The effect of an upwind non-attainment area on ozone in California's Sierra Nevada Mountains

The climate, topography, high population and associated emissions of California pose unique challenges in attaining the ozone (O3) National Ambient Air Quality Standard. California's Sierra Nevada Mountains are situated east, and downwind of, the San Joaquin Valley (SJV), which has some of the worst air quality in the US. This work reports surface and vertical O3 observations over the Sierra Nevada Mountains and the SJV from 1998 to 2018. Over this period, the O3 concentrations observed on highly polluted days (95th percentile) decreased, primarily because of summertime emission reductions. Analysis of surface site diurnal data highlights the impacts that the SJV has on the Sierra Nevada Mountains, particularly at lower elevations. O3 typically remains higher overnight at mountain sites, compared to SJV sites, and as a result mountain sites can have high cumulative exposure to O3. Evidence of local and transport effects on O3 trends across the Sierra Nevada Mountains is presented through examining of the timing of O3 exceedances and correlations with different surface sites and with O3 aloft. The data indicate a strong regional influence of emissions from the SJV over the Sierra Nevada Mountains and aloft up to 3 km. •Long-term compression of the O3 range in Sierra Nevada Mountains, with decreases in the 95th percentile and increases in the 5th percentile.•The O3 diurnal cycles are driven by transport of pollution, O3 remains elevated over night at elevated sites increasing cumulative exposure.•Sierra Nevada Mountain O3 show increased correlations with O3 aloft during spring, highlighting the increased influence the free-troposphere.