Fig. 4.
Trends in monitoring by the FRM or the FEM. The FRM and FEM monitors for ozone report continuously, year-round in some locations and during selected warm months at others. Measured “days” for ozone are plotted as reported hours/24. FRM and FEM monitors for PM2.5 collect 24-h samples year-round, daily or every third or sixth day. At sites with multiple monitors, only the one reporting the most observations was counted. Data were downloaded December 2007 from AirData, http://www.epa.gov/aqspubl1/annual_summary.html