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 (FNRsec) as measured by Pandora remote-sensing spectrometers located across Japan. Region-specific FNRsec 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 NOx-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 NOx-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 FNRsec retrieved from Pandora measurements.