1

EPA Technical Support Document in Review of Colorado Department of 

Public Health and Environment’s Demonstration Package for the 

April 28 & 29, 2010 Alamosa, Pagosa Springs, and Durango Exceptional 

Events

EXCEPTIONAL EVENTS RULE REQUIREMENTS 

The Environmental Protection Agency (EPA) promulgated the Exceptional Events Rule 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:

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

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;

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

including background; and

There would have been no exceedance or violation but 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 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, Colorado Department of Public Health and Environment (CDPHE) submitted a 

Technical Support Document (TSD) dated May 17, 2013, of exceptional event demonstrations for five

exceedances and four high readings of the 24-hour PM10 standard that occurred at several monitoring 

stations in southwestern Colorado on April 28 and 29, 2010. Table 1 summarizes the exceedances that 

 

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

CDPHE would like considered for removal from the Air Quality System (AQS) database, table 2 

summarizes the high sample readings that CDPHE would like considered in support of the existence of 

the high wind blowing dust event but not for removal from AQS.

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 28, 2010 Adams State College 08-003-0001 285 Yes

Alamosa Municipal Building 08-003-0003 236 Yes

Pagosa Springs School 08-007-0001 181 Yes

April 29, 2010 Pagosa Springs School 08-007-0001 162 Yes

Durango River City Hall 08-067-0004 226 Yes

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

High Sample 

Date

Monitor/Site Name AQS ID 24-hour Avg. 

(ug/m3)

CDPHE 

Request to 

Consider for 

AQS Removal?3

April 28, 2010 Mt. Crested Butte 08-051-0007 123 No

April 29, 2010 Adams State College 08-003-0001 92 No

Alamosa Municipal Building 08-003-0003 94 No

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 Cristo

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. To the 

southwest of Pagosa Springs and Durango are the deserts of the New Mexico, Utah, and Arizona Four 

Corners region. The areas are shown in Figure 1. 

 

2

“TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 3.

3

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

request their removal.
3

Figure 1: Map of Monitor Locations with PM10 NAAQS Exceedances

In its exceptional event demonstration package CDPHE describes the April 28 and 29, 2010 event as:

the consequence of strong southwesterly prefrontal surface winds which caused significant 

blowing dust across much of Arizona, northwest New Mexico, southeast Utah and southwest 

Colorado. These winds were the result of a significant surface low pressure and surface cold front 

associated with a major upper-level trough that was moving across the Western United States. 

This single storm system caused blowing dust during the transported afternoon and evening hours 

of April 28 that continued through the morning hours of April 29. It transported PM10 dust into 

the southwestern portion of Colorado.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 causing dust to blow affected air 

quality in this area.

 

4

“TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 2.
4

EPA Figures 2 and 3 from CDPHE’s demonstration5visually demonstrates wind speed and direction of 

blowing dust across northeastern Arizona, southeastern Utah, and northwestern New Mexico into 

southeastern Colorado. 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, and Pagosa Springs.

Figure 2: Map of Wind Speed on April 28, 2010 (CDPHE’s Figure 8 in the State’s Demonstration)

 

5

Ibid., 16 and 19. 
5

Figure 3: Map of Wind Speed on April 29, 2010 (CDPHE’s Figure 14 in the State’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 is also eligible to be considered to be nRCP if:

The anthropogenic sources of dust have reasonable controls in place;

The reasonable controls have been effectively implemented and enforced; and 

The wind speed was high enough to overwhelm the reasonable controls.6

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, and Durango. This 

includes 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:

 

6

“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.
6

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.

7

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.8

CDPHE provided the documentation listed in CPDHE tables 13, 14, 17, 18, 19, and 20 showing that 

sustained wind speeds associate with these events were above 25 mph.

9

For example, in Durango,

sustained surface winds of up to 33 mph with gusts up to 54 mph occurred on April 28, and sustained 

wind speeds of 31 mph with gusts up to 40 mph occurred on April 29. In Alamosa, sustained surface 

winds of up to 41 mph with gusts up to 56 mph occurred on April 28, and sustained wind speeds of 38 

mph with gusts up to 54 mph occurred on April 29. In Pagosa Springs sustained surface winds of up to 31 

mph with gusts up to 44 mph occurred on April 28, and sustained wind speeds of 32 mph with gusts up to 

47 mph occurred on April 29. CDPHE previously established in its Exceptional Event 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,

10 and appendix A of CDPHE’s analysis 

demonstrates a similar situation in Grand Junction. The wind speeds on April 28 and 29, 2010 clearly 

exceed the established high-wind threshold.

11

 

7

“TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 92.

8

Ibid., 92.

9

Ibid., 38-39, 42-45.

10 Ibid., 11.

11 Page 3 of EPA’s Interim Guidance for High Wind Events recommends air agencies establish wind thresholds 

based on local conditions above which reasonable dust controls might be overwhelmed and natural dust sources 

would be active. Had Colorado not been able to establish these thresholds at sustained winds of 30 mph and gusts of 

40 mph, EPA would have accepted a threshold of 25 mph with evidence that it was an appropriate wind level. 

However, EPA has not established 25 mph as a bright line threshold for high wind events. 
7

Figure 4: Wind Speed in Durango, CO on April 28, 2010 (CDPHE’s Table 13 in the State’s 

Demonstration)
8

Figure 5: Wind Speed in Durango, CO on April 29, 2010 (CDPHE’s Table 14 in the State’s 

Demonstration)12

 

12 While wind speed in Durango was only above 30 mph for 1 hour of April 29, haze and reduced visibility data for 

April 28 and 29 shows that regional dust impacts from wind in the region carried over from April 28 into April 29.
9

Figure 6: Wind Speed in Alamosa, CO on April 28, 2010 (CDPHE’s Table 17 in the State’s 

Demonstration)
10

Figure 7: Wind Speed in Alamosa, CO on April 29, 2010 (CDPHE’s Table 18 in the State’s 

Demonstration)
11

Figure 8: Wind Speed in Pagosa Springs, CO on April 28, 2010 (CDPHE’s Table 19 in the State’s 

Demonstration)
12

Figure 9: Wind Speed in Pagosa Springs, CO on April 29, 2010 (CDPHE’s Table 20 in the State’s 

Demonstration)
13

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;

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 Regulations 1, 3, 6, and 9.13

Alamosa has the following controls in place:

Natural Events Action Plan (NEAP)14 for high wind events, 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;

15

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.16

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 is working with the Natural Resources Conservation Service to 

 

13 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 92.

14 Ibid., Appendix C.

15 “EPA’s Natural Events Policy guidance requires the state to periodically reevaluate: 1) the conditions causing 

violations of the PM10 NAAQS in the area, 2) the status of implementation of the NEAP, and 3) the adequacy of the 

actions being implemented. The State will reevaluate the NEAP for Alamosa at a minimum of every 5 years and 

make appropriate changes to the plan accordingly.” TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and 

Durango Exceptional Event, May 17, 2013, Appendix C, page 18. 

16 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 94.
14

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

prevent wind erosion in the area.17

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 sites without confirmed irrigation in Figure 10 (Sites B, E, and H) are 

naturally vegetated, and satellite images show minimal soil disturbance.18

Figure 10: Alamosa Site Assessment (CDPHE’s Figure 47 in the State’s Demonstration)

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;

19

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. Figure 11 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;20 it is also an area where off-road 

 

17 Ibid., 96.

18 Ibid., 97-100.

19 Ibid., 102-106.

20 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.
15

recreational vehicles are prohibited by the parks department. Site D is an irrigated and maintained 

turf and stabilized clay baseball field since April 2009.21

Figure 11: Pagosa Springs Site Assessment (CDPHE’s Figure 49 in the State’s Demonstration)

On March 16, 2000, CDPHE’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]

22 and Pagosa Springs areas. No additional control measures are needed in these areas to 

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

23 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. 

Durango local controls include:

 

21 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

pages 104-106.

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

Pagosa Springs when removing the nonattainment designation on March 16, 2000.

23 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. 
16

Dust Control rules and ordinances found in Figure 13 (CDPHE’s table 30),

24

The City of 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;

25

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, 200726; and 

Vegetation, signage, and other dust control measures are in place for undeveloped parcels of land 

in town. Figure 12 shows identified areas of soil disturbance that 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 off-road 

recreational vehicles are prohibited by the parks department.

27

Figure 12: Durango Site Assessment (CDPHE’s Figure 64 in the State’s Demonstration)

 

24 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

pages 107-108.

25 Ibid., 108-109.

26 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

27 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

pages 110-113.
17

Figure 13: Durango and La Plata County Dust Control Rules and Ordinances (CDPHE’s Table 30 in the 

State’s Demonstration)
18

Sections 2 and 5 of CDPHE’s analysis 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 nRCP. Potential anthropogenic sources nearer 

the monitors were shown to have reasonable dust control in place. Table 3 summarizes the EPA’s 

evaluation of CDPHE’s nRCP EE demonstration. 
19

Table 3: EPA’s Analysis 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 28, 2010 Adams State 

College

Section 5.0: 

p.92-93 and 

Section 5.1: 

p.94-101

Section 2: p.11-61, 

esp. p.42-42

Yes Yes

Alamosa 

Municipal 

Building

Section 5.0: 

p.92-93 and 

Section 5.1: 

p.94-101

Section 2: p.11-61, 

esp. p.42-42

Yes Yes

Pagosa Springs 

School

Section 5.0: 

p.92-93 and 

Section 5.2: 

p.102-106

Section 2: p.11-61, 

esp. p.44-45

Yes Yes

April 29, 2010 Pagosa Springs 

School

Section 5.0: 

p.92-93 and 

Section 5.2: 

p.102-106

Section 2: p.11-61, 

esp. p.44-45

Yes Yes

Durango River 

City Hall

Section 5.0: 

p.92-93 and 

Section 5.3: 

p.107-113

Section 2: p.11-61, 

esp. p.38-39

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, CDPHE provided 7-year time series plots of 2,214 samples 

(Alamosa- Adams State College), 2,214 samples (Alamosa- Municipal Building), 2,287 samples (Pagosa

Springs), and 811 samples (Durango) of 24-hour averages. CDPHE explains that concentrations measured 

on April 28, 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 Alamosa Municipal Building monitor and the 

Pagosa Springs School monitor.28 The concentrations measured on April 29, 2010 were more rare than the 

99 percentile at the Pagosa Springs monitor and the 99.9 percentile at the Durango monitor.29 

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

consider the percentile ranks of high-wind blowing-dust data.30 The April 28 and 29, 2010 Pagosa Springs 

and April 29, 2010 Durango values fit into the range of 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 28 and 29, 2010 are extremely rare in the absence of high wind dust exceptional event 

 

28 Ibid., 63.

29 Ibid., 63, 69.

30 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.
20

impacts, and that it is unlikely that any sources of PM10 were involved in the April 28 and 29, 2010

exceedances other than wind-blown dust. Table 4 summarizes the EPA’s evaluation of CDPHE’s HF EE 

demonstration. 

Table 4: EPA’s Analysis of CDPHE’s Technical Support Documentation of HF

Exceedance 

Date

Exceedance 

Location

Historical Fluctuations 

Demonstration 

Citation

Sufficient Quality 

of Evidence?

EPA Criterion 

Met?

April 28, 2010 Adams State 

College Section 3: p.63-65

Yes Yes

Alamosa Municipal 

Building Section 3: p.63, 67

Yes Yes

Pagosa Springs 

School Section 3: p.63, 69-70

Yes Yes

April 29, 2010 Pagosa Springs 

School Section 3: p.63, 69-70

Yes Yes

Durango River City 

Hall Section 3: p.63, 72

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.31

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;

Maps of real-time weather data from NCAR;

Regional wind and weather by hour through MesoWest;

NOAA HYSPLIT modeling of wind trajectories;

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. 

 

31 Ibid., 21-22.
21

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 analysis, Section 2, Figures 29, 30, and 31 show a MODIS Terra satellite image of the blowing 

dust originating in Arizona.32 Figures 20-26 show NOAA HYSPLIT Model 18-hour forward trajectories 

and 12-hour backward trajectories that demonstrate transport of air from northeast Arizona, northwest 

New Mexico, and southeast Utah into Colorado on the dates in question.33

Confirming the April 28 and 29, 2010 high wind dust event, the Center for Snow and Avalanche Studies, 

an organization that has been studying 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, “lists April 28, 

2010, as one of nine Dust-on-Snow events for the 2009/2010 water year”.

34

Further confirmation from NOAA’s Satellite Services Division Text Narrative for Smoke/Dust Observed 

in Satellite Imagery describes the high wind dust event as, “A significant area of blowing dust is being 

picked up across parts of NE Arizona and NW New Mexico and moving ENE across a large section of 

Colorado and beginning to reach parts of W Nebraska and Kansas. There are very high winds across the 

region.”35

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

list April 28 and 29, 2010 as dust events with the following characterization:

A strong pacific storm front moved through the southwest today, bringing very strong winds cold 

temperatures and snow in the higher elevations. 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 mid morning on to well after sunset. Interstate 

40 in Arizona was closed from Belmont to Winslow for most of the day.36

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.37

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”.38 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 

 

32 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 56-58.

33 Ibid., 47-53.

34 Ibid., 54-55.

35 Ibid., 56.

36 Ibid., 59.

37 Ibid., 82-91.

38 Ibid., 11.
22

unnoticeable as compared to the dust transported in from the sources regions of the dust storm”.39 Table 5 

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

Table 5: EPA’s Analysis of CDPHE’s Technical Support Documentation of CCR40

Clear Causal Relationship Demonstration 

Citation 

Sufficient Quality of Evidence? EPA Criterion 

Met?

Section 2: p.11-61, Figures 1-35 Yes Yes

Section 2: p.11, 26-45, Tables 1-20 Yes Yes

Section 2: p.54-56 Yes Yes

Section 4: p.82-91 Yes Yes

Affects Air Quality (AAQ)

As stated in EPA’s 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 19 and 

20-21 of this (EPA’s) TSD, respectively, and cited in Table 6, “data and meteorological analysis 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 28 and 29, 2010”.

41 Table 6 summarizes the EPA’s 

evaluation of CDPHE’s AAQ EE demonstration.

Table 6: EPA’s Analysis of CDPHE’s Technical Support Documentation of AAQ

HF and

CCR

Exceedance Location Affects Air Quality 

Demonstration Citation 

Sufficient 

Quality of 

Evidence?

EPA 

Criterion 

Met?

HF Adams State College

Section 3: p.63-65

Yes Yes

Alamosa Municipal 

Building Section 3: p.63, 67

Yes Yes

Pagosa Springs School

Section 3: p.63, 69-70

Yes Yes

Durango River City 

Hall Section 3: p.63, 72

Yes Yes

CCR All Monitors Section 2: p.11-61, Figures 1-

35

Yes Yes

Section 2: p.11, 26-45, Tables 

1-20

Yes Yes

Section 2: p.54-56 Yes Yes

Section 4: p.82-91 Yes Yes

 

39 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 92.

40 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.

41 “TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, 

page 114.
23

Natural Event

As stated in EPA’s 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”42 and that “the 

prevalence of winds above blowing dust thresholds prove that this dust storm was a natural event that was 

not reasonably controllable or preventable”.

43 Table 7 summarizes the EPA’s evaluation of CDPHE’s 

Natural Event EE demonstration.

Table 7: EPA’s Analysis 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?

nRCPAnthropogenicAdams State 

College

Section 5.0: p.92-93 and 

Section 5.1: p.94-101

Yes Yes

Alamosa Municipal 

Building

Section 5.0: p.92-93 and 

Section 5.1: p.94-101

Yes Yes

Pagosa Springs 

School

Section 5.0: p.92-93 and 

Section 5.2: p.102-106

Yes Yes

Durango River City 

Hall

Section 5.0: p.92-93 and 

Section 5.3: p.107-113

Yes Yes

nRCP-Wind Speed Adams State 

College

Section 2: p.11-61, esp. 

p.42-42

Yes Yes

Alamosa Municipal 

Building

Section 2: p.11-61, esp. 

p.42-42

Yes Yes

Pagosa Springs 

School

Section 2: p.11-61, esp. 

p.44-45

Yes Yes

Durango River City 

Hall

Section 2: p.11-61, esp. 

p.38-39

Yes Yes

CCR All Monitors Section 2: p.11-61, Figures 

1-35

Yes Yes

Section 2: p.11, 26-45, 

Tables 1-20

Yes Yes

Section 2: p.54-56 Yes Yes

Section 4: p.82-91 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:

 

42 Ibid., 114.

43 Ibid., 115.
24

[t]he PM10 exceedances in Alamosa, Pagosa Springs, and Durango on April 28 and 29, 2010, 

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

northeastern Arizona, portions of extreme northwestern New Mexico, and portions of southern 

Colorado with 30-day precipitation totals below the thresholds for blowing dust; (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; and (c) friction velocities over the desert 

regions of New Mexico, Utah, Arizona, and much of Colorado that were high enough to allow 

entrainment of dust from natural sources with subsequent transport of the dust into (or within) 

Colorado in strong, southwesterly winds.44

While not explicitly stated in the documentation, EPA notes 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: EPA’s Analysis 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 28, 2010 Section 6: p.114 Yes Yes

April 29, 2010 Section 6: p.114 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. CDPHE received some 

minor non-substantive external comments during the 30 day comment period and appropriate corrections 

were made to the report. Table 8 outlines EPA’s evaluation of these requirements. 

 

44 Ibid., 114.
25

Table 9: EPA’s Analysis of 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.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

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, Durango, and Alamosa areas on April 28 and 29, 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 28 and 29, 2010. EPA has determined that the flagged exceedances at these 

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

Springs, 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. 

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.45

 

 

45 40 CFR 50.14(a)(1).
26

The monitoring values of 123 ug/m3at Mt. Crested Butte on April 28, 2010, and the values of 92 ug/m3 at 

Adams State College, and 94 ug/m3at the Alamosa Municipal Building monitors on April 29, 2010 are 

not exceedances or violations of the NAAQS and therefore not able to be considered under the 

Exceptional Events Rule. 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 285 ug/m3at Adams State College, 236 ug/m3at the Alamosa Municipal Building, and 

181 ug/m3at the Pagosa Springs School monitors on April 28, 2010, as well as the NAAQS exceedances 

of 162 ug/m3at the Pagosa Springs School Monitor and 226 ug/m3at the Durango River City Hall 

monitors on April 29, 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.