1

EPA Technical Support Document in Review of Colorado Department of 

Public Health and Environment’s Demonstration Package for the April 5, 

2010 Alamosa, Pagosa Springs, Durango, and Telluride 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 50.1(j) for the definition of an exceptional 

event:

The event affects air quality;

The event is not reasonably controllable or preventable; and

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 June 28, 2013, the Colorado Department of Public Health and Environment (CDPHE) submitted 

exceptional event demonstrations for six exceedances and four high readings of the 24-hour PM10 

standard that occurred at several monitoring stations in southwestern Colorado on April 5, 2010. Table 1 

summarizes these exceedances, Table 2 summarizes the high sample readings that CDPHE would like 

 

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

considered in support of the existence of the high wind blowing dust event but not for removal from the 

Air Quality System (AQS) database. 

Table 1: EPA PM10 Exceedance Summary of the 150 ug/m3 24-hour NAAQS

Exceedance 

Date

Monitor/Site Name AQS ID 24-hour Avg. 

(ug/m3)

CDPHE 

Request to 

Consider for 

AQS 

Removal?2

April 5, 2010 Telluride 08-113-0004 354 Yes

Pagosa Springs School 08-007-0001 349 Yes

Durango River City Hall 08-067-0004 320 Yes

Alamosa Adams State College 08-003-0001 185 Yes

Crested Butte 08-051-0004 174 Yes

Mt. Crested Butte 08-051-0007 168 Yes

Table 2: EPA PM10 High Samples Included in CDPHE’s TSD for Demonstration Purposes Only

High Sample 

Date

Monitor/Site Name 24-hour Avg. 

(ug/m3)

CDPHE 

Request to 

Consider for 

AQS Removal?

April 5, 2010 Delta 115 No3

Clifton 75 No4

Aspen 70 No5

Grand Junction Powell 51 No6

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. Durango 

lies along the Animas River at an elevation of 6,512 feet in a valley running north to south. Telluride sits 

at 8,750 feet at the top of a valley that runs downhill to the west. The east end of the valley is similar to a 

box canyon. Crested Butte and Mt. Crested Butte sit at 8,885 feet at the top of a valley that descends

southward and widens into the relatively flat mountain park through which the westward flowing Delta 

 

2

“Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 1.

3

CDPHE included these values in their report to demonstrate the regional nature of the event but did not specifically 

request their removal.

4

CDPHE included these values in their report to demonstrate the regional nature of the event but did not specifically 

request their removal.

5

CDPHE included these values in their report to demonstrate the regional nature of the event but did not specifically 

request their removal.

6

CDPHE included these values in their report to demonstrate the regional nature of the event but did not specifically 

request their removal.
3

River runs. To the southwest of all of these sites are the deserts of the New Mexico, Utah, and Arizona 

Four Corners region. 

Figure 1: Map of Desert, Four Corners, and Monitor Locations with PM10 NAAQS Exceedances

In its exceptional event demonstration package CDPHE describes the April 5, 2010 event as: 

the consequence of strong southwesterly prefrontal surface winds in combination with dry 

conditions which caused significant blowing dust across much of Arizona, northwest New 

Mexico, southeast Utah and southwest Colorado. These winds were the result of a strong short 

wave in the upper atmosphere that was moving across the Great Basin and the associated surface 

cold front and low pressure system. This dust storm originated in the desert regions of northern 

central Arizona and northwestern New Mexico. It transported PM10 dust into the southern and 

central portions of Colorado.

7

 

7

“Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 1-2.
4

CDPHE provides a comprehensive description and discussion of this overnight 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.

CDPHE’s inclusion of Figure 2 from pages 46 of the state’s demonstration8visually demonstrates wind 

speed and direction of blowing dust across northeastern Arizona, southeastern Utah, and northwestern 

New Mexico into southeastern Colorado. Figure 3 is a close-up of the plume in Arizona.

9

Furthermore, 

figures 1-35 from CDPHE’s demonstration use modeling, meteorology, and monitoring analysis to 

support the evidence of wind direction and reach of the dust storm into Durango, Alamosa, Pagosa 

Springs, Telluride, and Crested Butte. This EPA evaluation of the state’s demonstration finds that 

Durango, Alamosa, Pagosa Springs, and Telluride all have complete demonstrations that warrant 

concurrence on the flagged AQS data, while Crested Butte and Mt. Crested Butte are lacking sufficient 

analysis, as explained below. 

Figure 2: Satellite Image of Dust Storm (CDPHE’s Figure 30 in the state’s demonstration)

 

8

“Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 46. 

9

“Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 48.
5

Figure 3: Satellite Image of Dust Storm in Arizona (Figure 32 in CDPHE’s demonstration)

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:
6

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.10

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

and voluntary controls in the Alamosa, Pagosa Springs, and Durango areas, including information on 

implementation of State Regulations 1, 3, 6, and 9, rule effectiveness, compliance, and public notification 

activities that occurred on the event day. As CDPHE states, they:

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.11

CDPHE provided the documentation listed in Figures 4 & 5 (CDPHE tables 7 & 9)

12

, which EPA 

supplemented with Figure 6, showing that sustained wind speeds associated with these events were above 

25 mph. For example, in Alamosa sustained surface winds of up to 44 mph with gusts up to 60 mph were 

measured on April 5, 2010, while Durango saw sustained wind speeds up to 30 mph and gusts up to 43

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 

April 5, 2010 clearly exceed the established high-wind threshold.

13

 

10 “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,” May 2013, page 10.

11 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event” 

May 17, 2013, page 87.

12 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event” 

May 17, 2013, pages 28 and 30.

13 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.
7

Figure 4: Wind Speed in Durango, Colo. on April 5, 2010
8

Figure 5: Wind Speed in Alamosa, Colo. on April 5, 2010
9

Figure 6: Wind Speed in Pagosa Springs (left) and Telluride (right) on April 5, 2010

Pagosa Springs Monitor - MesoWest Telluride Monitor - MesoWest 

ID = KPSO

Wind 

mph

GUST 

mph ID = KTEX

Wind 

mph

GUST 

mph

4-5-2010 0:15 MDT 3.5 4-5-2010 0:15 MDT 10.4 41.4

4-5-2010 0:35 MDT 0 4-5-2010 0:33 MDT 15 32.2

4-5-2010 0:55 MDT 3.5 4-5-2010 0:53 MDT 10.4 20.7

4-5-2010 1:15 MDT 0 4-5-2010 1:13 MDT 13.8 19.6

4-5-2010 1:35 MDT 0 4-5-2010 1:35 MDT 18.4 29.9

4-5-2010 1:55 MDT 0 4-5-2010 1:53 MDT 13.8 21.9

4-5-2010 2:15 MDT 0 4-5-2010 2:13 MDT 15 29.9

4-5-2010 2:35 MDT 0 4-5-2010 2:33 MDT 19.6 32.2

4-5-2010 2:55 MDT 0 4-5-2010 2:55 MDT 19.6 34.5

4-5-2010 3:15 MDT 0 4-5-2010 3:13 MDT 6.9 27.6

4-5-2010 3:35 MDT 0 4-5-2010 3:35 MDT 17.3 25.3

4-5-2010 3:55 MDT 0 4-5-2010 3:53 MDT 12.7 32.2

4-5-2010 4:15 MDT 0 4-5-2010 4:15 MDT 8.1 18.4

4-5-2010 4:35 MDT 0 4-5-2010 4:33 MDT 6.9

4-5-2010 4:55 MDT 0 4-5-2010 4:52 MDT 10.4 18.4

4-5-2010 5:15 MDT 0 4-5-2010 5:13 MDT 16.1 24.2

4-5-2010 5:35 MDT 0 4-5-2010 5:35 MDT 18.4 24.2

4-5-2010 5:55 MDT 0 4-5-2010 6:16 MDT 8.1 25.3

4-5-2010 6:15 MDT 0 4-5-2010 6:33 MDT 17.3 31.1

4-5-2010 6:35 MDT 0 4-5-2010 6:53 MDT 3.5 9.2

4-5-2010 6:55 MDT 0 4-5-2010 7:13 MDT 9.2 40.3

4-5-2010 7:15 MDT 0 4-5-2010 7:33 MDT 8.1

4-5-2010 7:35 MDT 0 4-5-2010 7:53 MDT 17.3 31.1

4-5-2010 7:55 MDT 0 4-5-2010 8:13 MDT 18.4 29.9

4-5-2010 8:15 MDT 0 4-5-2010 8:35 MDT 19.6 35.7

4-5-2010 8:35 MDT 0 4-5-2010 8:53 MDT 13.8 28.8

4-5-2010 8:55 MDT 0 4-5-2010 9:15 MDT 15 33.4

4-5-2010 9:15 MDT 0 4-5-2010 9:33 MDT 18.4 32.2

4-5-2010 9:35 MDT 11.5 17.3 4-5-2010 9:55 MDT 19.6 34.5

4-5-2010 9:55 MDT 8.1 4-5-2010 10:13 MDT 28.8 39.1

4-5-2010 10:15 MDT 10.4 4-5-2010 10:36 MDT 15 50.6

4-5-2010 10:35 MDT 13.8 24.2 4-5-2010 10:54 MDT 21.9 43.7

4-5-2010 10:55 MDT 18.4 25.3 4-5-2010 11:13 MDT 17.3 35.7

4-5-2010 11:15 MDT 10.4 20.7 4-5-2010 11:33 MDT 23 51.8

4-5-2010 11:35 MDT 11.5 17.3 4-5-2010 11:53 MDT 17.3 35.7
10

4-5-2010 11:55 MDT 16.1 20.7 4-5-2010 12:13 MDT 29.9 44.9

4-5-2010 12:15 MDT 13.8 23 4-5-2010 12:34 MDT 16.1 35.7

4-5-2010 12:35 MDT 17.3 24.2 4-5-2010 12:53 MDT 11.5 27.6

4-5-2010 12:55 MDT 11.5 24.2 4-5-2010 13:33 MDT 28.8 49.5

4-5-2010 13:15 MDT 12.7 21.9 4-5-2010 13:53 MDT 24.2 49.5

4-5-2010 13:35 MDT 8.1 4-5-2010 14:13 MDT 27.6 50.6

4-5-2010 13:55 MDT 13.8 18.4 4-5-2010 14:33 MDT 25.3 61

4-5-2010 14:15 MDT 11.5 21.9 4-5-2010 14:53 MDT 24.2 59.8

4-5-2010 14:35 MDT 16.1 24.2 4-5-2010 15:13 MDT 33.4 58.7

4-5-2010 14:55 MDT 17.3 25.3 4-5-2010 15:33 MDT 24.2 59.8

4-5-2010 15:15 MDT 26.5 35.7 4-5-2010 15:53 MDT 18.4 46

4-5-2010 15:35 MDT 19.6 31.1 4-5-2010 16:13 MDT 24.2 50.6

4-5-2010 15:55 MDT 28.8 35.7 4-5-2010 16:33 MDT 23 52.9

4-5-2010 16:15 MDT 24.2 31.1 4-5-2010 16:55 MDT 12.7 25.3

4-5-2010 16:35 MDT 24.2 38 4-5-2010 17:13 MDT 20.7 35.7

4-5-2010 16:55 MDT 23 35.7 4-5-2010 17:33 MDT 12.7 25.3

4-5-2010 17:15 MDT 20.7 33.4 4-5-2010 17:53 MDT 15 29.9

4-5-2010 17:35 MDT 24.2 33.4 4-5-2010 18:15 MDT 16.1 39.1

4-5-2010 17:55 MDT 19.6 34.5 4-5-2010 18:33 MDT 16.1 38

4-5-2010 18:15 MDT 19.6 32.2 4-5-2010 18:53 MDT 15 28.8

4-5-2010 18:35 MDT 16.1 31.1 4-5-2010 19:12 MDT 17.3 32.2

4-5-2010 18:55 MDT 20.7 29.9 4-5-2010 19:35 MDT 15 31.1

4-5-2010 19:15 MDT 18.4 25.3 4-5-2010 19:53 MDT 20.7 31.1

4-5-2010 19:35 MDT 15 23 4-5-2010 20:13 MDT 17.3 40.3

4-5-2010 19:55 MDT 16.1 32.2 4-5-2010 20:54 MDT 24.2 31.1

4-5-2010 20:15 MDT 19.6 29.9 4-5-2010 21:13 MDT 16.1 34.5

4-5-2010 20:35 MDT 19.6 32.2 4-5-2010 21:33 MDT 18.4 28.8

4-5-2010 20:55 MDT 26.5 34.5 4-5-2010 21:53 MDT 18.4 39.1

4-5-2010 21:15 MDT 19.6 35.7 4-5-2010 22:13 MDT 24.2 38

4-5-2010 21:35 MDT 19.6 40.3 4-5-2010 22:34 MDT 11.5 24.2

4-5-2010 21:55 MDT 20.7 29.9 4-5-2010 22:53 MDT 20.7 41.4

4-5-2010 22:15 MDT 26.5 36.8 4-5-2010 23:13 MDT 20.7 35.7

4-5-2010 22:35 MDT 19.6 36.8 4-5-2010 23:36 MDT 25.3 40.3

4-5-2010 22:55 MDT 12.7 28.8 4-5-2010 23:54 MDT 15 32.2

4-5-2010 23:15 MDT 24.2 38

4-5-2010 23:35 MDT 21.9 34.5

4-5-2010 23:55 MDT 19.6 28.8
11

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);

Upper air sounding analysis from the University of Wyoming;

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;

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. 

Rather than catalogue the anthropogenic controls in place in Telluride, Crested Butte, and Mt. Crested 

Butte, CDPHE provided a comprehensive analysis of snow depth at elevation in early April. The analysis

demonstrates that any potential anthropogenic sources were covered by 70 inches of snow in Telluride 

and 20 or more inches of snow in Crested Butte and Mt. Crested Butte.14 The following NOAA figure 

indicates the depth of snow at the time of the event, an image supported by NOAA Snow Water 

Equivalent, Snow Depth, Snow Cover, and Snow Melt analysis charts and narrative.15 Based on the 

snowpack at the time of the event it is reasonable to conclude expected seasonal anthropogenic controls 

such as vegetation, irrigation, fencing, or clay stabilization of undeveloped lots were neither appropriate 

nor applicable. Additionally, satellite imagery of the valley in the spring of 2010 shows no potential 

undeveloped parcels of land of concern. 

 

14 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 40-41.

15 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 40-44.
12

Figure 7: NOAA Snow Depth in Southern Colo. on April 5, 2010

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 of Particulates, 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.16

 

16 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 87-88.
13

Alamosa has the following controls in place:

Natural Events Action Plan (NEAP)17, 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.18

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. Since 2008 the 

Mosca-Hooper Conservation District has been working with the Natural Resources Conservation Service 

to implement the Colorado State Forest Service’s program to plant between 2,805 and 7,432 trees per year 

since 2008 to prevent wind erosion in the area.19

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 

confirmed to be irrigated. The three remaining sites in the following figure 8 (Sites B, E, and H) are 

naturally vegetated and satellite images show minimal soil disturbance.20

Figure 8: Alamosa Site Assessment21

 

17 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, Appendix C.

18 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 89.

19 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 91.

20 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event” 

May 17, 2013, pages 92-94.

21 This image is from 2007, EPA review of Google Earth satellite images as recent as 2011 show that there is still 

adequate vegetation. 
14

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;

22

EPA approved PM10 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 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 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 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;23 it 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 2009.24

 

22 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 96-100.

23 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.

24 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 98-100.
15

Figure 9: Pagosa Springs Site Assessment

On March 16, 2000, Colorado’s State Implementation Plan (SIP) was revised and amended 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 and Pagosa Springs areas. No additional control measures are needed in these areas to 

demonstrate long-term maintenance of the PM-10 NAAQS”.

25 The dust control strategies described in 

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. 

Durango local controls include:

Dust Control rules and ordinances found in Figure 11 (CDPHE’s table 30) following;

26

The City or Durango, La Plata County, and the Southern Ute Indian Tribe work together to 

implement the following programs: street sweeping and sanding controls, dust suppressant for 

gravel roads, landfill dust control, and smoke mitigation for locomotives;

27

 

25 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. 

26 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 101-102.

27 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 102-103.
16

La Plata County and the Southern Ute Indian Tribe have surface operating standards to control 

dust from oil & gas development in the area. This includes maintaining roads to well pads, 

establishing speed limits, and designating gradient to minimize dust. Guidelines are found in the 

Bureau of Land Management’s (BLM) “Surface Operating Standards and Guidelines for Oil and 

Gas Exploration and Development, The Gold Book” 4th edition, 2007;

28 and

Vegetation, signage, and other 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. Site A is a former industrial site that was leveled and then compacted and allowed to 

return to a natural state. CDPHE confirmed that the soil remains compacted and vegetated and 

should not be considered an anthropogenic source. Site B is naturally vegetated and has not been 

developed. Site C is an off-leash dog park with signage to indicate that it is an area where offroad recreational vehicles are prohibited by the parks department.

29

Figure 10: Durango Site Assessment

 

28 Online version: 

http://www.blm.gov/pgdata/etc/medialib/blm/wo/MINERALS__REALTY__AND_RESOURCE_PROTECTION_/e

nergy/oil_and_gas.Par.18714.File.dat/OILgas.pdf

29 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 104-107.
17

Figure 11: Durango and La Plata County Dust Control Rules and Ordinances
18
19

Telluride local controls include:

Woodburning30 controls which the Town of Telluride and San Miguel County have adopted as 

wood and coal burning emission reduction measures. These wood and coal burning controls were 

adopted and implemented throughout the 1980s and early 1990s, were approved by EPA in 1994, 

and are defined in detail in Section II. A. of the “State Implementation Plan-Specific Regulations 

for Nonattainment – Attainment/Maintenance Areas (Local Elements).” All of these requirements 

remain effective, until they are removed or revised by a future SIP revision;

Emissions from stationary sources of pollution are regulated by several Air Quality Control 

Commission Regulations31;

Street Sanding Controls requiring that any user that applies street sanding material in the 

Telluride attainment/maintenance area must use materials containing less than two percent fines. 

This strategy was adopted in 1994 and approved by EPA in 1996, and it is defined in detail in 

Section II.B. of the “State Implementation Plan-Specific Regulations for Nonattainment –

Attainment/Maintenance Areas (Local Elements); and 

Control Measures Included in the Attainment/Maintenance Plan, including federal fuels and 

tailpipe standards and regulations32. 

 

30 Air Quality Control Commission Regulation No. 4 covers wood stoves, conventional fireplaces and woodburning 

on high pollution days, as approved by EPA as part of the federal SIP in 1997. This maintenance plan makes no 

changes to Regulation No. 4.

31 1) Regulation No. 1 regulates emissions of particulates, smoke, sulfur dioxide, and nitrogen oxides and establishes 

limits on these pollutants from covered sources. Sections I-IV, Sections VI-IX, and Appendices A and B are already 

included in the approved SIP. This maintenance plan incorporates the regulatory limits in calculations of maximum 

allowable emissions for stationary sources. No additional revisions are made to Regulation No. 1 as part of the 

maintenance plan revision.

2) Regulation No. 3 lays out provisions of the State of Colorado’s stationary source permitting program. Parts A and 

B of Regulation No. 3 are already included in the approved SIP. Part C implements the federal operating permit 

program and this reference to Part C of Regulation No. 3 shall not be construed to mean that these regulations are 

included in the SIP.

3) Regulation No. 6 implements the federal standards of performance for new stationary sources. This maintenance 

plan makes no changes to this regulation. This reference to Regulation No. 6 shall not be construed to mean that 

these regulations are included in the SIP.

4) The Common Provisions Regulation contains general provisions applicable to all emissions sources in Colorado. 

This maintenance plan makes no changes to this regulation.

The emission inventories for stationary sources supporting the maintenance demonstration have followed all 

relevant EPA rules and guidance documents for calculating such emissions. As an attainment/maintenance area 

since August 14, 2001, the State and federal attainment PSD permitting requirements remain in effect in the 

Telluride area. This program requires the application of Best Available Control Technology when constructing new 

or modified major stationary sources.

32 The Telluride area will rely on the control programs listed below to demonstrate maintenance of the 24-hour 

PM10 standard through 2021. No emission reduction credit has been taken in the attainment/maintenance 

demonstration for any other current State or local control programs and no other such programs, strategies, or 

regulations shall be incorporated or deemed as enforceable measures for the purposes of this maintenance 

demonstration.
20

Examination of available satellite imagery in Telluride has shown very limited construction and soil 

disturbance within the village since 2003, particularly south of the PM10 monitor. The monitor is only 2 

blocks north from the base of the ski mountain, and available land south of the monitor is essentially built 

out. One 1.4 acre project was constructed approximately 1⁄2 mile WSW of the monitor sometime between 

June 2005 and August 2011. Construction to the south of the monitor is unlikely to have played a 

significant role in the April 2013 exceedance.

 

Credit is taken in this attainment/maintenance plan for current federal regulations concerning motor vehicles, fuels, 

small engines, diesels, and non-road mobile sources. While credit is taken for these federal requirements, they are 

not part of the Colorado SIP. 
21

Figure 12: June 2005 and August 2011 GoogleEarth views of Telluride, with one 1.4 acre project

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

Durango, Crested Butte, and Mt. Crested Butte 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. Table 3 summarizes the EPA’s 

evaluation of CDPHE’s nRCP EE demonstration. 
22

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

Exceedance 

Date

Location Reasonable 

Controls 

Demonstration 

Citation

Wind Speed 

Demonstration 

Citation

Sufficient 

Quality of 

Evidence?

EPA 

Criterion 

Met?

April 5, 2010 Alamosa 

Adams State 

College

Section 4.0: 

p.87-88 and 

Section 4.1: 

p.89-95

Section 2: p.11-55, 

esp. p.22-31

Yes Yes

Pagosa Springs 

School

Section 4.0: 

p.87-88 and 

Section 4.2: 

p.96-100

Section 2: p.11-55, 

esp. p.22-31

Yes Yes

Durango River 

City Hall

Section 4.0: 

p.87-88and 

Section 4.3: 

p.101-107

Section 2: p.11-55, 

esp. p.22-31

Yes Yes

Telluride Section 2.0: 

p.40-44

Section 2: p.11-55, 

esp. p.22-31

Yes Yes

Crested Butte Section 2.0: 

p.40-44

Section 2: p.11-55, 

esp. p.22-31

Yes Yes

Mt. Crested 

Butte

Section 2.0: 

p.40-44

Section 2: p.11-55, 

esp. p.22-31

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 3 to 5 years. To 

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

(Alamosa Adams State College), 2,232 samples (Pagosa Springs), 811 samples (Durango) of daily 24

hour averages. CDPHE did not provide extensive analysis of the Telluride, Crested Butte, and Mt. 

Crested Butte monitors because the state believes that the attainment status of the sites will not be 

affected by the April 5, 2010 exceedances33. The EPA is interested in the historical fluctuations of the 

Telluride monitor since Telluride’s 10-year maintenance plan is currently under review by EPA.

Therefore, EPA is including a brief analysis here of the historical fluctuations at the monitor. 

The Interim Guidance for High Wind Exceptional Events asks for a times series concentration for the 

event and percentile rank of the concentration relative to annual and seasonal data. Figure 12 following is 

a time series concentration of the Telluride monitor from 2000 to 2012. There was a single exceedance of 

the NAAQS, and it occurred on April 5, 2010. With 1,380 samples collected in 2000 to 2012, the highest 

value is the equivalent of the 99.93 percentile, but more directly, one exceedance was recorded in this 13 

year period, showing that any exceedance of the 150 ug/m3 NAAQs in Telluride is well outside normal 

historical fluctuations.

 

33 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 56.
23

Figure 13: EPA Time Series Concentration for Telluride from 2000 to 2012

CDPHE explains that concentrations measured on April 5, 2010 were more rare than the 99.6 percentile 

level at the Alamosa Adams State College monitor and were the highest values ever recorded at the 

Pagosa Springs, Telluride, and Durango monitors.34

The EPA “Interim Guidance on High Wind Exceptional Events” recommends that demonstrations also 

consider percentile ranks when dust impacted data is excluded.35 EPA calculates this percentile as more 

rare than 99.68 percentile for the Alamosa monitors. The April 5, 2010 Pagosa Springs, Durango, and 

Alamosa values rank 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 April 5, 2010 

are extremely rare in the absence of high wind dust exceptional event impacts, and that it is unlikely that 

any sources of PM10 were involved in the April 5, 2010 exceedances other than wind-blown dust. Table 4 

summarizes the EPA’s evaluation of CDPHE’s HF EE demonstration. 

 

34 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 58.

35 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 20.
24

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

Exceedance 

Date

Exceedance 

Location

Historical Fluctuations 

Demonstration 

Citation

Sufficient Quality 

of Evidence?

EPA Criterion 

Met?

April 5, 2010 Alamosa Adams 

State College Section 3: p.56-59, 59-

62

Yes Yes

Pagosa Springs 

Section 3: p.56-59, 62-

65

Yes Yes

Durango 

Section 3: p.56-59, 65-

66

Yes Yes

Telluride

Section 3: p.56-59

Yes36 Yes

Crested Butte

Section 3: p.56-59

No37 No

Mt. Crested Butte

Section 3: p.56-59

No38 No

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.39

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 NOAA;

Upper air sounding analysis from the University of Wyoming;

 

36 As supplemented by EPA’s analysis. 

37 According to 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, pages 48-49, 

the state needed to include (but did not) the following analysis for sufficient HF quality of evidence:

1) A time series for concentration for the event area for the previous three years; 

2) Percentile rank of concentration relative to annual data; and

3) Percentile rank of concentration relative to seasonal data.

38 See footnote above.

39 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 pages 21-22.
25

Maps of real-time weather data from 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;

Maps establishing the lack of regional precipitation 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. 

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. 

CDPHE’s documentation also provides dust-on-snow data from the Center for Snow and Avalanche 

Studies, an organization that studies the effects of wind-blown desert dust from Arizona, New Mexico, 

and Utah on snowpack albedo and snowmelt in the San Juan Mountains of Colorado. Figure 29 of 

CDPHE’s documentation shows that April 5, 2010, was one of nine Dust-on-Snow events for the 

2009/2010 water year.40 This information helps to confirm that a regional blowing dust event with longrange transport caused the PM10 exceedances measured across portions of Colorado on April 5, 2010.

The U.S. Geological Survey (USGS) Southwest Geographic Science Team’s dust monitoring web pages 

list April 5, 2010 as a Dust Event day with the following characterization:

Another frontal system advanced into the interior from the Pacific on Monday April 5th, 2010 

bringing strong winds and gusts in excess of 50 mph. The dominant wind direction was out of the 

south-west. There was no apparent dust activity visible from satellite until mid-afternoon and 

only then in the 4 corners region of the southwest. From the images... it is apparent that yet again 

the regions just north of the Little Colorado River on the Navajo Nation were the predominant 

dust sources for this event. We noted an ancillary dust source in New Mexico on the eastern edge 

of the Navajo Nation. Both Thermal Differencing and Visible images show atmospheric dust 

moving well into southern Colorado.

41

Also included are high-wind blowing dust accounts from the Crestone Weather Center, the Durango 

Herald, the Arizona Daily Sun, The Denver Post, and the local NBC news affiliate in Grand Junction that 

describe low visibility, high winds, wind damage, and dust.42

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”.43 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 

 

40 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 44-45.

41 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 45-46.

42 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, pages 74-86.

43 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 12.
26

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”.44 Table 5 

summarizes the EPA’s evaluation of CDPHE’s CCR EE demonstration. 

Table 5: Evaluation of CDPHE’s Technical Support Documentation of CCR45

Clear Causal Relationship Demonstration 

Citation 

Sufficient Quality of Evidence? EPA Criterion 

Met?

Section 2: p.11-55, Figures 1-40 Yes Yes

Section 2: p.11-12, 22-31, Tables 1-10 Yes Yes

Section 2: p.44-46 Yes Yes

Section 4: p.74-86 Yes Yes

Affects Air Quality (AAQ)

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

events to have affected air quality if the CCR and HF requirements have been adequately demonstrated. 

CDPHE states that due to the information presented in the HF and CCR demonstrations on pages 17 and 

18-19 of this TSD, respectively, and cited in Table 6 below, “statistical and meteorological data clearly 

shows that but for this high wind blowing dust event, Alamosa, Pagosa Springs, and Durango would not 

have exceeded the 24-hour NAAQS on April 5, 2010”.46 EPA concludes this was also true for Telluride. 

Table 6 summarizes the EPA’s evaluation of CDPHE’s AAQ EE demonstration.

 

44 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 87.

45 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.

46 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 108.
27

Table 6: Evaluation of CDPHE’s Technical Support Documentation of AAQ

HF or CCR Exceedance Location Affects Air Quality 

Demonstration Citation 

Sufficient 

Quality of 

Evidence?

EPA 

Criterion 

Met?

HF Alamosa Adams State 

College Section 3: p.56-59, 59-62

Yes Yes

Pagosa Springs 

Section 3: p.56-59, 62-65

Yes Yes

Durango 

Section 3: p.56-59, 65-66

Yes Yes

Telluride

Section 3: p.56-59

Yes Yes

Crested Butte

Section 3: p.56-59

No47 No

Mt. Crested Butte

Section 3: p.56-59

No48 No

CCR All Monitors Section 2: p.11-55, Figures 1-

40

Yes Yes

Section 2: p.11-12, 22-31, 

Tables 1-10

Yes Yes

Section 2: p.44-46 Yes Yes

Section 4: p.74-86 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, “There has never been an exceedance that was not associated with high 

winds carrying PM10 dust from distant sources in these six areas since at least 2005”49, 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 and, more specifically, a significant natural dust storm 

originating in northeastern Arizona and northwestern New Mexico and spreading into 

southwestern and south-central Colorado but for the dust storm on April 5, 2010, this exceedance 

would not have occurred.50

Table 7 summarizes the EPA’s evaluation of CDPHE’s Natural Event EE demonstration.

 

47 See HF section in this document, especially footnotes to Table 4. 

48 See HF section in this document, especially footnotes to Table 4.

49 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 108.

50 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 109.
28

Table 7: Evaluation of CDPHE’s Technical Support Documentation of Natural Event

nRCP or CCR Exceedance 

Location

Natural Event 

Demonstration Citation 

Sufficient Quality 

of Evidence?

EPA 

Criterion 

Met?

nRCPAnthropogenicAlamosa Adams 

State College

Section 4.0: p.87-88 and 

Section 4.1: p.89-95

Yes Yes

Pagosa Springs 

School

Section 4.0: p.87-88 and 

Section 4.2: p.96-100

Yes Yes

Durango River 

City Hall

Section 4.0: p.87-88and 

Section 4.3: p.101-107

Yes Yes

Telluride Section 2.0: p.40-44 Yes Yes

Crested Butte Section 2.0: p.40-44 Yes Yes

Mt. Crested Butte Section 2.0: p.40-44 Yes Yes

nRCP-Wind Speed Alamosa Adams 

State College

Section 2: p.11-55, esp. 

p.22-31

Yes Yes

Pagosa Springs 

School

Section 2: p.11-55, esp. 

p.22-31

Yes Yes

Durango River 

City Hall

Section 2: p.11-55, esp. 

p.22-31

Yes Yes

Telluride Section 2: p.11-55, esp. 

p.22-31

Yes Yes

Crested Butte Section 2: p.11-55, esp. 

p.22-31

Yes Yes

Mt. Crested Butte Section 2: p.11-55, esp. 

p.22-31

Yes Yes

CCR All Monitors Section 2: p.11-55, 

Figures 1-40

Yes Yes

Section 2: p.11-12, 22-

31, Tables 1-10

Yes Yes

Section 2: p.44-46 Yes Yes

Section 4: p.74-86 Yes Yes

No Exceedance or Violation But For the Event 

Generally, the No Exceedance or Violation 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: 

The PM10 exceedances in Durango, Telluride, Pagosa Springs, Crested Butte, Mount Crested 

Butte and Alamosa on April 5, 2010, would not have occurred if not for the following: (a) dry soil 

conditions over northeastern Arizona, most of Utah, and parts of western Colorado with 30-day 

precipitation totals below the threshold identified as a precondition for blowing dust in 

northeastern Arizona; (b) a strong surface and upper-level low pressure system that caused 

widespread strong gusty winds through a deep layer of the atmosphere over the area of concern; 
29

and (c) friction velocities over the desert regions of northwest New Mexico, Utah, and Arizona 

that were high enough to allow entrainment of dust from natural sources with subsequent 

transport of the dust to Colorado in strong, southwesterly winds. These winds were the result of a 

strong short wave in the upper atmosphere that was moving across the Great Basin and the 

associated surface cold front and low pressure system.51

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 8 summarizes the EPA’s evaluation of CDPHE’s NEBF EE 

demonstration.

Table 8: 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?

April 5, 2010 Section 6: p.108-109 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 9 outlines EPA’s 

evaluation of these requirements. 

Table 9: 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)(l)(i) Section 1:p.9 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.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.10 Yes

 

51 “Technical Support Document for the April 5, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” 

May 17, 2013, page 108.
30

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 Telluride, Crested Butte, Mt. Crested Butte, Pagosa Springs, Alamosa, and 

Durango areas on April 5, 2010. These high wind events caused exceedances of the 24-hour PM10 

NAAQS at the locations identified in Table 1 of this document on April 5, 2010. EPA concurs that the 

flagged exceedances at the Alamosa, Pagosa Springs, Telluride, and Durango locations on these days 

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, Alamosa, Telluride, 

and Durango areas and subsequently overwhelmed reasonable controls within Alamosa, Pagosa Springs, 

Telluride, and Durango. 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 NEBF, and the measured exceedances are 

in excess of normal HFs. 

EPA will not take action at this time on the flagged exceedances at the Crested Butte and Mt. Crested 

Butte locations due to CDPHE’s decision not to submit an analysis of HF at the locations and at present 

the data for these monitors are not expected to influence any anticipated EPA rules. The exceedances are 

nRCP, and meet the definition of a natural event, but the AAQ is dependent upon sufficient evidence to 

determine HF and CCR. The EPA is interested in the historical fluctuations of the Telluride monitor since 

Telluride’s 10-year maintenance plan is currently under review by EPA. Due to the large number of EE 

demonstration packages that EPA Region 8 receives only those that occur at monitors which have a 

potential bearing on regulatory activities are being reviewed at this time. 

40 CFR 50.14(a)(1) states:

A State may request EPA to exclude data showing exceedances or violations of the national 

ambient air quality standard that are directly due to an exceptional event from use in 

determinations by demonstrating to EPA’s satisfaction that such event caused a specific air 

pollution concentration at a particular air quality monitoring location.52

 

The monitoring values of 115 ug/m3at Delta, of 75 ug/m3 at Clifton, of 70 ug/m3 at Aspen, and 51 ug/m3

at the Grand Junction Powell monitors on April 5, 2010 are not exceedances or violations of the NAAQS 

and therefore not able to be considered under the EER. CDPHE included these values in their report to 

demonstrate the regional nature of the event but did not specifically request their removal. 

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

exceedances of 185 ug/m3at Alamosa Adams State College, 349 ug/m3at Pagosa Springs, 354 ug/m3at 

Telluride, and 320 ug/m3at the Durango monitors on April 5, 2010. These concurrences do not constitute 

 

52 40 CFR 50.14(a)(1).
31

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.