Two Decades of Ground-Level Ozone–NOx–VOC Chemistry over U.S. Urban Areas Inferred from Satellite and Ground-Based Observations

Ozone (O₃) is produced from photochemical reactions involving its precursors: nitrogen oxides (NO_x= NO+NO₂) and volatile organic compounds (VOCs). While current satellites cannot retrieve the abundance of ground-level ozone, they have provided continuous global observations of O₃ precursors, namely tropospheric columns of NO₂ and formaldehyde (HCHO, a proxy for VOCs), for over two decades. We analyze long-term changes in non-linear surface O₃-NO_x-VOC chemistry using ground-based O₃ observations from U.S. urban areas and satellite observations of tropospheric columns of NO₂ and formaldehyde (HCHO, a proxy for VOCs) retrieved consistently from multiple satellites, including GOME, GOME-2, SCIAMACHY and OMI. These multi-instrument products can be used to document the long-term effectiveness of precursor emission controls for mitigating ground-level O₃ pollution, especially over megacities. As a result of continued NO_x emission reductions, both satellite and ground-based observations show large declines in NO_x over U.S. urban areas during the past two decades. As a result, the O₃ formation over U.S. urban areas has become less NO_x-saturated and more sensitive to NO_x emissions. We analyze the trends in reactivity-weighted VOCs from both satellite observations (for which HCHO trends are a proxy) and ground-based measurements of VOCs. Trends in reactivity-weighted VOCs are more spatially heterogeneous and less significant than the trends of NO_x. The areal extent of NO_x-saturated chemistry has declined, leading to a reversal of O₃ weekend effect (i.e. O₃ is higher on weekends than weekdays due to NO_x titration effects) over major U.S. cities (e.g. New York, Chicago, Los Angeles, Houston) in summer. The timing of this reversal, however, varies by city. A corollary finding is that the location of peak O₃ production has moved towards the city center over time. Our study aims to demonstrate how space-based HCHO/NO₂ can complement in situ O₃ networks and model simulations by providing information on the spatial heterogeneity and temporal evolution of O₃chemical regimes.