Fig. 3. Time series components by GAMs of the tropospheric NO₂ column over Beijing.

a NO₂ daily series of both OMI-measured and GAM-fitted data, as indicated by black dots and a green line, respectively. b The bar plot of the daily series of accumulated meteorological smooth terms, i.e., S(meteos), where positive and negative S(meteos) are indicated with red and blue, respectively, while the black solid line denotes the smoothed series using a moving average with a window of 15 days. c same as b but for the non-meteorological terms, i.e., S(non-meteos). d Interannual series of S(meteos), S(non-meteos), and relative NO_x variation compared to the overall mean, as shown with red, green, and blue dotted lines, respectively, while the triangular dots denote the MEIC NO_x emissions over Beijing, corresponding to the right y-axis