Inferring Changes in Summertime Surface Ozone–NOx–VOC Chemistry over U.S. Urban Areas from Two Decades of Satellite and Ground-Based Observations

Urban ozone (O 3 ) formation can be limited by NO x, VOCs, or both, complicating the design of effective O 3 abatement plans. A satellite-retrieved ratio of formaldehyde to NO 2 (HCHO/NO 2 ), developed from theory and modeling, has previously been used to indicate O 3 formation chemistry. Here, we connect this space-based indicator to spatiotemporal variations in O 3 recorded by on-the-ground monitors over major U.S. cities. High-O 3 events vary nonlinearly with OMI HCHO and NO 2, and the transition from VOC-limited to NO x -limited O 3 formation regimes occurs at higher HCHO/NO 2 value (3 to 4) than previously determined from models, with slight intercity variations. To extend satellite records back to 1996, we develop an approach to harmonize observations from GOME and SCIAMACHY that accounts for differences in spatial resolution and overpass time. Two-decade (1996–2016) multisatellite HCHO/NO 2 captures the timing and location of the transition from VOC-limited to NO x -limited O 3 production regimes in major U.S. cities, which aligns with the observed long-term changes in urban–rural gradient of O 3 and the reversal of O 3 weekend effect. Our findings suggest promise for applying space-based HCHO/NO 2 to interpret local O 3 chemistry, particularly with the new-generation satellite instruments that offer finer spatial and temporal resolution.