My computational studies investigate a novel reaction mechanism to produce HO2 radicals in the troposphere, from the relatively abundant tropospheric precursors, formaldehyde and acetaldehyde. Current atmospheric chemistry models are unable to explain HO2 radical concentrations in many parts of the world, suggesting there are formation mechanisms missing from the models. But what is the missing chemistry? When we look closely at atmospheric chemistry models, we see these models often neglect electronic ground state photochemistry, and assume excited state reactivity dominates. This is an incorrect assumption for formaldehyde and acetaldehyde. Following photoexcitation of formaldehyde or acetaldehyde by an actinic UV photon in the troposphere, efficient crossing back to the ground state produces a highly vibrationally excited (“hot”) molecule. There are several well-understood unimolecular dissociation processes available for these hot ground state aldehydes which are important in the atmosphere; many of which were incorrectly assigned as excited state processes until recently. For example, aldehyde dissociation to R + formyl radicals is an important source of HO2 radicals in the troposphere, and atmospheric chemistry models have assumed this dissociation is only an excited state process. Physical chemists have shown that the dissociation to R + formyl radicals can occur on both the ground state and the first excited triplet state at actinic UV energies, and that including the reactivity on the hot ground state affects the outcomes of atmospheric chemistry models. There are several other energetically accessible hot ground state reactions in these aldehydes, including, in acetaldehyde, phototautomerisation to vinyl alcohol, which reacts to form formic acid in the troposphere. But there are other hot ground state reactions, such dissociation via roaming (to produce a hot closed-shell hydrocarbon), whose atmospheric fates are unstudied. My investigations centre on reactions between these hot roaming species and O2 gas - a known atmospheric radical scavenger, and by far the most abundant reactive collision partner available in the troposphere. I will present theoretical evidence for the reaction of hot roaming products with O2 gas as a new mechanism for HO2 radical formation in the atmosphere. I will also present preliminary calculations showing the reaction between the hot ground state aldehyde itself with O2 is energetically accessible.

Hot ground state photochemistry of aldehydes in the atmosphere

2019 Fall National Meeting

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