The dependence on emissions of urban photochemistry is studied under meterological conditions favouring pollution episodes. We use a simple chemistry-transport model, yet realistic with respect to the main chemical and physical processes and considering the diurnal cycle of forcing parameters. For a range of NOx emissions, the diurnal equilibria of the system are searched for. It is shown that, if the photochemistry over a large city mostly exhibits a negative sensitivity to NOx emissions, there are meteorological situations under which it becomes (positively) NOx sensitive, the dispersion time scale being a key parameter. We show that, under specific conditions, the urban atmosphere might reach two very different diurnal equilibria. We demonstrate that the sensitivity to NOx emissions evolves with time: under specific conditions, the urban atmosphere becomes NOx sensitive within two days. ¿ 2000 American Geophysical Union