Surface ozone (O₃) air pollution in populated regions has been attributed to emissions of nitrogen oxides (NO + NO₂ = NO_x) and reactive volatile organic compounds (VOCs). These constituents react with hydrogen oxide radicals (OH + HO₂ = HO_x) in the presence of sunlight and heat to produce O₃. 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 O₃ sensitivity by analyzing the ratio of formaldehyde (HCHO, a marker of VOCs) to nitrogen dioxide (NO₂), 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 O₃). 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 NO₂ have distinct seasonal patterns, FNR also has a pronounced seasonality, consistent with the seasonal cycle of surface O₃. Examining trends from 2005 to 2013 indicates rapid growth in NO₂, especially over less‐developed areas where O₃ 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 O₃ season by 2030. However, in megacities such as Guangzhou, Shanghai, and Beijing, NO₂ 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, NO_x, and VOCs. The space‐based tropospheric column ratio of formaldehyde to NO₂ (FNR) has been used as an indicator to identify NO_x‐limited versus NO_x‐saturated O₃ formation regimes. Quantitative use of this indicator ratio is subject to three major uncertainties: (1) the split between NO_x‐limited and NO_x‐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 NO_x‐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 NO_x sensitivity implies that NO_x emission controls will improve O₃ air quality more now than it would have a decade ago.

Biomass burning emits an estimated 25 % of global annual nitrogen oxides (NO_x), an important constituent that participates in the oxidative chemistry of the atmosphere. Estimates of NO_x emission factors, representing the amount of NO_x per mass burned, are primarily based on field or laboratory case studies, but the sporadic and transient nature of wildfires makes it challenging to verify whether these case studies represent the behavior of the global fires that occur on earth. Satellite remote sensing provides a unique view of the earth, allowing for the study of emissions and downwind evolution of NO_x from a large number of fires. We describe direct estimates of NO_x emissions and lifetimes for fires using an exponentially modified Gaussian analysis of daily TROPOspheric Monitoring Instrument (TROPOMI) retrievals of NO₂ tropospheric columns. We update the a priori profile of NO₂ with a fine-resolution (0.25^∘) global model simulation from NASA's GEOS Composition Forecasting System (GEOS-CF), which largely enhances NO₂ columns over fire plumes. We derive representative NO_x emission factors for six fuel types globally by linking TROPOMI-derived NO_x emissions with observations of fire radiative power from Moderate Resolution Imaging Spectroradiometer (MODIS). Satellite-derived NO_x emission factors are largely consistent with those derived from in situ measurements. We observe decreasing NO_x lifetime with fire emissions, which we infer is due to the increase in both NO_x abundance and hydroxyl radical production. Our findings suggest promise for applying space-based observations to track the emissions and chemical evolution of reactive nitrogen from wildfires.