I don’t think that the amount of Ozone refers to the overall air quality. Respectively for high value of ground ozone must be present hydrocarbons and/or nitrogen oxides (both present eq. in exhausts) and must be high solar radiation/activity. I think that Carbon Dioxide sensor will be closer to overall air quality.
Nevertheless, these sensors have low sensitivity - at least I think - It starts from 10ppm. See datasheet for MQ 131 or MQ 135 and others similar.
I found great gas sensor test at Explore the World With US -
BTW: MQ-135 is also on e-bay http://www.ebay.co.uk/itm/MQ135-sensor-Air-quality-sensor-Hazardous-gas-detection-/180713997925?pt=LH_DefaultDomain_0&hash=item2a1364c665