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. The detection and attribution of high background ozone (O3) events inthe southwestern US is challenging but relevant to the effectiveimplementation of the lowered National Ambient Air Quality Standard (NAAQS;70 ppbv). Here we leverage intensive field measurements from the Fires,Asian, and Stratospheric Transport−Las Vegas Ozone Study (FAST-LVOS) inMay–June 2017, alongside high-resolution simulations with two globalmodels (GFDL-AM4 and GEOS-Chem), to study the sources of O3 duringhigh-O3 events. We show possible stratospheric influence on 4 out ofthe 10 events with daily maximum 8 h average (MDA8) surface O3above 65 ppbv in the greater Las Vegas region. While O3 produced fromregional anthropogenic emissions dominates pollution events in the Las VegasValley, stratospheric intrusions can mix with regional pollution to pushsurface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics ofdeep stratospheric intrusions consistent with ozonesondes, lidar profiles,and co-located measurements of O3, CO, and water vapor at Angel Peak,whereas GEOS-Chem has difficulty simulating the observed features andunderestimates observed O3 by ∼20 ppbv at the surface.On days when observed MDA8 O3 exceeds 65 ppbv and the AM4 stratosphericozone tracer shows 20–40 ppbv enhancements, GEOS-Chem simulates∼15 ppbv lower US background O3 than GFDL-AM4. The twomodels also differ substantially during a wildfire event, with GEOS-Chemestimating ∼15 ppbv greater O3, in better agreement withlidar observations. At the surface, the two models bracket the observed MDA8O3 values during the wildfire event. Both models capture thelarge-scale transport of Asian pollution, but neither resolves somefine-scale pollution plumes, as evidenced by aerosol backscatter, aircraft,and satellite measurements. US background O3 estimates from the twomodels differ by 5 ppbv on average (greater in GFDL-AM4) and up to 15 ppbvepisodically. Uncertainties remain in the quantitative attribution of eachevent. Nevertheless, our multi-model approach tied closely to observationalanalysis yields some process insights, suggesting that elevated backgroundO3 may pose challenges to achieving a potentially lower NAAQS level(e.g., 65 ppbv) in the southwestern US.

Characterizing sources of high surface ozone events in the southwestern US with intensive field measurements and two global models