Geostationary Air Quality Monitoring Substantiates Column NO ₂ as a Better O ₃ Formation Regime Indicator in China

Abstract China has become a global hotspot of ozone (O 3 ) pollution, and understanding O 3 formation regime is crucial for air quality management. Satellite‐observed formaldehyde‐to‐nitrogen‐dioxide ratio (HCHO/NO 2 , FNR) has long been demonstrated as an efficient way to infer O 3 formation regime. However, reported FNR thresholds that divide O 3 formation regimes into VOC‐limited, transitional, and NO x ‐limited regimes uncover considerable differences. Using data from the world's first geostationary air quality monitoring instrument Geostationary Environment Monitoring Spectrometer, we report the spatiotemporal variations, particularly diurnal features, of O 3 ‐FNR relationships and O 3 formation regimes in China. We find that O 3 ‐FNR relationships fluctuate significantly over time and across locations, with coefficients of variation (CVs) of 0.22 and 0.27, respectively. We identified the amount of HCHO as a key factor. Given the strong correlation between NO 2 and LRO x /LNO x (chemical loss of HO 2 +RO 2 (LRO x ) to chemical loss of NO x (LNO x )), we demonstrate that O 3 formation regime thresholds alter minimally when column NO 2 is used as the indicator in China, with the CVs reduced to around 0.1. This finding highlights its potential to provide a more reliable diagnosis for large‐scale, space‐based O 3 formation sensitivity, offering promising insights for advancing O 3 pollution management in China.