Ozone Weekend Effect Research Articles

Modeling the effect of weekday-weekend differences in motor vehicle emissions on photochemical air pollution in central California.

Ambient ozone concentrations vary by day of week in some locations, often with higher concentrations observed on weekends in urban and downwind areas. Emissions of ozone precursors appear to be lower on weekends, so the behavior of ozone concentrations on weekends may indicate the outcome of particular ozone control strategies. To examine the influence of day-of-week differences in motor vehicle emissions on ambient ozone concentrations, we combine a fuel-based motor vehicle emission inventory containing weekend-specific activity with an Eulerian photochemical airshed model applied to central California. Emissions of NOx on weekends are approximately 30% lower than on weekdays due to a large drop in heavy-duty diesel truck activity, and emissions of VOC are only slightly lower on weekends. In rural areas, passenger car traffic and the associated emissions are highest on Fridays and Sundays. The combination of VOC sensitivity and reduced emissions of NOx on weekends results in higher ozone concentrations on weekends. Changes in the timing of emissions also contribute to the weekend ozone effect, but sensitivity tests show that changes in emissions timing have a minor effect compared to changes in total mass of emissions on weekends. Even in situations where reductions in NOx emissions lead to higher ozone concentrations, NOx reductions may still be necessary for control of other air pollutants such as nitrogen dioxide, nitric acid, and aerosol nitrate.

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, NO x , 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 NO x 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 NO x , 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 NO x 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 NO x .