Evolution of NO_x in the Denver Urban Plume during the Front Range Air Pollution and Photochemistry Experiment

Abstract. As NO_x (NO_x ≡ NO + NO₂) is transported away from cities, it undergoes photochemical oxidation to peroxynitrates (RO₂NO₂, ΣPNs), alkyl nitrates (RONO₂, ΣANs), and nitric acid (HNO₃). These higher oxide species each have different lifetimes to permanent removal or conversion back to NO_x, resulting in nitrogen oxide chemistry that evolves as plumes are transported away from cities. Here, observations from the Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) are used to describe the evolution of NO_x and NO_y (NO_y ≡ NO_x + ΣPNs + ΣANs + HNO₃ + …) as the Denver urban plume flows outward from the city center. We evaluate the chemistry, dilution, and deposition rates in the plume to provide numerical constraints on the NO_x and NO_y,i lifetimes. We find that plume dilution with background air occurs with a lifetime of 3.5 hours. NO_x concentrations decrease more rapidly with a lifetime to chemical loss and dilution of 2 hours in the near field of the city center. NO_y has an effective lifetime of 3 hours and due to a combination of HNO₃ deposition and dilution. The results provide a useful test of conceptual and numerical models of chemistry during the evolution of urban plumes.