Observations of the temperature dependent response of ozone to NOx reductions in the Sacramento, CA urban plume
1Department of Chemistry, University of California, Berkeley, CA, USA
2Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA
3Department of Civil and Environmental Engineering, University of California, Berkeley, USA
4Department of Earth and Planetary Science, University of California, Berkeley, USA
*now at: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA, USA
Abstract. Observations of NOx 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 production rates in the outflow from the Sacramento metropolitan region and examine the O3 decrease resulting from reductions in NOx 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 NOx dependence for Ox (Ox = O3 + NO2) production is strongly coupled with temperature, suggesting that temperature-dependent biogenic VOC emissions can drive Ox production between NOx-limited and NOx-suppressed regimes. As a result, NOx reductions were found to be most effective at higher temperatures over the 7 year period. We show that violations of the California 1-hour O3 standard (90 ppb) in the region have been decreasing linearly with decreases in NOx (at a given temperature) and predict that reductions of NOx 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 NOx is expected by 2012, and an end to violations of the 1 h standard in the Sacramento region appears to be imminent.