Figure 3. The top panel shows the observed monthly average NO₂ column (grey circles) averaged over the eastern United Kingdom, Belgium, Luxembourg, and the Netherlands (area indicated by the white box in Figure 2(top left)) and their 12-month running mean (black line).

The red line indicates the corresponding average low-pass filtered signal. The bottom panel shows again the Benelux-UK average low-pass filtered NO₂ column (red line) with the fit of the regression model (Eqn. 1) to the data (grey). The uncertainty in the average observed NO₂ column (black error bar) represents the precision of the spatial average over the Benelux-UK area. The measurement precision in each grid cell that is propagated when taking the average is derived by assuming σ = 5% as discussed in the Supplementary Information – “Regression Modeling”. The standard error in the average regression model fit is commensurate with the thickness of the line.