Surface ozone (O 3 ) air pollution in populated regions has been attributed to emissions of nitrogen oxides (NO + NO 2 = NO x ) and reactive volatile organic compounds (VOCs). These constituents react with hydrogen oxide radicals (OH + HO 2 = HO x ) in the presence of sunlight and heat to produce O 3 . The question of whether to reduce NO x emissions, VOC emissions, or both is complicated by spatially and temporally heterogeneous ozone-NO x -VOC sensitivity. This study characterizes spatial and temporal variations in O 3 sensitivity by analyzing the ratio of formaldehyde (HCHO, a marker of VOCs) to nitrogen dioxide (NO 2 ), a metric known as the formaldehyde nitrogen ratio (FNR). Level 3 gridded retrievals from the Ozone Monitoring Instrument (OMI) aboard the NASA Aura satellite were used to calculate FNR, with our analysis focusing on China. Based on previous studies, we take FNR < 1.0 as indicating VOC-limited regimes, FNR > 2.0 as indicating NO x -limited regime, and FNR between 1.0 and 2.0 as indicating transitional regime (where either NO x reductions or VOC reductions would be expected to reduce O 3 ). We find that the transitional regime is widespread over the North China Plain (NCP), the Yangtze River Delta, and the Pearl River Delta during the ozone season (defined as having near-surface air temperatures >20°C at the early afternoon OMI overpass time). Outside of these regions, the NO x -limited regime is dominant. Because HCHO and NO 2 have distinct seasonal patterns, FNR also has a pronounced seasonality, consistent with the seasonal cycle of surface O 3 . Examining trends from 2005 to 2013 indicates rapid growth in NO 2 , especially over less-developed areas where O 3 photochemistry is NO x limited. Over this time period, HCHO decreased in southern China, where VOC emissions are dominated by biogenic sources, but increased slightly over the NCP, where VOC emissions are dominated by anthropogenic sources. A linear regression approach suggests that most of China (70% of grid cells) will be characterized by a transitional regime during the O 3 season by 2030. However, in megacities such as Guangzhou, Shanghai, and Beijing, NO 2 has decreased such that the chemical regime has shifted from VOC limited in 2005 to transitional in 2013.

Determining effective strategies for mitigating surface ozone (O3) pollution requires knowledge of the relative ambient concentrations of its precursors, NOx, and VOCs. The space‐based tropospheric column ratio of formaldehyde to NO2 (FNR) has been used as an indicator to identify NOx‐limited versus NOx‐saturated O3 formation regimes. Quantitative use of this indicator ratio is subject to three major uncertainties: (1) the split between NOx‐limited and NOx‐saturated conditions may shift in space and time, (2) the ratio of the vertically integrated column may not represent the near‐surface environment, and (3) satellite products contain errors. We use the GEOS‐Chem global chemical transport model to evaluate the quantitative utility of FNR observed from the Ozone Monitoring Instrument over three northern midlatitude source regions. We find that FNR in the model surface layer is a robust predictor of the simulated near‐surface O3 production regime. Extending this surface‐based predictor to a column‐based FNR requires accounting for differences in the HCHO and NO2 vertical profiles. We compare four combinations of two OMI HCHO and NO2 retrievals with modeled FNR. The spatial and temporal correlations between the modeled and satellite‐derived FNR vary with the choice of NO2 product, while the mean offset depends on the choice of HCHO product. Space‐based FNR indicates that the spring transition to NOx‐limited regimes has shifted at least a month earlier over major cities (e.g., New York, London, and Seoul) between 2005 and 2015. This increase in NOx sensitivity implies that NOx emission controls will improve O3 air quality more now than it would have a decade ago.

Urban ozone (O3) formation can be limited by NO x , VOCs, or both, complicating the design of effective O3 abatement plans. A satellite-retrieved ratio of formaldehyde to NO2 (HCHO/NO2), developed from theory and modeling, has previously been used to indicate O3 formation chemistry. Here, we connect this space-based indicator to spatiotemporal variations in O3 recorded by on-the-ground monitors over major U.S. cities. High-O3 events vary nonlinearly with OMI HCHO and NO2, and the transition from VOC-limited to NO x -limited O3 formation regimes occurs at higher HCHO/NO2 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/NO2 captures the timing and location of the transition from VOC-limited to NO x -limited O3 production regimes in major U.S. cities, which aligns with the observed long-term changes in urban–rural gradient of O3 and the reversal of O3 weekend effect. Our findings suggest promise for applying space-based HCHO/NO2 to interpret local O3 chemistry, particularly with the new-generation satellite instruments that offer finer spatial and temporal resolution.