## 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 ofClean 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 ofwhich must be met before EPA can concur under the EER on the exclusion ofair quality data from regulatory decisions. Under 40 CFR 50.14(c)(3)(iv), the air agency demonstration tojustify exclusion ofdata must provide evidence that:

• The event satisfies the criteria set forth in 40 CFR 50.1 

### (j) for the definition ofan 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 ofthe 24-hour PM 10 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". 1Table 1 : EPA PM 10 Exceedance Summary ofthe 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 ofa valley that runs downhill to the west. The east end ofthe valley is similar to a box canyon. The west end ofthe valley gives way to the high desert Colorado Plateau. To the southwest ofTelluride are the deserts ofthe New Mexico, Utah, and Arizona Four Comers region. The areas described are shown in Figure I.

· Figure I: Map ofDesert, Four Comers, and Monitor Location with the PM 10 NAAQS Exceedance

2In 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 ofArizona, 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 ofthis overnight event in Sections 2, 3, 4, 5, and 6 ofthe 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 ofwind direction and reach ofthe dust storm into Telluride. Figure 2 : Map of Blowing Dust on April 16, 2013 (CDPHE ' s Figure 30 in the State ' s Demonstration)



### Fig me 30: MODIS Aqua satellite afternoon image ofthe 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 ofdust, a high wind dust event can generally be considered to be nRCP ifwinds 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

. 3to cause emissions from natural undisturbed areas. For 

### anthropogenic sources of dust, a high wind dust event can be nRCP if:

• The anthropogenic sources ofdust 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 ofrequired 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 PM 10 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 ofCertain 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 ofthe source ofthe 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 of40 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 werejust 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. 4Figure 3: Wind Speed in Telluride, CO on April16, 2013 (CDPHE?s Tabl e 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 4 1 180 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 of40 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. MesoWest data show winds at Winslow, AZ first exceeding 65 mph, for example, between I0:56 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. 5Figure 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/mesowest O
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 3 1 43 2 10 1:56 

### 55 37 35 

### 45 220 2: 56 

### 55 32 3 1 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 2 10 11 :56 68 13 

### 47 66 

### 210 12 :56 7 1 11 

### 43 58 220 13 :56 7 1 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 2 10 18 :56 63 23 33 46 

### 220 19 :56 

### 59 29 29 40 

### 2 10 20:56 

### 56 35 16 220 2 1:56 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 ofthe event, including:

• Statewide PM 10 monitoring maps ; • Surface and upper air wind speed anal ysis from the National Oceanic and Atmospheric Administration (NOAA) ; • Maps ofreal-time weather data from the National Center for Atmospheric Research (NCAR); • Regional wind and weather by hour through MesoWest; • NOAA HYSPLIT modeling ofwind 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 ofregional precipitation and drought status from U.S : Department of Agriculture (USDA), NOAA, and the National Drought Mitigation Center; • Maps ofelevated 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 ofa high wind blowing dust event originating far southwest ofthe 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 oftwo 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. ofthe SIP Regulations for Nonattainment- Attainment/Maintenance Areas. 12 "TSD for the April 16, 2013 , Telluride Exceptional Event," October I, 2013 , pages 77-78

. 7Figure 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.

8On March 16, 2000, CDPHE' s SIP was revised and amended to designate Telluride as a PM 10 attainment area with the effect of deleting road pav ing 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 ofthe 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 ofthe 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 Apri l 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 ofwhether or not the city had recently swept, the street conditions would not have played a significant role in the dust storm PM 10 on April 16, 2013. Figures 6 and 7 below are EPA ' s meteorological demonstration. Figure 6 : EPA Investigation using NOAA data

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## Precipitation , Snow Water Eauivalent, and Snow Depth Modeled and Observed station: TELC 2
- TELLURIDE COOP Latitude : 37

.9492 N Longitude : 107

.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)

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.0 2013-04-1 4 00 0

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.0 2013-04-14 00 2013-04-15 00 D ate (U T C) Snow Precipitation Non-Snow Precipitation Unkn• l Pr it i n Snow Water Equivalent (Mode led) Snow Depth Modeled) Present Weat er (Observed)**ain ~**  "i=r

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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

. 1 4 State Implementation Plan , Colorado Dept. of Public Health and Environment, Regulation: NonattainmentAttainment!Maintenance Areas , Rule Title: Approval and Promulgation ofAir Quality Implementation Plans; Colorado ; Designation ofAreas for Air Quality Planning Purposes , Telluride and Pagosa Springs, Direct Final Rule, June 15 ,2001 , 66 FR 32556. 9Figure 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 PM 10 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 ofelevated PM 10 concentrations throughout the area and the wind speeds associated with the event contribute to EPA ' s evaluation ofwhether 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 ofthe 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 of24-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


### 10all ofthe month ofApril (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 ofthe PM 10 NAAQS , effective in September 1987 , there have been only three exceedances ofthe PM 10 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 I0 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 PM 10 sources implemented 

### in the Telluride area under the PM 10 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

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so Telluride PM 1 0, 1987-20 1 3

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15 " TSD for the April 16, 2013 , Telluride Exceptional Event," October I, 2013 , page 52-53 , table I0

. 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

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

## Telluride PM10 Data, 2013 300 ,------------------------------------------------------------------ 

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# ...... 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 ofhigh wind dust exceptional event impacts, and that it is unlikely that any sources ofPM 10 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



### 12Clear Causal Relationship
EPA considers a variety ofevidence 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 ofthe monitor(s) where measurements were recorded ; the size ofthe area affected by the transported emissions ; the relationship in time between the event, transport ofemissions, 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 ofthe event, including:

• Statewide PM 10 monitoring maps; • Surface and upper air wind speed analysis from NOAA ; • Wind speed maps from the North American Regional Analysis (NARR); • Maps ofreal-time weather data from NCAR; • Regional wind and weather by hour through MesoWest; • NOAA HYSPLIT modeling ofwind trajectories ; • NAAPS forecasted dust concentrations ; • MODIS Terra satellite images demonstrating the blowing dust across the region ; • Maps establishing the lack ofregional precipitation and drought status from the USDA, NOAA, and the National Drought Mitigation Center; • Maps ofelevated 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 ofevidence the existence ofa high wind blowing dust event originating far southwest ofthe 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 ofwind-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 PM 10 exceedances measured across portions ofColorado on April 16, 2013.

17 Ibid ., 21-22

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

. 13The 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 ofmoderate 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 ofthe 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 ofdust generated locally was easily overwhelmed by, and largely unnoticeable as compared to the dust transported in from the sources regions ofthe 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 ofEvidence? 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 ifthe 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" ofMay 2013 page 21. 

### 14have 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 ofAAQ 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 E xceptional 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 ofdata for the event in question and analyses ofpast 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 ofEvidence? 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. 

### 15No Exceedance or Violation But For the Event Generally, the No Exceedance or Violation But for the Event (NEBF) demonstration is similar to the demonstration ofthe nRCP and CCR requirements , and should show that the measured concentration would have been below the applicable NAAQS without the affect ofthe event. CDPHE provides a summary ofthe analysis and information presented in the demonstration that show both the nRCP and CCR requirements have been met, and states that: The PM 10 exceedances on April 16, 2013 , would not have occurred ifnot for the following : (a) dry soil conditions over southeastern Utah, northeastern Arizona, and portions ofextreme 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 ofconcern ; and (c) elevated friction velocities and the deep mixing ofthe 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 PM 10 NAAQS , providing further support for CDPHE ' s conclusion. Table 7 summarizes the EPA ' s evaluation ofCDPHE 's NEBF EE demon stration

. 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 ofthese requirements

.

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

### 16Table 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 ofthe 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 151 ofthe following year? Was the demonstration submitted within 40 CFR 50.14(c)(3)(i) Oct. 3, 2013 Yes 3 years ofthe end ofthe 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 PM 10 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 ofthe 24- hour PM 10 NAAQS at the Telluride monitor on April 16,2013

. EPA concurs that the flagged exceedance at the Telluride monitor meets the definition ofan exceptional event: the exceedance affected air quality, was nRCP, and meets the definition ofa natural event. Specifically, EPA has determined thatthe event was nRCP due to high wind conditions that transported PM 10 from sources outside ofthe Telluride area and subsequently overwhelmed reasonable controls within Telluride

. Also, regardless oftransport into the area, information pertaining to the controls implemented within the affected area, the spatial extent of elevated PM 10 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 ofevidence is sufficient to concur on the flagging ofthe data for the NAAQS exceedance of265 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 ofthe 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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