Abstract. We examine spatial variations in the weekday–weekend pattern of NO₂ over the Los Angeles metropolitan area using the Ozone Monitoring Instrument (OMI) and then compare the observations to calculations using the WRF-Chem model. We find that the spatial pattern of the weekday–weekend variations of the NO₂ column in the model is significantly different than observed. A sensitivity study shows that the contrasting spatial pattern of NO₂ on weekdays and weekends is a useful diagnostic of emissions and chemistry. These improvements suggest that constraints from space-based observations of the processes affecting urban photochemistry (e.g., spatial patterns of emissions, ratios of VOC to NO_x emissions, rate constants) are possible at a level of detail not previously described.