EPA Technical Support Document in Review of Colorado Department of 

Public Health and Environment's Demonstration Package for the 

April16, 2013 Telluride 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 U), (k) and (I); 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 ofthe 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 October 3, 2013, Colorado Department of Public Health and Environment (CDPHE) submitted a 

demonstration (termed a Technical Support Document or TSD) dated October I, 2013 for an exceptional 

event exceedance of the 24-hour PM10 standard that occurred at the Telluride monitoring station in 

southwestern Colorado on April 16, 2013. Table I summarizes the exceedance that CDPHE would like 

considered for exceptional event concurrence in the Air Quality System (AQS) database. 

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". 

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

Exceedance Monitor/Site Name AQSID 24-hour Avg. CDPHE 

Date (ug/m3

) Request to 

Consider for 

AQS Removal? 

April 16,2013 Telluride 08-113-0004 265 Yes 

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

is similar to a box canyon. The west end of the valley gives way to the high desert Colorado Plateau. To 

the southwest ofTelluride are the deserts of the New Mexico, Utah, and Arizona Four Comers region. 

The areas described are shown in Figure I. 

·Figure I: Map of Desert, Four Comers, and Monitor Location with the PM10 NAAQS Exceedance 

2 
In its exceptional event demonstration package CDPHE describes the April 16, 2013 event as: 

the consequence of strong southwesterly prefrontal winds in combination with dry conditions, 

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

Utah and southwest Colorado. Widespread restrictions to visibility occurred in southwest 

Colorado that day? 

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

5, and 6 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 demonstration visually demonstrates dust plumes and wind direction across 

northeastern Arizona, southeastern Utah, and northwestern New Mexico into southeastern Colorado. 3 

Figures 1-37 from CDPHE's demonstration use modeling, meteorology, and monitoring analysis to 

support the evidence of wind direction and reach of the dust storm into Telluride. 

Figure 2: Map of Blowing Dust on April 16, 2013 (CDPHE's Figure 30 in the State's Demonstration) 

Figme 30: MODIS Aqua satellite afternoon image of the Four Comers region on Aptill6, 2013. 

( sourct: http ://gt .ssec. wisc.tdu/modis-today/indtx.php) 

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 

2 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page I. 3 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page 44. 

3 
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 Telluride area, including information on implementation of State 

Regulations I, 3, 6, and 9, rule effectiveness, compliance, and public notification activities that occurred 

on the event day. 

Section 5 ofCDPHE's demonstration provides a thorough analysis ofthe 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 4- New Wood Stoves and Use of Certain Woodbuming Appliances During High 

Pollution Days; 

• 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.5 

CDPHE provided the documentation listed in Figures 3 and 4 below showing that sustained wind speeds 

associated with these events were above 25 mph (CDPHE tables 1 and 6)6

. Telluride measured sustained 

surface winds of up to 29 mph with gusts up to 53 mph on April 16, 2013, while sustained winds in the 

area of the source of the dust storm near Winslow, Arizona were up to 50 mph with gusts up to 66 mph. 

CDPHE previously established in its exceptional events demonstration for the January 19, 2009 event in 

Lamar, Colorado that sustained wind speeds of30 mph and/or gusts of 40 mph or higher have been 

shown to cause blowing dust in Colorado (attached as appendix A ofCDPHE's demonstration). While the 

sustained winds in Telluride were just below this Colorado threshold, gusts in Telluride were well above 

the threshold. Given the satellite evidence of Figure 2, 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. However, the high local gust speeds in Telluride would likely be capable of 

overwhelming some local controls. The wind gust speeds on April16, 2013 clearly exceed the established 

high-wind threshold. 

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 I 0. 5 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 74-76. 6 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 25 and 30. 

4 
Figure 3: Wind Speed in Telluride, CO on April16, 2013 (CDPHE?s Table 1 in the State's 

D . )7&8 emonstratwn 

Table 1: Weather observations for Telluride, Colorado on April16, 2013. 

(source: httn://www.met.utah.edu/mesowestD 

Wind \Vind 

Time Relative Speed \Vind Direction 

MST Temperatun Humidity in Gust in in 

April16 Desuee.s F in% mph mph Degrees 

0:55 41 38 13 39 200 

1:55 39 41 17 24 170 

2:55 39 41 18 31 180 

3:55 37 48 7 100 

4:55 37 44 20 31 180 

5:55 37 38 25 35 170 

6:55 37 38 24 38 170 

7:55 41 36 12 38 210 

8:55 43 31 17 43 150 

9:55 45 33 13 37 180 

10:55 46 34 9 30 130 

11:55 48 29 23 38 180 

12:55 50 23 29 53 180 

13:55 52 22 22 38 210 

14:55 52 22 24 47 180 

15:55 52 17 15 41 180 

16:55 52 17 12 25 200 

17:55 50 18 14 21 200 

18:55 46 25 14 23 210 

19:55 45 26 9 20 210 

20:55 43 31 9 18 210 

21 :55 41 41 15 28 190 

22:55 41 36 20 36 200 

23:55 39 41 10 32 150 

\Veather 

haze 

haze 

haze 

haze 

haze 

haze 

haze 

Visibility 

in miles 

10 

10 

7 

7 

5 

7 

10 

10 

7 

7 

10 

10 

10 

7 

7 

4 

3 

3 

3 

4 

5 

7 

10 

10 

7 Local wind speeds were above the gust threshold of 40 mph defined for Colorado for four hours in Telluride on 

April 16. Visibility degradation and reports of haze during other hours were highlighted by Colorado in the 

demonstration, and may be indicative of dust blowing into Telluride from desert areas upwind with higher wind 

speeds. Meso West data show winds at Winslow, AZ first exceeding 65 mph, for example, between I 0:56 and II :56 

am, MST. 8 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. 

5 
Figure 4: Wind Speed in Winslow, AZ on April 16, 2013 (CDPHE's Table 6 in the State's 

Demonstration) 

Table 6: Weather observations for Winslow, Arizona on April16, 2013. 

(source: http:/ :vww.met.utah.edu/mesowestO 

Time Wind 

MST Relative \Vind \Vind Dinctioo 

April Temperature Humidity Speed Gustin in 

16 Degrees F in o/o in mph mph De~rees 

0:56 56 34 31 43 210 

1:56 55 37 35 45 220 

2:56 55 32 31 43 210 

3:56 55 30 33 44 210 

4:56 54 32 36 47 200 

5:56 53 35 40 53 200 

6:56 54 35 40 52 200 

7:56 58 29 47 60 200 

8:56 62 22 47 61 200 

9:56 63 22 45 60 200 

10:56 65 17 50 62 210 

11 :56 68 13 47 66 210 

12:56 71 11 43 58 220 

13 :56 71 11 43 59 220 

14:56 70 13 43 60 220 

15:56 70 13 45 59 200 

16:56 69 17 37 52 190 

17:56 67 19 43 51 210 

18:56 63 23 33 46 220 

19:56 59 29 29 40 210 

20:56 56 35 16 220 

21:56 53 41 10 220 

22:56 51 42 10 270 

23:56 48 47 6 250 

\Veather 

haze 

haze 

haze 

haze 

haze 

Visibility 

in miles 

7 

10 

10 

10 

10 

10 

10 

5 

5 

3 

5 

8 

8 

5 

9 

10 

10 

10 

10 

10 

10 

10 

10 

10 

Section 2 ofCDPHE'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); 

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

• Regional wind and weather by hour through Meso West; 

• NOAA HYSPLIT modeling of wind trajectories; 

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

• MODIS Terra satell ite images demonstrating the blowing dust across the region; 

• GOES-11 Longwave Infrared Difference images; 

6 
• Maps establishing the lack of regional precipitation and drought status from U.S: Department of 

Agriculture (USDA), NOAA, and the National Drought Mitigation Center; 

• Maps of elevated friction velocities in the region from the NOAA National Centers for 

Environmental Prediction (NCEP) North American Model; and 

• Snow Depth Analysis by elevation from NOAA. 

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

southwest of the monitors in question. 

Section 5 ofCDPHE's demonstration provides a thorough analysis ofthe state and local controls in place 

to control PM 10 emissions. These controls include CDPHE's Regulations 1, 3, 6, and 9. 

9 

Telluride has the following controls in place: 10 

• Federal fuels and tailpipe standards and regulations; 

• Woodburning emission reduction measures in addition to CO Regulation 4 and approved by EPA 

in the 1994 State Implementation Plan (SIP) approval of section II.A.; 

• Controls on Stationary Sources per CO Regulations I, 3, 6, and the Common Provisions as well 

as State and Federal Prevention of Significant Deterioration (PSD) and Best Available Control 

Technology (BACT) requirements; 

• Street Sanding Controls applied 230 days per year which removes 750 to I ,200 cubic yards of 

sediment per year and mandates <2% fine particles in sanding material 11 ; and 

• Construction Mitigation Plan (CMP) outlined in Article 11 ofthe town's municipal code that 

reduces air and other pollution caused by dust and other pollutants from construction sites, as well 

as mandating revegetation of sites. 

Figure 5 is a map of two parcels of land in town that were developed recently, and are determined to be 

reasonably controlled. As CDPHE states in the demonstration, very limited construction has occurred 

since 2003, the construction sites in Figure 5 were completed prior to 20 II and therefore could not have 

contributed to the PM 10 levels in 2013 .

12 

9 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 74-76. 10 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 76-77. 11 This Strategy was adopted in 1994 and approved by EPA in 1996, see Section II.B. of the SIP Regulations for 

Nonattainment- Attainment/Maintenance Areas. 12 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 77-78. 

7 
Figure 5: Telluride Site Assessment (CDPHE's Figure 45 in the State's Demonstration) 

Figure 45: June 2005 and August 2011 Google Earth view: 

acre new construction. 

8 
On March 16, 2000, CDPHE's SIP was revised and amended to designate Telluride 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 [and Pagosa Springs] 13 areas. No 

additional control measures are needed in these areas to demonstrate long-term maintenance of the PM-10 

NAAQS" .

14 The dust control strategies described in bullet five for Telluride local construction controls 

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

maintenance plan. 

EPA investigation revealed that there was a small rain event ending with a little snow the morning of 

April 14, 2013, with the snow ending about 6:00AM MST. Temperatures were above freezing all night 

except for 6:00 to 8:00 AM when it dropped to 31 degrees; it is conceivable some street sanding may 

have occurred that morning. On April 15, there was a little rain at noon with the temperatures in the mid40s, so no sanding is expected to have occurred then, especially since temperatures remained above 

freezing all night. Regardless of whether or not the city had recently swept, the street conditions would 

not have played a significant role in the dust storm PM10 on April 16, 2013. Figures 6 and 7 below are 

EPA's meteorological demonstration. 

Figure 6: EPA Investigation using NOAA data 

c ..-. o'- '0·- Q) 

..!!!10 li 

-8:9.~ oo ..... ::::l!!a>O 

£t ,!;;; 

...... 

-u5~ Q)·- Q) 

>li li ~li~ ~n ..QO.,_ 

OQ)O 

D:§ 

2 

Precipitation, Snow Water Eauivalent, and Snow Depth Modeled and Observed 

station: TELC2 - TELLURIDE COOP 

Latitude: 37.9492 N 

Longitude: 1 07.8733 W 

Elevation: 8668 Feet 

start Date: 2013-04-14 00 UTC 

stop Date: 2013-04-1 7 00 UTC 

Forest Density: 29% 

Land Use: Deciduous Broadleaf Forest (5) 

0.2 

0.1 

0.0 

2013-04-1 4 00 

0.2 

0.1 

0.0 

2013-04-14 00 2013-04-15 00 

Date (UTC) 

Snow Precipitation Non-Snow Precipitation 

Unkn• l Pr it i n Snow Water Equivalent (Modeled) Snow Depth Modeled) 

Present Weat er (Observed) 

ain ~ "i=r.eezina) . ee 1na .OrJZzleiRtlin 

1xe ffilln/Snow 

now 

2013-04-16 00 

• 

• 

• 

.!::. 

1i 

Q) 

0 

~ 0 

c 

(/) 

13 Pagosa Springs is not under consideration for this Exceptional Event documentation but was considered along 

with Telluride when removing the nonattainment designation on March 16, 2000. 14 State Implementation Plan, Colorado Dept. of Public Health and Environment, Regulation: NonattainmentAttainment!Maintenance Areas, 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. 

9 
Figure 7: EPA Investigation using NOAA data 

5 

Snow Surface, Mean Snow Pack, and Air Temperatures Modeled and Observed 

..... 

~ 30 

2013-04-14 00 

Snow Surface Temperature 

Mean Snow Pack Temperature 

ft 

Observed Air Temperature 4-4 ---413 

Observed Dew Point Temperature J},e.----£>.8 

..... -,----;---c-------~---r---,----.------- 30 ~ 

201 3-04-15 00 2013-04-16 00 2013-04-1 7 00 

Date (UTC) 

Sections 2 and 5 ofCDPHE's demonstration show that the monitor in Telluride 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 the area and the wind speeds associated with the event contribute to 

EPA's evaluation of whether this event is nRCP in Telluride. 

Table 2: EPA's Analysis ofCDPHE's TSD ofnRCP 

Exceedance Location Reasonable Wind Speed Sufficient EPA 

Date Controls Demonstration Quality of Criterion 

Demonstration Citation Evidence? Met? 

Citation 

April 16, 2013 Telluride Section 5.0: Section 2: p.8-51 , Yes Yes 

p.74-78 esp. p.25 

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 626 samples 

of 24-hour averages. CDPHE explains that the concentration measured on April 16, 2013 was more rare 

than the 99.8 percentile level at the Telluride monitor overall, more rare than the 98.2 percentile level for 

10 
all of the month of April (every sample in any April), and the maximum value in the year of2013 

(through June ).

15 

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

consider percentile ranks when dust impacted data is excluded. 16 In regard to 50.14(c)(iv)(C) in excess of 

normal HF, since promulgation of the PM10 NAAQS, effective in September 1987, there have been only 

three exceedances of the PM10 NAAQS in Telluride. All have been flagged as high wind exceptional 

events. EPA provided concurrence on high wind flags for the March 31 , 1999 and April 5, 20 I 0 

exceedances. There is no evidence that local emissions or sources in the Telluride area have significantly 

contributed to any exceedances since 1987. The general decrease in numbers of PM 10 samples greater 

than 75 ug/m3 since 1993 shows the effectiveness of local controls on PM10 sources implemented in the 

Telluride area under the PM10 SIP. Overall levels remained relatively constant through the 2000's, 

barring some apparent increase in dust storm impacted data in the late 2000's. Figures 8 and 9 illustrate 

this historical data. 

Figure 8: EPA Data from AQS, 24-hour filter data only (no TEOM data) 

I ... 250 

e 

-.a 

I 

a zoo 

~ 

100 

so 

Telluride PM10, 1987-2013 

0 'f"'4 N M 

~ ":t '1"'4 .-;t 

! ! ! ! 

15 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page 52-53, table I 0. 16 See " Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude ~bient Air 

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

11 
Figure 9: EPA Data from AQS, 24-hour filter data only (no TEOM data) 

Telluride PM10 Data, 2013 

300 ,------------------------------------------------------------------

• 

250 +------------------------------------------------------------------

200 

150 I 

100 +------------------------------------------------------------------

so ··-------------------------------------

. • • ; . ..,. . .. . .. ~ 

..... , ......... ~· ..... · '"' ....... ., ...... 

12/7/2012 1/26/2013 3/17/2013 S/6/2013 6/25/2013 8/14/2013 

Percentile can be calculated for the April 16, 2013 value as the second maximum in the entire historical 

dataset. The fact that there have been no exceedances that were not high wind dust exceptional events is 

much more significant than a numeric calculation. 

This information indicates that values at the levels seen on April 16, 2013 are extremely rare in the 

absence of high wind dust exceptional event impacts, and that it is unlikely that any sources ofPM10 were 

involved in the April 16, 2013 exceedance other than wind-blown dust. Table 3 summarizes the EPA's 

evaluation ofCDPHE's HF EE demonstration. 

Table 3: EPA's Analysis ofCDPHE's TSD ofHF 

Exceedance Exceedance Historical Fluctuations Sufficient EPA Criterion 

Date Location Demonstration Citation Quality of Met? 

Evidence? 

April 16, 2013 Telluride Section 3: p.52-59 Yes Yes 

12 
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.

17 

Section 2 ofCDPHE's demonstration provides a complex meteorological analysis of the event, including: 

• 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 Meso West; 

• 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; 

• Maps of elevated friction velocities in the region from the NOAA NCEP North American Model; 

and 

• Snow Depth Analysis by elevation from NOAA. 

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-an-snow data from the Center for Snow and Avalanche 

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

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

CDPHE's documentation shows that April 16, 2013, was one often Oust-on-Snow events for the 

2012/2013 water year.

18 This information helps to confirm that a regional blowing dust event with longrange transport caused the PM10 exceedances measured across portions of Colorado on April 16, 2013. 

17 Ibid., 21-22. 18 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 42-43. 

13 
The Smoke Text Product from NOAA's Satellite Services Division described April 16, 2013 with the 

following characterization: 

Very strong Swly [southwesterly] winds are kicking up a large area of moderate dust/sand with 

numerous appox. I Km wide parallel bands with very dense conditions ... particularly in AZ along 

the flats/canyon rim SW and NE of the Little Colorado River, from around Tuba City to 

Winslow; also in the Chinle Valley (particularly SW sources); and in NM along US-491 from 

Shiprock to Gallup. This dust and sand then covers all of SE Utah and SW Colorado, moving 

NNE and more N toward the frontal zone across N UT/SW WY, but becomes thinner the further 

from the sources.

19 

Also included are high-wind blowing dust accounts from the Associated Press, AirportView.net, the 

Center for Snow & Avalanche Studies, the Denver Post, Weatherspark.com, and the Community 

Collaborative Rain, Hail & Snow Network? 0 

CDPHE generally summarizes that "the exceedances were the consequence of high winds from an intense 

area of surface low pressure system combined with a vigorous cold front. .. This single storm system 

caused blowing dust during various time intervals on April16, 2013 ... ".21 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".22 

Table 4 summarizes the EPA's evaluation ofCDPHE's CCR EE demonstration. 

Table 4: EPA's Analysis ofCDPHE's TSD ofCCR23 

Clear Causal Relationship Demonstration Sufficient Quality of Evidence? EPA Criterion 

Citation Met? 

Section 2: p.S-51 , Figures 1-3 7 Yes Yes 

Section 2: p.8, 25-32, Tables 1-8 Yes Yes 

Section 2: p.42-46 Yes Yes 

Section 4: p.60-73 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 summarized on 

pages 10 through 12 ofthis (EPA's) TSD, respectively, and cited in Table 5, " statistical and 

meteorological data clearly shows that but for this high wind blowing dust event, Telluride would not 

19 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page 46. 20 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, pages 60-73. 21 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page 8. 22 "TSD for the April 16,2013, Telluride Exceptional Event," October I, 2013, page 74. 23 See " Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air 

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

14 
have exceeded the 24-hour NAAQS on April 16, 2013".24 Table 5 summarizes the EPA's evaluation of 

CDPHE's AAQ EE demonstration. 

Table 5: EPA's Analysis ofCDPHE's TSD of AAQ 

Exceedance HFandCCR Affects Air Quality Sufficient EPA 

Location Demonstration Citation Quality of Criterion 

Evidence Met? 

HF Section 3: p.52-59 Yes Yes 

Section 2: p.8-51 , Figures 1-37 Yes Yes 

Telluride Section 2: p.8, 25-32, Tables 1- Yes Yes 

CCR 8 

Section 2: p.42-46 Yes Yes 

Section 4: p.60-73 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: 

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 Colorado .. . But for the dust storm on April 16, 2013, this exceedance would not 

have occurred? 5 

CDPHE has sufficiently demonstrated the CCR and the nRCP requirements for the April 16, 2013 event. 

Table 6 summarizes the EPA's evaluation ofCDPHE's Natural Event demonstration. 

Table 6: EPA's Analysis ofCDPHE's TSD ofNatural Event 

Exceedance nRCPorCCR Natural Event Sufficient Quality EPA 

Location Demonstration Citation of Evidence? Criterion 

Met? 

nRCP- Section 5.0: p.74-78 Yes Yes 

Anthropogenic 

nRCP-Wind Speed Section 2: p.8-51 , esp. Yes Yes 

p.25 

Telluride Section 2: p.8-51 , Yes Yes 

Figures 1-37 

CCR Section 2: p.8, 25-32, Yes Yes 

Tables 1-8 

Section 2: p.42-46 Yes Yes 

Section 4: p.60-73 Yes Yes 

24 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page 79. 25 "TSD for the April 16,2013, Telluride Exceptional Event," October I, 2013, pages 79 and 80. 

15 
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: 

The PM10 exceedances on April 16, 2013 , would not have occurred if not for the following: (a) 

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

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

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

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

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

northwest New Mexico, and southeast Utah? 6 

While not explicitly stated in the documentation, EPA notes that PM 10 concentrations before the period of 

high winds on the event day were below the 24-hour PM10 NAAQS, providing further support for 

CDPHE's conclusion. Table 7 summarizes the EPA's evaluation ofCDPHE's NEBF EE demonstration. 

Table 7: EPA's Analysis ofCDPHE's TSD ofNEBF 

Exceedance Date No Exceedance But Sufficient Quality of EPA Criterion Met? 

For the Event Evidence? 

Demonstration 

Citation 

Aprill6,2013 Section 6: p.79-80 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. 

26 "TSD for the April 16, 2013, Telluride Exceptional Event," October I, 2013, page 79. 

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

Reference Schedules and EPA 

Procedural Criterion 

Criteria Met? 

Demonstration 

Citation 

Oid the State provide prompt public 40 CFR 50.14(c)(l)(i) Section I :p.6 and Yes 

notification of the event? Appendix B 

Were flags and initial description placed 40 CFR 50.14(c)(2)(iii) Section 1 :p.6 Yes 

on the data by July 1

51 of the following 

year? 

Was the demonstration submitted within 40 CFR 50.14(c)(3)(i) Oct. 3, 2013 Yes 

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? 

Was the public comment process 40 CFR 50.14(c)(3)(v) Section 1 :p.7 Yes 

followed and documented? Submittal Lette.-27 

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 Telluride on April 16, 2013. This high wind event caused an exceedance of the 24-

hour PM10 NAAQS at the Telluride monitor on April 16,2013. EPA concurs that the flagged exceedance 

at the Telluride monitor meets the definition of an exceptional event: the exceedance affected air quality, 

was nRCP, and meets the definition of a natural event. Specifically, EPA has determined thatthe event 

was nRCP due to high wind conditions that transported PM10 from sources outside of the Telluride area 

and subsequently overwhelmed reasonable controls within Telluride. Also, regardless of transport into the 

area, information pertaining to the controls implemented within the affected area, 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 this event was nRCP. Furthermore, EPA has 

determined that there is a CCR between the event and the measured exceedance, there would have been 

NEBF, and the measured exceedance was 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 

exceedance of 265 ug/m3 in Telluride on April 16, 2013. 

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. 

27 Submittal Letter, Gordon E. Pierce, Program Manager, Technical Services Program, Air Pollution Control 

Division to Richard Payton, EPA Region 8 Air Program, submitting fmal demonstration, October 3, 2013; "This 

report was made available for public comment on our website from August 30 through September 30, 2013, and was 

also distributed to potentially interested parties. No external comments were received." 

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