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

We have observed lower nitrogen dioxide (NO 2 ) and ozone (O 3 ) during a hot weekend (summer 2010) from aircraft over the entire South Coast Air Basin (SoCAB). Surface concentrations of NO 2 , 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 NO x reductions are likely to be increasingly effective at reducing O 3 pollution initially under hot conditions in the coming decade.