UNITED STATES ENVIRONMENTAL PROTECTION 

AGENCY 

REGION 10 

1200 Sixth Avenue, Suite 900 

Seattle, WA 98101-3140 

orr. t ~ l013 

Mr. Curt Fransen, Director 

Idaho Department of Environmental Quality 

1410 North Hilton 

Boise, Idaho 83706 

Dear Mr. Fransen: 

OFFICE OF THE 

REGIONAL 

ADMINISTRATOR 

This letter responds to the Idaho Department of Environmental 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 concur that 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 by high winds. The Boise 

monitor exceeded the 150 Jlg/m3 PM10 24-hour NAAQS on February 15 and 16,2011, and the Nampa 

monitor exceeded the same PMw standard on February 15, 2011. 

After careful consideration ofthe information provided, we concur with the IDEQ's exceptional events 

flags for February 15 and 16, 2011, at the Boise Fire Station Number 5 monitor. The elevated value on 

February 15, 2011, at the Nampa Fire Station monitor was not reviewed as it does not have regulatory 

significance. The basis for our decision on the Boise events flags is set forth in the 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 of a final regulatory action subject to public notice and comment. Such 

actions could include decisions to exclude the affected data from use in an approval of a non-attainment 

plan, development of a maintenance plan or determination of attainment. 

Thank you for the IDEQ's timely submittal of this exceptional event documentation. If you have any 

questions or wish to discuss this matter further, please contact me or have your staff contact Justin 

Spenillo, State and Tribal Air Programs Unit in the Office of Air, Waste and Toxics at (206) 553-6125. 

Sincerely, 

~9-~~ 

Enclosure 

cc: Tiffany Floyd 

Administrator, Air Quality Division 

Dennis J. McLerran 

Regional Administrator 

Idaho Department of Environmental Quality 
Bruce Louks 

Manager, Modeling, Monitoring, and Emissions Inventory 

Idaho Department of Environmental Quality 

Robert Wilkosz 

Manager, Mobile and Area Source Program 

Idaho Department of Environmental Quality 
EPA Region 10 

Review of Exceptional 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 by Exceptional 

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 if a 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 to justify the exclusion. After considering the weight of evidence provided in the 

demonstration, the EPA decides whether or not to concur with each flag. 

IDEO's Request 

The Idaho Department of Environmental 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 Nampa monitors, respectively. The recorded 24-hour PM10 

levels for both monitors ranged from 156 J.tg/m3 to 183 J.tg/m3 for the data values during the 

exceptional events for which the IDEQ is requesting the EPA's concurrence. 

B. mse an dN amp a M om "t ore d V a 1 ues 

Date Boise monitor- Nampa monitorPM10 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/m3 PM10 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 of July 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 

1 
exceptional 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 PM10 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 PM10 NAAQS for February 15-16, 2011, at the monitor in Boise, Idaho, demonstrate 

that the requirements of the 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 of the NAAQS at the Nampa 

monitor. 

The table below summarizes the requirements of the 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 of Flagged Exceedances: 

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

and include an initial 2011 24-hour PM10 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 of the 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). 

2 
Technical Criteria: 

• The event satisfies Conceptual Model I Affected Air 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 quality by 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 of the 14th until the 

EPA to be an evening of the 15th. Dust remained in the air overnight 

exceptional event). and into the day of the 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 in the 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 of the five values above 150 J..lg/m3 

in the past 1 0 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 of newspaper articles 

and distributed IDEQ air quality alerts. 

3 
Natural Event and not Reasonably Controllable or 

Preventable 

The ID EQ 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 of airborne 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 of the 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-

3 0 mph, wind gusts exceeded 40 mph, and that the wind 

was blowing in the direction of the Boise area. The 

MODIS imagery in Figures 13-17 clearly identify dust 

entrainment during the period of winds in the 20-30 

mph range. 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 or preventable" (nRCP) criterion is 

established through a weight of evidence of MODIS 

imagery and a mix of wind measurements above 25 mph 

and between 15 mph and 25 mph. 1 

The IDEQ discusses anthropogenic sources of emissions 

in section 2.3 and 4.8 and identifies that they did not 

contribute to the event for a variety of reasons. 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 PM2.5 monitors show the event was 

regional in nature and not confined to Boise. These facts 

combined with evidence of no unusual road dusting or 

1 The documentation does not include specific and scientific justification for entrainment of the 

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 of precipitation to create the evaporites followed by a wind storm. 

4 
agricultural activities substantiate the claim that the dust 

was from the playas rather than from local emissions of 

geologic material. An annual event on the Black Rock 

Playa that may disturb a very small portion of the playas 

occurs in the summer not the winter when the 

exceedances were measured and so that anthropogenic 

event was deemed to not have 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 Nevada playas, and the 

event was not reasonably controllable or preventable. 

• There is a clear Overview 

causal relationship The IDEQ established the clear causal relationship by 

between the utilizing a series of MODIS satellite images (Figures 13-

exceedance and the 16) which show the lake beds prior to the event and 

claimed exceptional during the event. The images clearly show the creation 

event. 40 CFR of dust clouds from the playas that can be seen traveling 

50.14(c)(3)(iii)(B) northeast towards Boise. In Figure 17, MODIS false 

color image analyses support that the dust clouds seen in 

the images were in fact dust and not clouds/water vapor. 

Surface wind speed and regional wind models, seen in 

Figures 4, 5, 21 and 27, support the wind conditions for 

the time of the event by identifying high winds in the 

region that were blowing from the southwest to the 

northeast and which align with the times in which the 

monitor recorded high particulate matter levels on 

February 15 and 16. Section 4.3 corroborates the 

movement of the emissions seen in the MODIS imagery 

by using NOAA's HYSPLIT modeling tool on the days 

ofthe event, February 15-16; these can be seen in 

Figure 28-29. PM to monitors show the regional nature 

of the event and show a rise and fall in concentration 

consistent with the MODIS imagery, HYSPLIT 

trajectories, and synoptic and mesoscale weather 

analyses. The analysis of alternative hypotheses and 

nearly unprecedented levels of PM to from this event rule 

out other sources for the high PM to concentrations. 

Conclusion 

Based on the suite of evidence provided, including 

MODIS imagery, HYSPLIT back trajectories, surface 

wind speed and wind direction data, and no evidence of 

significant sources of anthropogenic PM10, the EPA 

concludes that there is a clear causal connection 

5 
between the Boise monitor values recorded on 

February 15-16,2011, and the high wind event that 

entrained and transported dust from the Nevada playas. 

• 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 of normal 25Jlg/m3 between 2002 and 2011. This shows that the 

historical event recorded values between 156 - 183 Jlg/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 have happened only a couple times at that 

magnitude in the past decade. 

Based on the presented data, the EPA concludes that the 

values recorded on February 15-16, 2011 are in excess 

of normal historical fluctuations including background. 

• There would have To show there would have been no exceedances "but 

been no exceedartces for" the event, the ID EQ 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 of dust on those days, and the 

PM10 values in Boise rose and fell consistent with 

transport of this 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 PM10 

concentrations in the Boise area would have been well 

below 150 Jlg/m3 if not for the dust contributed by the 

Nevada playas 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 of a clear causal relationship between the 

6 
high wind event and the elevated emissions, and that 

there were no other reported irregular emissions events 

during that time period, the EPA concludes that but for 

the high wind event entraining and transporting dust 

from Nevada to the Boise area, there would have been 

no exceedances of the 24-hour PM10 NAAQS at the 

monitor in Boise, Idaho. 

• Mitigation, 40 CPR The IDEQ provided public notification with Air Quality 

51.930 Advisories on February 15 and 16 and included 

associated education materials/ weblinks in the 

distributed advisories. These can be found in Appendix 

E. Given the short duration of the event, its unexpected 

occurrence, and the rare frequency of occurrence, the 

EPA concludes that the necessary mitigation steps were 

taken during this event. 

Conclusion 

Based on the documentation submitted by the IDEQ on September 23, 2013, the EPA concurs on 

the PM10 data values listed in following table which have been flagged by the IDEQ in AQS as 

exceptional events. 

Boise Monitored Values 

Date Boise monitor -

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 of evidence sufficient for the EPA concurrence on the flagged data from the 

Boise monitor on the dates listed above and as described in this document. Accordingly, we are 

placing a concurrence indicator in the EPA's AQS 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 as part of a final regulatory action subject to public notice and 

comment. Such actions would include, for example, decisions to exclude the affected data from 

use in an approval of a non-attainment plan, maintenance plan or other regulatory decision. 

7