O3 Attainment Status and Trends

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 ozone attainment status and design value trends and see NHDES’ State Implementation Plans for NHDES reports related to ozone attainment/nonattainment designations.

New Hampshire’s Attainment Status
Although ozone levels may exceed the standard on a few days per year in New Hampshire, all three-year design values for New Hampshire’s ozone monitors are below the NAAQS, and the entire state is in attainment for ozone. However, this attainment status is currently based on a NAAQS of 0.075 ppm, which was in effect from 2008 through 2015. EPA updated the NAAQS to the current standard of 0.070 ppm in October 2015, and attainment designations based on this standard have not yet been determined.

Former Nonattainment Areas

1979 1-Hour Standard
Not all of New Hampshire has always been in attainment for ozone. Following the 1990 CAA Amendments, EPA designated much of southern New Hampshire as nonattainment for the 1979 one-hour ozone standard of 0.112 parts per million (ppm); this area included Cheshire, Merrimack, Hillsborough, Rockingham, and Strafford County (see map). In June 2005, EPA revoked the one-hour ozone standard. Today, air quality in New Hampshire has improved such that the state would be in attainment of this standard even if it were still in place.

1997 8-Hour Standard
EPA replaced the one-hour standard with an eight-hour standard of 0.08 ppm; this was initially adopted in 1997, but not implemented until 2003 after a lengthy legal process. In April 2004, EPA designated parts of southeastern New Hampshire in nonattainment of this new 1997 eight-hour ozone standard; this area consisted of portions of Hillsborough, Merrimack, Rockingham, and Strafford Counties (see map).

March 2013, EPA reclassified this area of New Hampshire as attainment, leaving no remaining parts of the state in nonattainment. This occurred following the 2012 submittal of New Hampshire’s Request for Redesignation of the Boston-Manchester-Portsmouth (SE), NH Nonattainment Area. The Request for Redesignation described monitoring and other evidence that New Hampshire’s air quality meets and will likely continue to meet the 1997 eight-hour ozone standard.

Historical Trends
The current ozone design value is defined as the three-year average of each year’s fourth highest daily maximum eight-hour average at each monitoring site. In 2015, EPA lowered the eight-hour ozone standard from 0.075 ppm to 0.070 ppm. Depending on how future year design values compare to this new standard, some parts of New Hampshire may return to nonattainment.

The chart below compares New Hampshire’s maximum ozone design values to the standard in effect at that time. These data show that maximum design values have fallen significantly and are currently stable around the current NAAQS of 0.070 ppm, or 70 parts per billion (ppb).

chart of 8-hour new hampshire ozone design values