Observations of the temperature dependent response of ozone to NO_x reductions in the Sacramento, CA urban plume

Abstract. Observations of NO_x in the Sacramento, CA region show that mixing ratios decreased by 30 % between 2001 and 2008. Here we use an observation-based method to quantify net ozone (O₃) production rates in the outflow from the Sacramento metropolitan region and examine the O₃ decrease resulting from reductions in NO_x emissions. This observational method does not rely on assumptions about detailed chemistry of ozone production, rather it is an independent means to verify and test these assumptions. We use an instantaneous steady-state model as well as a detailed 1-D plume model to aid in interpretation of the ozone production inferred from observations. In agreement with the models, the observations show that early in the plume, the NO_x dependence for O_x (O_x = O₃ + NO₂) production is strongly coupled with temperature, suggesting that temperature-dependent biogenic VOC emissions and other temperature-related effects can drive O_x production between NO_x-limited and NO_x-suppressed regimes. As a result, NO_x reductions were found to be most effective at higher temperatures over the 7 year period. We show that violations of the California 1-h O₃ standard (90 ppb) in the region have been decreasing linearly with decreases in NO_x (at a given temperature) and predict that reductions of NO_x concentrations (and presumably emissions) by an additional 30 % (relative to 2007 levels) will eliminate violations of the state 1 h standard in the region. If current trends continue, a 30 % decrease in NO_x is expected by 2012, and an end to violations of the 1 h standard in the Sacramento region appears to be imminent.