Historically, Maryland ozone episodes have been regional and consistent. Essentially, yesterday's ozone would persist into the next day, resulting in another exceedance day. However, as background ozone concentrations (the ozone found in the free atmosphere) have faded, so too has the persistence of ozone across the region, and with it, multi-day ozone episodes are fewer, more sporadic and briefer. 

The Hart-Miller Island (HMI) monitoring project has continued to be a success story. 2017 was the first full ozone season measuring ozone and providing "ground truth" for air quality in the Chesapeake Bay. Similar to 2016, MDE found that generally, the air over the Bay does contain higher ozone concentrations than land sites but less than predicted by the air quality models. The HMI monitor was the highest reading ozone monitor in the state about 25% of all days it operated. Of the 17 exceedance days, HMI was the highest reading on 11 days.