Meteorological anomalies lead to elevated O3 in the western U. S. in June 2015

In winter through early summer of 2014-2015, temperatures in the western U. S. were substantially enhanced due to a persistent high-pressure ridge and warm water in the northeastern Pacific Ocean. Concurrently, monthly averaged maximum daily 8 h average (MDA8) O-3 in June 2015 was enhanced by 3-13 ppb across a large portion of the western U. S. At the Mount Bachelor Observatory (2.8 km above sea level) in central Oregon, O-3 in June 2015 was enhanced by 11 ppbv compared to the long-term mean. Some urban areas had many days in June 2015 with MDA8 values above the current air quality threshold of 70 ppbv. We show that the high O-3 was associated with enhanced temperatures, reduced cloud fraction, increased stagnation, and increased biogenic emissions. The data in June 2015 show enhanced.O-3/Delta temperature slopes at several sites, compared to previous June data, due to these multiple factors.