Cross-evaluating WRF-Chem v4.1.2, TROPOMI, APEX and in situ NO₂ measurements over Antwerp, Belgium

Abstract. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is employed as an intercomparison tool for validating Tropospheric Monitoring Instrument (TROPOMI) satellite NO2 retrievals against high-resolution Airborne Prism EXperiment (APEX) remote sensing observations performed in June 2019 in the region of Antwerp, a major hotspot of NO2 pollution in Europe. The model is first evaluated using meteorological and chemical observations in this area. Sensitivity simulations varying the model planetary layer boundary (PBL) parameterization were conducted for a 3-day period in June 2019, indicating a general good performance of most parameterizations against meteorological data (namely ceilometer, surface meteorology and balloon measurements), except for a moderate overestimation (~1 m s-1) of near-surface wind speed. On average, all but one PBL schemes reproduce fairly well the surface NO2 measurements at stations of the Belgian Interregional Environmental Agency, although surface NO2 is generally underestimated during the day (between -4.3 and -25.1 % on average) and overestimated at night (8.2–77.3 %). This discrepancy in the diurnal evolution arises despite (1) implementing a detailed representation of the diurnal cycle of emissions (Crippa et al., 2020), and (2) correcting the modelled concentrations to account for measurement interferences due to NOy reservoir species, which increases NO2 concentrations by about 20 % during the day. The model is further evaluated by comparing a 15-day simulation with surface NO2, NO, CO and O3 data in the Antwerp region. The modelled daytime NO2 concentrations are more negatively biased during weekdays than during weekends, indicating a misrepresentation of the weekly temporal profile applied to the emissions, obtained from Crippa et al. (2020). Using a mass-balance approach, we determined a new weekly profile of NOx emissions, leading to a homogenization of the relative bias among the different weekdays. The ratio of weekend to weekday emissions is significantly lower in this updated profile (0.6) than in the profile based on Crippa et al. (2020) (0.84). Comparisons with remote sensing observations generally show a good reproduction of the spatial patterns of NO2 columns by the model. Both APEX and TROPOMI columns are underestimated on the 27/6, whereas no significant bias is found on the 29/6. The two datasets are intercompared by using the model as an intermediate platform to account for differences in vertical sensitivity through the application of averaging kernels. The derived bias of TROPOMI v1.3.1 NO2 with respect to APEX is about -10 % for columns between (6–12)x1015 molec. cm-2. The obtained bias for TROPOMI v1.3.1 increases with the NO2 column, following CAPEX = 1.217 Cv1.3 - 0.783x1015 molec. Cm-2, in line with previous validation campaigns. The bias is slightly lower for the reprocessed TROPOMI v2.3.1, with CAPEX = 1.055 CPAL - 0.437x1015 molec. cm-2 (PAL). Finally, a mass balance approach was used to perform a crude inversion of NOx emissions, based on 15-day averaged TROPOMI columns. The emission correction is conducted only in regions with high columns and high sensitivity to emission changes, in order to minimize the errors due to wind transport. The results suggest emissions increases over Brussels-Antwerp (+20 %), Ruhr Valley (13 %), and especially Paris (+39 %), and emission decreases above a cluster of power plants in West Germany.