Urban centers in developing regions are increasingly important contributors to atmospheric greenhouse gas emissions and ozone-relevant air pollution, yet long-term observational analyses remain limited across sub-Saharan Africa. This study presents a comparative assessment of carbon dioxide (CO2), methane (CH4), and ozone (O3) over two major northern Nigerian cities Kano and Katsina using satellite and model-derived datasets from GEOS-Chem (CO2), the Atmospheric Infrared Sounder (AIRS; CH4), and MERRA-2 (O3) for the period 2015-2021. Monthly concentrations were evaluated using descriptive statistics, seasonal climatology, and the Modified Mann-Kendall (MMK) trend test with Sen’s slope estimator. CO2 exhibited statistically significant increasing trends in both Kano (Z = 8.421, p < 0.000001) and Katsina (Z = 8.376, p < 0.000001), with Sen’s slope magnitudes of 2.46 ppm yr⁻ ¹ and 2.45 ppm yr⁻ ¹, respectively. Mean concentrations increased from approximately 399 ppm in 2015 to about 417 ppm by 2021, consistent with reported global background growth rates. Methane also showed significant upward trends, with Kano (Z = 6.315, p < 0.000001; β = 0.0114 ppm yr⁻ ¹) and Katsina (Z = 7.022, p < 0.000001; β = 0.0127 ppm yr⁻ ¹) exhibiting sustained increases alongside stronger seasonal variability than CO2. In contrast, O3 displayed weak positive slopes that were not statistically significant (p > 0.05), although pronounced seasonal cycles were observed, with dry-season maxima and wet-season minima consistent with photochemical production under enhanced solar radiation and reduced wet scavenging. Kano systematically exhibited slightly higher CO2 and CH4 concentrations relative to Katsina, reflecting differences in urban scale, industrial activity, and traffic density. Inter-gas comparisons reveal concurrent long-term increases in CO2 and CH4, suggesting strengthening anthropogenic influence, while O3 variability appears primarily controlled by seasonal meteorological drivers. These findings contribute one of the few multi-year comparative urban atmospheric assessments in northern Nigeria and provide regionally relevant evidence to inform integrated greenhouse gas mitigation and ozone precursor management strategies across the Sahel.