SO2 Attainment Status and Trends

Overview
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 sulfur dioxide (SO2) attainment status and design value trends.

New Hampshire’s Sulfur Dioxide Attainment Statusmap of new hampshire 1-hour SO2 nonattainment area
A small area in central New Hampshire is currently designated nonattainment for SO2 (see map at right); see EPA’s Sulfur Dioxide (2010) Nonattainment Areas for the list of specific counties and towns. Also see NHDES’ State Implementation Plans page for related reports. Because there has never previously been a nonattainment designation for SO2 in New Hampshire, there are no maintenance areas.

NHDES expects the current SO2 nonattainment area to be re-designated as attainment in the near future, as SO2 levels have dropped greatly in the last few years (see below).

Historical Trends
The chart below compares New Hampshire’s maximum one-hour SO2 design values to the standard in effect at that time. The current one-hour SO2 design value, which has been in effect since 2010, is defined as the three-year average of each year’s 99th percentile based on hourly concentrations.

These data show that maximum design values have been higher than the NAAQS until recently. Design values were gradually decreasing until around 2004, after NHDES moved one of the SO2 monitoring stations closer to the Merrimack Station power plant. In 2012, NHDES required the Merrimack Station power plant to install air emission controls that dramatically cut emissions of SO2 and other air pollutants. This has brought statewide maximum levels below the NAAQS.

chart of new hampshire 1-hour sulfur dioxide design values