Potential of TROPOMI for understanding spatio-temporal variations in surface NO ₂ and their dependencies upon land use over the Iberian Peninsula

In orbit since late 2017, the Tropospheric Monitoring Instrument (TROPOMI) is offering new outstanding opportunities for better understanding the emission and fate of nitrogen dioxide (NOinline-formula₂) pollution in the troposphere. In this study, we provide a comprehensive analysis of the spatio-temporal variability of TROPOMI NOinline-formula₂ tropospheric columns (TrC-NOinline-formula₂) over the Iberian Peninsula during 2018–2021, considering the recently developed Product Algorithm Laboratory (PAL) product. We complement our analysis with estimates of NOinline-formula_x anthropogenic and natural soil emissions. Closely related to cloud cover, the data availability of TROPOMI observations ranges from 30 %–45 % during April and November to 70 %–80 % during summertime, with strong variations between northern and southern Spain. Strongest TrC-NOinline-formula₂ hotspots are located over Madrid and Barcelona, while TrC-NOinline-formula₂ enhancements are also observed along international maritime routes close the strait of Gibraltar, and to a lesser extent along specific major highways. TROPOMI TrC-NOinline-formula₂ appear reasonably well correlated with collocated surface NOinline-formula₂ mixing ratios, with correlations around 0.7–0.8 depending on the averaging time.

We investigate the changes of weekly and monthly variability of TROPOMI TrC-NOinline-formula₂ depending on the urban cover fraction. Weekly profiles show a reduction of TrC-NOinline-formula₂ during the weekend ranging from inline-formula−10 % to inline-formula−40 % from least to most urbanized areas, in reasonable agreement with surface NOinline-formula₂. In the largest agglomerations like Madrid or Barcelona, this weekend effect peaks not in the city center but in specific suburban areas/cities, suggesting a larger relative contribution of commuting to total NOinline-formula_x anthropogenic emissions. The TROPOMI TrC-NOinline-formula₂ monthly variability also strongly varies with the level of urbanization, with monthly differences relative to annual mean ranging from inline-formula−40 % in summer to inline-formula+60 % in winter in the most urbanized areas, and from inline-formula−10 % to inline-formula+20 % in the least urbanized areas. When focusing on agricultural areas, TROPOMI observations depict an enhancement in June–July that could come from natural soil NO emissions. Some specific analysis of surface NOinline-formula₂ observations in Madrid show that the relatively sharp NOinline-formula₂ minimum used to occur in August (drop of road transport during holidays) has now evolved into a much broader minimum partly de-coupled from the observed local road traffic counting; this change started in 2018, thus before the COVID-19 outbreak. Over 2019–2021, a reasonable consistency of the inter-annual variability of NOinline-formula₂ is also found between both datasets.

Our study illustrates the strong potential of TROPOMI TrC-NOinline-formula₂ observations for complementing the existing surface NOinline-formula₂ monitoring stations, especially in the poorly covered rural and maritime areas where NOinline-formula_x can play a key role, notably for the production of tropospheric Oinline-formula₃.