1

EPA Technical Support Document on Review of Colorado Department of 

Public Health and Environment’s Demonstration Package for the May 11, 

2010 Alamosa and Pagosa Springs Exceptional Event

EXCEPTIONAL EVENTS RULE REQUIREMENTS 

EPA promulgated the Exceptional Events Rule (EER) in 2007, pursuant to the 2005 amendment of Clean 

Air Act (CAA) Section 319. 42 U.S.C. §7619. The EER added 40 CFR 50.1(j), (k) and (l); 50.14; and 

51.930 to the Code of Federal Regulations (CFR). These sections contain definitions, criteria for EPA 

approval, procedural requirements, and requirements for air agency 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 demonstration to justify exclusion of data must provide 

evidence that:

A. The event satisfies the criteria set forth in 40 CFR 50.l(j) for the definition of an exceptional 

event; 

The event affects air quality;

The event is not reasonably controllable or preventable; and/or

The event is caused by human activity that is unlikely to recur at a particular location or 

[is] a natural event.1

B. There is a clear causal relationship between the measurement under consideration and the event 

that is claimed to have affected the air quality in the area; 

C. The event is associated with a measured concentration in excess of normal historical fluctuations,

including background; and

D. There would have been no exceedance or violation but for the event.

This review applies the EPA’s “Interim Guidance on the Preparation of Demonstrations in Support of 

Requests to Exclude Ambient Air Quality Data Affected 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 July 1, 2013, the Colorado Department of Public Health and Environment (CDPHE) submitted 

exceptional events demonstrations for 3 exceedances (one in Pagosa Springs and two in Alamosa) of the 

24 hour PM10 standard that occurred at the Pagosa Springs School (08-007-0001) monitor, the Alamosa 

Municipal Building (08-003-0003) monitor, and the Adams State College (08-003-0001) monitor. Table 1 

summarizes these exceedances. 

 

1 A natural event is further described in 40 CFR 50.1(k) as “an event in which human activity plays little or no direct 

causal role”. 
2

Table 1: EPA PM10 Exceedance Summary of the 150 ug/m324-hour NAAQS

Exceedance Date Monitor/Site Name AQS ID Measured Exceedance 

(ug/m3)

May 11, 2010 Pagosa Springs School 08-007-0001 200

Alamosa Municipal Building 08-003-0003 161

Adams State College 08-003-0001 160

Alamosa, Colorado lies in the broad, flat, high elevation San Luis Valley of the upper Rio Grande River. 

Alamosa is at an elevation of 7,600 feet, and is between the South San Juan and Sangre de Christo 

Mountain ranges. Alamosa is generally surrounded by agricultural lands within the San Luis Valley. 

Pagosa Springs lies along the San Juan River, to the west and south of the San Juan Mountains. To the 

southwest of Pagosa Springs are the deserts of the New Mexico and Arizona Four Corners region.

Figure 1: EPA Map of Monitor Locations
3

In its exceptional events demonstration package CDPHE describes the May 11, 2010 event as a high wind 

blowing dust event and explains that “[s]ince at least 2005, there has not been an exceedance that was not 

associated with high winds carrying PM10 dust from distant sources in these areas.2” CDPHE asserts that 

the exceedances would not have occurred but for:

(a) dry soil conditions over southeastern Utah, northeastern Arizona, and portions of extreme 

northwestern New Mexico; (b) a combination of strong surface low pressure and a cold front 

associated with an intense upper-level trough that was moving across the western United States 

that created conditions necessary for widespread strong gusty winds over the area of concern; (c) 

elevated friction velocities and the deep mixing of the blowing dust from desert regions of

Arizona, northwest New Mexico, and southeast Utah. . . These sources are not reasonably 

controllable during a significant windstorm under abnormally dry or moderate drought 

conditions.3

CDPHE provides a comprehensive description and discussion of the event in Sections 2, 3, 4, 5, and 6 of 

the demonstration. As noted below, the high wind event caused dust to blow and affected air quality in 

this area.

EPA Figure 2 from page 40 of the state’s demonstration (CDPHE’s Figure 20) visually demonstrates 

plumes of dust blowing across northeastern Arizona and northwestern New Mexico into southeastern 

Colorado. Figures 1-29 from CDPHE’s demonstration use modeling, meteorology, and monitoring 

analysis to support the wind direction and the reach of the dust storm into Alamosa and Pagosa Springs. 

 

2

“Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 1. 

3

“Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 82.
4

Figure 2: Satellite Image of Dust Storm (CDPHE’s Figure 20 in the state’s demonstration)

The following material documents our review of CDPHE’s June 30, 2013 exceptional event 

demonstration package for the May 11, 2010 exceptional event. 

Not Reasonably Controllable or Preventable

EPA evaluates whether an event was not Reasonably Controllable 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 generally be considered to be nRCP if winds are high enough 

to cause emissions from natural undisturbed areas. For anthropogenic sources of dust, a high wind dust 

event can be not reasonably controllable or preventable if:

1. The anthropogenic sources of dust have reasonable controls in place;

2. The reasonable controls have been effectively implemented and enforced; and,

3. The wind speed was high enough to overwhelm the reasonable controls. 

In addressing reasonable controls, CDPHE provided detailed information on the current set of required 

and voluntary controls in the Pagosa Springs PM10 maintenance area and in Alamosa, including 

information on implementation of State Regulations 1, 3, 6, and 9, implementation effectiveness, 

compliance, and public notification activities that occurred on the event day. As CDPHE states, they
5

“conducted thorough analyses and outreach with local governments to confirm that no unusual 

anthropogenic PM10-producing activities occurred in these towns and that despite reasonable control 

measures in place, high wind conditions overwhelmed all reasonably available controls”.

4

CDPHE provided the documentation listed in figures 3 & 4 (CDPHE tables 9 & 10) showing that 

sustained wind speeds associated with these events were above 25 mph. For example, in Alamosa 

sustained surface winds of up to 43 mph with gusts up to 58 mph were measured on May 11, 2010, while 

Pagosa Springs saw sustained wind speeds up to 30 mph and gusts up to 48 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 analysis). The wind speeds on May 11, 2010 clearly 

exceed the established high-wind threshold.

5

 

4

“Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 67.

5 The EPA’s minimum high wind threshold for wind speed in the West (although not intended to draw a bright line) 

is 25 mph sustained wind speeds, according to page 3 of the “Interim Guidance on the Preparation of 

Demonstrations in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the 

Exceptional Events Rule” of May 2013. CDPHE has established that 30 mph sustained winds and 40 mph gusts are 

an appropriate threshold in Colorado.
6

Figure 3: Wind Speed in Alamosa, Colo. On May 11, 2010
7

Figure 4: Wind Speed in Pagosa Springs, Colo. On May 11, 2010
8

Section 2 of CDPHE’s documentation provides a complex meteorological analysis of the event, 

including:

Statewide PM10 monitoring maps;

Surface and upper air wind speed analysis from the National Oceanic and Atmospheric 

Administration (NOAA);

Maps of real-time weather data from the National Center for Atmospheric Research (NCAR);

Regional wind and weather by hour through MesoWest;

NOAA HYSPLIT modeling of wind trajectories from NOAA HYSPLIT modeling runs;

MODIS Terra satellite images demonstrating the blowing dust across the region;

GOES-11 longwave split window time-lapse images showing dust originating in Arizona;

Maps establishing the lack of regional precipitation 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 Model. 

These analyses demonstrate persuasively through the weight of evidence the existence of a high wind 

blowing dust event originating far southwest of the monitors in question. 

Section 5 of CDPHE’s documentation provides a thorough analysis of the State and local controls in 

place to control PM10 emissions. These controls include CDPHE’s: 

Regulation 1- Emission Control for Particulate Matter, Smoke, Carbon Monoxide, and Sulfur 

Oxides;

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

Regulation 6- Standards of Performance for New Stationary Sources; and 

Regulation 9- Open Burning, Prescribed Fire, and Permitting applied to both Pagosa Springs and 

Alamosa. 

Alamosa has the following controls in place:

Natural Events Action Plan (NEAP), completed in May 2003, which addresses public health 

programs, public notification, and implements Best Available Control Measures (BACM). 

CDPHE ensured in 2007 and 2013 that the NEAP was implemented satisfactorily;

City Code of Ordinances Article VII of Section 21-140(5) which addresses dust control at homes;

Article V Sec. 17-87(3) requires pavement of new roads/alleys; and

Article VI Sec. 21-119(3) requires new commercial establishments to install underground 

irrigation. 

In addition, Alamosa conducts street sweeping, has irrigated and installed vegetative cover on all city 

parks, encourages low tilling and other soil conservation practices outside of town through the Colorado 

State University Co-Op Extension Office and the Natural Resources Conservation Service. 

Maps are included of eight undeveloped parcels of land in town that were determined to be reasonably 

controlled or considered natural sources. All eight are covered with vegetation and five of those eight are 
9

confirmed to be irrigated. The three remaining sites in the following figure (Sites B, E, and H), are 

naturally vegetated, and satellite images show minimal soil disturbance.6

Figure 5: Alamosa Site Assessment

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 to maintain cleanliness of private lots;

EPA approved PM10 Maintenance Plan, effective on August 14, 2001, requiring Prevention of 

Significant Deterioration (PSD) permitting requirements mandating Best Available Control 

Technology (BACT) at sources potentially emitting PM10; and 

Vegetation, irrigation, fencing, and stabilized clay dust control measures are in place for 

undeveloped parcels of land in town. The map following 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 Resort and Spa was 

completed in May 2009 and therefore couldn’t have impacted the May 11, 2010 event. Site C 

was naturally vegetated and receives groundwater from a nearby stream;7it is also an area where 

off-road recreational vehicles are prohibited by the parks department. Site D is an irrigated and 

maintained turf and stabilized clay baseball field since April 2009.8

 

6

“Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, pages 75-78.

7 Colorado State Reg. 1 Section III.D.2.b considers natural vegetation with regular groundwater availability due to 

low lying terrain to be the appropriate available and practical method that is technologically feasible and 

economically reasonable in order to minimize fugitive particulate emissions for this lot at this time.

8

“Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20,

2013, pages 79-84.
10

Figure 6: Pagosa Springs Site Assessment

On March 16th, 2000, Colorado’s State Implementation Plan (SIP) was revised and amended to designate 

Pagosa Springs as a PM10 attainment area with the effect of deleting road paving requirements, street 

sanding reporting requirements, and any other “unnecessary mandatory contingency measures in the 

[Telluride]9and Pagosa Springs areas. No additional control measures are needed in these areas to 

demonstration long-term maintenance of the PM-10 NAAQS.”10 The dust control strategies described in

previous bullet 3 for Pagosa Springs local controls are voluntary measures and are not intended to be 

federally enforceable elements of the SIP or maintenance plan. 

 

9 Telluride is not under consideration for this Exceptional Event documentation but was considered along with 

Pagosa Springs when removing the nonattainment designation on 3/16/2000.

10 State Implementation Plan, Colorado Dept. of Public Health and Environment, Regulation: NonattainmentAttainment/Maintenance Areas, Rule #1, Rule Title: Pagosa Springs Attainment/Maintenance Area, Notice of Final 

Rule date 6/15/2001, 66FR 32556. 
11

Sections 2 and 5 of CDPHE’s analysis clearly demonstrate that monitors in Pagosa Springs and Alamosa 

were affected by PM10 transport from outside the area, with the main source areas located to the 

southwest of the area in Arizona, southeastern Utah, and northwestern New Mexico. In addition to 

transport, the spatial extent of elevated PM10 concentrations throughout the area and the wind speeds 

associated with the event contribute to EPA’s evaluation of whether these events are not reasonably 

controllable or preventable. Potential anthropogenic sources nearer the monitors were shown to have 

reasonable dust control in place. 

Table 2: Evaluation of CDPHE’s Technical Support Documentation of nRCP

Location Reasonable Controls 

Demonstration Citation

Wind Speed 

Demonstration Citation

Sufficient 

Quality of 

Evidence?

EPA 

Criterion 

Met?

Pagosa Springs Section 5.2: p.67-68, 77-

81

Section 2: p.10-45, esp. p.17 

& 31

Yes Yes

Alamosa Municipal 

Building

Section 5.1: p.67-68-, 69-

76

Section 2: p.10-45, esp. p.17 

& 31

Yes Yes

Adams State 

College

Section 5.1: p.67-68, 69-

76

Section 2: p.10-45, esp. p.17 

& 31

Yes Yes

Historical Fluctuations

EPA evaluates whether a measured exceedance is in excess of Historical Fluctuation (HF) by taking into 

account the level of the exceedance in relation to historical data, which is typically at least 3 to 5 years. 

To demonstrate that this requirement was met, CDPHE provided 7-year time series plots of 2,214 samples 

(Alamosa) and 2,287 samples (Pagosa Springs) of daily 24-hour averages. CDPHE explains that 

concentrations measured on May 11, 2010 were more rare than the 99.5 percentile level in both Pagosa 

Springs11 and Alamosa,

12 indicating that on average PM10 this high recurs no more often than 2 times 

each year. The EPA “Interim Guidance on High Wind Exceptional Events”

13 recommends that 

demonstrations also consider percentile ranks when dust impacted data is excluded. The May 11, 2010 

Pagosa Springs value of 200 ug/m

3 ranks among the values exceeding the NAAQS that, since at least 

2005, are only found during high wind dust events. This information indicates that values at the levels 

seen on May 11, 2010 are extremely rare in the absence of high wind dust exceptional event impacts, and 

that it is extremely unlikely that any sources of PM10 were involved in the May 11, 2010 exceedances 

other than wind-blown dust.

 

11 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 57.

12 Ibid pages 51 & 54.

13 See “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air 

Quality Data Affected by High Winds Under the Exceptional Events Rule” of May 2013 p. 20.
12

Table 3: Evaluation of CDPHE’s Technical Support Documentation of HF

Exceedance Location Historical Fluctuations 

Demonstration Citation

Sufficient Quality of 

Evidence?

EPA Criterion 

Met?

Pagosa Springs School Section 3: p.54-55, Figure 38 Yes Yes

Alamosa Municipal Building Section 3: p.51-52, Figure 34 Yes Yes

Adams State College Section 3: p.48-49, Figure 30 Yes Yes

Clear Causal Relationship

EPA considers a variety of evidence when evaluating whether there is a Clear and 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 occurred 

and that emissions related to the event were transported in the direction of the monitor(s) where 

measurements were recorded; the size of the area affected by the transported emissions; the relationship 

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. 

Section 2 of CDPHE’s demonstration included a comprehensive conceptual model of the events, 

including a complete overview of the meteorology, climate, and drought conditions for the Four Corners 

region. The analysis also included a detailed discussion of the event that occurred on May 11, 2010, 

including 29 geospatial figures and 10 tables with statewide PM10 monitoring maps, surface and upper air 

wind speed analysis from NOAA, maps of real-time weather data from NCAR, regional wind and 

weather by hour through MesoWest, HYSPLIT modeling of wind trajectories from NOAA, MODIS Terra 

satellite images demonstrating the blowing dust across the region, GOES-11 longwave spilt window timelapse images showing dust originating in Arizona, maps establishing the lack of regional precipitation and 

drought status from USDA, NOAA, and the National Drought Mitigation Center, and lastly maps of 

elevated friction velocities in the region from the NOAA NCEP North American Model. Taken together, 

this analysis demonstrates a high wind blowing dust event originating far southwest of the monitors in 

question.

CDPHE’s analysis, Section 2 Figure 20, shows a MODIS Terra satellite image of the blowing dust 

originating in Arizona. Figures 21-23 include a GOES-11 longwave split window modeled time-lapse 

that, using color enhancement to highlight the negative temperature differences for select wavelengths 

associated with blowing dust, demonstrate the dust originating from outside of Colorado’s borders, 

approaching from the south and southwest. 

CDPHE’s demonstration includes the following information from NOAA and the U.S. Geological Survey 

(USGS) which helps to confirm that a regional blowing dust event with long-range transport caused the 

PM10 exceedances measured across portions of Colorado on May 11, 2010: 
13

NOAA- An area of moderately dense to locally dense blowing dust originating from point 

sources in northeastern Arizona and northwestern New Mexico moved northeastward across far 

southeastern Utah and southwestern Colorado toward central Colorado.14

USGS- A strong pacific storm front moved through the southwest today [May 11, 2010], bringing 

very strong winds but no precipitation. The dominant wind direction was out of the south-west. 

Severe dust was viewable from satellite by both visible band and thermal imaging in the Four 

Corners area. Activity was apparent from early morning on to well after sunset. Interstate 40 in 

Arizona was closed from Winona to Winslow for most of the day. Peak gusts in the Winslow area 

at the heart of the western dust sources reached 77 mph.15 USGS categorized May 11, 2010 as a 

Dust Event day. 

Also included are eyewitness accounts from the Community Collaborative Rain, Hail, & Snow Network 

that describe low visibility, high winds, wind damage, and dust; as well as a travel log from tourists in the 

region noting high wind and dust conditions in this area. 

CDPHE generally summarizes that “these exceedances were the consequence of strong southwesterly 

prefrontal surface winds over dry soils which caused significant blowing dust across much of Arizona, 

northwest New Mexico, southeast Utah, and southwest Colorado”.

16 CDPHE further states that “[w]hile it 

is likely that some dust was generated within the local communities as gusts from the regional dust storm 

passed through the area, the amount of dust generated locally was easily overwhelmed by, and largely 

unnoticeable as compared to the dust transported in from the sources regions of the dust storm”.

17

Table 4: Evaluation of CDPHE’s Technical Support Documentation of CCR

18

Clear Causal Relationship Demonstration 

Citation

Sufficient Quality of Evidence? EPA Criterion 

Met?

Section 2: p.10-45, Figures 1-29 Yes Yes

Section 2: p.10, 21-30, Tables 1-10 Yes Yes

Section 2: p.43-44 Yes Yes

Section 4: p.65-69 Yes Yes

Affects Air Quality

As stated in the Interim Guidance for High Wind Exceptional Events Section 3.4, EPA will consider 

events to have Affected Air Quality (AAQ) if the CCR and HF requirements have been adequately 

 

14 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 43.

15 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 43-44.

16 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 10.

17 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 67.

18 See “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air 

Quality Data Affected by High Winds Under the Exceptional Events Rule” of May 2013 page 21.
14

demonstrated. CDPHE states that due to the information presented in the HF and CCR demonstrations on 

pages 8-9 and 9-11 of this TSD, respectively, and cited in Table 5 below, “data clearly shows that but for 

this high wind blowing dust event, Alamosa and Pagosa Springs would not have exceeded the 24-hour 

NAAQS on May 11, 2010.19”

Table 5: Evaluation of CDPHE’s Technical Support Documentation of AAQ

HF or CCR Affects Air Quality Demonstration Citation Sufficient Quality 

of Evidence?

EPA Criterion 

Met?

HF Section 3: p.54-55, Figure 38 Yes Yes

HF Section 3: p.51-52, Figure 34 Yes Yes

HF Section 3: p.48-49, Figure 30 Yes Yes

CCR Section 2: p.10-45, Figures 1-29 Yes Yes

CCR Section 2: p.10, 21-30, Tables 1-10 Yes Yes

CCR Section 2: p.41 Yes Yes

CCR Section 4: p.62-66 Yes Yes

Natural Event

As stated in the Interim Guidance for High Wind Exceptional 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 generally states that, “[s]ince at least 2005, there has not been an exceedance that was not 

associated with high winds carrying PM10 dust from distant sources in these areas” and that “multiple 

sources of data for the event in question and analyses of past dust storms in this area prove that this was a 

natural event”.

20

Table 6: Evaluation of CDPHE’s Technical Support Documentation of Natural Event

nRCP or CCR Natural Event Demonstration 

Citation

Sufficient Quality 

of Evidence?

EPA Criterion 

Met?

nRCP Section 5.2: p.67-68, 77-81 Yes Yes

nRCP Section 5.1: p.67-68-, 69-76 Yes Yes

nRCP Section 5.1: p.67-68, 69-76 Yes Yes

CCR Section 2: p.10-45, Figures 1-29 Yes Yes

CCR Section 2: p.10, 21-30, Tables 1-10 Yes Yes

CCR Section 2: p.41 Yes Yes

CCR Section 4: p.62-66 Yes Yes

Conclusion Section 6: p.82-83 Yes Yes

 

19 CDPHE’s “Technical Support Document for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango 

Exceptional Event,” page 85.

20 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 85.
15

No Exceedance or Violation But For the Event

Generally, the No Exceedance But For the 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 information presented in the documentation that 

demonstrate both the nRCP and CCR requirements have been met and states that:

[t]he PM10 exceedances in Alamosa and Pagosa Springs on May 11, 2010, would not have 

occurred if not for the following: (a) dry soil conditions over southeastern Utah, northeastern

Arizona, and portions of extreme northwestern New Mexico; (b) a combination of strong surface 

low pressure and a cold front associated with an intense upper-level trough that was moving 

across the western United States that created conditions necessary for widespread strong gusty 

winds over the area of concern; and (c) elevated friction velocities and the deep mixing of the 

blowing dust from desert regions of Arizona, northwest New Mexico, and southeast Utah.21

While not explicitly stated in the documentation, EPA acknowledges that PM10 concentrations before the 

periods of high winds on the event days were below the 24-hour PM10 NAAQS, providing further support 

for CDPHE’s conclusion. 

Table 7: Evaluation of CDPHE’s Technical Support Documentation of NEBF

Exceedance Date No Exceedance But 

For the Event 

Demonstration 

Citation

Sufficient Quality of 

Evidence?

EPA Criterion Met?

May 11, 2010 Section 6: p.85-86 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 to request data exclusion. Table 8 outlines EPA’s 

evaluation of these requirements. 

 

21 “Technical Support Document for the May 11, 2010, Alamosa and Pagosa Springs Exceptional Event” May 20, 

2013, page 85.
16

Table 8: Schedules and Procedural Criteria

Reference Schedules and 

Procedural 

Criteria 

Demonstration 

Citation

Criterion 

Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14(c)(l)(i) Section 1:p.8 Yes

Were flags and initial description placed

on the data by July 1st of the following 

year?

40 CFR §50.14(c)(2)(iii) Section 1:p.8-9 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 28, 2013 Yes

Was the public comment process 

followed and documented?

40 CFR §50.14(c)(3)(v) Section 1:p.9 Yes

CONCLUSION

EPA has reviewed documentation provided by CDPHE to support claims that dust emissions generated 

by high winds resulted in PM10 transport from arid regions in Arizona, northwest New Mexico, and 

southeast Utah into the Pagosa Springs and Alamosa areas on May 11, 2010. These high wind events 

caused exceedances of the 24-hour PM10 NAAQS at the locations identified in Table 1 of this document

on May 11, 2010. EPA has determined that the flagged exceedances at these locations on this day meet

the definition of an exceptional event: the exceedances affected air quality, were nRCP, and meet the 

definition of a natural event. Specifically, EPA has determined that events were nRCP due to high wind 

conditions that transported PM10 from sources outside of the Pagosa Springs and Alamosa areas and 

subsequently overwhelmed reasonable controls within Alamosa and Pagosa Springs. Also, regardless of 

transport into the area, information pertaining to the controls implemented within the affected areas, the 

spatial extent of elevated PM10 concentrations measured in the area, and the wind speeds 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 measured exceedances, 

there would have been no exceedance but for the events, and the measured exceedances are in excess of 

normal HF. 

EPA finds that the weight of evidence is sufficient to concur on the flagging of the data for these monitors 

on May 11, 2010. These concurrences do not constitute final EPA action regarding any matter on which 

EPA is required to provide an opportunity for public comment. In particular, this applies to 

determinations regarding the attainment status or classification of the area. Final actions will take place 

only after EPA completes notice and comment rulemaking on those determinations.