ABSTRACT
This study has produced an improved percentile and seasonal (median) trend estimate of free tropospheric ozone above western North America (WNA), through a data fusion of ozonesonde, lidar, commercial aircraft, and field campaign measurements. Our method combines heterogeneous data sets according to the consensus data characteristics and inherent uncertainty in order to produce our best fused product. In response to different data collection environments (in situ or ground‐based), we investigate the ozone variability based on a wide range of percentiles, which is preferable for trend detection due to tropospheric ozone's high degree of heteroscedasticity (i.e., inconsistent trends and variability between different ozone percentiles). We then compare the ozone trends and variability above the California sub‐domain to the full WNA region for better understanding of the correlations between different regional scales. In California, the 1995–2021 percentile (from the 5th to 95th) and seasonal trends are clearly positive in terms of high signal‐to‐noise ratios. The magnitude of the trends is generally weaker over WNA compared to California, but reliable positive trends can still be found between the 10th and 70th percentiles, as well as winter and summer, whereas autumn shows a negative trend over the same period. In addition, dozens of rural surface sites across the region are selected to represent the boundary layer variability. In contrast to increasing free tropospheric ozone, we find overall strong negative surface trends since 1995, with the greatest divergence found in summer. Throughout the analysis implications of the COVID‐19 economic downturn on ozone variability are discussed.Alternate :Plain Language SummaryFree tropospheric ozone above western North America has increased since the mid‐1990s. Despite an observed drop of ozone in 2020 due to the COVID‐19 economic downturn, this observation‐based study shows the overall free tropospheric ozone trends have not been offset and continued to increase over 1995–2021, mainly driven by strong positive trends in winter and summer. In combination with the strong negative trends observed at rural surface sites over the same period, this study adds to the growing body of evidence that surface trends are frequently disconnected from the general increases observed in the free troposphere.
ABSTRACT
We analyzed NO2 and O3 data from 32 U.S. non‐attainment areas (NAAs) for 1995–2020. Since 1995, all regions have shown steady reductions in NO2 and the weekend‐weekday pattern indicates that the O3 production regime in most NAAs has transitioned to a NOx‐limited regime, while a few NAAs remain NOx‐saturated. In the eastern U.S., all NAAs have made steady progress toward meeting the current (70 ppb) O3 standard, but this is less true in midwestern and western NAAs, with most showing little improvement in peak O3 concentrations since about 2010. Due to COVID‐19 restrictions, NO2 concentrations were substantially reduced in 2020. In the eastern NAAs, we see significant reductions in both NO2 and peak O3 concentrations. In the midwestern U.S., results were more variable, with both higher and lower O3 values in 2020. In the western U.S. (WUS), we see variable reductions in NO2 but substantial increases in O3 at most sites, due to the influence from huge wildland fires. The recent pattern over the past decade shows that the large amount of wildland fires has a strong influence on the policy‐relevant O3 metric in the WUS, and this is making it more difficult for these regions to meet the O3 standard. Key Points We found a strong relationship between long‐term concentrations of nitrogen dioxide (NO2) and peak O3 in all U.S. non‐attainment areas In 2020, O3 in the eastern U.S. declined due to nitrogen oxide reductions, but increased in the western U.S. due to large wildfires Since 2010, inter‐annual variations in wildland fires in the western U.S. are significantly correlated with policy relevant O3 concentrations