UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION 10 1200 Sixth Avenue, Suite 900 Seattle, WA 98101-3140 

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## l013 Mr. Curt Fransen, Director Idaho Department ofEnvironmental Quality 1410 North Hilton Boise, Idaho 83706
DearMr. Fransen: OFFICE OFTHE REGIONAL ADM IN ISTRATOR

This letterresponds to the Idaho Department ofEnvironmental Quality's September 23, 2013, submittal regarding the elevated 24-hour 

### PMw National Ambient Air Quality Standard levels at the Boise and Nampa air monitors during February 2011. The IDEQ has requested that the Environmental Protection Agency concurthat the PM10 levels for the two submitted dates were elevated due to an exceptional event where dust from a natural area (playa) was entrained and transported byhigh winds. The Boise monitor exceeded the 150 Jlg/m3 PM 10 24-hourNAAQS on February 15 and 16,2011, and the Nampa monitor exceeded the same 

### PMw standard onFebruary 15, 2011.
Aftercareful consideration oftheinformationprovided, we concurwith the IDEQ's exceptional events flags for February 15 and 16, 2011, atthe Boise Fire StationNumber 5 monitor. The elevated value on February 15, 2011, at the Nampa Fire Stationmonitor was not reviewed as it does nothave regulatory significance

. Thebasis for ourdecision ontheBoise events flags is setforth inthe enclosed document.
Note that the EPA's decisions on exceptional event exclusions are not considered final agency action until they are acted upon as part ofa final regulatory action subject to public notice and comment. Such actions could include decisions to excludethe affected data from use in an approval ofa non-attainment plan, development ofa maintenanceplan ordetermination ofattainment.
Thank you for the IDEQ' s timely submittal ofthis exceptional event documentation. Ifyou have any questions orwish to discuss this matter further, please contactme orhave your staffcontact Justin Spenillo, State and Tribal AirPrograms Unit in the Office ofAir, Waste and Toxics at (206) 553-6125. Sincerely,

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Enclosure

cc: Tiffany Floyd Administrator, Air Quality Division Dennis J. McLerran Regional Administrator

Idaho Department ofEnvironmental QualityBruce Louks Manager, Modeling, Monitoring, and Emissions Inventory Idaho Department ofEnvironmental Quality
Robert Wilkosz Manager, Mobile and Area Source Program Idaho Department ofEnvironmental QualityEPA Region 10 Review ofExceptional Event Request Boise and Nampa, ID 24-hour PM10 NAAQS Dates Analyzed: February 15 and 16, 2011

Background
On March 22, 2007, the EPA adopted a final rule, Treatment of Data Influenced byExceptional Events (Exceptional Events Rule or EER at 72 FR 13560) to govern the review and handling of certain air quality monitoring data for which the normal planning and regulatory processes are not appropriate. Under the EER, the EPA may exclude data from use in determinations of National Ambient Air Quality Standard (NAAQS) exceedances and violations ifa state demonstrates that an "exceptional event" caused the exceedances. Before the EPA can exclude data from these regulatory determinations, the state must flag the data in the EPA's Air Quality System (AQS) database and, after notice and opportunity for public comment, submit a demonstration tojustifythe exclusion. After considering the weight ofevidence provided in the demonstration, the EPA decides whether ornot to concur with each flag.
IDEO's Request
The Idaho Department ofEnvironmental Quality (IDEQ) requested concurrence on flagged 24- hour PM10data on February 15,2011, and February 16,2011, at the Boise Fire Station No.5, Ada County, Idaho monitor (AQS site 16-001-0009 POC3) and on February 15, 2011 at the Nampa Fire Station, Canyon County, Idaho monitor (AQS site 16-027-0002 POC2); herein these will be referred to as the Boise and Nampamonitors, respectively. The recorded 24-hour PM10 levels for both monitors ranged from 156 J.tg/m3to 183 J.tg/m3for the data values during the exceptional events for which the IDEQ is requesting the EPA's concurrence. B.

mse andNampa M "t d V 1 om ore a ues Date Boise monitor- NampamonitorPM10 concentration PM10 concentration (J.tg/m3) (J.tg/m3) February 15, 2011 183 174 February 16, 2011 156 Not submitted

PM10 levels from the submitted days exceeded the 150 J.tg/m3PM10 NAAQS. Documentation provided by IDEQ demonstrates that preceding and during those days there were high winds entraining dust particles from the playas (dry lake beds) in Nevada and transporting the playa dust northwest towards the Boise and Nampa area in southwest Idaho.
The IDEQ flagged the monitored values as high wind exceptional events before the statutory deadline ofJuly 1, 2012. The agency made the documentation available for public comment for 30 days starting on August 5, 2013, and received no comments. The IDEQ submitted the


### 1exceptional event demonstration package to the EPA Region 10 on September 23, 2013, for the three dates at two sites as described above. IDEQ is requesting concurrence from the EPA on these days, based on the agency's conclusion that the data has regulatory significance with regard to the Northern Ada County PM 10 maintenance plan which relies on data from these monitors.
The EPA's Exceptional Event Evaluation
The EPA evaluated whether the documentation provided by the IDEQ for the flagged values of the 24-hour PM 10NAAQS for February 15-16, 2011, at the monitor in Boise, Idaho, demonstrate that the requirements ofthe Exceptional Events Rule were met. The elevated value on February 15, 2011, at Nampa Fire Station monitor was not reviewed as it did not have regulatory significance because the elevated value does not affect attainment ofthe NAAQS at the Nampa monitor

. The table below summarizes the requirements ofthe Exceptional Events Rule and describes how the IDEQ met each requirement. All references to page numbers, tables, and figures relate to the IDEQ's September 23, 2013 submittal.

Procedural Requirements: EPA's Evaluation ofFlagged Exceedances:

• The data are flagged The IDEQ flagged and described the February 15-16,

and include an initial 2011 24-hour PM 10 values as a high wind entrained dust event description in exceptional event in the EPA's AQS database prior to the EPA's AQS the July 1, 2012 deadline. database. 40 CFR 50.14(c)(2)(i) and (iii)

• The public had an The IDEQ provided a 30-day public comment period on

opportunity to review the documentation for the multi-day claimed exceptional and comment on event. The public comment period began on August 5, demonstration 2013 and the IDEQ received no comments. justifying data exclusion. 40 CFR 50.14(c)(3)(i) and (iv)

• Demonstration The EPA received demonstration documentation from justifying data the IDEQ on September 23, 2013 before the deadline of exclusion submitted not later than 3 years following the end ofthe calendar timely to the EPA. 40 quarter in which the flagged concentration was recorded CFR 50.14(c)(3)(i) (in this case, March 31, 2014).



### 2Technical Criteria:



## • The event satisfies ConceptualModelIAffectedAir Quality the criteria in 40 The IDEQ developed a conceptual model that shows CFR 50.10) (i.e., how a high wind event affected air qualityby entraining affects air quality, is dust from large natural lake beds in Nevada and not reasonably transporting it northeast into Idaho on February 14-16, controllable or 2011. The playas in northern Nevada were uncovered preventable, is an and rainfall in the prior month, as shown in Figure 18, event caused by made the lakebed surface dust susceptible to high wind human activity that entrainment. The high winds on February 14-16 were is unlikely to recur at the first ones after the rain that were strong enough to a particular location entrain the evaporites (playa dust) and had transport
or a natural event, winds pointed towards Boise. Dust was transported to and is determined by the Boise area from the evening ofthe 14th until the EPA to be an evening ofthe 15th. Dust remained in the air overnight exceptional event). and into the day ofthe 16th due to light winds at the 40CFR surface. Strong winds associated with the cold front 50.14(c)(3)(iii)(A) cleared out the dust late inthe morning on the 16th and ended the event.
As evidence that the event affected air quality, the IDEQ documented that the event exceeded historical fluctuations and demonstrated that there was a clear causal relationship between the event and the exceedances at the monitors. In the executive summary, Table A identifies that the two monitors on the submitted days recorded values above the PM10 24-hr standard. In section 3, Figures 11 identifies the event as responsible for three ofthe five values above 150 J..lg/m3 in the past 10 years, compared to several thousand values below 100 J..lg/m3• Table 1 identifies the event recorded values as being several standard deviations above the mean annual concentration at the monitors of between 22 and 25 J..lg/m3• The clear causal relationship criterion (described below) also supports the conclusion that the event affected air quality. This section describes the event using HYSPLIT back trajectories, hourly PM2

.s levels, observed meteorological data, MODIS imagery, and text. Appendices E - F also provide supporting information in the form ofnewspaper articles and distributed IDEQ air quality alerts.



### 3Natural Event and notReasonably Controllable or Preventable The IDEQ described the Nevada lake beds area as naturallakebeds, primarily the Black Rock Playa and the Carson Sink Playa, covering approximately 4,000 square kilometers, and includes discussion in section 1.3 explaining the natural phenomenon ofairborne dust formation in the playas. Figure 18 supports how the surface received significant precipitation in January 2011. Between the natural erosion process described in section 1.3 and the weakening ofthe surface layer via precipitation and drying, the playas were highly susceptible to erosion in February 2011. In Figure 5, the IDEQ plots surface wind speeds, wind gust speeds, and wind directions. These charts identify that at some weather stations the surface wind speeds exceeded 20- 30 mph, wind gusts exceeded 40 mph, and that the wind
was blowing in the direction ofthe Boise area. The MODIS imagery in Figures 13-17 clearly identify dust entrainment during the period ofwinds in the 20-30 mphrange. The description supports the assertion that the event occurred over a natural area and carried nonanthropogenic dust northeast towards the monitors that recorded the event. For this case the "not reasonably controllable orpreventable" (nRCP) criterion is established through a weight ofevidence ofMODIS imagery and a mix ofwind measurements above 25 mph and between 15 mph and 25 mph. 1

The IDEQ discusses anthropogenic sources ofemissions in section 2.3 and 4.8 and identifies that they did not contribute to the event for a variety ofreasons. The particles measured at the monitor were coarse and geologic in origin, which rules out combustion sources. In addition, the specific geological minerals in the particles are more abundant on the playas than locally in Boise. Regional PM2
.5monitors show the event was regional in nature and not confined to Boise. These facts combined with evidence ofno unusual road dusting or

1 The documentation does not include specific and scientificjustification for entrainment ofthe playa dust at wind speeds below 25 mph. The EPA, therefore, does not agree that the documentation establishes 15 mph or 20 mph as the default entrainment velocity for these playas. Rather, the submitted documentation indicates that this was an infrequent string of events: just the right amount ofprecipitation to create the evaporites followed by a wind storm.

4agricultural activities substantiate the claim that the dust was from the playas ratherthan from local emissions of geologic material. An annual event onthe Black Rock Playa that may disturb a very small portion ofthe playas
occurs inthe summernot the winter whenthe exceedances were measured and so that anthropogenic event was deemed to nothave contributed to the event.
The above information supports the conclusion that this was a natural event, the dust emissions were primarily natural and derived from the Nevadaplayas, and the event was not reasonably controllable orpreventable.



## • There is a clear Overview causal relationship The IDEQ established the clear causal relationship by betweenthe utilizing a series ofMODIS satellite images (Figures 13- exceedance and the 16) which show the lake beds priorto the event and claimed exceptional during the event. Theimages clearly show the creation event. 40 CFR ofdust clouds from the playas that canbe seen traveling 50.14(c)(3)(iii)(B) northeast towards Boise. InFigure 17, MODIS false color image analyses support that the dust clouds seen 

### in the images were 

### in fact dust and not clouds/watervapor. Surface wind speed and regional wind models, seen in Figures 4, 5, 21 and 27, support the wind conditions for the time ofthe eventbyidentifyinghighwinds in the region that wereblowing from the southwest to the northeast and which align with the times in which the monitorrecorded highparticulatematterlevels on February 15 and 16. Section 4.3 corroborates the movement ofthe emissions seenintheMODIS imagery byusing NOAA's HYSPLIT modeling tool onthe days ofthe event, February 15-16; these canbe seen in Figure 28-29. PMto monitors show theregional nature ofthe event and show a rise and fall in concentration consistent with theMODIS imagery, HYSPLIT trajectories, and synoptic and mesoscale weather analyses. The analysis ofalternative hypotheses and nearlyunprecedented levels ofPMto from this event rule out other sources for the high PMto concentrations

. Conclusion Based onthe suite ofevidenceprovided, including MODIS imagery, HYSPLIT back trajectories, surface wind speed and wind direction data, and no evidence of
significant sources ofanthropogenic PM10, the EPA concludes that there is a clear causal connection



### 5between the Boise monitor values recorded on February 15-16,2011, and the high wind event that entrained and transported dust from the Nevadaplayas.


# • The event is To show PM10 concentrations during the 2011 high wind associated with event exceeded historical fluctuations, the IDEQ measured showed in section 3 that the average particulate matter concentrations in concentrations historically were between 22 and
excess ofnormal 25Jlg/m3 between 2002 and 2011. This shows that the historical event recorded values between 156

- 183Jlg/m3 were fluctuations several standard deviations above the average. This is including expanded upon in Figures 7-12 which use a variety of background. 40 CFR charts to show that these values were higher than normal 50.14 (c)(3)(iii)(C) and havehappened only a couple times at that magnitude in thepast decade.
Based on the presented data, the EPA concludes that the values recorded on February 15-16, 2011 are in excess ofnormal historical fluctuations including background.


# • There would have To show there would have been no exceedances "but been no exceedartces for" the event, the IDEQ demonstrated that no sources "but for" the event. other than wind-blown dust have been able to generate 40CFR concentrations above the standard of 150 Jlg/m3 in the 50.14(c)(3)(iii)(D) recent past. Chemical data and PM2. 5/PM10 ratios demonstrate that the high values are from geologic particles. Local sources that have in the recent past created wind-blown dust events in Boise were not contributing on February 14-16, 2011, and the chemical data suggest evaporite material. Information provided in the clear, causal relationship section identifies the playas
as a very large source ofdust onthose days, and the PM10 values in Boise rose and fell consistent with transport ofthis dust from the playa. In addition, there
were no unusual changes in combustion-related emissions. There is sufficient qualitative and semiquantitative evidence to conclude that PM 10 concentrations in the Boise area would have been well below 150 Jlg/m3 ifnot for the dust contributed bythe Nevadaplayas and thus there would have been no exceedance but for the event.
Given that the exceedances submitted were above the 150 Jlg/m3 standard (ranging from 156

- 183 Jlg/m3) and well above the background concentration levels, the demonstration ofa clear causal relationship between the



### 6high wind event and the elevated emissions, and that there wereno otherreported irregular emissions events during that timeperiod, the EPA concludes that but for thehigh wind event entraining and transporting dust from Nevadato the Boise area, there would havebeen no exceedances ofthe 24-hourPM 10NAAQS at the monitor inBoise, Idaho.



## • Mitigation, 40 CPR The IDEQ provided public notification with Air Quality 51.930 Advisories onFebruary 15 and 16 and included associated education materials/ weblinks inthe distributed advisories. These canbe found inAppendix E. Given the shortduration ofthe event, its unexpected occurrence, and therare frequency ofoccurrence, the EPA concludes that thenecessarymitigation steps were taken during this event.

Conclusion
Based on the documentation submitted bythe IDEQ on September23, 2013, the EPA concurs on thePM10data values listed in following table which havebeenflagged bythe IDEQ inAQS as exceptional events.
Boise Monitored Values Date Boisemonitor

- 

### PMw concentration (J.!g/m3) February 15, 2011 183 February 16, 2011 156
The information and analyses presented in the IDEQ's exceptional event demonstration package provided weight ofevidence sufficient for the EPA concurrence onthe flagged data from the Boisemonitor onthe dates listed above and as described inthis document. Accordingly, we are placing a concurrence indicator inthe EPA'sAQS database for these dates at this monitor.
Note that the EPA's decisions on exceptional event exclusions are not considered final agency action until they are acted upon aspart ofa final regulatory action subject to public notice and comment. Such actions would include, for example, decisions to excludethe affected data from use inan approval ofa non-attainment plan, maintenanceplan orother regulatory decision.



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