A new offline technique for retrieving tropospheric NO2 slant columns from space over urban regions is proposed in this work. An Earth radiance reference measurement from a remote unpolluted region is used instead of the standard solar irradiance measurement used by operational retrieval algorithms. Testing with spectra measured by the Ozone Monitoring Instrument (OMI) shows that the technique largely removes the presence of stratospheric NO2 and also greatly suppresses instrumental biases.