Enhanced ozone concentrations are deteterious to human health and vegetation. The site shows how surface ozone responds to both trends in emissions and to the interannual variability in weather. Ozone at most sites, especially in the East, show decreases over time due to the powerful Clean Air Act and its amendments, which reduced emissions of ozone precursors. But a very hot summer can upend the trend. Try clicking on the sites in Illinois or southern Indiana and you will see the influence of the very hot summer of 2012.
Ozone observations are downloaded from CASTNET (https://java.epa.gov/castnet/clearsession.do)
June-July-August maximum daily 8-hour average (MDA8) ozone concentrations (CASTNET, 1990-2016)