the hydroxyl radical, and nitrates, however, these heterogeneous chemical reactions are relatively poorly understood and studied. In the current study, reactivity of the seven-member ring PAH coronene to oxidation sources ozone and hydroxyl radical is examined. To probe the extent of chemical reaction, product formation, and change in surface morphology as a function of reaction, we examine coronene adsorbed onto various substrates, from both a surface and bulk perspective, with ambient pressure photoemission spectroscopy (APPES) and aerosol mass spectrometry (AMS), respectively. In bulk on-line analysis, a 20nm thick layer of coronene adsorbed onto NaCl seed particles and reacted with either oxidant in a flow tube showed very little reactant conversion to product in the AMS. However, surface analysis by the APPES of the same reaction where coronene was adsorbed onto model substrates showed up to 90% conversion of the carbon species to volatilized or oxidized carbon. Data obtained with these two complimentary bulk and surface techniques provide evidence for a surface selective reaction. Using APPES, we are able observe the two oxidation reactions transforming on different timescales and through differing pathways, resulting in dissimilar final states.