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Area Designations and Implementation of the Sulfur Dioxide National Ambient Air Quality Standard

Posted: February 22nd, 2013

Author: All4 Staff 

On February 6, 2013, U.S. EPA issued a white paper outlining the agency’s updated strategy for completing initial area designations under the June 2010 1-hour primary sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS).  The white paper is not intended as final rulemaking or to be legally binding, but rather offers U.S. EPA’s current line of the thinking on the NAAQS implementation process.  According to U.S. EPA, their implementation strategy will incorporate input received from air agencies and stakeholders, other NAAQS designation approaches, and the historical presence and importance of SO2 modeling in the past and in the Prevention of Significant Deterioration (PSD) permitting program.  U.S. EPA also believes that the strategy will place an emphasis on prioritizing sources based on magnitude of emissions and proximity to large populations, advance an appropriate time scale for developing sufficient data needed to make determinations, and account for responsibilities of state and federal air agencies set forth in the designation process under the Clean Air Act (CAA).  Received with mixed reviews, the issuance of the white paper largely represents a shift away from U.S. EPA’s focus on emissions modeling and towards emissions monitoring as the primary method of SO2 NAAQS compliance.

At its surface, the updated strategy revolves around the principal of providing flexibility for air agencies to determine the most appropriate and effective approach for determining NAAQS designations.  Specifically, U.S. EPA is proposing that air agencies prescribe to a dual-pathway approach to the designation process in which individual agencies are allowed to use either monitoring, modeling, or some mix thereof for the characterization of SO2 concentrations.  U.S. EPA believes that by utilizing such a dual-pathway approach air agencies will benefit from the cost- and time-effectiveness that modeling techniques offer in circumstances that prohibit the more accurate use of ambient monitoring networks.  However, as stated in the paper, the U.S. EPA’s preference is for SO2 designations to be made in the same fashion as with other NAAQS designations (i.e., through the use of monitoring networks).  Indeed, it appears U.S. EPA’s new strategy will focus on monitoring for emissions categorization as a starting point and use modeling as a secondary means when factors such as funding become an issue.  

In addition to introducing the dual-pathway approach, the U.S. EPA’s new strategy places an emphasis on prioritizing which sources should be assessed in regards to attainment of the 1-hour standards.  To do so, U.S. EPA proposes to identify sources based on the magnitude of their emissions and their proximity to large populations.  U.S. EPA has issued information which indicates that approximately 540 of the 20,000 national SO2 sources nationwide have actual emissions which exceed 1,900 tons per year or, in other terms, account for roughly 90% of all SO2 emissions in the United States.  To address these statistics, U.S. EPA is proposing the use of a two-pronged emission threshold for SO2 designation which would place a lower threshold of approximately 2,000-3,000 tons per year for sources located in more densely populated areas and a higher threshold of approximately 5,000-10,000 tons per year for sources located in less densely populated areas.  Sources falling within these categories would need to be assessed relative to NAAQS compliance via the installation of ambient monitors of a dispersion modeling evaluation as determined by the applicable state agency.  U.S. EPA has identified a number of factors that it intends to consider in selecting the appropriate thresholds for the two-pronged system, including: the comprehensiveness of the total emissions represented; the comparability of source coverage under the two-pronged approach with the typical source coverage of an ambient monitoring network; emission levels for sources in areas with monitored violations; and emission levels associated with sources that are well-controlled. 

U.S. EPA has also proposed an implementation timeline for future designations that utilize the dual-pathway and two-pronged approaches discussed above.  The proposed implementation timeline is designed to take effect in two stages so that a first round of modeling-based designations takes place prior to a subsequent round of monitoring-based designations.  In general, the U.S. EPA would issue final technical assistance documents in July of 2013, identify which sources and areas will deploy new ambient monitors and which will conduct dispersion modeling in 2015, deploy new monitors and require completed dispersion modeling analyses for selected sources and nonattainment areas in January of 2017, require SIP designations for nonattainment modeled areas in 2019, issue final designations for the rest of the country in December of 2020, and require SIP attainment demonstrations for areas designated as nonattainment in 2022.

If you facility’s actual emissions are within or exceed the ranges of emissions referenced above it would be prudent to consider exploratory dispersion modeling and to understand the possible locations and costs of ambient monitors.


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