Commonly found in fertilizer, urea, (NH₂)₂C=O, is a well-known and well-studied pollutant in groundwater and soil, but its fate in the atmosphere remains virtually unknown. Recent measurements have provided some evidence of urea being present in the atmosphere in greater quantities than previously expected. So what happens when urea enters the atmosphere? And can the atmosphere serve as a sink of urea? Using computational chemistry, we studied hydrogen abstraction reactions from urea by common atmospheric reactants including OH, NO₃, Cl, O₃, and O. These all produce a resonance stabilized NH₂C(=O)NH radical, as shown in the scheme below. Next, this study has looked at possible reactions of this radical, including addition reactions with NO, NO₂, O₂, and C₂H₄, hydrogen abstraction reactions with CH₄, and an oxygen abstraction by CO. Pseudo-first order rate constants have been calculated at 298 K for each hydrogen abstraction reaction using typical oxidant concentrations. These indicate that hydrogen abstraction proceeds mostly via reaction with OH, with minor contributions from Cl and NO₃. The spin density of the urea radical product indicates that the resonance structure with the unpaired electron on the nitrogen atom dominates over the structure with the radical center on the oxygen atom. This nitrogen centered radical can then undergo barrierless addition reactions with NO and NO₂, as well as addition of O₂ over a modest barrier. We plan to compute temperature dependencies of rate constants for each reaction, as well as rate constants for the barrierless NO_x reactions to better determine which reaction pathways control the fate of urea in the atmosphere.

Lyra Sauer

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