Satellite NO₂ data are essential for monitoring air pollution, but they are affected by several sources of uncertainty. This study evaluates the major error contributors in TROPOMI (TROPOspheric Monitoring Instrument) NO₂ retrievals by analyzing each retrieval step and comparing with ground-based observations. Key findings include biases in the separation of stratospheric and tropospheric NO₂, representation and smoothing errors, and substantial variability among the ground-based instruments used for validation.