EPA Proposes SO2 One-Hour NAAQS

On December 8, 2009, EPA officially proposed revisions to the primary SO2 standard to a level between 50 and 100 parts per billion (ppb) measured over 1-hour.  This proposed primary NAAQS is based on a three-year average of the annual 99th percentile (or 4th highest) of 1-hour daily maximum concentrations.  EPA is also proposing to revoke the current 24-hour and annual primary SO2 standards (because it anticipates that the new proposed 1-hour standard would better protect public health) as well as maintain the current 24-hour and annual standards.  The proposed changes would not impact the SO2 secondary 3-hour standard.  EPA is accepting public comments until February 8, 2010, and plans to hold a public hearing January 5, 2010 in Atlanta. EPA plans to issue a final rule by June 2010. 

EPA is taking comment on alternative levels for the 1-hour standard outside the 50 to 100 ppb range, up to 150 ppb and on the proposed range of 50-100 ppb.  The proposed range was set based on the Administrator’s findings from both epidemiologic and controlled human exposure studies as well as the results of air quality and exposure analyses.  It is the Administrator’s opinion that the proposed range would protect public health with an adequate margin of safety, but EPA acknowledges the subjectivity in interpretation of these studies.  EPA specifically solicits comments on the weight placed on the epidemiologic evidence, controlled human exposure evidence, and air quality, exposure, and risk information, the benchmark used to select the proposed range, and the uncertainties associated with each.  It was noted that if a 1-hour standard outside the 50 to 100 ppb range was selected, the 24-hour standard would be retained, recognizing that a 1-hour standard greater than 100 ppb at the 99th percentile would not have the effect of maintaining 24-hour average SO2 concentrations below the current 24-hour standard.  In the scientific studies, there was little evidence to support an annual standard for the purpose of protecting against health effects so, independent of the final 1-hour standard, EPA proposed to revoke the current annual standard.

Additionally, EPA proposes a new Federal Reference method (FRM) for measuring SO2 in the ambient air.  The new FRM for SO2 would be an automated method based on ultraviolet fluorescence (UVF) specified in the form of a reference measurement principle and a calibration procedure.  Older manual FRMs and Federal Equivalent Methods (FEMs) will be phased out over time, to avoid creating an economic burden on states and monitoring firms.  However, 348 additional SO2 monitors are anticipated  in order to report both a maximum five- minute and one hour average for each hour of the day,  and to  capture concentrations at locations of expected maximum hourly concentrations, based on population weighted emission index and the state-level contribution to the national SO2 emissions inventory.

Lastly, to ensure that anti-backsliding provisions and principles are met and applied if EPA revokes a current standard, EPA is proposing that the current SO2 NAAQS would remain in effect for one year following the effective date of the revised SO2 NAAQS.