Diurnal variations of the relative bias between IAGOS and urban background stations. Results are shown for CO (top panel) and O₃ (bottom panel) at Frankfurt. Hours are in LT. The filled circles indicate a statistically significant difference (at a 95% confidence level) while the empty circles indicate that the difference is not significant (too low number of points and/or too small difference). Both the correlation (r, at the top of each panel) and the number of points included in the calculation (at the bottom of the panel, in brackets) are reported on the Figure. For both CO and O₃, the bias between IAGOS and urban background stations is found to be higher during the morning and early evening, and lower during the afternoon. DOI: https://doi.org/10.1525/elementa.280.f4