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## EPA Technical Support Document in Review of Colorad o Department of Public Health and Environment’s Demonstration Packa ge for the April 3, 2009, Pagosa Springs Exceptional Event
EXCEPTIONAL EVENTS RULE REQUIREMENTS

The Environmental Protection Agency (EPA) promulgat ed the Exceptional Events Rule (EER) in 2007, pursuant to the 2005 amendment of Clean Air Act (CA A) Section 319. 42 U.S.C. §7619. The EER added 40 CFR 50.1(j), (k) and (l); 50.14; and 51.930 to t he Code of Federal Regulations (CFR). These section s contain definitions, criteria for EPA approval, pro cedural requirements, and requirements for air agen cy demonstrations, all of which must be met before EPA can concur under the EER on the exclusion of air quality data from regulatory decisions.
Under 40 CFR 50.14(c)(3)(iv), the air agency demons tration to justify exclusion of data must provide evidence that:

• The event satisfies the criteria set forth in 40 CF R 50.1(j) for the definition of an exceptional event;

• The event affects air quality;

• The event is not reasonably controllable or prevent able; and

• The event is caused by human activity that is unlik ely to recur at a particular location or [is] a natural event. 1

• There is a clear causal relationship between the me asurement under consideration and the event that is claimed to have affected the air quality in the area;

• The event is associated with a measured concentrati on in excess of normal historical fluctuations, including background; and

• There would have been no exceedance or violation bu t for the event.
This review applies the EPA’s “Interim Guidance of the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air Quality Data Affect ed by High Winds Under the Exceptional Events Rule” of May 2013, (Interim Guidance for High Wind Exceptional Events) in addition to the applicable statutory and regulatory provisions.


## SUMMARY

Overview
On June 30, 2012, Colorado Department of Public Hea lth and Environment (CDPHE) submitted a demonstration (termed a Technical Support Document or TSD) dated May 29, 2012 for an exceptional event exceedance of the 24-hour PM 10 standard that occurred at the monitoring station in Pagosa Springs on April 3, 2009. CDPHE supplemented that demonstra tion with additional information in an addendum

1 A natural event is further described in 40 CFR 50. 1(k) as “an event in which human activity plays lit tle or no direct causal role”.2

dated September 30, 2013. Table 1 summarizes the ex ceedance that CDPHE would like considered for exceptional event concurrence in the Air Quality Sy stem (AQS) database.

Table 1: EPA PM 10 Exceedance Summary of the 150 ug/m 3 24-hour NAAQS Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (ug/m 3 ) CDPHE

Request to Consider for AQS Removal? April 3, 2009 Pagosa Springs 08-007-0001 225 Yes

Pagosa Springs lies along the San Juan River, to th e west and south of the San Juan Mountains. To the southwest of Pagosa Springs are the deserts of the New Mexico, Utah, and Arizona Four Corners region. The areas described are shown in Figure 1.
Figure 1: Map of Desert, Four Corners, and Monitor Location with the PM 10 NAAQS Exceedance

In its exceptional event demonstration package CDPH E describes the April 3, 2009 event as:
the consequence of strong gusty winds ahead of a de ep low pressure with a trailing cold front, in combination with dry conditions, which caused signi ficant blowing dust across parts of Arizona, New Mexico and Colorado. These winds were partly th e result of a developing low pressure centered over southwestern Utah with a cold front t railing to the south as well as a second low pressure system over south-central Wyoming. These l ow pressures merged and centered over Colorado on April 3, 2009. Surface weather analyses show an area of low pressure affecting the3

Four Corners region. Winds in the dust source regio ns of northeast Arizona and in Flagstaff, Arizona were 74 mph on April 3, 2009. Surface winds of 20 to 54 mph with gusts of 25 to 91 mph were recorded across the Four Corners region on April 3, 2009. These speeds are above the thresholds for blowing dust identified in EPA draft guidance and in detailed analyses completed by the State of Colorado. 2
CDPHE provides a comprehensive description and disc ussion of this overnight event in Sections 2, 3, 4, 5, 6, and 7 of the demonstration. As noted in the f igure below, the high wind event caused dust to blo w and affected air quality in this area.
EPA Figure 2 from CDPHE’s demonstration 3 visually demonstrates blowing dust across northeas tern Arizona headed towards southwestern Colorado. Figur es 1-30 from CDPHE’s demonstration use modeling, meteorology, and monitoring analysis to s upport the evidence of the wind direction and reach of the dust storm into Pagosa Springs.

2 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 5. 3 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 20.4

Figure 2: Image of Blowing Dust on April 3, 2009 (C DPHE’s TSD Figure 14)5

Not Reasonably Controllable or Preventable
EPA evaluates whether an event was not Reasonably C ontrollable or Preventable (nRCP) at the time of the event by taking into account controls in place and wind speed, along with other factors. For natural sources of dust, a high wind dust event can general ly be considered to be nRCP if winds are high enoug h to cause emissions from natural undisturbed areas. For anthropogenic sources of dust, a high wind dust event can be nRCP if:

• The anthropogenic sources of dust have reasonable c ontrols in place;

• The reasonable controls have been effectively imple mented and enforced; and,

• The wind speed was high enough to overwhelm the rea sonable controls. 4
In addressing reasonable controls, CDPHE provided d etailed information on the current set of required and voluntary controls in the Pagosa Springs area, including information on implementation of State Regulations 1, 3, 6, and 9, rule effectiveness, com pliance, and public notification activities that oc curred on the event day. As CDPHE states:

There are very few anthropogenic local sources of g eologic PM 10 sized dust in the area as most of the area is covered in coniferous forest vegetation . Also, the anthropogenic sources are well controlled due to a State Implementation Plan (SIP) / PM 10 Maintenance Plan. See section 5.0 Local Dust Controls for a comprehensive list of PM 10 control measures employed in Pagosa Springs through the PM 10 SIP / Maintenance Plan. 5

CDPHE’s demonstration provides an analysis of the s tate and local controls in place to control PM 10 emissions. Although specifically focusing on Regula tions 1, 3 and 6, these controls include CDPHE’s:

• Regulation 1- Emission Control for Particulate Matt er, Smoke, Carbon Monoxide, and Sulfur Oxides;

• Regulation 3- Stationary Source Permitting and Air Pollutant Emission Notice Requirements;

• Regulation 6- Standards of Performance for New Stat ionary Sources; and

• Regulation 9- Open Burning, Prescribed Fire, and Pe rmitting. 6

CDPHE provided the documentation listed in Figures 3 and 4 showing that wind speeds associated with these events were above 30 mph for sustained and 40 mph for gusts (CDPHE tables 4 and 10) 7 . For example, southwest of Pagosa Springs in Farmington, NM, upwind of the monitor registering an exceedance, sustained surface winds of up to 35 mph with gusts up to 47 mph were measured on April 3, 2009, while Pagosa Springs saw gusts up to 41 mph. CDPHE previously established in its exceptional events demonstration for the January 19, 2009 event in Lamar, Colorado that wind speeds of 30 mph and/or gusts of 40 mph or higher have been shown to cause blowing dust in Colorado (attached as appendix A of CDPHE’s demonstration). While the sus tained winds in Pagosa Springs were below this Colorado threshold, gusts in Pagosa Springs briefly were above the threshold. Given the satellite evid ence

4 “Interim Guidance on the Preparation of Demonstrat ions in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Exceptional E vents Rule,” May 2013, page 10. 5 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 52. 6 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, pages 69-70. 7 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, pages 35-36 and 44-46.6

of Figure 2 above, transported dust from the upwind desert region appears to be a more significant contribution to the exceedance than was local winds overwhelming implemented local controls. EPA notes that the high local gust speeds in Pagosa Spr ings would likely be capable of overwhelming some local controls. The supplemental addendum provided by CDPHE supports this conclusion by describing in detail “Best Available Control Measures (BACM), other reasonable control measures, applicable federal, state, and local regulations, appropriate land use management, and an in-depth analysis of potential areas of local soil disturbance for the a ffected community.” 8

8 “Addendum to the Technical Support Document for the April 3, 2009 Pagosa Springs Exceptional Event,” September 30, 2013, page 3.7

Figure 3: Wind Speed in Farmington, NM (southwest o f Pagosa Springs) on April 3, 2009 (CDPHE’s TSD Table 4)8

Figure 4: Wind Speed in Pagosa Springs, CO on April 3 2009 (CDPHE’s TSD Table 10)

Section 2 of CDPHE’s demonstration provides a compl ex meteorological analysis of the event, including:

• Statewide PM 10 monitoring maps;

• Surface and upper air wind speed analysis from the National Oceanic and Atmospheric Administration (NOAA);

• Meteorological Sounding Analysis from the Universit y of Wyoming;

• Maps of real-time weather data from the National Ce nter for Atmospheric Research (NCAR);9

• Regional wind and weather by hour, as well as wind direction, through MesoWest;

• NOAA HYSPLIT modeling of wind trajectories;

• Naval Advanced Aerosol Prediction System (NAAPS) fo recasted dust concentrations;

• MODIS Terra satellite images demonstrating the blow ing dust across the region;

• GOES-11 Longwave Infrared Difference images;

• Maps establishing the lack of regional precipitatio n and drought status from U.S. Department of Agriculture (USDA), NOAA, and the National Drought Mitigation Center; and

• Maps of elevated friction velocities in the region from the NOAA National Centers for Environmental Prediction (NCEP) North American Mode l.
These analyses demonstrate persuasively through the weight of evidence the existence of a high wind blowing dust event originating far southwest of the monitor in question.
CDPHE’s Addendum to the TSD for April 3, 2009 Pagos a Springs Exceptional Event and Section 5 of CDPHE’s May 29, 2012 demonstration provide a thorou gh analysis of the state and local controls in plac e to control PM 10 emissions. These controls include CDPHE’s Regulatio ns 1, 3, 6, and 9. 9
Pagosa Springs local controls include:

• Land Use Development Code 6.6.3(h) and 6.6.3(m)(i) requiring paved streets, street sanding and sweeping/cleaning controls, encourages businesses t o maintain cleanliness of private lots; 10

• EPA approved PM 10 Maintenance Plan for the Pagosa Springs Attainment /Maintenance Area, effective on August 14, 2001, requiring Prevention of Significant Deterioration (PSD) permitting requirements mandating Best Available Control Techn ology (BACT) at sources potentially emitting PM 10 ;

• Vegetation, irrigation, fencing, and stabilized cla y dust control measures are in place for undeveloped parcels of land in town. Figure 5 below shows that identified areas of soil disturbance have reasonable controls. For example, Site A was seeded in 2006 and has since been covered by vegetation;

• Site B was the recent construction of the Springs R esort and Spa and was completed in May 2009 CDPHE states that:
The construction project was awarded “Gold Status” (second highest award) for complying with the Leadership in Energy & Environme ntal Design (LEED) 1 standards for New Construction Version 2.2 ... The Town of Pago sa Springs has provided Building Inspection Reports demonstrating that on February 2 7, 2009, the exterior lath was inspected. The building inspection report from Apri l 29, 2009 shows that the hotel was approved for temporary occupancy for 60 days. The f inal building inspection was completed on June 18, 2009. From the dates of the T own’s Building inspections it is clear

9 “Addendum to the Technical Support Document for th e April 3, 2009 Pagosa Springs Exceptional Event,” September 30, 2013, page 3-4. 10 Ibid., 5-6.10 that by April 2009, the entire construction site wa s paved and the building was completely constructed; the interior was just being finished; 11

• Site C was naturally vegetated and receives groundw ater from a nearby stream; 12 it is also an area where off-road recreational vehicles are prohibited by the parks department; and

• Site D is an irrigated and maintained turf and stab ilized clay baseball field. 13
Figure 5: Pagosa Springs Site Assessment (CDPHE’s F igure 1 in the Addendum to the TSD for the April 3, 2009 Pagosa Springs Exceptional Event)

On March 16, 2000, CDPHE’s SIP was revised and amen ded by the State (followed by a Notice of Final Rule on June 15, 2001) to designate Pagosa Springs as a PM 10 attainment area with the effect of deleting road paving requirements, street sanding reporting requirements, and any other “unnecessary mandatory contingency measures in the [Telluride] 14 and Pagosa Springs areas. No additional control me asures are

11 “Addendum to the Technical Support Document for the April 3, 2009 Pagosa Springs Exceptional Event,” September 30, 2013, page 8. 12 Colorado State Reg. 1 Section III.D.2.b considers natural vegetation with regular groundwater availab ility due to low lying terrain to be the appropriate available a nd practical method that is technologically feasibl e and economically reasonable in order to minimize fugiti ve particulate emissions for this lot at this time. 13 “Addendum to the Technical Support Document for the April 3, 2009 Pagosa Springs Exceptional Event,” September 30, 2013, page 10. 14 Telluride is not under consideration for this Exce ptional Event documentation but was considered alon g with Pagosa Springs when removing the nonattainment desi gnation on March 16, 2000.11

needed in these areas to demonstrate long-term main tenance of the PM-10 NAAQS”. 15 The dust control strategies described in bullets three through six a bove for Pagosa Springs local controls are voluntar y measures and are not intended to be federally enfor ceable elements of the SIP or maintenance plan.

Sections 2 and 5 of CDPHE’s demonstration show that the monitor in Pagosa Springs was affected by PM 10 transport from outside the area, with the main sou rce areas located to the southwest in Arizona, southeastern Utah, and northwestern New Mexico. In addition to transport, the spatial extent of elevat ed PM 10 concentrations throughout southwest Colorado and t he wind speeds associated with the event contribute to EPA’s evaluation of whether this even t is nRCP in Pagosa Springs. Table 2 summarizes the EPA’s evaluation of CDPHE’s nRCP EE demonstration.
Table 2: EPA’s Analysis of CDPHE’s TSD of nRCP Exceedance Date Location Reasonable Controls Demonstration Citation Wind Speed

Demonstration Citation Sufficient

Quality of Evidence? EPA

Criterion Met? April 3, 2009 Pagosa Springs Section 5: p. 69-70, and Addendum to the TSD For the April 3, 2009 Pagosa Springs Exceptional Event: p.3-12 Section 2: p.7-

50, esp. p.44 Yes Yes

Historical Fluctuations

EPA evaluates whether a measured exceedance is in e xcess of Historical Fluctuation (HF) by taking into account the level of the exceedance in relation to historical data, which is typically 3 to 5 years. To demonstrate that this requirement was met, CDPHE provided annual data series plots of 2009 samples of 24-hour averages, 5-year seasonal data samples o f 24-hour averages for the winters of 2004-2009, an d monthly data showing that most readings are consist ently below the NAAQS threshold. 16 CDPHE explains that for the concentration measured on Apr il 3, 2009:
The PM 10 sample on April 3, 2009 at Pagosa Springs of 225 u g/m 3 exceeds the 99 th percentile value (87 ug/m 3 ) for all April data, is greater than the 99 th percentile value (138 ug/m 3 ) for all 2009 data, and exceeds the 99 th percentile value (56 ug/m 3 ) for the entire dataset. Overall, this sample is the fourth highest sample in the dataset, the second highest sample in 2009, and maximum sample in any April...There are 6,096 samples in this dataset. 17

15 State Implementation Plan, Colorado Dept. of Publi c Health and Environment, Regulation: Nonattainment

Attainment/Maintenance Areas, Rule #1, Rule Title: Approval and Promulgation of Air Quality Implementa tion Plans; Colorado; Designation of Areas for Air Quali ty Planning Purposes, Telluride and Pagosa Springs, Direct Final Rule, June 15, 2001, 66 FR 32556. 16 “TSD for the April 3, 2009, Pagosa Springs Exceptio nal Event,” May 29, 2012, pages 53, 59, and 60. 17 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, pages 61-62.12

The EPA “Interim Guidance on High Wind Exceptional Events” recommends that demonstrations also consider percentile ranks when dust impacted data i s excluded. 18 Since at least 2005 in Pagosa Springs, values exceeding the NAAQS are only found during hi gh wind dust events, including April 3, 2009. This information indicates that values at the level seen on April 3, 2009 are extremely rare in the absence of high wind dust exceptional event impacts, and that it is unlikely that any sources of PM 10 were involved in the April 3, 2009 exceedance other than wind-blo wn dust. Table 3 summarizes the EPA’s evaluation of CDPHE’s HF EE demonstration.
Table 3: EPA’s Analysis of CDPHE’s TSD of HF Exceedance Date Exceedance

Location Historical Fluctuations

Demonstration Citation Sufficient Quality of Evidence? EPA Criterion

Met?
April 3, 2009 Pagosa Springs Section 3: p.51-64 Yes Yes
Clear Causal Relationship
EPA considers a variety of evidence when evaluating whether there is a Clear Causal Relationship (CCR) between the measurement under consideration and the event that is claimed to have affected the air quality in the area. Demonstrations should include documentation showing that the event in fact occurr ed and that emissions related to the event were transp orted in the direction of the monitor(s) where measurements were recorded; the size of the area af fected by the transported emissions; the relationsh ip in time between the event, transport of emissions, and recorded concentrations; and, as appropriate, pollutant species-specific information supporting a CCR between the event and the measured concentration. 19
Section 2 of CDPHE’s demonstration provides a compl ex meteorological analysis of the event, including:

• Statewide PM 10 monitoring maps;

• Surface and upper air wind speed analysis from NOAA ;

• Meteorological Sounding Analysis from the Universit y of Wyoming;

• Wind speed maps from the North American Regional Re analysis (NARR);

• Maps of real-time weather data from NCAR;

• Regional wind and weather by hour through MesoWest;

• NOAA HYSPLIT modeling of wind trajectories;

• NAAPS forecasted dust concentrations;

• MODIS Terra satellite images demonstrating the blow ing dust across the region;

• Maps establishing the lack of regional precipitatio n and drought status from the USDA, NOAA, and the National Drought Mitigation Center; and

• Maps of elevated friction velocities in the region from the NOAA NCEP North American Model.

18 See “Interim Guidance on the Preparation of Demons trations in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Excep tional Events Rule” of May 2013 page 20. 19 See “Interim Guidance on the Preparation of Demons trations in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Excep tional Events Rule” of May 2013 pages 21-22.13

These analyses demonstrate the existence of a high wind blowing dust event originating far southwest of the monitors in question.
CDPHE’s demonstration also provides dust-on-snow da ta from the Center for Snow and Avalanche Studies, an organization that studies the effects o f wind-blown desert dust from Arizona, New Mexico, and Utah on snowpack albedo and snowmelt in the San Juan Mountains of Colorado. Figure 30 of CDPHE’s demonstration shows that the Center for Sno w and Avalanche Studies “lists April 3, 2009, as one of twelve Dust-on-Snow events for the 2008/2009 water year.” 20, 21 This information helps to confirm that a regional blowing dust event with long-range transport caused the PM 10 exceedances measured across portions of Colorado beginning on April 3, 2 009.
The National Aeronautics and Space Administration’s (NASA) MODIS Rapid Response Team described the April 3, 2009 event as follows, “A dust storm s truck northeastern Arizona on April 3, 2009. With winds over 145 kilometers (90 miles) per hour repor ted new Meteor Crater, east of Flagstaff, the storm reduced visibility and forced the temporary closure of part of Interstate 40, according to the Arizona Republic .” 22

Also included are high-wind blowing dust accounts f rom the Southwest Climate Outlook, and the National Science Foundation. 23
CDPHE generally summarizes that “This exceedance wa s the consequence of strong gusty winds ahead of a deep low pressure with a trailing cold front, in combination with dry conditions, which caused significant blowing dust across parts of Arizona, N ew Mexico, and Colorado”. 24 CDPHE further states that “but for the dust storm on April 3, 2009, this exceedance would not have occurred”. 25 Table 4 summarizes the EPA’s evaluation of CDPHE’s CCR EE d emonstration.
Table 4: EPA’s Analysis of CDPHE’s TSD of CCR 26 Clear Causal Relationship Demonstration

Citation Sufficient Quality of Evidence? EPA Criterion Met? Section 2: p.7-50, Figures 1-30 Yes Yes Section 2: p.7, 35-47, Tables 4-10 Yes Yes Section 2: p.25, 31-34 Yes Yes Section 4: p.65-68 Yes Yes
Affects Air Quality (AAQ)
As stated in EPA’s Interim Guidance for High Wind E xceptional Events Section 3.4, EPA will consider events to have affected air quality if the CCR and HF requirements have been adequately demonstrated.

20 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 25. 21 “TSD for the April 3, 2009, Pagosa Springs Exceptio nal Event,” May 29, 2012, page 34.
22 “TSD for the April 3, 2009, Pagosa Springs Exceptio nal Event,” May 29, 2012, page 31. 23 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, pages 65-68. 24 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 72. 25 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 73. 26 See “Interim Guidance on the Preparation of Demons trations in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Excep tional Events Rule” of May 2013 page 21.14

The information presented in the HF and CCR demonst rations on the preceding three pages of this (EPA’s) TSD are cited in Table 5. Additionally, CDP HE states that:
The Pagosa Springs concentration is greater than th e 99 th percentile, and a conservative estimate of the dust storm contribution to the total concent ration is 182 – 188 μ g/m3. This is evidence that the event was associated with a measured concentrat ion in excess of normal historical fluctuations including background

. 27

Table 5 summarizes the EPA’s evaluation of CDPHE’s AAQ EE demonstration.
Table 5: EPA’s Analysis of CDPHE’s TSD of AAQ HF and CCR Exceedance Location Affects Air Quality Demonstration Citation Sufficient Quality of Evidence EPA

Criterion Met? HF Pagosa Springs Section 3: p.51-64 Yes Yes

CCR Pagosa Springs Section 2: p.7-50, Figures 1-30 Yes Yes Section 2: p.7, 35-47, Tables 4-

10 Yes Yes
Section 2: p.25, 31-34 Yes Yes Section 4: p.65-68 Yes Yes

Natural Event
As stated in EPA’s Interim Guidance for High Wind E xceptional Events Section 6.3.6 EPA will consider an event to be a natural event if both the nRCP and CCR requirements have been adequately demonstrated. CDPHE states that , “The high friction velocities and the data on soi l moisture conditions presented elsewhere in this report prove that this dust storm was a natural event that was not reasona bly controllable or preventable

. 28

CDPHE has sufficiently demonstrated the CCR and nRC P requirements for the April 3, 2009 event. Table 6 summarizes the EPA’s evaluation of CDPHE’s Natura l Event EE demonstration.

27 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 72. 28 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 72.15

Table 6: EPA’s Analysis of CDPHE’s TSD of Natural E vent nRCP or CCR Exceedance Location Natural Event

Demonstration Citation Sufficient Quality of Evidence? EPA

Criterion Met?nRCPAnthropogenic Pagosa Springs Section 5: p. 69-70, and Addendum to the TSD For the April 3, 2009 Pagosa Springs Exceptional Event: p.3-

12 Yes Yes

nRCP-Wind Speed Pagosa Springs Section 2: p.7-50, esp. p.44 Yes Yes

CCR Pagosa Spings Section 2: p.7-50, Figures 1-30 Yes Yes
Section 2: p.7, 35-47, Tables 4-10 Yes Yes
Section 2: p.25, 31-34 Yes Yes Section 4: p.65-68 Yes Yes

No Exceedance or Violation But For the Event
Generally, the No Exceedance or Violation But for t he Event (NEBF) demonstration is similar to the demonstration of the nRCP and CCR requirements, and should show that the measured concentration would have been below the applicable NAAQS without the affect of the event.
CDPHE provides a summary of the analysis and inform ation presented in the demonstration that show both the nRCP and CCR requirements have been met an d states that:

Satellite imagery shows large plumes of southwest t o northeast trending blowing dust in the Painted Desert and Little Colorado River Valley reg ion of northeastern Arizona on April 3, 2009. Satellite imagery also shows this dust moving acros s the Four Corners into southwest Colorado. Backward trajectories, wind streamline analyses, an d surface and upper-level wind patterns show that this dust would have been transported into Col orado on April 3, 2009. Multiple sources of data and analyses of past dust storms in this area prove that this was a natural event and, more specifically, a significant natural dust storm orig inating in northeastern Arizona and northwestern New Mexico. But for the dust storm on April 3, 2009 , this exceedance would not have occurred. 29

While not explicitly stated in the documentation, E PA notes that PM 10 concentrations before the periods of high winds on the event days were below the 24-h our PM 10 NAAQS, providing further support for CDPHE’s conclusion. Table 7 summarizes the EPA’s ev aluation of CDPHE’s NEBF EE demonstration.

29 “TSD for the April 3, 2009, Pagosa Springs Excepti onal Event,” May 29, 2012, page 72.16

Table 7: EPA’s Analysis of CDPHE’s TSD of NEBF Exceedance Date No Exceedance But For the Event Demonstration Citation Sufficient Quality of Evidence? EPA Criterion Met?

April 3, 2009 Section 7: p.72-73 Yes Yes
Schedule and Procedural Requirements
In addition to technical demonstration requirements , 40 CFR 50.14(c) specifies the schedule and procedural requirements an air agency must follow t o request data exclusion. CDPHE received no external comments during the 30 day comment period, but minor corrections were made to the report based on internal staff review. Table 8 outlines EP A’s evaluation of these requirements.
Table 8: EPA’s Analysis of Schedules and Procedural Criteria Reference Schedules and Procedural Criteria Demonstration Citation EPA

Criterion Met?

Did the State provide prompt public notification of the event? 40 CFR 50.14(c)(1)(i) No real-time samplers were in use at the time of this event 30 Yes

Were flags and initial description placed on the data by July 1 st

of the following year? 40 CFR 50.14(c)(2)(iii) CDPHE Letter to EPA dated June 30, 2012 31 Yes

Was the demonstration submitted within 3 years of the end of the quarter in which the event occurred and 12 months prior to the date that any regulatory decision must be made by EPA? 40 CFR 50.14(c)(3)(i) June 30, 2012 Yes

Was the public comment process followed and documented? 40 CFR 50.14(c)(3)(v) CDPHE Letter to EPA dated June 30, 2012 32 Yes

30 According to 40 CFR 50.14(c)(1)(i): “All States and, where appli cable, their political subdivisions must notify the public promptly whenever an event occurs or is reas onably anticipated to occur which may result in the exceedance of an applicable air quality standard.” CDPHE’s Add endum to the Technical Support Document explains th at the event was not captured by real time samplers but ra ther from filter-based samplers analyzed weeks afte rward. 31 CDPHE letter accompanying TSD from Gordon E. Pierc e, Program Manager, Technical Services Program, Air Pollution Control Division to Richard Payton of EPA dated June 30, 2012. 32 CDPHE letter accompanying TSD from Gordon E. Pierc e, Program Manager, Technical Services Program, Air Pollution Control Division to Richard Payton of EPA dated June 30, 2012.17



## CONCLUSION

EPA has reviewed documentation provided by CDPHE to support claims that dust emissions generated by high winds resulted in PM 10 transport from arid regions in Arizona, northwest New Mexico, and southeast Utah into the Pagosa Springs area on Apri l 3, 2009. This high wind event caused an exceedanc e of the 24-hour PM 10 NAAQS at the location identified in Table 1 of this document on April 3, 2009, in Pagosa Springs. EPA concurs that the flagged exceed ance at the Pagosa Springs monitor on this day meets the definition of an exceptional event: the e xceedance affected air quality, was nRCP, and met t he definition of a natural event. Specifically, EPA ha s determined that the event was nRCP due to high wi nd conditions that transported PM 10 from sources outside of the Pagosa Springs area and subsequently overwhelmed reasonable controls within Pagosa Sprin gs. Also, regardless of transport into the area, information pertaining to the controls implemented within the affected areas, the spatial extent of el evated PM 10 concentrations measured in the area, and the wind s peeds associated with the high wind dust event provide sufficient evidence to conclude that these events were nRCP. Furthermore, EPA has determined that there is a CCR between the events and the meas ured exceedances, there would have been NEBF, and the measured exceedances are in excess of normal HF s. EPA finds that the weight of evidence is sufficient to concur on the flagging of the data for the NAAQ S exceedance of 225 ug/m 3 the Pagosa Springs monitor on April 3, 2009.

This concurrence does not constitute final EPA acti on regarding any matter on which EPA is required to provide an opportunity for public comment. In parti cular, this applies to determinations regarding the attainment status or classification of the area. Fi nal actions will take place only after EPA complete s notice and comment rulemaking on those determinatio ns.