Quantification of the magnitude and long-term changes of ozone concentrations transported into the US is important for effective air quality policy development. We synthesize multiple published trend analyses of western US baseline ozone, and show that all results are consistent with an overall, non-linear change – rapid increase during the 1980s that slowed in the 1990s, maximized in the mid-2000s, and was followed by a slow decrease thereafter. This non-linear change accounts for ~2/3 of the variance in the published linear trend analyses; we attribute the other 1/3 to unquantified autocorrelation in the analyzed data sets. Recent systematic changes in baseline ozone at the US West Coast have been relatively small - the standard deviation of the 2-year means over the 1990-2017 period is 1.5 ppb. International efforts to reduce anthropogenic precursor emissions from all northern mid-latitude sources could possibly reduce baseline ozone concentrations, thereby improving US ozone air quality.

We investigate the role of wildfire smoke on ozone photochemical production (P(O3)) and atmospheric boundary layer (ABL) dynamics in California’s Central Valley during June-September, 2016-2020. Wildfire events are identified by the Hazard Mapping System (HMS) and Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT). Air quality and meteorological data are acquired from 10 monitoring sites operated by the California Air Resources Board (CARB) across the Central Valley. During wildfire influenced periods, maximum daily 8h averaged (MDA8) O3 was enhanced by about 5 ppb (~10%) across the entire valley after the temperature correction. The photochemical ozone production rate calculated from a modified Leighton relationship was also found to be higher by 35% on average compared to non-fire periods despite the average diminution of by ~7% due to the shading effect of the wildfire plumes. Furthermore, the in-situ ozone production rates are found to be elevated due to an increase of both peroxyl radicals (~24%) and NO (~11%). Surface heat flux measurements from two AmeriFlux sites in the Northern San Joaquin Valley show midday surface buoyancy fluxes decrease by 30% on average when influenced by wildfire smoke. Further, ABL height measured from a radio acoustic sounding system (RASS) located in Visalia in the Southern San Joaquin Valley were found to decrease 80 m and virtual potential temperatures in ABL are higher on average by ~1.5 K when wildfire smoke is present. The increased temperature is likely the result of shortwave-radiation absorption by the additional aerosols in the wildfire smoke.