1 

EPA Technical Support Document in Review of Colorado Department of 

Public Health and Environment’s Demonstration Package for the 

March 22, 2009, Pagosa Springs Exceptional Event 

EXCEPTIONAL EVENTS RULE REQUIREMENTS 

The Environmental Protection Agency (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: 

• 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 March 30, 2012, Colorado Department of Public Health and Environment (CDPHE) submitted a 

demonstration (termed a Technical Support Document or TSD) for an exceptional event exceedance of 

the 24-hour PM10 standard that occurred at the monitoring station in Pagosa Springs on March 22, 2009. 

CDPHE supplemented that demonstration with additional information in an addendum dated September 

 

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 

30, 2013. Table 1 summarizes the exceedance that CDPHE would like considered for exceptional event 

concurrence in 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? 

March 22, 2009 Pagosa Springs 08-007-0001 255 Yes 

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

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

The areas described are shown in Figure 1. 

Figure 1: Map of Desert, Four Corners, and Monitor Location with the PM10 NAAQS Exceedance 

In its exceptional event demonstration package CDPHE describes the March 22, 2009 event as: 

the consequence of strong southwesterly wind in combination with dry conditions which caused 

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

Colorado. These winds were partly the result of a developing low pressure over the Great Basin 

and strong winds aloft which mixed down to the surface in the deeply-mixed surface boundary 

layer.... Multiple sources of data and analyses of past dust storms in this area prove that this was 
3 

a natural event and, more specifically, a significant natural dust storm originating in northeastern 

Arizona and northwestern New Mexico.2 

CDPHE provides a comprehensive description and discussion of this overnight event in Sections 2, 3, 4,

5, 6, and 7 of the demonstration. As noted in the figure below, the high wind event caused dust to blow 

and affected air quality in this area. 

EPA Figure 2 from CDPHE’s demonstration3 visually demonstrates blowing dust across northeastern 

Arizona headed towards southwestern Colorado. Figures 1-32 from CDPHE’s demonstration use 

modeling, meteorology, and monitoring analysis to support the evidence of the wind direction and reach

of the dust storm into Pagosa Springs. 

 

2

 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, pages 5 and 6. 

3

 Ibid., 44. 
4 

Figure 2: Image of Blowing Dust on March 22, 2009 (CDPHE’s TSD Figure 30) 

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, 
5 

• 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 Pagosa Springs area, 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: 

There are very few anthropogenic local sources of geologic PM10 sized dust in the area as most of 

the area is covered in vegetation. Also, the anthropogenic sources are well controlled due to a 

State Implementation Plan (SIP) / PM10 Maintenance Plan. See section 5.0 Local Dust Controls 

for a comprehensive list of PM10 control measures employed in Pagosa Springs through the PM10 

SIP / Maintenance Plan.5

CDPHE’s demonstration provides an analysis of the state and local controls in place to control PM10 

emissions. Although specifically focusing on Regulations 1, 3 and 6, 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.6

CDPHE provided the documentation listed in Figures 3 and 4 showing that wind speeds associated with 

these events were above 30 mph for sustained and 40 mph for gusts (CDPHE tables 1 and 10)7. For 

example, 260 miles southwest in Winslow, AZ, near where the storm originated, sustained surface winds 

of up to 48 mph with gusts up to 62 mph were measured on March 22, 2009, while the Sandoval Mesa 

weather monitoring station 20 miles southwest Pagosa Springs saw gusts up to 45 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 TSD). While the sustained winds in Pagosa 

Springs were below this Colorado threshold, gusts around Pagosa Springs briefly were above the 

threshold. Given the satellite evidence of Figure 2 above, transported dust from the upwind desert region 

appears to be a more significant contribution to the exceedance than was local winds overwhelming 

implemented local controls, but, the high local gust speeds around Pagosa Springs would likely be 

capable of overwhelming some local controls. The supplemental addendum provided by CDPHE supports 

this conclusion by describing in detail “Best Available Control Measures (BACM), other reasonable 

control measures, applicable federal, state, and local regulations, appropriate land use management, and 

an in-depth analysis of potential areas of local soil disturbance for the affected community.”8

 

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. 

5

 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, page 48. 

6

 Ibid., 75-76. 

7

 Ibid., 29 and 38. 

8

“Addendum to the Technical Support Document for the March 22, 2009 Pagosa Springs Exceptional Event,” 

September 30, 2013, page 3.
6 

Figure 3: Wind Speed in Sandoval Mesa, CO (20 miles southwest of Pagosa Springs) on March 22, 2009 

(CDPHE’s TSD Table 1) 
7 

Figure 4: Wind Speed in Winslow, AZ on March 22, 2009 (CDPHE’s TSD Table 10) 

Section 2 of CDPHE’s demonstration 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); 

• Meteorological 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, as well as wind direction, 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. 
8 

These analyses demonstrate the existence of a high wind blowing dust event originating far southwest of 

the monitor in question. 

CDPHE’s Addendum to the TSD for the March 22, 2009 Pagosa Springs Exceptional Event and Section 5 

of CDPHE’s March 30, 2012 demonstration provide 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 9.9 

Pagosa Springs local controls include: 

• Land Use Development Code 6.6.3(h) and 6.6.3(m)(i) requiring paved streets, street sanding and 

sweeping/cleaning controls, encourages businesses to maintain cleanliness of private lots;10

• 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; 

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

undeveloped parcels of land in town. Figure 5 below shows that identified areas of soil 

disturbance have reasonable controls. For example, Site A was seeded in 2006 and has since been 

covered by vegetation; 

• Site B was the recent construction of the Springs Resort and Spa and was completed in May 2009 

CDPHE states that: 

The construction project was awarded “Gold Status” (second highest award) for 

complying with the Leadership in Energy & Environmental Design (LEED)1 standards 

for New Construction Version 2.2 ... The Town of Pagosa Springs has provided Building 

Inspection Reports demonstrating that on February 27, 2009, the exterior lath was 

inspected. The building inspection report from April 29, 2009 shows that the hotel was 

approved for temporary occupancy for 60 days. The final building inspection was 

completed on June 18, 2009. From the dates of the Town’s Building inspections it is clear 

that by March 2009, the entire construction site was paved and the building was 

completely constructed; the interior was just being finished;11

• Site C was naturally vegetated and receives groundwater from a nearby stream;12 it is also an area 

where off-road recreational vehicles are prohibited by the parks department; and 

• Site D is an irrigated and maintained turf and stabilized clay baseball field.13 

Figure 5: Pagosa Springs Site Assessment (CDPHE’s Figure 1 in the State’s Addendum to the TSD for 

the March 22, 2009 Pagosa Springs Exceptional Event)

 

9

 “Addendum to the Technical Support Document for the March 22, 2009 Pagosa Springs Exceptional Event,”

September 30, 2013, page 3-4. 

10 Ibid., 5-6. 

11 Ibid., 8.

12 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.

13“Addendum to the Technical Support Document for the March 22, 2009 Pagosa Springs Exceptional Event,” 

September 30, 2013, page 10. 
9 

On March 16, 2000, CDPHE’s SIP was revised and amended by the State (followed by a Notice of Final 

Rule on June 15, 2001) 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]14 and Pagosa Springs areas. No additional control measures are 

needed in these areas to demonstrate long-term maintenance of the PM-10 NAAQS”.15 The dust control 

strategies described in bullets three through six above for Pagosa Springs local controls are voluntary 

measures and are not intended to be federally enforceable elements of the SIP or maintenance plan.

Sections 2 and 5 of CDPHE’s demonstration show that the monitor in Pagosa Springs was affected by 

PM10 transport from outside the area, with the main source areas located to the southwest in Arizona, 

southeastern Utah, and northwestern New Mexico. In addition to transport, the spatial extent of elevated 

PM10 concentrations throughout southwest Colorado and the wind speeds associated with the event 

contribute to EPA’s evaluation of whether this event is nRCP in Pagosa Springs. Table 2 summarizes the

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

 

 

14 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. 

15 State Implementation Plan, Colorado Dept. of Public Health and Environment, Regulation: NonattainmentAttainment/Maintenance Areas, Rule #1, Rule Title: Approval and Promulgation of Air Quality Implementation 

Plans; Colorado; Designation of Areas for Air Quality Planning Purposes, Telluride and Pagosa Springs, Direct 

Final Rule, June 15, 2001, 66 FR 32556. 
10 

Table 2: EPA’s Analysis of CDPHE’s TSD of nRCP 

Exceedance 

Date 

Location Reasonable 

Controls 

Demonstration 

Citation

Wind Speed 

Demonstration 

Citation

Sufficient 

Quality of 

Evidence?

EPA 

Criterion 

Met?

March 22, 

2009 

Pagosa Springs Section 5: p. 75-

76, 

and 

Addendum to the 

TSD For the 

March 22, 2009 

Pagosa Springs 

Exceptional 

Event: p.3-12

Section 2: p.8-46, 

esp. p.29 

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 annual data series plots of 2009 samples 

of 24-hour averages, 5-year seasonal data samples of 24-hour averages for the winters of 2004-2009, and 

monthly data showing that most readings are consistently below the NAAQS threshold.16 CDPHE 

explains that for the concentration measured on March 22, 2009: 

The PM10 sample on March 22, 2009 at Pagosa Springs of 255 ug/m3 exceeds the 99th percentile 

value (107 ug/m3

) for all March data, exceeds the 99th percentile value (138 ug/m3) for all 2009 

data, and exceeds the 99th percentile value (56 ug/m3) for the entire dataset. Overall, this sample is 

the third highest sample in the dataset, the highest sample in 2009, and maximum sample in any 

March, and exceeds the 99th percentile sample value for this site for all samples...There are 6,096 

samples in this dataset.17

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

consider percentile ranks when dust impacted data is excluded.18 Since at least 2005 in Pagosa Springs, 

values exceeding the NAAQS are only found during high wind dust events, including March 22, 2009. 

This information indicates that values at the level seen on March 22, 2009 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 March 22, 2009 exceedance other than wind-blown dust. Table 3 summarizes the EPA’s 

evaluation of CDPHE’s HF EE demonstration. 

 

 

16 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, pages 51,52, and 61. 

17 Ibid., 60. 

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

Quality Data Affected by High Winds Under the Exceptional Events Rule” of May 2013 page 20. 
11 

Table 3: EPA’s Analysis of CDPHE’s TSD of HF 

Exceedance 

Date 

Exceedance 

Location 

Historical Fluctuations 

Demonstration Citation 

Sufficient 

Quality of 

Evidence? 

EPA Criterion 

Met? 

March 22, 2009 Pagosa Springs Section 3: p.47-63 Yes Yes 

Clear Causal Relationship 

EPA considers a variety of evidence when evaluating whether there is a Clear Causal Relationship (CCR)

between the measurement under consideration and the event that is claimed to have affected the air 

quality in the area. Demonstrations should include documentation showing that the event in fact 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.19 

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

• Statewide PM10 monitoring maps; 

• Surface and upper air wind speed analysis from NOAA; 

• Meteorological Sounding Analysis from the University of Wyoming; 

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

• Maps of real-time weather data from NCAR; 

• Regional wind and weather by hour through MesoWest; 

• NOAA HYSPLIT modeling of wind trajectories; 

• NAAPS forecasted dust concentrations; 

• MODIS Terra satellite images demonstrating the 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 the existence of a high wind blowing dust event originating far southwest of 

the monitors in question. 

CDPHE’s demonstration also provides dust-on-snow 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 demonstration shows that the Center for Snow and Avalanche Studies “lists March 22, 2009, as 

one of twelve Dust-on-Snow events for the 2008/2009 water year.”20, 21 This information helps to confirm 

 

19 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. 

20 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, page 43. 

21 Ibid., 41.
12 

that a regional blowing dust event with long-range transport caused the PM10 exceedances measured 

across portions of Colorado beginning on March 22, 2009. 

The Smoke Text Product from NOAA’s Satellite Services Division described March 22, 2009 with the 

following characterization, “Arizona: In Navajo and Apache counties, blowing dust is moving quite fast

into northwest New Mexico, far southeastern Utah, and southwest Colorado.”22

Also included are high-wind blowing dust accounts from the Pitkin County Environmental Manager, 

Crested Butte Town Manager, the Aspen Times, and the Vail Daily.23

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

in combination with dry conditions which caused significant blowing dust across much of northeast 

Arizona, northwest New Mexico, and southwest Colorado”.24 CDPHE further states that “but for the dust 

storm described in detail in this report, there would have been no exceedance on this day in Pagosa 

Springs”.25 Table 4 summarizes the EPA’s evaluation of CDPHE’s CCR EE demonstration. 

Table 4: EPA’s Analysis of CDPHE’s TSD of CCR26

Clear Causal Relationship Demonstration 

Citation 

Sufficient Quality of Evidence? EPA Criterion 

Met? 

Section 2: p.12-46, Figures 1-32 Yes Yes 

Section 2: p.8-9, 28-38, Tables 1-10 Yes Yes 

Section 2: p.41-43, 45 Yes Yes 

Section 4: p.64-74 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. 

The information presented in the HF and CCR demonstrations on the preceding three pages of this 

(EPA’s) TSD are cited in Table 5. Additionally, CDPHE states that: 

The Pagosa Springs concentration exceeds the 99th percentile, and a conservative estimate of the 

dust storm contribution to the total concentration is 199-211 ug/m3

. This is evidence that the 

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

including background.27 

Table 5 summarizes the EPA’s evaluation of CDPHE’s AAQ EE demonstration. 

 

 

22 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, page 45. 

23 Ibid., 64-74. 

24 Ibid., 80. 

25 Ibid., 80. 

26 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. 

27 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, page 80. 
13 

Table 5: EPA’s Analysis of CDPHE’s TSD of AAQ 

HF and 

CCR 

Exceedance Location Affects Air Quality 

Demonstration Citation 

Sufficient 

Quality of 

Evidence 

EPA 

Criterion 

Met? 

HF Pagosa Springs Section 3: p.47-63 Yes Yes

CCR Pagosa Springs 

Section 2: p.12-46, Figures 1-

32 

Yes Yes 

Section 2: p.8-9, 28-38, Tables 

1-10 

Yes Yes 

Section 2: p.41-43, 45 Yes Yes 

Section 4: p.64-74 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 states that: 

Multiple sources of data and analyses of past dust storms in this area prove that this was a natural 

event and, more specifically, a significant natural dust storm originating in northeastern Arizona 

and northwestern New Mexico. But for the dust storm on March 22, 2009, this exceedance would 

not have occurred.28 

CDPHE has sufficiently demonstrated the CCR and nRCP requirements for the March 22, 2009 event. 

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

Table 6: EPA’s Analysis of CDPHE’s TSD of Natural Event 

nRCP or CCR Exceedance 

Location 

Natural Event 

Demonstration Citation 

Sufficient Quality 

of Evidence? 

EPA 

Criterion 

Met? 

nRCPAnthropogenic Pagosa Springs Section 5: p. 75-76, 

and 

Addendum to the TSD 

For the March 22, 2009 

Pagosa Springs 

Exceptional Event: p.3-

12 

Yes Yes 

nRCP-Wind Speed Pagosa Springs Section 2: p.8-46, esp. 

p.29 

Yes Yes 

CCR Pagosa Spings 

Section 2: p.12-46, 

Figures 1-32 

Yes Yes 

Section 2: p.8-9, 28-38, 

Tables 1-10 

Yes Yes 

Section 2: p.41-43, 45 Yes Yes 

Section 4: p.64-74 Yes Yes 

 

28 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, page 80. 
14 

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 demonstration that show 

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

Satellite imagery shows large plumes of southwest to northeast trending blowing dust in the 

Painted Desert and Little Colorado River Valley region of northeastern Arizona on March 22, 

2009. Forward and backward trajectories, wind streamline analyses, and surface and upper-level 

wind patterns show that this dust would have been transported into Colorado on March 22, 2009. 

An analysis run of a high-resolution meteorological model also shows winds flowing across 

northeast Arizona and Northwest New Mexico into Colorado. This analysis indicates that the 

zone of highest speeds targeted the Painted Desert and Little Colorado River Valley areas of 

northeastern Arizona ... to be a significant source region for blowing dust transported into 

Colorado. Multiple sources of data and analyses of past dust storms in this area prove that this 

was a natural event and, more specifically, a significant natural dust storm originating in 

northeastern Arizona and northwestern New Mexico. But for the dust storm on March 22, 2009, 

this exceedance would not have occurred.29

 

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 TSD of NEBF 

Exceedance Date No Exceedance But 

For the Event 

Demonstration 

Citation 

Sufficient Quality of 

Evidence? 

EPA Criterion Met? 

March 22, 2009 Section 7: p.80-81 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 no 

external comments during the 30 day comment period, but minor corrections were made to the report 

based on internal staff review. Table 8 outlines EPA’s evaluation of these requirements. 

 

 

29 “TSD for the March 22, 2009, Pagosa Springs Exceptional Event,” March 30, 2012, page 80. 
15 

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) No real-time 

samplers were in 

use at the time of 

this event 30

Yes 

Were flags and initial description placed 

on the data by July 1st of the following 

year? 

40 CFR 50.14(c)(2)(iii) CDPHE Letter to 

EPA dated March 

30, 201231

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) March 30, 2012 Yes 

Was the public comment process 

followed and documented? 

40 CFR 50.14(c)(3)(v) CDPHE Letter to 

EPA dated March 

30, 201232

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 area on March 22, 2009. This high wind event caused an 

exceedance of the 24-hour PM10 NAAQS at the location identified in Table 1 of this document on 

March 22, 2009, in Pagosa Springs. EPA concurs that the flagged exceedance at the Pagosa Springs 

monitor on this day meets the definition of an exceptional event: the exceedance affected air quality, was 

nRCP, and met the definition of a natural event. Specifically, EPA has determined that the event was 

nRCP due to high wind conditions that transported PM10 from sources outside of the Pagosa Springs area 

and subsequently overwhelmed reasonable controls within 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. 

 

30 According to 40 CFR 50.14(c)(1)(i): “All States and, where applicable, their political subdivisions must notify the

public promptly whenever an event occurs or is reasonably anticipated to occur which may result in the exceedance 

of an applicable air quality standard.” CDPHE’s Addendum to the Technical Support Document explains that the 

event was not captured by real time samplers but rather from filter-based samplers analyzed weeks afterward.

31 CDPHE letter accompanying TSD from Gordon E. Pierce, Program Manager, Technical Services Program, Air

Pollution Control Division to Richard Payton of EPA dated March 30, 2012. 

32 Ibid. 
16 

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

exceedance of 255 ug/m3 the Pagosa Springs monitor on March 22, 2009. 

This concurrence does 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.