Wednesday, 17 January 2007
Characterization of surface ozone concentrations in the United States
Exhibit Hall C (Henry B. Gonzalez Convention Center)
The distribution, and spatial and temporal variability of surface ozone reported into EPA's air quality system (AQS) are characterized. The data were collected in a variety of environments ranging from urban to relatively clean continental background. Emphasis is placed on the 1-h and 8-h average daily maxima, although 24-h average values are well correlated with these metrics. This was done because the 1-h and 8-h averages are most closely linked to adverse health effects. Ozone is monitored during specified “ozone seasons”, during which monitoring is mandatory. The lengths of these seasons vary across the U.S. In the Northeast, it ranges typically from April to October and in many southwestern states it extends all year round. The maximum number of stations record data from May to September and so this time frame is used in the analyses, although it should be noted that there are a number of instances of high ozone concentrations at other times of the year. The mean daily 8-h maximum O3 concentration for May to September from 2000 to 2004 at all sites was 49 ppbv and only 5 % of the site-wise means exceeded 57 ppbv. Although there were sizable declines at the upper end of the O3 distribution, O3 concentrations nearer the center of the distribution showed much smaller changes from 1990 to 2004, with noticeable dips in 2003 and 2004. The dips in mean O3 levels were likely associated with cooler than normal conditions in the eastern U.S., where most O3 monitors are located. Diurnal variations in O3 are controlled by a number of factors, resulting in differences in different environments across the U.S. In general, the maximum daily 1-h average O3 occurs in mid afternoon and the 8-h max between 10 a.m. and 6 p.m. However, there are a number of cases where the highest values occur either early in the morning or at night. These cases result from transport of ozone rich air from above the planetary boundary layer. Although most attention is given to ozone episodes occurring in the summer, there are a number of cases in which high ozone values are found in other seasons. These occurrences could be due to causes as diverse as stratospheric-tropospheric exchange (more likely in northern states) or photochemical activity (more likely in southern states). O3 concentrations are spatially heterogeneous in urban areas, largely due to its reaction with NO emitted by motor vehicles. However, O3 concentrations tend to be well correlated across monitoring sites in these areas. Characteristics of O3 specific to several urban areas will be presented.