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.