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

Abstract. Nitrogen oxides (NO_x) 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 NO_x 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.