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. UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION IX 75 Hawthorne Street San Francjf,

. OFFICE OFTHE REGIONAL ADMINISTRATOR
Mr. Lewis Wallenmeyer Director Clark County Department ofAir Quality 4701 W. Russell Road Suite 200 Las Vegas, Nevada 89118 DearMr. Wallenmeyer: This letterresponds to ClarkCountyDepartmentofAir Quality’s (DAQ’s) April 11, 2014 submittal regarding three exceedances ofthe 24-hour PM 10 standardthatoccurred at monitoring stations within Clark County on July 3, 2011. DAQ’s submittal included documentation thatthese exceedances were causedbyhigh wind exceptional events. EPA has reviewed the documentation providedbyDAQ to demonstratethat the exceedances on these days meetthe criteria for an exceptional event in the Exceptional Events Rule (EER). EPA concurs based on the weight ofthe evidence thatthe exceedances were causedbyhigh wind exceptional events and finds that DAQ has successfullymade the demonstrations referredto in 40 CFR §50.14 to EPA’s satisfaction. In addition, DAQ has metthe schedule andprocedural requirements in section 50.14(c) with respectto the same data. A more detailed assessmentofDAQ’s demonstration is enclosed. My staff has or shortlywill enter “concurrence flags” forthese data into EPA’s AQS data system. Based on these determinations, EPA will exclude these data fromthe following types ofcalculations and activities:

• EPA’s Air Quality Data system (AQS) will not countthese days as exceedances when generating userreports, or include them in design values estimates, unless the AQS user specifically indicates thatthey shouldbe included.

• EPA will acceptthe exclusion ofthese data forthe purposes ofselecting appropriate background concentrations forNew Source Review air quality analyses.’

• EPA will acceptthe exclusion ofthese data forthe purposes ofselecting appropriate background concentrations fortransportation conformity hot spot analyses.:

• The data will continue to be publicly available, butEPAs publications andpublic information statements on the status ofair qualityin the affected areawill notreflectthese data in any summary statistic ofpotential regulatory application, unless such inclusion is specifically noted.

Ifwe 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 PM 10 and PM 25

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

Printed on Recycled Paperin 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 PM 10 nonattainment area has met the PM 10 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 PM 10 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 PM 10 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 PM 10 EXCEEDANCES MONITORED IN CLARK COUNTY ON JULY 3, 2011 AS EXCEPTIONAL EVENTS

EXCEPTIONAL EVENTS RULE REQUIREMENTS

EPApromulgated the Exceptional Events Rule (EER) in 2007, pursuantto the 2005 amendment ofClean AirAct (CAA) Section 319. The EERadded40 CFR §50.1(j), (k) and (1); §50.14; and §51.930 to the Code ofFederal Regulations (CFR). These sections contain definitions, criteria for EPA approval, procedural requirements, andrequirements for air agency demonstrations, all ofwhichmust be metbefore EPA can concurunderthe EER onthe exclusion ofairquality data from regulatory decisions. Under40 CFR §50.14(c)(3)(iv),the air agency demonstrationtojustify exclusion ofdatamust provide evidence that: A. “The event satisfies the criteria set forth in 40 CFR §50.1(j)” forthe definition ofan exceptional event;

• The event “affects airquality.”

• The event “is notreasonably controllable orpreventable.”

• The eventis “causedby human activity thatis unlikelyto recur at a particular location or [is] anatural event.” B. There is a clearcausal relationship betweenthe measurementunder consideration andthe eventthat is claimedto have affectedthe airquality inthe area;” C. “The event is associatedwith ameasured concentration in excess ofnormal historical fluctuations, including background;” and D. “There wouldhave been no exceedance orviolationbutforthe event.” Not Reasonably Controllable or Preventable (nRCP) EPA evaluates whether an eventwas notreasonably controllable orpreventable atthe time ofthe eventby taking into account controls inplace and wind speed, along with other factors.2 For natural sources ofdust, ahighwind dust event can generallybe consideredto be notreasonably controllable orpreventable ifwinds are high enoughto 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 ofImplementation Plans: Designation ofAreas for Air Quality Planning Purposes: State ofCalifornia: PM-l0; Affirmation ofDetermination ofAttainment forthe San Joaquin Valley Nonattainment Area.” 73 FR 14691 (March 19. 2008).
1areas. 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 of the area affected by the transported emissions; the relationship in time between the event, transport of emissions, and recorded concentrations; and, as appropriate, pollutant species-specific information supporting a 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. 3 This 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).2Generally, forhighwind dustevents, theNEBF demonstrationis similarto and informed by the demonstration ofthe nRCP and CCRrequirements, and is expectedto showthatthe measured concentration would havebeenbelowthe applicableNAAQS withoutthe effect ofthe event.



### OVERVIEW OF EVENTS On letter dated April 1 1 th, 2014, ClarkCounty Department ofAir Quality (DAQ) submitted an exceptional event demonstration for 3 exceedances ofthe 24-hourPM 10 standardthat occurred at monitoring stations within Clark County, NV onJuly 3’, 2011. Table 1 summarizes these exceedances. DAQ states thatthe three exceedances measured onJuly 31, 2011 were associatedwith ahighwind-generated dust eventwhich “was causedby thunderstorms inthe source area, which came up throughthe Colorado Rivercorridor.” DAQ provided acomprehensive description and discussion ofthe eventin the demonstration. Table 1: EPA PM 10 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 32-003-0601 242
Not Reasonably Controllable or Preventable (nRCP) Two ofthe exceeding monitoring sites, Sunrise Acres andJ. D. Smith, are located inthe Las Vegas Valley. In addressing reasonable controls inthe Las Vegas Valley, DAQ provided references to the current set ofBACM-requiredcontrols inthe Clark County (partial) Las Vegas PM 10 nonattainrnent area. Theserules, AQR Sections 90-94 “require stabilization ofopen areas and disturbed vacant lands; stabilization ofunpavedparking lots; stabilization ofunpaved shoulders onpavedroads; and use ofsoil-specific bestmanagementpractices for construction activities.” The Eldorado Valley, where the exceeding BoulderCity monitoris located, is not included in anonattainment areabutDAQ states thatthe rule concerning construction activities (Section 94) does apply in this area. As part ofDAQ’s Natural Events Action Plan forHigh-WindEvents (NEAP, April 2005), DAQ forecasts thepotential for elevatedPM 10 concentrations, however, the models didnot indicate potential elevated PMio concentrations on July 3 rd 2011. The normal procedure following forecasted elevated PM 10 concentrations includes ahigh-wind eventnotification systemthat includes an early warning and enhancedenforcement and complianceprograms to reduce emissions. This unforeseen eventnecessitated alternate methods to determine ifBACM was implemented and iflocal sources contributed significantly. In lieu ofthe normalprocedure, DAQ conducted an assessment ofvalley-wide activities, which indicated thatthere were no unique or unusual activities onthe event day, and reliedupon informal staffobservations, which indicated thatthe dust enteredthe Las Vegas Valley fromthe southeast, and was transported from outside 3the 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 PM 10 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 PM 10 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 PM 10 generated in the aforementioned source area to the monitoring sites in the Las Vegas Valley. All of the monitoring PM 10 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 of the 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 PM 10 transport described above. The analysis indicates that monitors in the Las Vegas and Eldorado Valleys were affected by PM 10 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. 4Table 3: Documentation ofnRCP 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 demonstratethatthis requirementwas met, DAQ provided 6-yeartime series plots ofall Clark County PM 1 o 24-hour averages in Figures 29-36 ofthe demonstration. DAQ stated that for Boulder City, July 3l, 2011 was the “highestrecorded 24-hour average PM 10 concentration recorded inthe Clark County PMio monitoring networkbetween 2006 and 2011.” Forthe Sunrise Acres and J.D. Smithmonitoring sites, July 3K1, 2011 was the highest concentration monitored at eachrespective site between 2006 and 2011, andwas the second andthird (respectively) highestconcentrationmonitored in the network overthe last 6 years. DAQ’s analysis sufficiently establishes thatthe 24-hourPM 10 concentrations measured on July 3 rd 2011 were in excess ofnormal historical fluctuations. Table 4: Documentation ofHF 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 ofDAQ’s demonstration included a comprehensive conceptual model ofthe events, including a general overview ofthe geographic setting ofthe monitors, local climate information, wind speed and direction data, and surface andupper airweathercharts forthe event for ClarkCounty. The conceptual model also included avery detailed discussion ofthe eventthat occurred on July 3’, 2011 and atime series graph forthe eventthatincluded hourly PM 10 concentrations for all the monitors in ClarkCounty. The demonstrationincluded site maps ofthe monitors with corresponding wind and pollution roses, hysplittrajectories indicating the origin and movement ofthe storm, and anumberof visibilityphotos attheNorth Las Vegas Airport showing the eventmove into and outofthe area through time. Before reaching the monitoring sites inthe Eldorado and Las Vegas Valleys, the dustmoved through the LaughlinlBullhead City Airport areawhich corresponds temporally with the NWS weatherobservations ofdecreasedvisibility inthe morning hours beginning at approximately 5:30 AM. TheNWS station attheNorth Las Vegas Airport andthe McCarran International Airport reported haze (HZ) onthe eventday withthe first indications occurring around 10 AM, once againtemporallymatching the description ofthe event and increased io concentrations. The National Weather Service (NWS) ForecastDiscussion forJuly 2011 describedthat amoisture surge fromthe southeastbrought acloud ofdust andthatwebcams show reduced visibility. The discussion furtherdescribes the event as “blowing dust which is not teclmically correctbut is the closest option with the available tools for Clark [County].” This description is indicative ofthe nature ofthis event as itwas causedby high winds, thoughupon arrival into the area, winds were light. Also included inthe demonstration were multiple videos ofmedia coverage concerning the dustevent. 5DAQ stated that the evidence presented shows a clear connection between the exceedance and event that is demonstrated by “the dramatic increase in hourly PM 10 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 PM 10 laden airflow” to the northeast of this site, resulting in lower particulate measurements. The analysis in Section 3, specifically, the PM 10 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 Demonstration Citation Exceedance Quality of Criterion Date Evidence Met? July 3, 2011 App. A Sufficient Yes 6No Exceedance orViolation But Forthe Event (NEBF) DAQ provided several reasons thatthe exceedance would nothave occurred but forthe event. Concentrations were low at all three violating monitors in the morning and the evening ofJuly 3 rd 2011, before the arrival ofthe dustfromthe southwestunderlow wind conditions andthe subsequent dispersion due to increasedwind speeds. DAQprovided a summary ofthe analysis and informationregarding boththe nRCP and CCRrequirements and also included atime series graph that includedhourly PM 1 o, hourly wind speeds (where available), andwind gusts (where available) indicating that PM 10 concentrations before afterthe eventwere belowthe 24-hour PM 10 NAAQS. DAQ’s demonstrationregarding the nRCP and CCRrequirements sufficiently establishes thattheNEBF criterionhas beenmet. 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 demonstrationrequirements, 40 CFR §50.14(c) specifies the schedule and procedural requirements an air agency must follow to request dataexclusion. Table 9 outlines EPA’s evaluation ofthese requirements. The promptpublic notification criterion was complicated due to the lack ofthis eventbeing forecasted. Normally, DAQ performs meteorological forecastingto predict apotential high wind event, and wouldbroadcast an advisoryto the public throughmediachannels in orderto alertthepublic. This was not done as it was notpredicted. However, DAQ provides documentation for abroadcastedholiday/smoke advisory on July l forJuly 4 th and forfireworks whichreasonably allowedthe public to make health-based decisions due to potentially high concentrations ofairpollution forthe day prior, July 31• Also, AQI (Air Quality Index) provides additional, publicly-available forecasting/prediction that is available online to the generalpublic. Table 9: Schedules and Procedural Criteria Demonstration Reference Citation Criterion Met? Didthe State provide prompt public 40 CFR §50.14 Section 1: P 3 Yes notification ofthe event? (c)(1)(i) Were flags and initial description placed 40 CFR §50.14 Section 1: p.3, Yes on the databyJuly l ofthe following (c)(2)(iii) Section 3: p. year? 117 Was the demonstration submitted within 40 CFR §50.14 April 17, 2014 Yes 3 years ofthe end ofthe quarterin (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

7Conclusion 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 PM 10 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 PM 10 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 PM 10 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. 8