Validation of TROPOMI and WRF-Chem NO₂ across seasons using SWING+ and surface observations over Bucharest

Nitrogen oxides (NOx) are key pollutants involved in ozone and particulate matter formation, with strong spatial variability near urban sources. Accurate monitoring of tropospheric nitrogen dioxide (NO2) is essential for air quality management and relies on validated chemistry transport models and multi-scale observations. This study evaluates the WRF-Chem model v4.5.1, run at 1 km resolution over Bucharest, Romania, using in situ meteorological data and surface chemical measurements, as well as airborne NO2 columns from 17 SWING+ flights conducted between 2021 and 2022. The model successfully captures key atmospheric processes and NO2 variability across all but one observation period. Our results indicate that anthropogenic NOx emissions from CAMS-REG v7.0 are underestimated, with satisfactory agreement with observations achieved when the emissions are scaled by a factor of 1.5. We also assess TROPOMI tropospheric NO2 columns v2.4.0 using SWING+ as reference, with WRF-Chem used as an intercomparison platform to account for differences in sampling and vertical sensitivity. TROPOMI biases range from +20 % at low concentrations (1015 molec. cm-2) to –13 % at higher levels (15 × 1015 molec. cm-2). Additionally, we provide seasonal diagnostics, a detailed treatment of uncertainty estimates, and contextualize our findings through a review of recent TROPOMI NO2 validation studies.