UV (390nm) light was used to modulate the rubidium partial pressure by inducing desorption of Rb atoms from the inner walls of the vacuum cell. A few mW/cm2 of UV light was enough to increase the rubidium partial pressure by a factor of 30 in about 1 s. After the source was turned off, the pressure quickly decayed back to its previous value with a time constant as short as 120 ms, allowing for long tapping lifetimes after the MOT holding phase. The number-lifetime product of the magnetic trap was shown as a function of the duration of the MOT holding phase, the maximum value was achieved after a MOT holding time of 1.25 s.