Interannual variability of baseline O3 at Mt. Bachelor Observatory and Trinidad Head. Nighttime observations of baseline O3 at Mt. Bachelor Observatory (blue) and baseline O3 with daytime onshore wind conditions at Trinidad Head (orange) for the 2nd (triangles), 50th (squares), and 98th percentiles (circles). The range of values for the 98th percentile at Mt. Bachelor over the period 2004–2016 is 64–86 ppb during spring and 61–84 ppb during summer. The range of values for the 98th percentile at Trinidad Head over the period 2005–2016 is 41–58 ppb during spring and 22–41 ppb during summer. DOI: https://doi.org/10.1525/elementa.309.f4