UC Berkeley Library

Air quality in the outflow from urban centers affects millions of people, as well as, natural and managed ecosystems downwind. In locations where there are large sources of biogenic VOCs downwind of urban centers, the outflow is characterized by a high VOC reactivity due to biogenic emissions and low NO_x. However most field and chamber studies have focused on limiting cases of high NO_x or of near zero NO_x. Recent measurements of a wide suite of VOCs, O₃ and meteorological parameters at several locations within the Sacramento urban plume have provided a detailed benchmark for testing our understanding of chemistry in a plume transitioning from high NO_x to low NO_x and high VOC reactivity. As an additional simplification, the strong mountain valley circulation in the region makes this urban plume a physical realization of a nearly idealized Lagrangian plume. Here, we describe a model of this plume. We use a Lagrangian model representing chemistry based on the Master Chemical Mechanism (MCM) v3.1 along with mixing and deposition. We discuss the effects of entrainment of background air, the branching ratio for the production of isoprene nitrates and the effects of soil NO_x emissions on the composition of the evolving plume. The model predicts that after 2–3 h of chemical processing only 45% of the peroxynitrates (ΣPNs) are PAN and that most (69%) RONO₂ are secondary alkyl nitrate products of the reaction of OH with RONO₂. We find the model is more consistent with the observations if: a) the yield of ΣPNs from large and multi-functional aldehydes is close to zero; and b) the reaction between OH and RONO₂ produces multifunctional nitrates as opposed to either HNO₃ or NO₂ as is typical in most currently adopted reaction mechanisms. Model results also show that adding NO_x emissions throughout the transect increases the available NO_x in the downwind regions, but modeled ozone concentrations were little affected by the increased NO_x.