Weekday/weekend ozone differences: What can we learn from them?

A national analysis of weekday/weekend ozone (O-3) differences demonstrates significant variation across the country. Weekend 1-hr or 8-hr maximum O-3 varies from 15% lower than weekday levels to 30% higher. The weekend O-3 increases are primarily found in and around large coastal cities in California and large cities in the Midwest and Northeast Corridor. Both the average and the 95th percentile of the daily 1-hr and 8-hr maxima exhibit the same general pattern. Many sites that have elevated O-3 also have higher O-3 on weekends even though traffic and O-3 precursor levels are substantially reduced on weekends. Detailed studies of this phenomenon indicate that the primary cause of the higher O-3 on weekends is the reduction in oxides of nitrogen (NOx) emissions on weekends in a volatile organic compound (VOC)-limited chemical regime. In contrast, the lower O-3 on weekends in other locations is probably a result of NOx reductions in a NOx-limited regime. The NOx reduction explanation is supported by a wide range of ambient analyses and several photochemical modeling studies. Changes in the timing and location of emissions and meteorological factors play smaller roles in weekend O-3 behavior. Weekday/weekend temperature differences do not explain the weekend effect but may modify it.