1



## EPA Technical Support Document i n Review of Colorado Department of
Public Health and Environment’s Demonstration Package for the April 28 & 29, 2010 Alamosa, Pagosa Springs, and Durango Exceptional
Event s

EXCEPTIONAL EVENTS RULE REQUIREMENTS

The Environmental Protection Agency ( EPA ) promulgated the Exceptional Events Rule in 2007, pursuant to the 2005 amendment of Clean Air Act (CAA) Section 319. 42 U.S.C. 7619. The EER added 40 CFR
50.1 (j), (k) and (l); 50.14; and 51.930 to the Code of Federal Re gulations (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 criter ia 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 measuremen t under consideration and the event
that is claimed to have affect ed the air quality in the area;
The event is associated with a measured concentration in excess of normal historical fluc tuations, including background; and
There would have been no exceeda nce 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, ( Inte rim Guidance for High Wind Exceptional Events) in addition to the applicable statutory and regulatory provisions.



## SUMMARY

Overview

On June 28, 2013, Colorado Department of Public Health and Environment ( CDPHE ) submitted a Technical Support Document (TSD) dated May 17, 201 3

, of excepti onal event demonstrations for five
exceedances and four high readings of the 24

- hour PM 10 standard that occurred at several monitoring stations in southwestern Colorado on April 28 and 29, 2010. Table 1 summarizes the exceedan ces that

1 A natural event is further described in 40 CFR 50.1(k) as “an event in which human activity plays little or no direct causal role”.2

CDPHE would like considered for removal from the Air Quality System ( AQS ) database , table 2 summarizes the high sample readings that CDPHE would like considered in support of the existence of the high wind blowing dust event but not for removal from AQS.

Table 1: EPA PM 10 Exceedance Summary of the 150 ug/m 3 24

- hour NAAQS
Exceedance Date Monitor/Site Name AQS ID 24

- hour Avg. (ug/m 3 ) CDPHE

Request to Consider for AQS Removal? 2 April 28, 2010 Adams State College 08

- 003

- 0001 285 Yes

Alamosa Municipal Building 08

- 003

- 0003 236 Yes Pagosa Springs School 08

- 007

- 0001 181 Yes April 29, 2010 Pagosa Springs School 08

- 007

- 0001 162 Yes

Durango River City Hall 08

- 067

- 0004 226 Yes

Table 2: EPA PM 10 High Samples Included in CDPHE’s TSD for Demonstration Purposes Only High Sample Date Monitor/Site Name AQS ID 24

- hour Avg. (ug/m 3 ) CDPHE

Request to Consider for AQS Removal? 3 April 28, 2010 Mt. Crested Butte 08

- 051

- 0007 123 No April 29, 2010 Adams State College 08

- 003

- 0001 92 No

Alamosa Municipal Building 08

- 003

- 0003 94 No

Alamosa, Colorado lies in the broad, flat, high elevation San Luis V alley of the upper Rio Grande R iver.
Alamosa is at an elevation of 7

, 600 feet, and is between the South San Juan and Sangre de Cristo
M ountain ranges. Alamosa is generally surrounded by agricultural lands within the San Luis Valley. Pagosa S prings lies along the San Juan R iver, to the west and south of the San Juan M ountains. Durango lies along the Animas R iver at an elevation of 6,512 feet in a valley running north to south. To the southwest of Pagosa Springs and Durango are the deserts of the New Mexico , Utah, and Arizona Four Corners region. The areas are shown in Figure 1.

2 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Excep tional Event,” May 17, 2013,
page 3

. 3 CDPHE included these values in their report to demonstrate the regional nature of the event but did not specifically request their removal.3

Figure 1: Map of Monitor Locations with PM 10 NAAQS Exceedances

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

the consequence of strong southwesterly prefrontal surface winds which caused significant blowing dust across much of Arizona, northw est New Mexico, southeast Utah and southwest Colorado. These winds were the result of a significant surface low pressure and surface cold front associated with a major upper

- level trough that was moving across the Western United States. This single storm s ystem caused blowing dust during the transported afternoon and evening hours of April 28 that continued through the morning hours of April 29. It transported PM 10 dust into the southwestern portion of Colorado. 4

CDPHE provides a comprehensive description and discussion of this overnight event in Sections 2, 3, 4, 5, and 6 of the demonstration. As noted below, the high wind event causing dust to blow affected air quality in this area.

4 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 2.4

EPA Figures 2 and 3 from CDPHE’s demonstration 5 visually demonstrates wind speed and direction of blowing dust across northeastern Arizona, southeastern Utah, and northwestern New Mexico into

sou theastern Colorado. Figures 1

- 35 from CDPHE’s d emonstration use modeling, meteorology, and monitoring analys is to support the evidence of wind direction and reach of the dust storm into Durango, Alamosa

, and Pagosa Springs.

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

5 Ibid., 16 and 19.5

Figure 3: Map of Wind Speed on April 29, 2010 (CDPHE’s Figure 14 in the State’s Demonstration)

Not Reasonably Controllable or Preventable

EPA evaluates whether an event was n ot R easonably C ontrollable or P reventable (nRCP) at the time of the event by taking into account controls in place and wind speed, along with other factors. For natural sources of dust, a high wind dust event can generally be considered to be nRCP if winds are high enough
to cause emissions from natural undisturbed areas. For anthropogenic sources of dust, a high wind dust event is also eligible to be considered to be nRCP if:

The anthropogenic sources of dust have reasonable controls in place ; The reasonable controls have been effectively implemented and enforced ; and The wind speed was high enough to overwhelm the reasonable controls. 6

In addressing reasonable controls, CDPHE provided detailed information on the current set of required and voluntary controls in the Pagosa Springs PM 10 maintenance area, and in Alamosa, and Durango

. This
includes information on implementation of State Regulations 1, 3, 6, and 9, implementation effectiveness, compliance, and public notification activities that occurred on the event day. As CDPHE states, they :

6 “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule,” May 2013, page 10.6

conducted th orough analyses and outreach with local governments to confirm that no unusual anthropogenic PM 10 producing activities occurred in these towns and that despite reasonable control measures in place, high wind conditions overwhelmed all reasonably available controls

. 7

Section 5 of CDPHE’s documentation provides a thorough analysis of the State and local controls in place to control 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 6

- Standards of Performance for New Stationary Sources ; and Regulation 9

- Open Burning, Prescribed Fire, and Permitting applied to bo th Pagosa Springs and
Alamosa. 8

CDPHE provided the documentation listed in CPDHE tables 13, 14, 17, 18 , 19, and 20 showing that sustained wind speeds associate with these events were above 25 mph

. 9 For example, in Durango

, sustained surface winds of up to 33 mph with gusts up to 54 mph occurred on April 28, and sustained wind speeds of 31 mph with gusts up to 40 mph occurred on April 29. In Alamosa

, sustained surface winds of up to 41 mph with gusts up to 56 mph occurred on April 28, and sustained wind speeds of 38 mph with gusts up to 54 mph occurred on April 29. In Pagosa Springs sustained surface winds of up to 31 mph with gusts up to 44 mph occurred on April 28, and sustained wind speeds of 32 mph with gusts up to 47 mph occurred on April 29. CDPHE previously established in its Exceptional Event Demonstration for the January 19, 2009 event in Lamar , Colorado that wind speeds of 30 mph and/or gusts of 40 mph or higher have been shown to cause blowing dust in Colorado

, 10 and appendix A of CDPHE’s analysis
demonstrates a similar situation in Grand Junction

. The wind speeds on April 28 and 29 , 2010 clearly
exceed the established high

- wind threshold

. 11

7 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 92.
8 Ibid. , 92.
9 Ibid. , 38

- 39, 42

- 45.
10 Ibid. , 11.
11 Page 3 of EPA’s Interim Guidance for High Wind Events recommends air agencies establish wind thresholds based on local conditions above which reasonable dust controls might be overwhelmed and natural dust sources would be active. Had Colorado not been able to establish these thresholds at sustained winds of 30 mph and gusts of 40 mph, EPA would have accepted a threshold of 25 mph with evidence that it was an appropriate wind level. However, EPA has not established 25 mph as a bright line threshold for high wind events.7

Figure 4: Wind Speed in Durango, CO on April 28, 2010 (CDPHE’s Table 13 in the State’s Demonstration)8

Figure 5: Wind Speed in Durango, CO on April 29, 2010 (CDPHE’s Table 14 in the State’s Demonstration) 12

12 While wind speed in Durango was only above 30 mph for 1 hour of April 29, haze and reduced visibility data for April 28 and 29 shows that regional dust impacts from wind in the region carried over from April 28 into April 29.9

Figure 6: Wind Speed in Alamosa, CO on April 28, 2010 (CDPHE’s Table 17 in the State’s
Demon stration)10

Figure 7: Wind Speed in Alamosa, CO on April 29, 2010 (CDPHE’s Table 1 8 in the State’s Demonstration)11

Figure 8: Wind Speed in Pagosa Springs, CO on April 28, 2010 (CDPHE’s Table 1 9 in the State’s Demonstration)12

Figure 9: Wind Speed in Pagosa Springs, CO on April 29, 2010 (CDPHE’s Table 20 in the State’s Demonstration)13

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

S tatewide PM 10 monitoring maps ; S urface and upper air wind speed analysis from the National Oceanic and Atmospheric Administration (NOAA) ; M aps of real

- time weather data from the National Center for Atmospheric Research (NCAR) ; R egional wind and weather by hour through MesoWest ; NOAA HYSPLIT modeling of wind trajectories ; MODIS Terra satellite images demonstrating the blowing dust across the region ; GOES

- 11 longwave spl i t window time

- lapse images showing dust originating in Arizona ; M aps establishing the lack of regional precipitation and drought status from U.S. Department of Agriculture (USDA)

, NOAA, and the National Drought Mitigation Center ; and
M aps of elevated friction velocities in the region from the NOAA National Centers for Environmental Prediction (NCEP) North American Model.

These analyses demonstrate persuasively through the weight of evidence the existence of a high wind blowing dust event originating far southwest of the monitors in question.

Section 5 of CDPHE’s documentation provides a thorough analysis of the State and local controls in place to control PM 10 emissions . These controls include CDPHE’s Regulations 1, 3, 6, and 9. 13

Alamosa has the following controls in place:

Natural Events Action Plan (NEAP) 14 for high wind events , completed in May 2003, which
addresses public health programs, public notification, and implements Best Available Control Measures (BACM). CDPHE ensured in 2007 and 2013 that the NEAP was implemented satisfactorily ; 15 City Code of Ordinances Article VII of Section 21

- 140(5) which addresses dust control at homes ; Article V Sec. 17

- 87(3) requires pavement of new roads/alleys ; and
Article VI Sec. 21

- 119(3) requires new commercial establishments to install underground irrigation. 16

In addition, Alamosa conducts street sweeping, has irrigated and inst alled vegetative cover on all city parks, encourages low tilling and other soil conservation practices outside of town through the Colorado State University Co

- Op Extension Office and the Natural Resources Conservation Service. Since 2008,

t he Mosca

- Hooper Conservation District is working with the Natural Resources Conservation Service to

13 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 92.
14 Ibid. , Appendix C.
15 “EPA’s Natural Events Policy guidance requires the state to periodically reevaluate: 1) the conditions causing violations of the PM 10 NAAQS in the area, 2) the status of implementation of the NEAP, and 3) the adequacy of the actions being implemented. The State will reevaluate the NEAP for Alamosa at a minimum of every 5 years and make appropriate changes to the plan accordingly.” TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event, May 17, 2013, Appendix C, page 18.
16 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 94.14

implement the Colorado State Forest Service’s program to plant between 2,805 and 7,432 trees per year to prevent wind erosion in the area. 17

Maps are included of eight und eveloped parcels of land in town that were determined to be reasonably controlled or considered natural sources. All eight are covered with vegetation and five of those eight are confirmed to be irrigated. The three sites without confirmed irrigation in F i gure 10 (Sites B, E, and H) are naturally vegetated, and satellite images show minimal soil disturbance. 18

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

Pagosa Springs local controls include:

Land Use Development Code 6.6.3(h) and 6.6.3(m)(i) requiring paved streets, street sanding and sweeping/cleaning controls, encourages businesses to maintain cleanliness of private lots ; 19 EPA approved PM 10 Maintenance Plan, effective on August 14, 2001 , requiring Prevention of Sign ificant Deterioration (PSD) permitting requirements mandating Best Available Control Technology (BACT) at sources potentially emitting PM 10 ; and Vegetation, irrigation, fencing, and stabilized clay dust control measures are in place for undeveloped parc els of land in town. Figure 11 show s 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 r ecent construction of the Springs Resort and Spa was c ompleted in May 2009 and therefore couldn’t have impacted the May 11, 2010 event. Site C was naturally vegetated and receives groundwater from a nearby stream; 20 it is also an area where off

- road

17 Ibid., 96.
18 Ibi d., 97

- 100.
19 Ibid., 102

- 106.
20 Colorado State Reg. 1 Section III.D.2.b considers natural vegetation with regular groundwater availability due to low lying terrain to be the appropriate available and practical method that is technologically feasible and

ec onomically reasonable in order to minimize fugitive particulate emissions for this lot at this time.15

recreational vehicles are prohibited by the parks department. Site D is an irrigated and maintained turf and stabilized clay baseball field since April 2009. 21

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

On March 16, 2000, CDPHE’s State Implementation Plan (SIP) was revised and amended to designate Pagosa Springs as a PM 10 attainment area with the effect of deleting road paving requirements, street sanding reporting requirements, and any other “unnecessary mandatory contingency measures in the [ Telluride ] 22 and Pag osa Springs areas. No additional control measures are needed in these areas to
demonstrat e long

- term maintenance of the PM

- 10 NAAQS ”

. 23 The dust control strategies described in previous bullet 3 for Pagosa Springs local controls are voluntary measures and a re not intended to be federally enforceable elements of the SIP or maintenance plan

.
Durango local controls include:

21 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, pages 104

- 106.
22 Telluride is not under consider ation for this Exceptional Event documentation but was considered along with Pagosa Springs when removing the nonattainment designation on March 16, 2000

. 23 State Implementation Plan, Colorado Dept. of Public Health and Environment, Regulation: Nonattainme nt

Attainment/Maintenance Areas, Rule #1, Rule Title: Pagosa Springs Attainment/Maintenance Area, Notice of Final Rule date 6/15/2001, 66FR 32556.16

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

, 24 The City o f Durango, La Plata County, and the Southern Ute Indian Tribe work together to implement the following programs: street sweeping and sanding controls, dust suppressant for gravel roads, landfill dust control, and smoke mitigation for locomotives ; 25

La Plata County and the Southern Ute Indian Tribe have surface operating standards to control
du st f r o m oil & gas development in the area. This includes maintaining roads to well pads, establishing speed limits, and designating gradient to minimize dust. Guidelines are found in the Bureau of Land Management’s (BLM) “Surface Operating Standards and Guidelines for Oil and Gas Exploration and Development, The Gold Book” 4 th edition, 2007 26 ; and Vegetation, signage, and other dust control measures are in plac e for undeveloped parcels of land
in town. Figure 12 shows identified areas of soil disturbance that have reasonable controls. Site A is a former industrial site that was leveled and then compacted and allowed to return to a natural
state. CDPHE confirmed that the soil remains compacted and vegetated and should not be considered an anthropogenic source. Site B is naturally vegetated and has not been developed.
Site C is an off

- leash dog park with signage to indicate that it is an area where off

- road

recreat ional vehicles are prohibited by the parks department

. 27

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

24 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013,
p ages 107

- 108.
25 Ibid., 108

- 109.
26 Online version: http://www.blm.gov/pgdata/etc/medialib/blm/wo/MINERALS __REALTY__AND_RESOURCE_PROTECTION_/e nergy/oil_and_gas.Par.18714.File.dat/OILgas.pdf
27 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, pages 110

- 113.17

Figure 13: Durango and La Plata County Dust Control Rules and Ordinances (CDPHE’s Table 30 in the
State’s Demonstration)18

Sections 2 and 5 of CDPHE’s analysis demonstrate that monitors in Pagosa Springs and Alamosa were affected by PM 10 transport from outside the area, with the main source areas located to the southwest of the area in Arizona, southeastern Utah, and northwestern New Mexico. In addition to transport, the spatial
extent of elevated PM 10 concentrations throughout the area and the wind speeds associated with the event contribute to EPA’s evaluation of whether these events are nRCP

. Potential anthropogenic sources nearer the monitors were shown to have reasonable dust control in place. Table 3 summarizes the EPA’s evaluation of CDPHE’s nRCP EE demonstration.19

Table 3: EPA’s Analysis of CDPHE’s Technical Support Docum entation of nRCP
Exceedance Date Location Reasonable Controls Demonstration Citation Wind Speed

Demonstration Citation Sufficient

Quality of Evidence ? EPA

Criterion Met?

April 28, 2010 Adams State

College Section 5.0:

p.92

- 93 and Section 5.1: p.94

- 101 Section 2: p.11

- 61,

esp. p.42

- 42 Yes Yes

Alamosa Municipal Building Section 5.0:

p.92

- 93 and Section 5.1: p.94

- 101 Section 2: p.11

- 61,

esp. p.42

- 42 Yes Yes

Pagosa Springs School Section 5.0:

p.92

- 93 and Section 5.2: p.102

- 106 Section 2: p.11

- 61,

esp. p.44

- 45 Yes Yes

April 29, 2010 Pagosa Springs

School Section 5.0:

p.92

- 93 and Section 5.2: p.102

- 106 Section 2: p.11

- 61,

esp. p.44

- 45 Yes Yes

Durango River City Hall Section 5.0:

p.92

- 93 and Section 5.3: p.107

- 113 Section 2: p.11

- 61,

esp. p.38

- 39 Yes Yes

Historical Fluctuations

EPA evaluates whether a measured exceedance is in excess of H istorical F luctuation (HF) by taking in to account the level of the exceedance in relation to historical data, which is typically 3 to 5 years. To demonstrate that this requirement was met, CDPHE provided 7

- year time series plots of 2,214 samples (Alamosa

- Adams State College), 2,214 samples (Alam osa

- Municipal Building), 2,287 samples (Pagosa Springs), and 811 samples (Dur a n go) of 24

- hour averages. CDPHE explains that concentrations measured on April 28, 2010 were more rare than the 99.6 percentile level at the Alamosa Adams State College
monitor and were the highest values ever recorded at the Alamosa Municipal Building monitor and the Pagosa Springs School monitor. 28 The concentrations measured on April 29, 2010 were more rare than the 99 percentile at the Pagosa Springs monitor and the 99.9 perce ntile at the Durango monitor. 29

The EPA “Interim Guidance on High Wind Exceptional Events” recommends that demonstrations also
consider the percentile ranks of high

- wind blowing

- dust data. 30 The April 28 and 29, 2010 Pagosa Springs and April 29, 2010 Durango values fit into the range of values exceeding the NAAQ S that, since at least 2005, are only found during high wind dust events. This information indicates that values at the levels

seen on April 28 and 29, 2010 are extremely rare in the absence of high wind dust exceptional event

28 Ibid., 63.
29 Ibid., 63, 69.
30 See “Interim Guida nce 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 p age 20.20

impacts, and that it is unlikely that any sources of PM 10 were involved in the April 28 and 29 , 2010 exceedances other than wind

- blown dust. Table 4 summarizes the EPA’s evaluation of CDPHE’s HF EE
demonstration.

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

Location Historical Fluctuations

Demonstration Citation Sufficient Quality

of Evidence ? EPA Criterion

Met?

April 28, 2010 Adams State

College Section 3: p.63

- 65 Yes Yes

Alamosa Municipal Building Section 3: p.63, 67 Yes Yes

Pagosa Springs School Section 3: p.63, 69

- 70 Yes Yes

April 29, 2010 Pagosa Springs

School Section 3: p.63, 69

- 70 Yes Yes

Durango River City Hall Section 3: p.63, 72 Yes Yes

Clear Causal Relationship

EPA considers a variety of evidence when evaluating whether there is a C lear and C ausal R elationship (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 transp orted 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, pol lutant species

- specific information supporting a CCR between the event and the measured
concentration. 31

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

S tatewide PM 10 monitoring maps ; S urface and up per air wind speed analysis from NOAA ; Upper air sounding analysis from the University of Wyoming ; M aps of real

- time weather data from NCAR ; R egional wind and weather by hour through MesoWest ; NOAA HYSPLIT modeling of wind trajectories ; MODIS Terra sat ellite images demonstrating the blowing dust across the region ; M aps establishing the lack of regional precipitation and drought status from the USDA

, NOAA, and the National Drought Mitigation Center ; and
M aps of elevated friction velocities in the region from the NOAA NCEP North American Model.

31 Ibid., 21

- 22.21

These analyses demonstrate persuasively through the weight of evidence the existence of a high wind blowing dust event originating far southwest of the monitors in question.

CDPHE’s analysis, Section 2

, Figure s 29 , 30, and 31 show a MODIS Terra satellite image of the blowing dust originating in Arizona. 32 Figures 20

- 26 show NOAA HYSPLIT Model 18

- hour forward trajectories
and 12

- hour backward trajectories that demonst rate transport of air from northeast Arizona, northwest New Mexico, and southeast Utah into Colorado on the dates in question. 33

Confirming the April 28 and 29, 2010 high wind dust event, the Center for Snow and Avalanche Studies, an organization that has been studying the effects of wind

- blown desert dust from Arizona, New Mexico, and Utah on snowpack albedo and snowmelt in the San Juan Mountains of Colorado , “ lists April 28, 2010, as one of nine Dust

- on

- Snow events for the 2009/2010 water year ”

. 34

Further confirmation from NOAA ’s Satellite Services Division Text Narrative for Smoke/Dust Observed in Satellite Imagery describes the high wind dust event as

, “ A significant area of blowing dust is being picked up across parts of NE Arizona and NW New Mexico and moving ENE across a large section of Colorado and beginning to reach parts of W Nebraska and Kansas. There are very high winds across the region.” 35

The U.S. Geological Survey (USGS) Southwest Geographic Science Team’s dust monitoring web pages list April 28 and 29, 2010 as dust events with the following characterization:

A strong pacific storm front moved through the southwest today, bringing very strong winds cold temperatures and snow in the higher elevations. The dominant wind direction was out of the

s outh

- west. Severe dust was viewable from satellite by both visible band and thermal imaging in the Four Corners area. Activity was apparent from mid morning on to well after sunset. Interstate
40 in Arizona was closed from Belmont to Winslow for most of th e day. 36

Also included are eyewitness accounts from the Community Collaborative Rain, Hail, & Snow Network that describe low visibility, high winds, wind damage, and dust; as well as a travel log from tourists in the region noting high wind and dust conditi ons in this area. 37

CDPHE generally summarizes that “these exceedances were the consequence of strong southwesterly prefrontal surface winds over dry soils which caused significant blowing dust across much of Arizona, northwest New Mexico, southeast Utah, a nd southwest Colorado”. 38 CDPHE further states that “[w]hile it is likely that some dust was generated within the local communities as gusts from the regional dust storm passed through the area, the amount of dust generated locally was easily overwhelmed by , and largely

32 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 56

- 58.
33 Ibid., 47

- 53.
34 Ibid., 54

- 55.
35 Ibid., 56.
36 Ibid., 59.
37 Ibid., 82

- 91.
38 Ibid., 11.22

unnoticeable as compared to the dust transported in from the sources regions of the dust storm”. 39 Table 5 summarizes the EPA’s evaluation of CDPHE’s CCR EE demonstration.

Table 5 : EPA’s Analysis of CDPHE’s Technical Support Documentation of CCR 40 Clear Causal Relationship Demonstration

Citation Sufficient Quality of Evidence ? EPA Criterion Met? Section 2: p.11

- 61, Figures 1

- 35 Yes Yes Section 2: p.11, 26

- 45, Tables 1

- 20 Yes Yes Section 2: p.54

- 56 Yes Yes Section 4: p.82

- 91 Yes Yes

Affects Air Quality (AAQ)

As stated in EPA’s Interim Guidance for High Wind Exceptional Events Section 3.4, EPA will consider events to have affected air quality if the CCR and HF requirements have been adequately demonstrated. CDPHE states that due to the information presented in the HF and CCR demo nstrations on pages 19 and
20

- 21 of this (EPA’s) TSD, respectively, and cited in Table 6 , “data and meteorological analysis clearly shows that but for this high wind blowing dust event, Alamosa, Pagosa Springs , and Durango would not
have exceeded the 24

- ho ur NAAQS on April 28 and 29 , 2010 ”

. 41 Table 6 summarizes the EPA’s evaluation of CDPHE’s AAQ EE demonstration.

Table 6 : EPA’s Analysis of CDPHE’s Technical Support Documentation of AAQ
HF and CCR Exceedance Location Affects Air Quality Demonstration Citation Sufficient Quality of Evidence ? EPA

Criterion Met? HF Adams State College Section 3: p.63

- 65 Yes Yes

Alamosa Municipal Building Section 3: p.63, 67 Yes Yes

Pagosa Springs School Section 3: p.63, 69

- 70 Yes Yes

Durango River City Hall Section 3: p.63, 72 Yes Yes

CCR All Monitors Section 2: p.11

- 61, Figures 1

- 35 Yes Yes

Section 2: p.11, 26

- 45, Tables

1

- 20 Yes Yes

Section 2: p.54

- 56 Yes Yes Section 4: p.82

- 91 Yes Yes

39 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 92.
40 See “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule” of May 2013 p age 21.
41 “ TSD for the April 28 & 29, 2010, Alamosa, Pagosa Springs, and Durango Exceptional Event,” May 17, 2013, page 114.23

Natural Event

As stated in EPA’s Interim Guidance for High Wind Exceptional Events Section 6.3.6

, EPA will consider an event to be a natural event if both the nRCP and CCR requirements have been adequately demonstrated. CDPHE generally states that, “[s]ince at least 2005, there has not been an exceedance that was not associated with high winds carrying PM 10 dust from distant sources in these areas” 42 and that “ the prevalence of winds above blowing dust thresholds prove that this dust storm was a natural event that was not reasonably c ontrollable or preventable”

. 43 Table 7 summarizes the EPA’s evaluation of CDPHE’s
Natural Event EE demonstration.

Table 7 : EPA’s Analysis of CDPHE’s Technical Support Documentation of Natural Event
nRCP or CCR Exceedance Location Natural Event

Demonstration Citation Sufficient Quality of Evidence ? EPA

Criterion Met? nRCP

Anthropogenic Adams State College Section 5.0: p.92

- 93 and

Section 5.1: p.94

- 101 Yes Yes

Alamosa Municipal Building Section 5.0: p.92

- 93 and

Section 5.1: p.94

- 101 Yes Yes

Pagosa Springs School Section 5.0: p.92

- 93 and

Section 5.2: p.102

- 106 Yes Yes

Durango River City Hall Section 5.0: p.92

- 93 and

Section 5.3: p.107

- 113 Yes Yes

nRCP

- Wind Speed Adams State College Section 2: p.11

- 61, esp. p.42

- 42 Yes Yes

Alamosa Municipal Building Section 2: p.11

- 61, esp. p.42

- 42 Yes Yes

Pagosa Springs School Section 2: p.11

- 61, esp. p.44

- 45 Yes Yes

Durango River City Hall Section 2: p.11

- 61, esp.

p.38

- 39 Yes Yes

CCR All Monitors Section 2: p.11

- 61, Figures 1

- 35 Yes Yes

Section 2: p.11, 26

- 45, Tables 1

- 20 Yes Yes

Section 2: p.54

- 56 Yes Yes Section 4: p.82

- 91 Yes Yes

No Exceedance or Violation But For the Event

Generally, the No Exceedance or Violation But for the Event ( NEBF ) demonstration is similar to the demonstration of the nRCP and CCR requirements, and should show that the measured concentration would have been below the applicable NAAQS without the affect of the event.

CDPHE provides a summary of the analysis and infor mation presented in the documentation that demonstrate both the nRCP and CCR requirements have been met and states that :

42 Ibid., 114.
43 Ibid., 115.24

[t]he PM 10 exceedances in Alamosa, Pagosa Springs, and Durango on April 28 and 2 9, 2010, would not have occurred if not for the follo wing: (a) dry soil conditions over southeastern Utah, northeastern Arizona, portions of extreme northwestern New Mexico, and portions of southern
Colorado with 30

- day precipitation totals below the thresholds for blowing dust; (b) a strong surface and uppe r

- level low pressure system that caused widespread strong gusty winds through a deep layer of the atmosphere over the area of concern; and (c) friction velocities over the desert regions of New Mexico, Utah, Arizona, and much of Colorado that were high eno ugh to allow entrainment of dust from natural sources with subsequent transport of the dust into (or within) Colorado in strong, southwesterly winds. 44

While not explicitly stated in the documentation, EPA notes that PM 10 concentrations before the periods of high winds on the event days were below the 24

- hour PM 10 NAAQS, providing further support for CDPHE’s conclusion. Table 8 summarizes the EPA’s evaluation of CDPHE’s NEBF EE demonstration.

Table 8 : EPA’s Analysis of CDPHE’s Technical Support Documentati on of NEBF
Exceedance Date No Exceedance But For the Event Demonstration Citation Sufficient Quality of Evidence ? EPA Criterion Met?

April 28, 2010 Section 6: p.114 Yes Yes April 29, 2010 Section 6: p.114 Yes Yes

Schedule and Procedural Requirements

In addition to technical demonstration requirements, 40 CFR 50.14(c) specifies the schedule and procedural requirements an air agency must follow to request data exclusion. CDPHE received some
minor non

- substantive external comments during the 30 day comme nt period and appropriate corrections were made to the report. Table 8 outlines EPA’s evaluation of these requirements.

44 Ibid., 114.25

Table 9 : EPA’s Analysis of Schedules and Procedural Criteria
Reference Schedules and Procedural Criteria Demonstration Citation Criterion

Met?

Did the State provide prompt public notification of the event? 40 CFR §50.14(c)(l)(i) Section 1:p.9 Yes

Were flags and initial description placed on the data by July 1 st of the following year? 40 CFR §50.14(c)(2)(iii) Section 1:p. 9 Yes

Was the demonstration submitted within 3 years of the end of the quarter in which the event occurred and 12 months prior to the date that any regulatory decision must be made by EPA? 40 CFR §50.14(c)(3)(i) June 28, 2013 Yes

Was the public comment process followed and documented? 40 CFR §50.14(c)(3)(v) Section 1:p.10 Yes



## CONCLUSION

EPA has reviewed documentation provided by CDPHE to support claims that dust emissions generated by high winds resulted in PM 10 transport from arid regions in Arizona, northwest New Mexico, and
southeast Utah into the Pagosa Springs , Durango, and Alamosa areas on April 28 and 29, 2010 . These high wind events caused exceedances of the 24

- hour PM 10 NAAQS at the location s identifi ed in Table 1
of this document on April 28 and 29, 2010. EPA has determined that the flagged exceedances at these locations on these days meet the definition of an exceptional event: the exceedances affected air quality,

were nRCP , and meet the definition of a natural event. Specifically, EPA has d etermined that events were
nRCP due to high wind conditions that transported PM 10 from sources outside of the Pagosa Springs, Alamosa, and Durango areas and subsequently overwhelmed reasonable controls within Alamosa, Pagosa Springs, and Durango. Also, reg ardless of transport into the area, information pertaining to the controls implemented within the affected areas, 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 these events were nRCP . Furthermore, EPA has determined that there is a CCR between the
events and the measured e xceedances, there would have been NEBF , and the measured exceedances are
in excess of normal HFs

.
40 CFR 50.14(a)(1) states :

A State may request EPA to exclude data showing exceedances or violations of the national ambient air quality standard that are di rectly due to an exceptional event from use in determinations by demonstrating to EPA’s satisfaction that such event caused a specific air pollution concentration at a particular air quality monitoring location. 45

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

The monitoring values of 123 ug/m 3 at Mt. Crested Butte on April 28, 2010, and the values of 92 ug/m 3 at Adams State College, and 94 ug/m 3 at the Alamosa Municipal Building monitors on April 29, 2010 are not exceedances or violations of the NAAQS and therefore not able to be considered und er the
E xceptional E vents R ule. CDPHE included these values in their report to demonstrate the regional nature of the event but did not specifically request their removal.

EPA finds that the weight of evidence is sufficient to concur on the flagging of th e data for the NAAQS exceedances of 285 ug/m 3 at Adams State College, 236 ug/m 3 at the Alamosa Municipal Building, and 181 ug/m 3 at the Pagosa Springs School monitors on April 28, 2010, as well as the NAAQS exceedances of 162 ug/m 3 at the Pagosa Springs School Monitor and 226 ug/m 3 at the Durango River City Hall monitors on April 29, 2010. These concurrences do not constitute final EPA action regarding any matter on which EPA is required to provide an opportunity for public comment. In particular, this ap plies 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.