Wintertime Overnight NOx Removal in a Southeastern United States Coal‐fired Power Plant Plume: A Model for Understanding Winter NOx Processing and its Implications

Nitric oxide (NO) is emitted in large quantities from coal‐burning power plants. During the day, the plumes from these sources are efficiently mixed into the boundary layer, while at night, they may remain concentrated due to limited vertical mixing during which they undergo horizontal fanning. At night, the degree to which NO is converted to HNO₃ and therefore unable to participate in next‐day ozone (O₃) formation depends on the mixing rate of the plume, the composition of power plant emissions, and the composition of the background atmosphere. In this study, we use observed plume intercepts from the Wintertime INvestigation of Transport, Emissions and Reactivity campaign to test sensitivity of overnight NOx removal to the N₂O₅ loss rate constant, plume mixing rate, background O₃, and background levels of volatile organic compounds using a 2‐D box model of power plant plume transport and chemistry. The factor that exerted the greatest control over NOx removal was the loss rate constant of N₂O₅. At the lowest observed N₂O₅ loss rate constant, no other combination of conditions converts more than 10% of the initial NOx to HNO₃. The other factors did not influence NOx removal to the same degree.f