Influence of daily variations in baseline ozone on urban air quality in the United States Pacific Northwest

We examine the influence of daily variations in baseline ozone (O 3 ) on urban air quality in the U.S. Pacific Northwest (PNW) during 2004 to 2010 through two analyses: (1) transport of free tropospheric (FT) O 3 from Mount Bachelor Observatory (MBO) to Boise, Idaho; and (2) transport of marine boundary layer (MBL) O 3 from Cheeka Peak (CP) to Enumclaw, Washington. Both Boise and Enumclaw experience days with maximum daily 8 hour averages of O 3 (MDA8) exceeding U.S. standards. Backward trajectory cluster analyses identify days when FT and MBL O 3 strongly influence MDA8 in Boise and Enumclaw. On these days, MBO and CP O 3 observations explain 40% and 69% of the variations in Boise and Enumclaw MDA8, respectively. Bivariate regressions for Boise/MBO and Enumclaw/CP have slopes of 0.52 ± 0.16 and 1.04 ± 0.08, respectively, representing the differing interplay of O 3 dilution, production, and loss during FT to boundary layer transport (Boise/MBO) and fast boundary layer transport (Enumclaw/CP). AIRPACT‐3/CMAQ (Air Indicator Report for Public Access and Community Tracking version 3/Community Multi‐scale Air Quality model) high‐resolution air‐quality simulation results demonstrate how transport of O 3 from the FT above MBO contributes to elevated O 3 at Boise. Average MDA8 O 3 in Boise is higher than in Enumclaw due to site elevation and greater entrainment of FT air masses, a finding likely applicable to other PNW sites. Days with high baseline influence at Boise and Enumclaw have lower average MDA8 O 3 than other days; however, some of these days would still exceed the U.S. standard if it is substantially tightened in 2013, highlighting the increasing importance of FT O 3 influence on urban MDA8.