NO₂ maps from airborne imaging remote sensing often appear much smoother than one would expect from high-resolution model simulations of NO₂ over cities, despite the small ground-pixel size of the sensors. Our case study over Zurich, using the newly implemented building module of the MYSTIC radiative transfer solver, shows that the 3D effect can explain part of the smearing and that building shadows cause a noticeable underestimation and noise in the measured NO₂ columns.