Cross-evaluating WRF-Chem v4.1.2, TROPOMI, APEX, and in situ NO2 measurements over Antwerp, Belgium
Authors: Poraicu Catalina , Müller Jean-François , Stavrakou Trissevgeni , Fonteyn Dominique , Tack Frederik , Deutsch Felix , Laffineur Quentin , Van Malderen Roeland , Veldeman Nele 
Published: January 2023

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 d period in June 2019, indicating
a generally 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 of the PBL schemes reproduce the surface NO2 measurements at stations of the Belgian Interregional Environmental Agency fairly well, 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 modeled 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 d simulation with surface NO2, NO, CO, and O3 data in the Antwerp region. The modeled 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. The
model underestimated both APEX (by ca. −37 %) and TROPOMI columns (ca. −25 %) on 27 June, whereas no significant bias is found on 29 June. 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) × 1015 molec. cm−2. The obtained bias for TROPOMI v1.3.1 increases with the NO2 column, following CAPEX=1.217Cv1.3-0.783 × 1015 molec. cm−2, in line with previous validation campaigns. The bias is slightly lower for the reprocessed TROPOMI v2.3.1, with CAPEX=1.055CPAL-0.437 × 1015 molec. cm−2 (PAL). Finally, a mass balance approach was used to perform a crude inversion of
NOx emissions based on 15 d 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 that emissions increase over Brussels–Antwerp (+20 %), the Ruhr Valley (13 %), and especially Paris (+39 %), and emissions decrease above a cluster of power plants in western Germany.