Weekday vs. Weekend Ambient Ozone Concentration Comparisons: Observations in Northern California and Implications for Ozone Control

Since the early 1970s researchers and data analysts have reported the difference 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 were written that focused on weekdaylweekend diierences in air quality in the South Coast @.os Angeles) Air Basin. Analysis of ozone concentrations measured at a number of locations within northern California reveal that weekend average ozone concentrations are frequently higher than on weekdavs. Violations of the California 0.09 oom 1-hour air aualitv standard for ozone also occur in disproportionately greater numbers on weekends. We hypothesize that this phenomenon is based largely on the difference between weekday and weekend emissions patterns and believe that;he observed differences may provide information regarding which pollutant reductions strategy, NO, or VOC control, may be more effective in reducing ambient ozone concentrations. For the northern California region, the presence of higher weekend ozone concentrations suggests the need for greater VOC control than NO, control. This phenomenon emphasizes the importance of using observation-based data to observe ambient ~ollution concentration relationshios. given the fact that current emission inventories are greatly underestimated. This natural experiment of varying emissions provides an interesting test case for sophisticated model performance and evaluation. Using Bay Area emissions and an estimate of their change from weekday to weekend combined with the classic EKMA diagram, we demonstrate the weekend effect