IAGOS has been measuring Ozone (O₃) and Carbon Monoxide (CO) since 1994 and 2002 respectively. This dataset allows us to calculate O₃ and CO trends in the upper troposphere across nine regions of the Northern Hemisphere. Trends are computed over multiple time periods to highlight recent changes.</p> <p>O₃ growth in the upper troposphere (UT) of the Northern Hemisphere was more pronounced between 1994 and 2013 (e.g 0.34 &plusmn; 0.09 ppb/y in East US and 0.32 &plusmn; 0.07 ppb/y in Europe). These trends are significantly higher than those calculated from 1994 to 2023 (0.17 &plusmn; 0.07 ppb/y in East US and 0.16 &plusmn; 0.05 ppb/y in Europe). This difference in trend of O₃ appears to be associated with high positive anomalies of O₃ in the early 2010s, as well as significantly lower O₃ levels after 2020, at least partially linked to the COVID-19 crisis.</p> <p>For CO, we observe a gradual reduction in its rate of decline over the years, until a point where trends are no longer detected or are associated with low positive values since the 2010s at mid-latitudes in the Northern Hemisphere.</p> <p>Further studies will be necessary to constrain the causes of recent O₃ and CO variations. Continued IAGOS monitoring over the coming years will be essential, to further update this analysis and to determine whether the recent change in the observed trends are temporary or represent a sustained change in the upper tropospheric air composition.