An analysis of ozone extreme events over the United States in the past three decades

Observed surface ozone for the 1980 through 2012 time period is analyzed for extreme ozone pollution event frequency and trends throughout the contiguous United States. The data show significantly reduced ozone pollution in terms of days above the 75 ppbv (maximum 8 hour daily average) EPA standard and in the duration of individual episodes, owing most likely to clean air initiatives mandated by EPA regulation. Geospatial and temporal variations in ozone episodes are analyzed by EPA region, EPA category (rural, suburban, urban and unclassified), over all 12 months of each year in the dataset. The geospatial behavior is categorized by the six continental EPA regions, with one added to separately treat California because of its unique ozone history in the Los Angeles and central valley regions. The 95th and 5th percentile ozone concentrations are studied at each observation station that has a statistically significant set of data and used to examine geospatial and temporal trends useful in understanding extreme and background ozone patterns.

The ozone season is usually assumed to be the three summer months of June through August based on the photochemical genesis of surface ozone pollution. The observed data as well as the 95th percentile studies confirm that this is indeed the most significant period for high ozone episodes, but the data also show that many events occur in an extended warm season from May through October. Mean ozone data excluding California shows the highest values occurring along the eastern coastline of the United States at mid-Atlantic latitudes with an underlying inter-annual meander of maximum mean and latitude of maximum mean. Stations north and east of this region of highest mean values tend to follow the traditional three month summer season while many stations to the south and west tend to have events throughout a longer six-month ozone season.

Previous studies documented high or geographically specific ozone episodes and identified contributions from anticyclone, transport, or sub-decadal to decadal time scale patterns pertinent to the events. Here, an ensemble analysis of all events from single day to multi-day episodes in the 33 year period places all episodes into a continuum of time and geospatial coordinates. Inter-annual patterns linked to source concentrations and seasonal transport are evident, but anomalies such as unseasonable and persistent anticyclones to winter events over snow cover can also be identified.

An EOF analysis was conducted to examine how regional and hemispheric climate variability affects the ozone extreme events. A question explored here is how well EOF analysis that links ozone concentrations to climate variables explains the temporal and geospatial variability of extreme ozone events (days and episodes >= 75 ppbv).