Validating tropospheric NO2 column density from GEMS version 3.0 and Tropospheric Chemistry Reanalysis version 2 from background to urban conditions over Japan: with a focus on diurnal variations

Abstract This study validates tropospheric NO2 vertical column density (TropNO2 VCD) data from the Geostationary Environment Monitoring Spectrometer (GEMS) version 3.0 and the Tropospheric Chemistry Reanalysis version 2 (TCR-2) against Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) and Tropospheric Monitoring Instrument (TROPOMI) data at three Japanese sites: Yokosuka (urban), Fukue (rural-remote), and Cape Hedo (remote). The GEMS v3.0 dataset showed markedly improved agreement with MAX-DOAS compared to v2.0, especially at Fukue and Cape Hedo remote islands, with significantly reduced normalized mean bias from 142–319% to 8–18%. GEMS v3.0 showed agreement with MAX-DOAS observations at Yokosuka (R = 0.75) and Fukue (R = 0.50). It performed particularly well in autumn and winter. There were positive biases in autumn at Yokosuka (NMB = 3.5%), but negative biases in the other seasons (NMB = −0.4% to −51.2%). The reanalysis TCR-2 and aggregated TROPOMI TropNO2 VCD data (both at 1 × 1° resolution) showed moderate to good correlation at all three sites (R = 0.72 at Yokosuka; R = 0.44 at Fukue; R = 0.57 at Cape Hedo). While the spatial resolution of TCR-2 (1.1 × 1.1°) precluded meaningful comparisons with MAX-DOAS at urban sites such as Yokosuka, TCR-2 exhibited lower biases than TROPOMI at Cape Hedo, which represents a background environment (NMB = 38% vs. 87%). GEMS v3.0 data effectively captured midday NO2 reductions at Cape Hedo, consistent with TCR-2 and MAX-DOAS, marking this as the first study to explore daytime NO2 decreases in remote regions using GEMS. NO2 loss pathways at Cape Hedo were examined using TCR-2 data and box-model simulations. NO2 levels from GEMS exhibited a decline between 10 AM and 3 PM in autumn, aligning with patterns from MAX-DOAS and TCR-2, with NO2 decay rates of 16 ± 10h from GEMS, 21 ± 5h from MAX-DOAS, and 22 ± 20h from TCR-2. The box-model NOx budget analysis revealed that the NO2 + OH reaction was the dominant pathway for NOx loss. The markedly improved agreement of GEMS v3.0 data with MAX-DOAS and TCR-2 data will allow potential exploration of the oxidizing capacity of the atmosphere over the clean marine regions, after further characterization of NOx chemistry.