Weakening of the weekend ozone effect over California's South Coast Air Basin

We have observed lower nitrogen dioxide (NO2) and ozone (O-3) during a hot weekend (summer 2010) from aircraft over the entire South Coast Air Basin (SoCAB). Surface concentrations of NO2, O-3, and temperature from 1996 to 2014 corroborate that this lower O-3 on weekends is increasingly likely in recent years. While higher surface O-3 on the weekends (weekend ozone effect, WO3E) remains widespread, the spatial extent and the trend in the probability of WO3E occurrences (P-WO3E) have decreased significantly compared to a decade ago. This decrease is mostly the result of lower O-3 on hot weekends in recent years. P-WO3E is lowest in the eastern SoCAB. The major decrease happened during the 2008 economic recession, after which P-WO3E has stabilized at a 15-25% lower level throughout most of the basin. Future NOx reductions are likely to be increasingly effective at reducing O-3 pollution initially under hot conditions in the coming decade.