What is the role of NO in production of SO3 from SO2

In the manufacture of sulphur trioxide from sulphur dioxide, nitrogen monoxide can be used in a two-stage process to increase the rate of production.
NO (g) + 1/2 O2 (g) -> NO2 (g)
NO2 (g) + SO2 (g) -> NO (g) + SO3 (g)
State and explain fully the role of NO in this process.

Nitric oxide acts as a catalyst for the oxidation of SO2 to SO3 since its concentration is neither depleted nor consumed.