This study quantifies the impact of the fires in California in fall 2007 on regional air quality and especially on surface ozone by analyzing surface observations of ozone concentrations together with global chemistry transport model simulations. The latter include a synthetic tracer providing information about the amount of ozone produced from the fires. It is shown that the global model is well suited for simulating the overall fire impact and a valuable tool for extracting information about the fire influence from the observations. A clear increase in observed ozone is found when the model predicts a strong impact of pollution from the fires, where measured afternoon 8-hour concentrations increased, on average, by about 10 ppb. The findings demonstrate that intense wildfire periods can significantly increase the frequency of ozone concentrations exceeding current U.S. health standards, and might cause violations also during photochemically less active seasons. The study also demonstrates the far-reaching impact of ozone production from the fires.