D Sr 41.

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

REGION IX

75 Hawthorne Street

San Francjf, .

OFFICE OF THE

REGIONAL ADMINISTRATOR

Mr. Lewis Wallenmeyer

Director

Clark County Department of Air Quality

4701 W. Russell Road Suite 200

Las Vegas, Nevada 89118

Dear Mr. Wallenmeyer:

This letter responds to Clark County Department of Air Quality’s (DAQ’s) April 11, 2014 submittal

regarding three exceedances of the 24-hour PM10 standard that occurred at monitoring stations within

Clark County on July 3, 2011.

DAQ’s submittal included documentation that these exceedances were caused by high wind exceptional

events. EPA has reviewed the documentation provided by DAQ to demonstrate that the exceedances on

these days meet the criteria for an exceptional event in the Exceptional Events Rule (EER). EPA concurs

based on the weight of the evidence that the exceedances were caused by high wind exceptional events

and finds that DAQ has successfully made the demonstrations referred to in 40 CFR §50.14 to EPA’s

satisfaction. In addition, DAQ has met the schedule and procedural requirements in section 50.14(c)

with respect to the same data. A more detailed assessment of DAQ’s demonstration is enclosed. My staff

has or shortly will enter “concurrence flags” for these data into EPA’s AQS data system.

Based on these determinations, EPA will exclude these data from the following types of calculations and

activities:

• EPA’s Air Quality Data system (AQS) will not count these days as exceedances when

generating user reports, or include them in design values estimates, unless the AQS user

specifically indicates that they should be included.

• EPA will accept the exclusion ofthese data for the purposes of selecting appropriate

background concentrations for New Source Review air quality analyses.’

• EPA will accept the exclusion of these data for the purposes ofselecting appropriate

background concentrations for transportation conformity hot spot analyses.:

• The data will continue to be publicly available, but EPAs publications and public

information statements on the status of air quality in the affected area will not reflect these

data in any summary statistic of potential regulatory application, unless such inclusion is

specifically noted.

If we are the permitting authority, we will propose permits on this basis. lfwe are commenting on another permitting

authority’s proposed action, our comments will be consistent with the determinations in this letter.

Applicable only to PM10 and PM25.

These data may be included in statistics intended to describe trends in actual air quality in the area.

Printed on Recycled Paper
in addition, EPA will rely on calculated values that exclude these data in proposed regulatory actions,

such as a proposed designation, classification, attainment demonstration, or finding as to whether the

Las Vegas PM10 nonattainment area has met the PM10 NAAQS. These regulatory actions require EPA to

provide an opportunity for public comment prior to taking a final Agency action. If EPA is pursuing one

of these actions for the Las Vegas PM10 nonattainment area, EPA will open a new comment period

during which EPA may receive comments on the exceptional event submission you have made and the

determinations conveyed in this letter. If so, we must consider and respond to those comments before

taking final regulatory action. Accordingly, the determinations conveyed in this letter do not constitute

final EPA action regarding any matter on which EPA is required to provide an opportunity for public

comment. In particular, this point 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. As an additional clarification, the determinations conveyed in this

letter are applicable only to determinations incorporating the submitted data relative to the PM10

NAAQS.

If you have any questions or wish to discuss this matter further, please contact Deborah Jordan. Director,

Air Division at (415) 947-8715.

Sincerely,

Jared Blumenfèld

Enclosure

cc: Ms. Jasmine Mehta, Nevada Division of Environmental Protection
ENCLOSURE: TECHNICAL SUPPORT DOCUMENT FOR EPA CONCURRENCE ON

PM10 EXCEEDANCES MONITORED IN CLARK COUNTY ON JULY 3, 2011 AS

EXCEPTIONAL EVENTS

EXCEPTIONAL EVENTS RULE REQUIREMENTS

EPA promulgated the Exceptional Events Rule (EER) in 2007, pursuant to the 2005 amendment

of Clean Air Act (CAA) Section 319. The EER added 40 CFR §50.1(j), (k) and (1); §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:

A. “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.”

• The event is “caused by human activity that is unlikely to recur at a

particular location or [is] a natural event.”

B. 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;”

C. “The event is associated with a measured concentration in excess of normal historical

fluctuations, including background;” and

D. “There would have been no exceedance or violation but for the event.”

Not Reasonably Controllable or Preventable (nRCP)

EPA evaluates whether an event was not reasonably controllable or preventable at the time of the

event by taking into account controls in place and wind speed, along with other factors.2 For

natural sources of dust, a high wind dust event can generally be considered to be not reasonably

controllable or preventable if winds are high enough to cause emissions from natural undisturbed

‘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 See e.g., EPA, Final rule. “Approval and Promulgation of Implementation Plans: Designation of Areas for Air Quality Planning

Purposes: State of California: PM-l0; Affirmation of Determination of Attainment for the San Joaquin Valley Nonattainment

Area.” 73 FR 14691 (March 19. 2008).

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areas. For anthropogenic sources of dust, a high wind dust event is also eligible to be considered

to be not reasonably controllable or preventable if

1. The anthropogenic sources of dust have reasonable controls in place.

2. The reasonable controls have been effectively implemented and enforced, and

3. The wind speed was high enough to overwhelm the reasonable controls.

Historical Fluctuations (HF)

EPA evaluates whether a measured exceedance is in excess of historical fluctuation by taking

into account the level of the exceedance in relation to historical data, which is typically 3 to 5

years.

Clear Causal Relationship (CCR)

EPA considers a variety of evidence when evaluating whether 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. Demonstrations typically 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 elevated concentration measurements were recorded; the size ofthe 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 causal relationship between the event and the measured concentration.

Affects Air Quality (AAQ)

Generally, EPA will consider events to have affected air quality if the CCR and HF requirements

have been adequately demonstrated.

Natural Event

Generally, EPA will consider a high wind dust event to be a natural event in cases where

windblown dust is entirely from natural sources or where all significant anthropogenic sources of

windblown dust have been reasonably controlled. 3This typically involves adequately

demonstrating both the nRCP and CCR requirements.

No Exceedance or Violation But For the Event (NEBF)

The EPA will generally consider human activity to have played little or no direct role in causing emissions of the dust generated

by high wind for purposes of the regulatory definition of”natural event” if contributing anthropogenic sources of the dust are

reasonably controlled, regardless of the amount of dust coming from these reasonably controlled anthropogenic sources, and thus

the event could be considered a natural event. In such cases, the EPA believes that it would generally he a reasonable

interpretation of its regulations to find that the anthropogenic source had “little” direct causal role. If anthropogenic sources of

windblown dust that are reasonably controllable hut that did not have those reasonable controls applied at the time of the high

wind event have contributed significantly to a measured concentration, the event would not be considered a natural event. See

preamble to the EER. 72 FR 13560. at 13566. Fn. 11 (March 22. 2007).

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Generally, for high wind dust events, the NEBF demonstration is similar to and informed by the

demonstration of the nRCP and CCR requirements, and is expected to show that the measured

concentration would have been below the applicable NAAQS without the effect of the event.

OVERVIEW OF EVENTS

On letter dated April 1 1th, 2014, Clark County Department of Air Quality (DAQ) submitted an

exceptional event demonstration for 3 exceedances of the 24-hour PM10 standard that occurred at

monitoring stations within Clark County, NV on July 3’, 2011. Table 1 summarizes these

exceedances.

DAQ states that the three exceedances measured on July 31, 2011 were associated with a highwind-generated dust event which “was caused by thunderstorms in the source area, which came

up through the Colorado River corridor.” DAQ provided a comprehensive description and

discussion of the event in the demonstration.

Table 1: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (ig/m 3)

Sunrise Acres 32-003-0561 191

July 3,2011 J. D. Smith 32-003-2002 185

Boulder City 32-003-060 1 242

Not Reasonably Controllable or Preventable (nRCP)

Two ofthe exceeding monitoring sites, Sunrise Acres and J. D. Smith, are located in the Las

Vegas Valley. In addressing reasonable controls in the Las Vegas Valley, DAQ provided

references to the current set of BACM-required controls in the Clark County (partial) Las Vegas

PM10 nonattainrnent area. These rules, AQR Sections 90-94 “require stabilization of open areas

and disturbed vacant lands; stabilization of unpaved parking lots; stabilization of unpaved

shoulders on paved roads; and use ofsoil-specific best management practices for construction

activities.” The Eldorado Valley, where the exceeding Boulder City monitor is located, is not

included in a nonattainment area but DAQ states that the rule concerning construction activities

(Section 94) does apply in this area.

As part of DAQ’s Natural Events Action Plan for High-Wind Events (NEAP, April 2005), DAQ

forecasts the potential for elevated PM10 concentrations, however, the models did not indicate

potential elevated PMio concentrations on July 3rd 2011. The normal procedure following

forecasted elevated PM10 concentrations includes a high-wind event notification system that

includes an early warning and enhanced enforcement and compliance programs to reduce

emissions. This unforeseen event necessitated alternate methods to determine if BACM was

implemented and if local sources contributed significantly. In lieu of the normal procedure, DAQ

conducted an assessment of valley-wide activities, which indicated that there were no unique or

unusual activities on the event day, and relied upon informal staff observations, which indicated

that the dust entered the Las Vegas Valley from the southeast, and was transported from outside

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the non-attainment area. This informal observation corroborates with the wind patterns recorded.

DAQ stated that “BACM was effectively implemented for all applicable emissions sources and

that local sources did not contribute to the elevated PM10 concentrations measured at the Boulder

City. Sunrise Acres, and J.D. Smith monitoring sites.”

DAQ used a weight of evidence approach and concluded that “the activity assessment. informal

field observations, and moderate local wind speeds validate BACM rule penetration, rule

effectiveness, and overall BACM control measures contained in the 2001 PM10 SIP.” DAQ cited

a study (June 2006) which concluded that 25 mph could serve as a threshold value for

overwhelming BACM and this wind speed was surpassed in the source area, which corroborates

with the 25 mph threshold determined to be sufficient in EPA’s draft guidance on high-wind

events in the west (EPA, June 20l2). DAQ provided documentation indicating that wind speeds

associated with the source area of the event were above 25 mph and that the high winds

“follow[ed] in time and magnitude from the Blythe Airport area to the Needles Airport area and

up through the Laughlin, NevadalBullhead City, Arizona airport in a northeasterly direction to

affect the Eldorado and Las Vegas Valleys.” The l3lythe Airport in Blythe, California recorded

wind speeds of up to 37mph and wind gusts of 48 mph and the Needles Airport in Needles,

California recorded wind speeds of up to 24 mph and wind gusts up to 40 mph. As the front

approached the Bullhead City/Laughlin Airport Nevada, the winds speeds decreased to 20 mph

with gusts of up to 31 mph. As the wind-blown generated dust entered the Eldorado Valley,

followed by the Las Vegas Valley, wind speeds were relatively low, but were sufficient to

transport PM10 generated in the aforementioned source area to the monitoring sites in the Las

Vegas Valley.

All of the monitoring PM10 monitoring sites in Las Vegas Valley exhibited elevated

concentrations, though only two exceeded the NAAQS. The only monitoring site in the Eldorado

Valley also exceeded the NAAQS, while the monitoring site in the Ivanpah Valley was not

affected as it was not in the predominant wind and dust flow corridor. The path ofthe storm as it

‘progressed through the Las Vegas Valley remain[ed] predominately to the eastern portion of the

valley, hugging the mountain range directly to the east of the valley and exited to the northeast of

the Las Vegas Valley” which accounts for the spatial extent of the exceeding monitors.

DAQ’s documentation included analysis of the event that supports the PM10 transport described

above. The analysis indicates that monitors in the Las Vegas and Eldorado Valleys were affected

by PM10 transported by high winds from outside the Valleys. In addition to transport.

information supplied by DAQ pertaining to controls implemented within the area, the spatial

extent of elevated io concentrations with respect to the path of the dust, and the wind speeds

associated with the event sufficiently establishes that the event was not reasonably controllable

or preventable.

Wacaser. eta!. Summary of Refined PMIO Aeo!ian Emission Factors for Native Desert and Disturbed Vacant Land Areas.

Department of C’ivil and Environmental Engineering. University of Nevada, Las Vegas. June 2006.

Draft 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. United States Environmental Protection Agency. June 2012.

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Table 3: Documentation of nRCP

Exceedance Demonstration Citation Quality of Criterion

Date Evidence Met?

July 3,2011 Section 1: p. 15-18, 82, Section 5: p. 234-235 Sufficient Yes

Historical Fluctuations (HF)

To demonstrate that this requirement was met, DAQ provided 6-year time series plots of all

Clark County PM1o24-hour averages in Figures 29-36 ofthe demonstration. DAQ stated that for

Boulder City, July 3l, 2011 was the “highest recorded 24-hour average PM10 concentration

recorded in the Clark County PMio monitoring network between 2006 and 2011.” For the

Sunrise Acres and J.D. Smith monitoring sites, July 3K1, 2011 was the highest concentration

monitored at each respective site between 2006 and 2011, and was the second and third

(respectively) highest concentration monitored in the network over the last 6 years. DAQ’s

analysis sufficiently establishes that the 24-hour PM10 concentrations measured on July 3rd 2011

were in excess of normal historical fluctuations.

Table 4: Documentation of HF

Exceedance Demonstration Citation Quality of Criterion

Date Evidence Met?

July 3,2011 Section 2: 82-83, 85-92 Sufficient Yes

Clear Causal Relationship (CCR)

Section 3 of DAQ’s demonstration included a comprehensive conceptual model of the events,

including a general overview of the geographic setting of the monitors, local climate

information, wind speed and direction data, and surface and upper air weather charts for the

event for Clark County. The conceptual model also included a very detailed discussion of the

event that occurred on July 3’, 2011 and a time series graph for the event that included hourly

PM10 concentrations for all the monitors in Clark County.

The demonstration included site maps ofthe monitors with corresponding wind and pollution

roses, hysplit trajectories indicating the origin and movement of the storm, and a number of

visibility photos at the North Las Vegas Airport showing the event move into and out of the area

through time. Before reaching the monitoring sites in the Eldorado and Las Vegas Valleys, the

dust moved through the LaughlinlBullhead City Airport area which corresponds temporally with

the NWS weather observations of decreased visibility in the morning hours beginning at

approximately 5:30 AM. The NWS station at the North Las Vegas Airport and the McCarran

International Airport reported haze (HZ) on the event day with the first indications occurring

around 10 AM, once again temporally matching the description ofthe event and increased io

concentrations. The National Weather Service (NWS) Forecast Discussion for July 2011

described that a moisture surge from the southeast brought a cloud of dust and that webcams

show reduced visibility. The discussion further describes the event as “blowing dust which is not

teclmically correct but is the closest option with the available tools for Clark [County].” This

description is indicative of the nature of this event as it was caused by high winds, though upon

arrival into the area, winds were light. Also included in the demonstration were multiple videos

of media coverage concerning the dust event.

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DAQ stated that the evidence presented shows a clear connection between the exceedance and

event that is demonstrated by “the dramatic increase in hourly PM10 concentrations that

coincided with the high-wind event transported dust from the multiple storm cells in the source

area and the outflow boundary that occurred in the source area that blew through Bullhead City,

Arizona, up the Colorado River corridor into the Eldorado and Las Vegas Valleys.” DAQ

accounted for the reason that only three monitors in Clark County exceeded, though all monitors

En the Las Vegas and Eldorado Valleys exhibited elevated concentrations during the same time

period. The non-exceeding monitors in the Las Vegas Valley were not in the path of the storm,

except for the Green Valley monitor, which did not exceed due to a small mountain ridge that

‘funne1ed a portion of the PM10 laden airflow” to the northeast ofthis site, resulting in lower

particulate measurements.

The analysis in Section 3, specifically, the PM10 time series graph, winds speed and direction

measurements, photographic evidence, media coverage, NWS forecast descriptions, and NWS

station reports of reduced visibility and haze, sufficiently establishes that there was a clear causal

relationship between uncontrollable emissions generated from high-winds in the source area and

the exceedances measured in the Las Vegas and Eldorado Valley monitors.

Table 5: Documentation of CCR

Exceedance Demonstration Citation Quality of Criterion

Date Evidence Met?

July 3, 201 I Section 3: p 117-193 Sufficient Yes

Affects Air Quality (AAQ)

Based on the documentation presented for both the CCR and HF requirements, it has been

adequately demonstrated that this event affected air quality. DAQ’s demonstration regarding the

CCR and HF requirements sufficiently establishes that the event affected air quality.

Table 6: Documentation of AAQ

Exceedance Demonstration Citation Quality of Criterion

Date Evidence Met?

July 3, 201 I Section 2: p 15-18 Sufficient Yes

Natural Event

Based on the documentation presented for both the nRCP and CCR requirements. it has been

adequately demonstrated that the event is a natural event caused by high-wind-generated dust.

DAQ’s demonstration regarding the CCR and HF requirements sufficiently establishes that the

event was a natural event.

Table 7: Documentation of Natural Event

Exceedance Demonstration Citation Quality of Criterion

Date Evidence Met?

July 3, 2011 App. A Sufficient Yes

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No Exceedance or Violation But For the Event (NEBF)

DAQ provided several reasons that the exceedance would not have occurred but for the event.

Concentrations were low at all three violating monitors in the morning and the evening of July

3rd 2011, before the arrival of the dust from the southwest under low wind conditions and the

subsequent dispersion due to increased wind speeds. DAQ provided a summary of the analysis

and information regarding both the nRCP and CCR requirements and also included a time series

graph that included hourly PM1o, hourly wind speeds (where available), and wind gusts (where

available) indicating that PM10 concentrations before after the event were below the 24-hour

PM10 NAAQS. DAQ’s demonstration regarding the nRCP and CCR requirements sufficiently

establishes that the NEBF criterion has been met.

Table 8: Documentation ofNEBF

Exceedance Demonstration Citation Quality of Criterion

Date Evidence Met?

July 3,2011 Section 2: p. 83-84 Sufficient 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. Table 9

outlines EPA’s evaluation of these requirements. The prompt public notification criterion was

complicated due to the lack of this event being forecasted. Normally, DAQ performs

meteorological forecasting to predict a potential high wind event, and would broadcast an

advisory to the public through media channels in order to alert the public. This was not done as it

was not predicted. However, DAQ provides documentation for a broadcasted holiday/smoke

advisory on July lfor July 4th and for fireworks which reasonably allowed the public to

make health-based decisions due to potentially high concentrations of air pollution for the day

prior, July 31• Also, AQI (Air Quality Index) provides additional, publicly-available

forecasting/prediction that is available online to the general public.

Table 9: Schedules and Procedural Criteria

Demonstration

Reference Citation Criterion Met?

Did the State provide prompt public 40 CFR §50.14 Section 1: P3 Yes

notification of the event? (c)(1)(i)

Were flags and initial description placed 40 CFR §50.14 Section 1: p.3, Yes

on the data by July l of the following (c)(2)(iii) Section 3: p.

year? 117

Was the demonstration submitted within 40 CFR §50.14 April 17, 2014 Yes

3 years of the end of the quarter in (c)(3)(i) Letter

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 Section 1: plO, Yes

tbllowed and documented? (c)(3)(v) App. C

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Conclusion

EPA has reviewed documentation provided by DAQ to support claims that dust emissions

generated by high winds that were transported into the Las Vegas and Eldorado Valleys caused

exceedances of the 24-hour PM10 NAAQS at the Boulder City, Sunrise Acres, and J. D. Smith

monitoring sites on July 2011. EPA has determined that the flagged exceedances at this

location on this day meet the definition of an exceptional event: the exceedance affected air

quality, was not reasonably controllable or preventable, and meets the definition of a natural

event. Specifically, EPA has determined that the event was not reasonably controllable and

preventable due to high wind conditions that transported PM10 up the Colorado River from

southeastern California and northwestern Arizona outside of the Las Vegas Valley and Eldorado

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

area, information pertaining to the controls implemented within the nonattainment area, the

spatial extent of elevated PM10 concentrations measured in the area, and the wind speeds

associated with the high-wind event provide sufficient evidence to conclude that the event was

not reasonably controllable or preventable. Furthermore, EPA has determined that there is a clear

causal relationship between the event and the measured exceedance, there would have been no

exceedance but for the event, and the measured excecdance is in excess of normal historical

fluctuations.

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