We have constructed a photochemical equilibrium model of minor constituent chemistry in the troposhere to investigate the photochemistry of troposheric ozone. We find that photochemical production must be supplemented by additional sources to account for observed ozone levels in clean air. While photochemical loss is more important than surface destruction as an ozone loss mechanism. Photochemically produced ozone is sensitive to the NOx background in the model, exhibiting a maximum at about 0.5 ppb of NOx.