Changes in ozone sensitivity to precursor emissions on diurnal, weekly, and decadal time scales

Despite efforts to improve air quality over the past several decades, ozone remains a stubborn threat to human health, agriculture, and forests. Emissions of the ozone precursors—volatile organic compounds (VOC) and nitrogen oxides (NO_x)—have been affected by regulation and growth in population, transportation, and industry.; Spatial and temporal variations in ambient pollutant concentrations contain information that has been overlooked by the regulatory-driven focus on one-hour maxima at individual monitoring sites. This work describes the history and spatial distribution of day-of-week differences in ozone and precursor concentrations through the analysis of two decades of observations from sites located throughout California.; Emission inventories, especially for motor vehicles, are a large source of uncertainty in current models of photochemical air pollution. Improved estimates of motor vehicle emissions in 1990 and 2000, containing weekend-specific descriptions of vehicle activity, have been developed as part of this research. Emissions of VOC and NO_x from motor vehicles are ∼5 and ∼25% lower on weekends compared to weekdays; the 70–80% drop in heavy-duty diesel truck traffic on weekends is the main source of weekday-weekend differences in NO_x emissions. Reductions in NO_x emissions from light-duty gasoline-powered vehicles have been offset by large increases in diesel truck traffic and a lack of improvement in the diesel truck emission factor for NO_x.; Photochemical modeling provides further insight into spatial and temporal variations in ozone sensitivity to precursor emissions. This photochemical modeling study is the first to use an emission inventory that describes the different temporal patterns of diesel trucks versus cars by hour and day of week. The reduced activity of heavy-duty diesel trucks, an important source of NO_x, on weekends is shown to be a primary cause of higher weekend ozone concentrations. Modeling results also show that long-term changes in emissions patterns have resulted in a shift towards increasing VOC-sensitivity. This shift, combined with the increased contribution of heavy-duty diesel trucks to the emission inventory and correspondingly larger weekday-weekend difference in NO_x emissions, explains why the weekend ozone effect has spread throughout California between 1980 and the present.; The major implications of this work are that a disproportionate share of exposure to ozone occurs on weekends and that weekday-weekend differences in ambient ozone concentrations can be used as a diagnostic tool to help determine whether ozone formation in an area is VOC- or NO_x-sensitive. Ozone formation cannot be considered in isolation of other air quality problems. (Abstract shortened by UMI.)...