1

EPA Technical Support Document in Review of Colorado Department of 

Public Health and Environment’s Demonstration Package for the 

May 22 & 23, 2010 Alamosa, Pagosa Springs, and Grand Junction

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 20, 2013, of exceptional events demonstrations for five 

exceedances of the 24-hour PM10 standard that occurred at several monitoring stations in southwestern 

 

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

Colorado on May 22 and 23, 2010. Table 1 summarizes the exceedances that CDPHE would like 

considered 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?

May 22, 2010 Alamosa-Adams State College 08-003-0001 260 Yes

Alamosa Municipal Building 08-003-0003 194 Yes

Pagosa Springs School 08-007-0001 187 Yes

May 23, 2010 Grand Junction- Clifton 

Sanitation Building

08-077-0019 189 Yes

Grand Junction- Mesa County 

Powell Building

08-077-0017 155 Yes

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. Grand 

Junction is located on the northeastern corner of the high desert Colorado Plateau. It is situated where the 

Gunnison River meets the Colorado River at an elevation of 4,579 feet, and at the midpoint of the 30 mile 

long and 5 mile wide Grand Valley. To the southwest of Pagosa Springs, Alamosa, and Grand Junction

are the deserts of the New Mexico, Utah, and Arizona Four Corners region. The areas described are 

shown in Figure 1. 
3

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

In its exceptional event demonstration package CDPHE describes the May 22 and 23, 2010 event as: 

the consequence of strong southwesterly prefrontal winds beginning on May 22 and extending 

through May 23 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 storm system 

transported PM10 dust into the southwestern portions of Colorado.2

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.

 

2

“TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, pages 2-3.
4

EPA Figures 2 and 3 from CDPHE’s demonstration3visually demonstrates wind speed and direction of 

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

southeastern Colorado. Figures 1-36 from CDPHE’s demonstration use modeling, meteorology, and 

monitoring analysis to support the evidence of wind direction and reach of the dust storm into Alamosa, 

Pagosa Springs, and Grand Junction. 

Figure 2: Map of Wind Speed on May 22, 2010 (CDPHE’s Figure 8 in the State’s Demonstration)

 

3

Ibid., 17 and 21. 
5

Figure 3: Map of Wind Speed on May 23, 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 can 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.4

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

and voluntary controls in the Alamosa, Pagosa Springs, and Grand Junction 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:

 

4

“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.5

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.6

CDPHE provided the documentation listed in Figures 4, 5, and 6 showing that sustained wind speeds 

associated with these events were above 25 mph (CDPHE tables 14, 15, and 21)

7

. For example, in 

Alamosa sustained surface winds of up to 41 mph with gusts up to 56 mph were measured on May 22, 

2010, while Pagosa Springs saw gusts up to 44 mph. On May 23, 2010 Grand Junction experienced 

sustained winds up to 31 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 May 22 and 23, 2010 clearly exceed the 

established high-wind threshold.

8

 

5

“TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, page 103.

6

Ibid., 103-104.

7

Ibid., 42, 43, and 49.

8

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. EPA 

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

Figure 4: Wind Speed in Alamosa, CO on May 22, 2010 (CDPHE’s Table 14 in the State’s 

Demonstration)
8

Figure 5: Wind Speed in Pagosa Springs, CO on May 22, 2010 (CDPHE’s Table 15 in the State’s 

Demonstration)9

 

9 Local wind speeds were above the gust threshold of 40 mph defined for Colorado for only one hour in Pagosa 

Springs on May 22. Visibility degradation and reports of haze during other hours were highlighted by Colorado in 

the demonstration, and may be indicative of dust blowing into Pagosa Springs from desert areas upwind with higher 

wind speeds. MesoWest data show winds at Winslow, AZ first exceeding 30 mph, for example, between 10:56 and 

11:56 am, MST. 
9

Figure 6: Wind Speed in Grand Junction, CO on May 23, 2010 (CDPHE’s Table 21 in the State’s 

Demonstration)10

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);

 

10 Colorado highlighted both hours where local winds were above the Colorado wind threshold of 30 mph sustained 

or 40 mph gusts, as well as periods with reported haze or limited visibility. The fact that poorest visibility is not 

coincident with peak local winds is supportive evidence for remote rather than local dust sources having significant 

impact. In evaluating nRCP, however, exceedance level PM at wind speeds lower than those expected to overwhelm 

implemented reasonable controls can be evidence of ineffective implementation or enforcement. The need for 

implementation evidence is discussed later in this TSD. 
10

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;

Naval Advanced Aerosol Prediction System (NAAPS) forecasted dust concentrations;

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

GOES-11 Longwave Infrared Difference images;

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. 11

Alamosa has the following controls in place:

Natural Events Action Plan (NEAP)12 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;

13

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.14

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 

to prevent wind erosion in the area.15

 

11 “TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, pages 103-104.

12 Ibid., Appendix C.

13 “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. 

14 “TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, page 105.

15 Ibid., 107.
11

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 Figure 7 (Sites B, E, and H) are naturally vegetated 

and satellite images show minimal soil disturbance.16

Figure 7: 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;

17

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. Figure 8 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 and 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;18 it is also an area where off-road 

 

16 Ibid., 108-110.

17 Ibid., 113-117.

18 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.
12

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

turf and stabilized clay baseball field since April 2009.19

Figure 8: 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 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]20 and Pagosa Springs areas. No additional control measures are needed in these areas to 

demonstrate long-term maintenance of the PM-10 NAAQS”.21 The dust control strategies described in 

bullet three for Pagosa Springs local controls are voluntary measures and are not intended to be federally 

enforceable elements of the SIP or maintenance plan. 

 

15“TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, pages 115-117.

20 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.

21 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, 66 FR 32556. 
13

Grand Junction local controls include22:

County Dust Control rules and ordinances found in Figure 11 (CDPHE’s table 32) following23; 

City Dust Control rules and ordinances found in Figure 12 (CDPHE’s table 33) following24;

Guidance from the Mesa County Air Quality Planning Committee made up of representatives 

from government, industry, education, medical, and legal sectors;

Mesa County Solid Waste Management’s composting project to provide an alternative to open 

burning;

Controlling road dust through municipal street sweeping as well as de-icing using a product 

composed of magnesium chloride, potassium chloride, and rust inhibitors as an alternative to 

spreading sand on roads;

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

in Grand Junction Clifton. Figure 9 shows that identified areas of soil disturbance have 

reasonable controls near the Grand Junction Clifton monitor. Site A is owned by the sanitation 

district and is vegetated. Site D is partially maintained for agriculture, and the remainder is 

naturally vegetated private property. Site E is zoned for agriculture and is vegetated. Sites G, H, 

and I are irrigated and vegetated agricultural land that is privately owned; 

Vegetation and dust control measures are discussed for the following parcels in Figure 9: Site B is 

owned by Halliburton and is partially landscaped and irrigated, graded about once per year as 

needed, treated with magnesium chloride every 3-5 years, with the un-vegetated area under 

contract to have the dust controlled by a third party with a watering truck that visits up to twice 

daily to water the lot depending on conditions. Site C is a gravel pit with a comprehensive 

Particulate Emissions Control Plan, is subject to Colorado’s Regulation 1 Section III.D, and has 

had a Conditional Use Permit “APCD Construction Permit #87ME343F”.25 Site F is a gravel pit 

and with a comprehensive Particulate Emissions Control Plan, is subject to Colorado’s 

Regulation 1 Section III.D, and has had a Conditional Use Permit “APCD Construction Permit 

#86ME024F”.26 The Interim Guidance for High Wind Exceptional Events, page 10, requires for 

demonstration of nRCP a showing that reasonable controls were in place, effectively 

implemented and enforced, and that wind speeds were high enough to overwhelm the controls. 

Since winds were only moderately above the Colorado threshold in Grand Junction, and some 

identified controls are sporadic or “as needed,” additional demonstration material on effective 

implementation and enforcement at the time of the event is needed at sites B, C, and F; 

 

22 “TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, pages 119-122.

23 Ibid., 118-119.

24 Ibid., 120-121.

25 Ibid., 124.

26 Ibid., 125-126.
14

Figure 9: Grand Junction Clifton Site Assessment (CDPHE’s Figure 67 in the State’s 

Demonstration)

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

in Grand Junction Powell. Figure 10 shows that identified areas of soil disturbance have 

reasonable controls near the Grand Junction Powell monitor. Site A is owned by Union Pacific 

Railroad Co. and controls dust through limiting vehicles to 5 mph, covering all dirt roads with 

gravel in 2010, and is undergoing paving projects, the remainder being naturally vegetated or 

undisturbed compacted soil. Site C is naturally vegetated park land. Sites E and F are the city’s 

municipal cemetery with fencing to control access and is maintained. No major disturbances 

occurred on this soil until 2012 (well after the EE) when it became a soil source for trail 

maintenance. The disturbance has since been re-vegetated; and

Vegetation and dust control measures are discussed for the following parcels in Figure 10: Site B 

is owned by IPE Holdings, LLC and is fenced to prevent dust from off road vehicles, has been 

covered by a layer of crushed limestone over top soil to control dust, has been compacted, and 

treated with magnesium chloride. Site D is a combination of public open space for non-motorized 

recreational use and a landscaped and maintained subdivision. The Interim Guidance for High 

Wind Exceptional Events, page 10, requires for demonstration of nRCP a showing that 

reasonable controls were in place, effectively implemented and enforced, and that wind speeds 

were high enough to overwhelm the controls. Since winds were only moderately above the 

Colorado threshold in Grand Junction, and some identified controls are sporadic or “as needed,” 

additional demonstration material on effective implementation and enforcement at the time of the 

event is needed at sites B and D. 
15

Figure 10: Grand Junction Powell Site Assessment (CDPHE’s Figure 75 in the State’s Demonstration)
16

Figure 11: Mesa County Dust Control Rules and Ordinances (CDPHE’s Table 32 in the State’s 

Demonstration)
17

Figure 12: City of Grand Junction Dust Control Rules and Ordinances (CDPHE’s Table 33 in the State’s 

Demonstration)
18

Sections 2 and 5 of CDPHE’s analysis demonstrate that monitors in Grand Junction, 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 in Pagosa 

Springs and Alamosa. Based on the information provided in the demonstration, EPA cannot determine 
19

that sites B, C, and F near the Grand Junction Clifton monitor and sites B and D near the Grand Junction 

Powell monitor satisfy the expectation that, “reasonable controls have been effectively implemented and 

enforced.”27 CDPHE confirmed on August 20, 2013, that references to April 5, 2010 in Section 5 on 

pages 103, 110, 116, and 117 of the state’s demonstration were typos due to the inadvertent reuse of 

language from a previous demonstration, and were intended to read as May 22 and 23, 2010. Table 2

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

Table 2: 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?

May 22, 2010 AlamosaAdams State 

College

Section 5.0: 

p.103-104 and 

Section 5.1: 

p.105-112

Section 2: p.12-67, 

esp. p.42

Yes Yes

Alamosa 

Municipal 

Building

Section 5.0: 

p.103-104 and 

Section 5.1: 

p.105-112

Section 2: p.12-67, 

esp. p.42

Yes Yes

Pagosa Springs 

School

Section 5.0: 

p.103-104 and 

Section 5.2: 

p.113-117

Section 2: p.12-67, 

esp. p.43

Yes Yes

May 23, 2010 Grand 

JunctionClifton 

Sanitation 

Building

Section 5.0: 

p.103-104 and 

Section 5.3: 

p.118-130

Section 2: p.12-67, 

esp. p.47

No No

Grand 

Junction- Mesa 

County Powell 

Building

Section 5.0: 

p.103-104 and 

Section 5.3: 

p.118-130

Section 2: p.12-67, 

esp. p.47

No No

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), 2,168 samples (Alamosa- Municipal Building), 2,287 samples (Pagosa 

Springs), 790 samples (Grand Junction Powell), and 507 samples (Grand Junction Clifton) of 24-hour 

averages. CDPHE explains that concentrations measured on May 22, 2010 were more rare than the 99.6 

percentile level at the Alamosa Adams State College monitor and the Alamosa Municipal Building 

 

27 “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air Quality 

Data Affected by High Winds Under the Exceptional Event Rule” of May 2012, page 10.
20

monitor, and were more rare than the 99.7 percentile at the Pagosa Springs School monitor.28 The 

concentrations measured on May 23, 2010 were more rare than the 99.8 percentile at the Grand Junction 

Powell monitor and were the maximum calculated value at the Grand Junction Clifton monitor.29 

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

consider percentile ranks when dust impacted data is excluded.30 The May 22, 2010 Alamosa and Pagosa 

Springs, and May 23, 2010 Grand Junction 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 May 22 and 23, 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 May 22 and 23, 2010 

exceedances other than wind-blown dust. Table 3 summarizes the EPA’s evaluation of CDPHE’s HF EE 

demonstration. 

Table 3: 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?

May 22, 2010 Adams State 

College Section 3: p.68-73

Yes Yes

Alamosa Municipal 

Building Section 3: p.68-70, 74-77

Yes Yes

Pagosa Springs 

School Section 3: p.68-70, 77-80

Yes Yes

May 23, 2010 Grand Junction 

Powell Section 3: p.68-70, 80-83

Yes Yes

Grand Junction 

Clifton Section 3: p.68-70, 84-86

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:

 

28 “TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, page 69, table 23.

29 Ibid., 69, table 24.

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.

31 Ibid., 21-22.
21

Statewide PM10 monitoring maps;

Surface and upper air wind speed analysis from NOAA;

Wind speed maps from the North American Regional Analysis (NARR);

Maps of real-time weather data from NCAR;

Regional wind and weather by hour through MesoWest;

NOAA HYSPLIT modeling of wind trajectories;

Navy Aerosol Analysis and Prediction System (NAAPS) forecasted dust concentrations; 

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 28 of 

CDPHE’s documentation shows that May 22, 2010, was one of nine Dust-on-Snow events for the 

2009/2010 water year.32 This information helps to confirm that a regional blowing dust event with longrange transport caused the PM10 exceedances measured across portions of Colorado beginning on May 22, 

2010.

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

list May 22 and 23, 2010 as Dust Event days with the following characterization, “The first of a two day 

long change in pressure systems brought consistently high winds to the Four Corners Region again this 

year. The resulting significant dust emissions from the Navajo Reservation reached well into southern 

Colorado and Utah each day.”

33

The Smoke Text Product from NOAA’s Satellite Services Division characterized May 22, 2010 with the 

following characterization:

A large area of blowing dust has been kicked up due to strong winds associated with a potent 

storm system. The blowing dust is emanating from northeast Arizona, northwest New Mexico 

and southwest/central Colorado. The dust is moving quickly to the northeast and extends from 

northeast Arizona, extreme southeast Utah and northwest New Mexico across Colorado and into 

the Nebraska panhandle.34

 

32 “TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, page 59.

33 Ibid., 63.

34 Ibid., 63.
22

Also included are high-wind blowing dust accounts from the Fox21 News, the Denver Post, Grand 

Junction NBC11 news, and the Community Collaborative Rain, Hail & Snow Network.

35

CDPHE generally summarizes that “the 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 west-central and southwest Colorado”.36 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”.37 Table 4 summarizes the EPA’s evaluation of CDPHE’s CCR EE 

demonstration. 

Table 4: EPA’s Analysis of CDPHE’s Technical Support Documentation of CCR38

Clear Causal Relationship Demonstration 

Citation 

Sufficient Quality of Evidence? EPA Criterion 

Met?

Section 2: p.12-67, Figures 1-36 Yes Yes

Section 2: p.12, 28-49, Tables 1-21 Yes Yes

Section 2: p.58-60, 63 Yes Yes

Section 4: p.91-102 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 5, “statistical and meteorological data clearly 

shows that but for this high wind blowing dust event, Alamosa, Pagosa Springs, and Grand Junction

would not have exceeded the 24-hour NAAQS on May 22 and 23, 2010”.39 Table 5 summarizes the 

EPA’s evaluation of CDPHE’s AAQ EE demonstration.

 

35 Ibid., 91-102.

36 Ibid., 12.

37 Ibid., 103.

38 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.

39 “TSD for the May 22 and 23, 2010, Alamosa, Pagosa Springs, and Grand Junction Exceptional Event,” May 20, 

2013, page 131.
23

Table 5: 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.68-73

Yes Yes

Alamosa Municipal 

Building Section 3: p.68-70, 74-77

Yes Yes

Pagosa Springs School

Section 3: p.68-70, 77-80

Yes Yes

Grand Junction Powell

Section 3: p.68-70, 80-83

Yes Yes

Grand Junction Clifton

Section 3: p.68-70, 84-86

Yes Yes

CCR All Monitors Section 2: p.12-67, Figures 1-

36

Yes Yes

Section 2: p.12, 28-49, Tables 

1-21

Yes Yes

Section 2: p.58-60, 63 Yes Yes

Section 4: p.91-102 Yes Yes

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, “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”40, 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 May 22 and 23, 2010, this 

exceedance would not have occurred.41

Although CDPHE has sufficiently demonstrated the CCR requirements for both May 22 and 23, 2010

events and the nRCP requirements for May 22, 2010 event, the state has not sufficiently demonstrated the 

nRCP requirements for May 23, 2010 event in Grand Junction. Table 6 summarizes the EPA’s evaluation 

of CDPHE’s Natural Event EE demonstration.

 

40 Ibid., 131.

41 Ibid., 132.
24

Table 6: 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.103-104 

and Section 5.1: p.105-

112

Yes Yes

Alamosa 

Municipal 

Building

Section 5.0: p.103-104 

and Section 5.1: p.105-

112

Yes Yes

Pagosa Springs 

School

Section 5.0: p.103-104 

and Section 5.2: p.113-

117

Yes Yes

Grand Junction 

Powell

Section 5.0: p.103-104 

and Section 5.3: p.118-

130

No No

Grand Junction 

Clifton

Section 5.0: p.103-104 

and Section 5.3: p.118-

130

No No

nRCP-Wind Speed Adams State 

College

Section 2: p.12-67, esp. 

p.42

Yes Yes

Alamosa 

Municipal 

Building

Section 2: p.12-67, esp. 

p.42

Yes Yes

Pagosa Springs 

School

Section 2: p.12-67, esp. 

p.43

Yes Yes

Grand Junction 

Powell

Section 2: p.12-67, esp. 

p.47

Yes Yes

Grand Junction 

Clifton

Section 2: p.12-67, esp. 

p.47

Yes Yes

CCR All Monitors Section 2: p.12-67, 

Figures 1-36

Yes Yes

Section 2: p.12, 28-49, 

Tables 1-21

Yes Yes

Section 2: p.58-60, 63 Yes Yes

Section 4: p.91-102 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 Alamosa, Pagosa Springs, and Grand Junction on May 22 and 23, 

2010, would not have occurred if not for the following: (a) dry soil conditions over northeast 
25

Arizona, northwest New Mexico, 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 surface low pressure system and cold front that were associated with a 

strong upper-level trough that caused strong prefrontal surface winds over the area of concern; 

and (c) friction velocities over regions of northern Arizona, northwest New Mexico, and 

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

subsequent transport of the dust to Colorado in strong winds.42

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 7 summarizes the EPA’s evaluation of CDPHE’s NEBF EE demonstration.

Table 7: 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?

May 22, 2010 Section 6: p.108-109 Yes Yes

May 23, 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. 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. 

Table 8: EPA’s Analysis of Schedules and Procedural Criteria

Reference Schedules and 

Procedural 

Criteria 

Demonstration 

Citation

EPA 

Criterion 

Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14(c)(l)(i) Section 1:p.10 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.10 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.11 Yes

 

42 Ibid., 131.
26

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 22, 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 22, 2010, in Pagosa Springs and Alamosa. EPA concurs that the flagged exceedances at the 

Pagosa Springs and Alamosa monitors 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 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 NEBF, and the measured 

exceedances are in excess of normal HFs. 

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

exceedances of 260 ug/m3at Alamosa Adams State College, 194 ug/m3at the Alamosa Municipal 

Building, and 187 ug/m3at the Pagosa Springs monitors on May 22, 2010. 

In Grand Junction, CDPHE included a comprehensive description of the controls at each potential 

anthropogenic source and the wind speed in the area, but did not describe whether the controls were 

implemented on, or immediately prior to May 23, 2010. Specifics describing the date of the last 

application of magnesium chloride, grading of the lots, or that the watering protocols were actually 

followed on the day in question are lacking. Section 3.1.3. of the guidance states that, “Cases where 

relevant control measures were not being fully implemented or properly enforced, but reasonably could 

and should have been, are unlikely to be eligible for data exclusion under the Exceptional Events Rule.”43

EPA welcomes the submittal of further information on those controls employed on or immediately prior 

to May 23, 2010 at these sites that will allow the agency to concur on the flagged data. 

Therefore, the flags on AQS data of 189 ug/m3at Grand Junction Clifton Sanitation Building and 155

ug/m3at the Grand Junction Mesa County Powell Building monitors on May 23, 2010 will not receive 

concurrence or non-concurrence at this time. 

The Alamosa and Pagosa Springs 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.

 

43 “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air Quality 

Data Affected by High Winds Under the Exceptional Event Rule” of May 2012, Section 3.1.3.