Investigating the Processes Affecting Surface Air-Quality in California's Sierra Nevada Mountain Range

The climate, topography, and high population 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 long-term (1998 to 2018) surface observations for 13 sites within the Sierra Nevada Mountains and 22 sites in the SJV together with 7 years of airborne O₃ observations (2011 to 2018). Over this period, there is a compression of the surface O₃ range, with O₃ decreases on high O₃ days (95^th percentile) and increases on low O₃ days (5^th percentile). Analysis of diurnal data shows O₃ in the Sierra Nevada Mountains is driven mainly by transport of polluted airmasses from the SJV. Overall, we see a trend of decreasing magnitude in the amplitude of the O₃ diurnal cycle and a broadening of the O₃ peak with increasing elevation. The fact that O₃ mixing ratios typically remain higher overnight at Sierra Nevada Mountain sites, compared to the SJV, means the mountain sites can have higher cumulative exposures to O₃ and elevated W126 indices relative to the SJV. These areas of high ozone exposure also coincide with areas of high tree mortality in the Southern Sierra. Analyses of the timing of the O₃ exceedances, and correlation comparisons between different surface sites and with O₃ aloft provide further evidence that regional transport is the key driver of O₃ trends across the Sierra Nevada Mountains. These analyses show the strong regional influence of transported emissions from the SJV, which dominates O₃ in Sierra Nevada Mountains and aloft up to 3 km.