Spectral analysis of weekday–weekend differences in ambient ozone, nitrogen oxide, and non-methane hydrocarbon time series in California

We describe the history and spatial distribution of day-of-week differences in ambient ozone, NOx, and VOC concentrations through the analysis of two decades of measurements from sites located throughout California. Spectral analysis of the concentration time series shows that weekly patterns in ozone concentrations, typically with higher values of ozone on weekends, have become more widespread in California between 1980 and 1999. In contrast, a strong weekly pattern in NOx concentrations has been present throughout the entire period, and weekly patterns in VOC concentrations, though not as evident, have also been present during the entire 20-yr period. We examine 8-h average ozone concentrations, which appear to be a more sensitive measure of day-of-week differences in ozone than are 1-h averages. At sites with significant weekly cycles, fluctuations in pollutant concentrations that occur on a weekly time scale account for 6.6±3.5%, 3.0±1.7%, and 2.1±0.9% of the total variance in NOx, NMHC, and ozone concentrations, respectively. Concentrations of all three pollutants have been declining in most locations over the past 20 yr. Our results support the hypothesis that the weekend ozone effect is due to a combination of VOC-sensitivity and reduced NOx emissions on weekends. The spread of the weekend ozone effect may be due to a shift in ozone formation towards VOC-sensitivity, as control programs have reduced emissions of VOC more than NOx.