Ozone formation sensitivity based on the secondary formaldehyde-to-nitrogen_dioxide ratio (FNR_sec) derived from ground-based remote sensing measurements and a chemical transport model

Abstract. Sensitivity analysis is essential for developing effective ozone (O₃) mitigation strategies. This study aims to extensively investigate the diurnal, seasonal, and vertical chemical sensitivity of O₃ using a photochemical indicator, the secondary formaldehyde (HCHO)-to-nitrogen_dioxide (NO₂) ratio (FNR_sec) as measured by Pandora remote-sensing spectrometers located across Japan. Region-specific FNR_sec thresholds were determined using the GEOS-Chem chemical transport model. Surface concentrations and vertical column amounts of HCHO and NO₂ were obtained from in situ measurements and Pandora spectrometers. The concentrations of HCHO and NO₂ varied with time of day, season, and altitude. Moreover, external pollution transport affected the vertical profiles and likely contributed to elevated concentrations. Seasonally, the ozone sensitivity analysis showed that NO_x-limited conditions were dominant in summer, transitional regimes in spring and fall, and VOC-limited regimes in winter. Vertically, VOC-limited conditions typically formed near the surface layers, followed by transitional regimes in the mid-levels, and NO_x-limited regimes aloft. Therefore, O₃ mitigation strategies should target not only the surface level but also elevated altitudes. This study contributes to fostering a comprehensive understanding of O₃ sensitivity in the troposphere using FNR_sec retrieved from Pandora measurements.