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National Ambient Air Quality Standards for Sulfur Oxides (Sulfur Dioxide); Availability of Information

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Environmental Protection Agency (EPA).




The EPA is announcing today the following actions: The availability of new information on 5-minute average sulfur dioxide (SO2) concentrations in the ambient air; The status of EPA's ongoing activities to characterize and address 5-minute peak SO2 levels that may pose risk to sensitive individuals with asthma, including plans to consider taking final action on the proposed intervention level program (ILP) for the reduction of SO2 emissions published on January 2, 1997 and to respond to the remand of the final decision on the national ambient air quality standards (NAAQS) for SO2 published on May 22, 1996; The solicitation of comments on the new air quality information.


Comments should be submitted on or before March 12, 2001.


Comments should be submitted to Susan Lyon Stone, U.S. Environmental Protection Agency (MD-15), Research Triangle Park, NC 27711; email

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Susan Lyon Stone at the same address; e-mail; telephone (919) 541-1146.

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On May 5, 1998, EPA announced its plans for responding to a remand of its final decision not to revise the SO2 NAAQS (61 FR 25566, May 22, 1996) and for final action on the proposed ILP (62 FR 210, January 2, 1997); identified interim actions that we planned to take to address 5-minute peak SO2 levels that may pose risk to sensitive individuals; and solicited new information and analyses on 5-minute peak SO2 levels (63 FR 24782). The sensitive population for the effects of 5-minute peaks of SO2 consists of children, adolescents and adults with mild or moderate asthma who are physically active outdoors. As discussed in this 1998 notice, the primary issue in our SO2 NAAQS decision was whether a new 5-minute NAAQS was appropriate to protect sensitive individuals with asthma from the risk posed by exposure to 5-minute SO2 levels of 0.6 ppm or above. Given the available health effects information; information as to the localized, infrequent, and site-specific nature of the risk involved; and the advice of the Clean Air Scientific Advisory Committee (CASAC), the Administrator concluded that short-term (i.e., 5-minute) peak concentrations of SO2 do not constitute a public health problem for which the establishment of a NAAQS would be appropriate.

Consistent with our final SO2 NAAQS decision, and to supplement the protection provided by the existing SO2 Start Printed Page 1666NAAQS, we subsequently proposed an ILP to assist States in determining whether 5-minute peak concentrations of SO2 posed a significant health risk in the local population, and if so, to identify appropriate remedial measures. A key element of the proposed ILP was the establishment of a concern level of 0.6 parts per million (ppm), 5-minute average SO2 concentration, and an endangerment level of 2.0 ppm, 5-minute average. The proposed ILP would require that State and tribal plans contain the authority to take whatever action is necessary to prevent further exceedances of such concern and endangerment levels when the State/tribe determines that intervention is appropriate. The proposed ILP includes factors that the State/tribe should consider in making such determinations, including the magnitude and frequency of peak concentrations exceeding these levels, the history and nature of any citizen complaints, available information on potential exposure of sensitive individuals with asthma, and information about the source(s) causing the peak SO2 concentrations. Based on these factors, the proposed ILP provides for flexibility for the State/tribe to determine the nature and degree of intervention that is warranted in any area and to relocate existing SO2 monitors to areas where 5-minute peak concentrations may be of concern.

On January 30, 1998, the Court of Appeals for the District of Columbia issued a decision in a case brought by the American Lung Association (ALA) and the Environmental Defense Fund, American Lung Association v. Browner, No. 134 F.3d 388 (D.C. Cir. 1998) (ALA) that challenged our decision not to establish a new 5-minute SO2 NAAQS. The court found that we had failed to provide an adequate explanation for our determination that no revision to the SO2 NAAQS was appropriate, and remanded the decision to us to more fully explain our decision. id. In the absence of any court-established deadline for EPA action, EPA agreed with ALA to finalize our response to the remand by the end of the year 2000 (63 FR 24782). Subsequently, in light of a decision by the court in another case relating to EPA's 1997 revisions of the NAAQS for ozone and particulate matter, American Trucking Associations v. EPA, 175 F.3d 1027, 195 F.3d 4, cert. granted 120 S. Ct. 2003 (U.S. May 22, 2000) (No. 9901257) (ATA), the ALA agreed to extend the time for us to respond to the remand of the SO2 NAAQS decision to accommodate our need for additional time and pending additional court action in the ATA case.

In conjunction with extension of this schedule, we committed to take a number of actions, building upon the actions discussed in our 1998 notice (63 FR 24782). These ongoing actions focus on broadening our efforts to collect and analyze data on 5-minute average SO2 concentrations, providing further guidance to States on monitoring 5-minute SO2 concentrations around industrial sources, and addressing specific situations relating to short-term SO2 exposures that are of concern in local communities. c

Availability of Information on 5-Minute SO2 Concentrations

This section discusses new information that is now available on 5-minute SO2 concentrations, and includes descriptions of the nature of such data in EPA's Aerometric Information Retrieval System (AIRS) and data in other formats, and analyses that we have conducted of these data. Our 1998 solicitation of new information and analyses on 5-minute peak SO2 levels (63 FR 24782) resulted in the submission of relatively little additional 5-minute SO2 monitoring data. On June 30, 2000, we directly requested the assistance of EPA's Regional Offices and the State and Territorial Air Pollution Program Administrators/Association of Local Air Pollution Control Officers (STAPPA/ALAPCO) in obtaining any additional 5-minute SO2 monitoring data that may have been collected but not submitted to AIRS.[1] In response to this request, we have received from nine States additional 5-minute SO2 monitoring data from more than 48 monitoring sites, recorded during the period 1994-2000. We note, however, that the newly submitted data generally have a number of limitations, such that neither EPA nor the States express any opinions about the validity of these data at this time. More specifically, much of the data has been provided to us in a variety of formats not directly compatible with AIRS; only one State and the District of Columbia submitted their additional data into AIRS. Most of the data files lacked information on monitor location and type, or nearby source types, and a number of States have warned us that the data have not been subjected to appropriate quality assurance/quality control (QA/QC) procedures.[2] To the extent possible and appropriate, we are working with States to address these limitations. However, at this time, we do not believe it is appropriate to disseminate or rely on these data. At such time as the data are validated, they will be available to the public in AIRS.

To supplement prior assessments [3] and improve our understanding of the frequency, magnitude, and number of locations at which high 5-minute concentrations of SO2 may be occurring, we have undertaken analyses of the data in AIRS that include the following activities:

(1) We have evaluated monitoring data from 83 monitoring sites reporting 5-minute concentrations in 14 States to determine the frequency of peak concentrations greater than or equal to 0.6 ppm, the variations of such frequencies across locations, whether there are industrial sources located nearby that may be contributing to measured peak concentrations, and the size of the surrounding population within a 5-km radius of the monitor.

(2) Since we have AIRS data from far more monitoring sites (695) recording 1-hour average concentrations than from monitors recording 5-minute concentrations, we have constructed and applied mathematical models to aid in estimating the potential for the occurrence of 5-minute peak concentrations at and above 0.6 ppm SO2 at locations where only 1-hour average concentrations are available. These models are based on determining the relationships of 5-minute peak concentration distributions [4] to 1-hour mean concentration distributions and evaluating the statistical strength of these relationships.

Although we intend to extend these analyses to include additional data to the extent they become certified for inclusion in AIRS, we have substantially completed the analyses described above. A draft report summarizing our preliminary findings has been placed on EPA's website at​ttn/​amtic.

Status of Ongoing Activities

This section discusses the status of our ongoing activities to characterize and address 5-minute peak SO2 levels Start Printed Page 1667that may pose risk to sensitive individuals with asthma. These activities include: (1) Efforts to obtain State certification of newly submitted 5-minute SO2 monitoring data and related information, to be followed by analysis of this additional certified data, as appropriate; (2) development of guidance on monitoring 5-minute SO2 concentrations; (3) additional 5-minute SO2 air quality monitoring, in coordination with States' and industry's monitoring activities; (4) consideration of taking final action on the proposed ILP; and (5) consideration of our response to the remand of our 1996 SO2 NAAQS decision.

We are now in the process of working with States who submitted new 5-minute SO2 monitoring data to facilitate their certification of the data. We are also working to obtain related information, as appropriate, such as monitor location, nearby source types, and surrounding population. To the extent that such information warrants further analysis, we intend to extend the analyses discussed above to include these data, and to complete these analyses by mid-2001.

In a separate but related effort, we are evaluating our ambient air monitoring regulations and approaches. As part of a broad, integrated monitoring strategy for all the criteria pollutants, which we expect to propose late Spring 2001, we also expect to propose regulatory changes necessary to reflect current data needs, which in part will involve SO2 monitoring. We initially proposed revisions to regulations at 40 CFR parts 53 and 58 to modify reference and equivalent methods for SO2 and to revise the minimum requirements for ambient monitoring in compliance with the SO2 NAAQS in order to facilitate additional monitoring of 5-minute concentrations (60 FR 58959, March 7, 1995). We will consider the input received from the earlier proposal in developing these changes.

In addition, we also intend to issue a guideline specifically on SO2 monitoring. We have already developed a draft guideline, which is intended to assist State and local air pollution control agencies in evaluating their networks and the appropriateness of revising those networks to better address the potential for 5-minute concentrations of concern. The draft guideline provides relevant background information, summarizes recommended procedures for network review, suggested procedures for review of available ambient data to determine the potential for high 5-minute concentrations, and recommendations for short-term monitoring network design, including cost estimation procedures to help assess the costs of network revisions. This draft document may be obtained at EPA's website at​ttn/​amtic. In addition to these efforts, we will work with the States to facilitate implementation of the SO2 monitoring guideline and the broader integrated monitoring strategy.

We are also starting to develop plans for collecting additional 5-minute SO2 air quality monitoring data. We intend to work with States and industry to elicit their support and participation in this project, which we expect will provide important new information as to the likelihood and nature of 5-minute peak SO2 concentrations that may now be occurring around various types of industrial facilities. We anticipate that this project will take approximately two years, including planning, coordination, data collection and analysis. We expect that this information will help inform our response to the remand of the SO2 NAAQS decision as well as the next periodic review of the SO2 NAAQS.

In our consideration of taking final action on the proposed ILP (62 FR 210, January 2, 1997), we will take into account comments received in response to this notice as well as comments received on our 1997 proposed action. We received 62 comments on the proposed ILP, of which 11 comments were from State and local agencies and a related organization, 38 comments were from individual industry commenters and trade groups, four were from public advocacy groups, and four comments were from private citizens. Many commenters supported the proposed ILP and its flexible implementation strategy, while others commented that States already have sufficient regulatory authority to deal with sources emitting high 5-minute peaks of SO2 that may pose a risk to the health of asthmatic individuals living nearby, and therefore an additional regulatory program is not necessary. The commenters disagreed about the significance of the health effects associated with exposure to short-term peaks of SO2, particularly at the concern level (0.6 ppm SO2, 5-minute average). Some expressed the view that the health effects associated with exposures at this level are not significant enough to warrant remedial action, while others expressed the view that this level was not sufficiently health protective and urged us to set the concern level at a lower concentration (e.g., 0.3 ppm SO2, 5-minute average). Many commenters expressed concern about the costs associated with implementing the proposed ILP, especially when compared to the relatively small size of the sensitive population (i.e., individuals with asthma who are active outdoors) that might be affected. In addition, some State and local agency commenters expressed concern about the costs associated with the additional source-based monitoring (e.g., for monitor purchase, monitor relocation, or additional staff members) that might be needed to implement the proposed ILP.

With regard to moving forward with a final ILP, we note that the results of the data analyses completed to date continue to suggest that there may be a number of locations in the country where repeated exposures to 5-minute peak SO2 levels of 0.60 ppm and above could pose a risk of significant health effects. Taking into account this information, the results of planned additional analyses, and public comments, we will consider taking final action on an ILP. We anticipate reaching a final decision on an ILP by the summer of 2001, as a separate matter from our consideration of our response to the remand of the 1996 SO2 NAAQS decision.[5]

Since the court decision in the ALA case, remanding our decision not to revise the SO2 NAAQS, we have continued to take a number of actions relating to 5-minute SO2 peaks of 0.6 ppm or greater, including the solicitation and review of additional information and analyses described above. Although we continue to evaluate this and other information in light of the ALA remand, the ruling of the Court of Appeals for the District of Columbia Circuit in the ATA case, and its subsequent appeal to the United States Supreme Court, has created potential uncertainty regarding the appropriate framework for decisions under section 109. As a result, we believe that the better course of action is to await a decision from the Supreme Court, which is expected during the spring of 2001, before responding to the ALA remand of our SO2 NAAQS decision. This will better enable us to Start Printed Page 1668review and assess all relevant information, including the court's opinion and any additional analyses and technical information, that could shed additional light on the appropriate response to the remand. We intend to publish our schedule for considering the relevant information and responding to the remand by mid-2001.

Request for Comments

We are soliciting comments on the data analyses and preliminary findings in our draft report that is now available, to better inform future actions to reduce the health risk that may be posed by potential exposures of exercising asthmatics to short-term peaks of SO2. More specifically, we solicit comments on the following: the appropriateness of using 1-hour average SO2 monitoring data as one element in our efforts to estimate the potential for 5-minute peak concentrations greater than or equal to 0.6 ppm SO2; the usefulness of these types of analyses in identifying the need for additional monitoring or other actions and the sources likely to contribute to high 5-minute SO2 concentrations; and, for the purpose of assessing the need for additional monitoring around SO2 sources, the appropriateness of using just the hourly maximum 5-minute average SO2 concentrations, rather than all the 5-minute averages in an hour, including any relevant data storage and management considerations. We will consider this information in the context of taking final action on the proposed ILP, conducting future analyses of 5-minute SO2 air quality data, responding to the ALA remand and conducting the next periodic review of the SO2 NAAQS.

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Dated: January 3, 2001.

Robert Perciasepe,

Assistant Administrator for Air and Radiation.

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1.  Robert Perciasepe, Assistant Administrator for Air and Radiation, to S. William Becker, Executive Director, STAPPA/ALAPCO, June 30, 2000; Robert Perciasepe, Assistant Administrator for Air and Radiation, to EPA Regional Administrators, Regions I-X, June 30, 2000.

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2.  For example, in our preliminary review, we have noted that a number of recorded values appear to have been automatically flagged by the data loggers as reflecting monitor malfunctions and calibration measurements.

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3.  Prior assessments were done as part of our 1996 review of the SO2 NAAQS and are available in the docket for that rulemaking (Docket No. A-84-25).

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4.  Five-minute peak concentrations are taken to be the maximum 5-minute block average within each hour.

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1.  Although we are in the process of considering whether to move forward with an ILP during the same time that we continue to consider our response to the remand in the ALA case, it is important to note that we view our ILP activities and our response to the ALA remand as independent actions. Our consideration of taking final action on the proposed ILP is not intended as a substitute for a decision on the ALA remand, nor is it intended to indicate that we have reached any particular outcome regarding the need for a revised SO2 NAAQS. Regardless of our decision in response to the ALA remand, we believe that it is appropriate at this time to consider taking final action on an ILP to provide any supplementary protection from exposures of concern to short-term SO2 peaks that may be appropriate.

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[FR Doc. 01-565 Filed 1-8-01; 8:45 am]