Temporal evolution of the [O III] and Hβ intensity following a drop in the nuclear radiation field for a spherical NLR of radius 1 kpc, with an initial emissivity law dropping as r^-2, and a density n_e = 100      cm^-3. Left panel: emissivity at each radius after  ~1500 years. Right panel: temporal evolution of the total lines intensity and of their ratio. We also report the changes of ratio with time for different emissivity laws in the form of r^−2 + α, with α =  ± 1.