Evaluation of the MONARCH-simulated NO2 tropospheric columns against Sentinel-5P TROPOMI observations over the Iberian Peninsula

Despite a relatively simple chemistry, nitrogen dioxide (NO₂) remains challenging to reproduce accurately in current air quality models, notably due to persistent uncertainties affecting the representation of its anthropogenic and natural emission sources. Another reason for these uncertainties lies is the lack of surface in-situ observations in many rural areas where nitrogen oxides (NO_X) are yet playing a strong role in ozone formation.

In this study, we take benefit from the TROPOMI NO₂ tropospheric columns (TrC-NO₂) observations complemented by surface NO₂ observations to comprehensively evaluate the MONARCH chemical weather model. We focus our analysis over the Iberian Peninsula during the year 2019. MONARCH reproduces relatively well the spatial variability of TrC-NO₂ observed by TROPOMI, although with a negative bias. A lack of NOx emission in rural areas over the Iberian Peninsula is strongly suggested by the results. In order to investigate more deeply the corresponding underestimation of rural NOx, different sensitivity analyses are performed, notably on anthropogenic NO_X and natural soil nitric oxide (NO) emissions. The sensitivity analysis showed a spatially and seasonally dependent response of the emissions used as inputs in MONARCH on the modeled TrC-NO₂. The combined evaluation on surface and TrC NO₂ provide some interesting insights on the vertical distribution and its representation in MONARCH.

This study thus illustrates the strong interest of such satellite-based observations to complement surface monitoring stations, especially in areas such as croplands where almost no surface observations are available.