WEEKDAY VS WEEKEND AMBIENT OZONE CONCENTRATIONS - DISCUSSION AND HYPOTHESES WITH FOCUS ON NORTHERN CALIFORNIA

Since the early 1970s, researchers and data analysts have reported dif ferences between weekday and weekend ozone concentrations, with higher ozone concentrations occurring on Sundays in some locations. At that time, the phenomenon was referred to as the ''Sunday effect.'' In the late 1980s, additional papers focused on weekday/weekend differences i n air quality in the South Coast (Los Angeles) Air Basin. Analyses of ozone concentrations measured at a number of locations in northern Cal ifornia reveal that average ozone concentrations are frequently higher on weekends than on weekdays. Violations of the California 0.09 ppm 1 -hour air quality standard for ozone also occur in disproportionately greater frequency on weekends. We hypothesize that this phenomenon is based largely on the differences between weekday and weekend emission patterns. We believe that the observed differences may provide informa tion regarding which pollutant reduction strategy, NOx or ROG control, may be more effective in reducing ambient ozone concentrations. For t he northern California region, the pres ence of higher weekend ozone c oncentrations suggests the need for ROG control is greater than for NO x control. If both NOx and ROG are to be controlled, it is important t o understand the interdependence of the two pollutants in forming ozon e. With the current uncertainty and debate regarding official vehicula r emission inventories, this phenomenon emphasizes the importance of u sing observation-based data to examine ambient pollution and emission relationships. This natural experiment of varying emissions provides a n interesting test case for sophisticated air pollution model performa nce and evaluation. Using a Bay Area emission inventory and an estimat e of its change from weekday to weekend, combined with a generic Empir ical Kinetic Modeling Approach (EKMA) diagram, we demonstrate the week end effect. In addition, changes in the Bay Area emission inventory fr om 1980 to 1990, when combined with the EKMA diagram, also show why th e weekend effect is more evident in the 1990s. It is our hypothesis th at the presence of the weekend effect, positive or negative, combined with changes in emission changes, provides a simple clue to whether an area is NOx or ROG limited with respect to ozone formation.