An optical apparatus includes an illumination system configured to form a pulsed radiation beam, an optical element with a surface on which the radiation beam is incident in operation, and a gas source arranged to supply a mixture of a first type of gas and a second type of gas to a space adjacent the surface. Particles of the first and second types of gas are capable of reacting with the surface, when activated by the radiation beam. The gas source is configured to generate a combination of surface occupation numbers of molecules of the first and second types of gas on the surface under operating conditions, at least prior to pulses of the radiation beam, the combination of surface occupation numbers lying in a range in which reactions of particles with the surface during pulses of the radiation beam are in majority reversed.