U.S. EPA Proposes Ozone Season Extension

On July 16, 2009, U.S. EPA proposed revisions to its ambient ozone (O3) monitoring network design requirements.  The revisions are in response to the amended primary and secondary National Ambient Air Quality Standards (NAAQS) for O3 that were promulgated on March 27, 2008.  Among the revisions to the O3 monitoring network design requirements is a proposed lengthening of the O3 monitoring season in select states.

Ozone is unique among criteria pollutants in that it is only required to be monitored during those parts of the year that are most conducive to O3 formation.  The length of the O3 monitoring season varies from place to place according to a number of seasonally-dependent factors (e.g., ambient temperature, strength of incoming solar radiation, length of day, etc.) that affect O3 formation.  The current O3 monitoring seasons range from four months of the year in states with cooler climates to 12 months of the year in those places with warmer climates.  Based on a recent analysis, U.S. EPA believes that ambient O3 concentrations could approach or exceed the 8-hour NAAQS of 0.075 parts per million (ppm) more frequently and during more months of the year, and that longer O3 monitoring seasons are therefore appropriate in the following 29 states and regions:

• The States/Regions of Delaware, District of Columbia, Georgia, Idaho, Louisiana AQCR 019 and 022, Maryland, Massachusetts, Missouri, Montana, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Pennsylvania, Tennessee, Vermont, Virginia, and Wisconsin will have a revised ozone monitoring season of March – October, which is an increase of one month.
• The States/Regions of Connecticut, Indiana, South Carolina, South Dakota, Utah, and  Washington will have a revised ozone monitoring season of March – October, which is an increase of two months.
• The States/Regions of Florida, Mississippi, and Texas will have a revised ozone monitoring season of January – December, which is an increase of four months.
• Wyoming’s revised ozone monitoring season will run January – December, which is an increase of five months.

Given the increased duration of the ozone monitoring seasons, one effect could be increased difficulty for currently designated O3 nonattainment areas to meet an attainment designation and the increased potential for current attainment areas to monitor a violation of the O3 standard and thus become nonattainment.  Continued nonattainment or increased areas of nonattainment could result in additional rules for reducing oxides of nitrogen (NOx) and volatile organic compounds (VOC) from existing sources, which are the two O3 precursor pollutants.  In addition, there could be an increase in the demand for emission offsets for these two precursor pollutants. U.S. EPA is proposing that the revised O3 monitoring seasons become effective on January 1, 2011.