The city is wide spread but the sensors are located not throughout the city but only in places known to create a high ozone count, and yes I mean O3. This issue is closely related to AGW.

Quote Originally Posted by Thorne View Post
I think you're confusing normal atmospheric ozone, such as the ozone layer, with man made ozone, that you get from the burning of fossil fuels. "Ozone days" are, I presume, a measure of air pollution, generally caused by weather conditions holding such pollution close to the ground. You place your sensors where the pollution tends to accumulate, not out in the countryside where it will generally be always low. These measurements are for local consideration only, and are not of global interest.