The Clean Air Act defines an area as attainment or nonattainment based on a “design value” (see Criteria Pollutants). Please read below for information about PM2.5 and PM10 attainment status and design value trends.
New Hampshire’s PM2.5 and PM10 Attainment Status
All of New Hampshire is currently in attainment for both PM2.5 and PM10 on a statewide basis. Because there has never been a nonattainment designation for either of these pollutants in New Hampshire, there are no maintenance areas.
The current PM2.5 design value is the average of the 98th percentiles over a three-year period based on 24-hour average concentrations. The chart below compares New Hampshire’s maximum 24-hour PM2.5 design values to the standard in effect at that time.
These data show that maximum design values have declined slightly since the early 2000s and been fairly stable in recent years, remaining several microns/m3 below the current NAAQS of 35 microns/m3. Over this period, Keene has had the highest PM2.5 values because of wintertime buildup of wood smoke in the valley city.
For PM10, the standard is met when no more than an average of one exceedance occurs in a three-year period. In the entire history of PM10 monitoring in New Hampshire, only one daily value has gone over the standard of 150 microns/m3. The chart below shows New Hampshire’s maximum 24-hour PM10 averages. These data reveal a steady decline in PM10 levels since the mid-1980s.