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The Richard E. Honrath Memorial Lecture
Nitrogen oxides and new-particle formation
The interplay of nitrogen oxides and hydrocarbon chemistry in the remote atmosphere was central to Richard Honrath’s research; this is a story he would have loved. Nitrogen oxides play a critical role in new-particle formation. First, they govern branching in hydrocarbon oxidation mechanisms. Second, gas-phase nitric acid can drive rapid growth of newly formed particles as small as a few nanometers in diameter and even greatly enhance nucleation itself in the free troposphere. We have been able to isolate key processes governing new-particle formation and growth in the CLOUD collaboration at CERN, where we have explored both topics. Nucleation and growth by organics generally requires highly oxygenated organic molecules (HOMs) and even covalently bound dimers formed from peroxy-radical association reactions. HOM and dimer formation requires autoxidation involving organic peroxy radicals, and nitrogen oxides can short circuit this chemistry under high-NO conditions. However, when it is cold enough, nitric acid and ammonia can develop a high enough supersaturation to drive particle formation and growth via inorganic routes (often involving sulfuric acid as well). Thus, the overall role of nitrogen oxides in atmospheric new-particle formation is nuanced.
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