Draft Guidance on the Preparation of 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
Draft for Public Notice and Comment 

Revision Date: June 2012 ii 

Table of Contents

Acronyms ....................................................................................................................................... iii

1. Highlights .................................................................................................................................... 1

2. Overview of Exceptional Events Rule ........................................................................................ 5

2.1 Definition of the “Event” for High Wind Dust Events .................................................... 5

2.2 Evidence Necessary to Support Exceptional Events Requests ........................................ 5

2.3 Mitigation Requirement ................................................................................................... 7

2.4 Process Requirements per EER ........................................................................................ 7

3. Evidence to be Included in a High Wind Dust Event Demonstration Package ......................... 8

3.1 Not Reasonably Controllable or Preventable (nRCP) .................................................... 10

3.1.1 Controls on Natural versus Anthropogenic Sources 10

3.1.2 Factors Considered in Determining the Reasonableness of Controls 12

3.1.3 Implementation and Enforcement of High Wind Dust Control Measures 15

3.1.4 Consideration of Wind Speed 16

3.1.5 Controls Analysis for Individual Events 17

3.1.6 Consideration of Controls on Tribal Lands 19

3.2 Historical Fluctuations (HF) ........................................................................................... 19

3.3 Clear Causal Relationship (CCR) .................................................................................. 20

3.4 Affects Air Quality (AAQ) .......................................................................................... 21

3.5 Caused by Human Activity Unlikely to Recur at a Particular Location (HAURL) or a 

Natural Event (Natural Event) .................................................................................................. 22

3.6 No Exceedance or Violation But For the Event (NEBF) ............................................... 23

3.7 Optional Analyses for nRCP Requirement .................................................................... 24

3.7.1 Prospective Controls Analysis 24

3.7.2 High Wind Action Plan 25

4. Mitigation .................................................................................................................................. 30

5. Process Issues for Exceptional Events Including High Wind Dust Events .............................. 31

5.1 Demonstrations Package Submittal and Review ............................................................ 31

5.2 Timeframes ..................................................................................................................... 31

5.3 Public Comment ............................................................................................................. 33

6. Recommendations for Preparing High Wind Dust Exceptional Event Demonstrations .......... 34

6.1 Framework for Preparing Evidence to Support a High Wind Dust Exceptional Event . 34

6.2 Preparing a Prospective Controls Analysis (Optional) .................................................. 36

6.3 Recommended Methods for the Technical Elements of a High Wind Dust Exceptional 

Events Package ......................................................................................................................... 37

6.3.1 Step 1: Develop a Conceptual Model 38

6.3.2 Step 2: Address not Reasonably Controllable or Preventable (nRCP) 39

6.3.3 Step 3: Present Historical Fluctuations (HF) Analyses 47

6.3.4 Step 4: Address Clear Causal Relationship (CCR) 49

6.3.5 Address Affects Air Quality (AAQ) 63

6.3.6 Address [HAURL] / Natural Event 63

6.3.7 Step 5: Address No Exceedance But For the Event (NEBF) 63

6.4 Prepare High Wind Action Plan (optional) .................................................................... 64

Appendix A1. Summary of Studies on Windblown Dust Emissions ........................................... 66

Appendix A2. Summary of Available Relevant Literature Related to Establishing Area-Specific 

High Wind Thresholds .................................................................................................................. 70

Appendix A3. Methods for Establishing Area-Specific High Wind Thresholds .......................... 73

Appendix B1. Checklist for High Wind Exceptional Events Demonstration Submission ........... 75

Appendix B2. Flowchart of Potential Review Process ................................................................. 78

Appendix C. Sample Letter of Intent ............................................................................................ 79
Draft for Public Notice and Comment 

Revision Date: June 2012 iii 

Acronyms 

AAQ Affects Air Quality 

ADEQ Arizona Department of Environmental Quality 

AQS Air Quality System 

BACM Best Available Control Measures 

BMP Best Management Practice 

CAA Clean Air Act 

CCR Clear Causal Relationship 

CFR Code of Federal Regulation 

CLASS Clean Air Support System 

DAQEM Department of Air Quality and Environmental Management (Clark County, NV) 

DRI Desert Research Institute 

EER Exceptional Events Rule 

EPA Environmental Protection Agency 

FEM Federal Equivalent Method 

FRM Federal Reference Method 

GIS Geographic Information System 

HAURL Human Activity Unlikely to Recur at a particular Location 

HF Historical Fluctuations 

MAG Maricopa Association of Governments (Arizona) 

MODIS Moderate Resolution Imaging Spectroradiometer 

MSM Most Stringent Measure 

nRCP not Reasonably Controllable or Preventable 

NAAQS National Ambient Air Quality Standards 

NEBF No Exceedance But For the event 

NSR New Source Review 

NWS National Weather Service 

RACM Reasonably Available Control Measures 

PM Particulate Matter 

SCAQMD South Coast Air Quality Management District (California) 

SIL Significant Impact Level 

SJV San Joaquin Valley 

SIP State Implementation Plan 

TOEM Tapered Element Oscillating Microbalance 

UNLV University of Nevada, Las Vegas 

WEG Wind Erodibility Group 

WGA Western Governors’ Association 

WRAP Western Regional Air Partnership 
Draft for Public Notice and Comment 

Revision Date: June 2012 1

1. Highlights 

The EPA developed this document to assist air agencies1 in meeting the requirements of the 

Exceptional Events Rule2

 (EER) for high wind dust (i.e., particulate matter) events3 and to 

provide example recommended elements for exceptional event demonstrations. High winds can 

entrain and transport particulate matter (PM) to a monitoring site. These particles can consist of 

both PM10 (i.e., particles less than 10 micrometers (μm) in diameter) and PM2.5 (i.e., particles 

less than 2.5 μm in diameter). High wind dust events can include both PM10 and PM2.5.

Purpose of this Document

The purpose of this document is to provide assistance and illustration to air agencies 

implementing the EER for high wind dust events.4

 

The EPA recognizes the limited resources of the air agencies that prepare and submit exceptional 

event demonstration packages and of the EPA regional offices that review these demonstration 

packages. One of the EPA’s goals in developing this document and the other exceptional event 

implementation guidance5

 is to establish clear expectations to enable affected agencies to better 

manage resources as they prepare the documentation required under the EER. The EPA expects 

submitters to prepare and submit the appropriate level of supporting documentation, which will 

vary on a case-by-case basis using the weight-of-evidence approach. The EPA expects the 

resources required to prepare (and review) high wind dust exceptional event packages, and 

demonstrations for other event types, to decrease as we continue to identify ways to streamline 

the process and continue to build our database of example demonstrations and analyses. In 

addition, the EPA acknowledges that certain extreme exceptional event cases may require more 

limited demonstration packages. 

To Whom does this Document Apply?

High wind dust events are typically a phenomenon experienced in the western United States 

where rainfall is seasonal, creating dry and dusty landscapes. Therefore, this document may be of 

most use to the states from the Great Plains (North Dakota, South Dakota, Nebraska, Kansas, 

Oklahoma, and Texas) and west. Generally, this includes the states that comprise the Western 

Regional Air Partnership, which is most of EPA Regions 6, 7, 8, 9, and 10. 

 

 

1

 References to “air agencies” are meant to include state, local, and tribal air agencies responsible for implementing 

the EER. 

2

 “Treatment of Data Influenced by Exceptional Events; Final Rule”, 72 FR 13560, March 22, 2007. 

3

 The term “high wind dust event” is used in this document to refer to the same type of event that was discussed as a 

“high wind event” in the EER. The EPA believes the term “high wind dust event” more clearly describes the 

referred-to event. 

4

 This draft guidance document presents examples to illustrate specific points. These examples are not necessarily 

required for all demonstrations. 

5

 Other draft exceptional event guidance documents include the following: “Draft Guidance to Implement 

Requirements for the Treatment of Air Quality Monitoring Data Influenced by Exceptional Events,” the “Draft 

Exceptional Events Rule Frequently Asked Questions,” and “Request for Comments on the Draft Guidance 

Documents on the Implementation of the Exceptional Events Rule.” Additional information and examples of 

exceptional event submissions and best practice components can be found at the EPA’s Exceptional Events website 

located at http://www.epa.gov/ttn/analysis/exevents.htm. 
Draft for Public Notice and Comment 

Revision Date: June 2012 2 

Guiding Principles for the Development of this Document

1. Air agencies should not be held accountable for exceedances due to events that were 

beyond their control at the time of the event. 

2. It is desirable to implement reasonable controls to protect public health.6

3. Clear expectations will enable the EPA and other air agencies to better manage resources 

related to the exceptional events process. 

Definition of a High Wind Dust Event

A high wind dust event includes both the high wind and the dust that the wind entrains and 

transports to a monitoring site. The event is not merely the occurrence of the high wind. 

Elements for the Technical Demonstration of High Wind Dust Events

 Air agencies’ demonstrations must address the following six technical elements under the 

EER before the EPA can concur on a high wind dust event demonstration: 

1. whether the event was not reasonably controllable or preventable (nRCP), 

2. whether there was a clear causal relationship (CCR), 

3. whether there would have been no exceedance or violation but for the event (NEBF), 

4. whether the event affects air quality (AAQ), 

5. whether the event was caused by human activity unlikely to recur or was a natural event 

(HAURL / Natural Event), and 

6. whether the event was in excess of normal historical fluctuations (HF). 

If a demonstration fails to sufficiently address any one of the above, the EPA will not be able to 

concur with the request to exclude data under the EER. 

 During the EPA’s review of several high wind dust events flagged by air agencies as 

exceptional events, the EPA has found that the following EER elements play a significant 

role in air agencies’ supporting documentation: nRCP, CCR, and NEBF. 

 The EPA has also found for a high wind dust event that satisfying the requirements for 

nRCP, CCR, and HF criteria also generally satisfies the requirements for two elements 

identified by statute, AAQ and Natural Event. 

 The EPA has not set pass/fail statistical criteria for the HF element, but will use a weight-ofevidence approach to assess each demonstration on a case-by-case basis. The air agency’s 

role in satisfying this element is to provide analyses and statistics as recommended by the 

EPA in this document and conclude that the provided data show that the event was in excess 

of normal historical fluctuations. The EPA will review the information provided by the air 

agency. Events do not necessarily have to be rare to satisfy this element. 

 

6

 With respect to exceptional events, Section 319 of the Clean Air Act states the following guiding principles 

(among others); 

(i) the principle that protection of public health is the highest priority 

*** 

(iv) the principle that each State must take necessary measures to safeguard public health regardless of the 

source of the air pollution 
Draft for Public Notice and Comment 

Revision Date: June 2012 3 

 While not listed as a technical element required by the EER, wind data (e.g., wind speed and 

direction) will generally play a vital role in informing the EPA’s decision on elements such 

as whether the event was not reasonably controllable or preventable and establishing a clear 

causal relationship. 

Not Reasonably Controllable or Preventable

 Exceedances caused in whole or in part by anthropogenic dust sources within the air 

agency’s control are unlikely to be eligible for treatment as exceptional events under the 

EER, even under conditions of elevated winds, unless the air agency shows that the event, 

including the emissions from the anthropogenic dust sources, was not reasonably controllable 

or preventable. The EPA intends to evaluate whether an event was not reasonably 

controllable or preventable at the time of the event by taking into account the wind speed; the 

controls in place; the controls required in the State Implementation Plan (SIP), which 

depends on an area’s attainment status; the frequency and severity of exceedances; 

contributing sources; benefits of the controls; costs of controls; and other factors. 

 The EPA also judges the reasonableness of controls based on the technical information that 

was available to the air agency at the time the event occurred. The EPA generally expects air 

agencies to already have the technical information needed to reasonably control sources 

within nonattainment areas. 

 

 The degree of event-specific information and data necessary for demonstrating “not 

reasonably controllable or preventable” will generally be less for sustained wind speeds at or 

above the high wind threshold and greater for speeds below that the threshold. The high wind 

threshold is the minimum threshold wind speed capable of overwhelming reasonable controls 

on anthropogenic sources (i.e., significant emissions from controlled sources) or causing 

emissions from natural undisturbed areas. The EPA recommends that air agencies establish 

area-specific high wind thresholds based on local or applicable conditions and information. If 

an agency is unable to develop an area-specific high wind threshold, the EPA will accept a 

threshold of a sustained wind of 25 mph for areas in the West provided the agencies submit 

evidence of this as the level at which they expect stable surfaces (i.e., controlled 

anthropogenic and undisturbed natural surfaces) to be overwhelmed. In identifying a high 

wind threshold, the EPA does NOT intend to set a bright line as to what speed constitutes a 

high wind dust event or to categorically concur with all events with sustained winds above a 

given threshold. 

 An air agency has the option of submitting a prospective controls analysis in advance or with 

a demonstration package. Described in more detail in Section 3.7.1, a prospective controls 

analysis is a generic7

 review of an area’s existing windblown dust controls and high wind 

threshold. In the prospective controls analysis, the air agency would provide information on 

attainment status, identify natural and anthropogenic windblown dust sources and emissions, 

provide the status of SIP submittals (if applicable), and identify the high wind threshold up to 

which the collective windblown dust controls are expected to be effective. Once the EPA 

approves the prospective controls analysis, an air agency’s subsequent high wind exceptional 

event packages could reference the approved set of controls in the prospective controls 

 

7

 “Generic” means a general review rather than a review specific to an identified event. 
Draft for Public Notice and Comment 

Revision Date: June 2012 4 

analysis and show that the wind speed for the event in question is at or above the high wind 

threshold established in the prospective controls analysis. Air agencies would also include in 

their demonstration some positive showing that control requirements were being met on the 

day in question. An EPA-approved prospective controls analysis would generally be effective 

for a minimum of three years. 

 The EPA and the submitting air agency may also consider developing a voluntary High Wind 

Action Plan. Air agencies can develop High Wind Action Plans to document their plans to 

implement needed controls on newly-identified sources that could emit dust during 

subsequent high wind events. A High Wind Action Plan is an optional mechanism to 

implement necessary controls more expeditiously than with the normal regulatory planning 

process. Preparation of such a plan and its approval by the EPA could promote a common 

understanding between the air agency and the EPA about whether subsequent high wind dust 

events are not reasonably controllable or preventable. 

Clear Causal Relationship

As described in Section 3.3, air agencies can use the following example analyses to establish a 

clear causal relationship: 

 analyses showing that the event in fact occurred and that emissions were transported in the 

direction of the monitors where measurements were recorded 

 the size of the area affected by the emissions 

 comparison to non-event days 

 the spatial and temporal relationship between the event, transport of emissions, and recorded 

concentrations 

No Exceedance But For the Event

The NEBF demonstration may be relatively straightforward for areas with typical concentrations 

on non-event days well below the applicable National Ambient Air Quality Standards (NAAQS). 

However, demonstrating NEBF becomes increasingly difficult if concentrations on non-event 

days during the same season exceed the standard and/or if the contribution of non-event pollution 

sources produce concentrations near the applicable NAAQS. 

 

Disclaimer 

The Exceptional Events Rule is the source of the regulatory requirements for exceptional events 

and exceptional event demonstrations. This document provides guidance and illustrations of 

events for the Exceptional Events Rule rather than imposing any new requirements. Further, any 

determination that an event is exceptional made on the basis of this guidance will need the facts 

in the documentation to support the decision. 
Draft for Public Notice and Comment 

Revision Date: June 2012 5 

2. Overview of Exceptional Events Rule 

The EER and the preamble describe specific criteria for an event to be considered an 

“exceptional event” for purposes of exclusion of air quality data from regulatory decisions and 

acknowledge that “natural events” can be recurring. 

2.1 Definition of the “Event” for High Wind Dust Events 

In high wind dust events, the meteorological phenomenon (i.e., wind) is purely natural but the 

pollution from the event may be a mixture of natural sources (e.g., undisturbed soil) and 

anthropogenic sources (e.g., soil disturbed by human activity, emissions from sand and gravel 

facilities, etc.). The EPA classifies high wind dust events as “natural events” in cases where 

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

windblown dust have been reasonably controlled.8

 This long-standing policy of considering both 

the wind and the dust as part of the event was established in the Natural Events Policy where 

high winds were defined as: 

“Ambient PM-10 concentrations due to dust raised by unusually high winds will be 

treated as due to uncontrollable natural events under the following conditions: (1) the 

dust originated from nonanthropogenic sources, or (2) the dust originated from 

anthropogenic sources controlled with best available control measures (BACM).”9

A high wind dust event is defined as the combination of high wind and uncontrollable dust 

emissions. Uncontrollable windblown dust emissions only occur in the presence of high wind. 

Therefore, for exceptional events purposes, it is necessary to consider both the emissions and the 

corresponding high wind as the “event.” 

2.2 Evidence Necessary to Support Exceptional Events Requests 

The EPA promulgated the Exceptional Events Rule in 2007, pursuant to the 2005 amendment10

of Clean Air Act (CAA) Section 319. The EER added 40 CFR §50.1(j), (k) and (l); §50.14; and 

§51.930 to the Code of Federal Regulations. These sections contain definitions, criteria for EPA 

approval, procedural requirements, and requirements for air agency demonstrations, all of which 

 

8

 Human activity would be considered to have played little or no direct causal role in causing emissions of the dust 

by high wind 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 would be considered a 

natural event. If anthropogenic sources of windblown dust that are reasonably controllable but 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. 

9

 “Areas Affected by PM10 Natural Events” (the PM10 Natural Events Policy), memorandum from Mary D. 

Nichols, Assistant Administrator for Air and Radiation, to EPA Regional Offices, May 30, 1996. The EPA intended 

to uphold the principle that windblown emissions from controlled sources would be considered natural, however, 

due to a technical drafting error, the language in the section of the EER preamble titled “B. High Wind Events” 

beginning on 72 FR at 13576 was ruled legally null in NRDC v. EPA, No. 07-1151 (D.C. Cir. 3/20/09). 10 Safe, Accountable, Flexible, Efficient Transportation Equity Act: a Legacy for Users (SAFETEA-LU), section 

6013 amending CAA §319, became law August 10, 2005; available at http://thomas.loc.gov/cgibin/query/z?c109:H.R.3: 
Draft for Public Notice and Comment 

Revision Date: June 2012 6 

must be met before the EPA can concur under the EER on the exclusion of air quality data from 

regulatory decisions. 

The definition of an exceptional event given in 40 CFR §50.1(j) parallels the statutory definition 

of Section 319 of the CAA and itself contains certain criteria for approval by the EPA: 

 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.”11

Additional criteria for the EPA approval to exclude data affected by a high wind dust event are 

given (with some repetition of key phrases) in 40 CFR §50.14(a) and (b)(1).12 Under these 

provisions the air agency must: 

 “demonstrat[e] to EPA’s satisfaction that such event caused a specific air pollution 

concentration at a particular air quality monitoring location.” 

 “demonstrate a clear causal relationship between the measured exceedance or violation of 

such standard and the event ...” 

 “demonstrat[e] to EPA’s satisfaction that an exceptional event caused a specific air 

pollution concentration in excess of one or more national ambient air quality standards at 

a particular air quality monitoring location and otherwise satisfies the requirements of 

this section [regarding schedules, procedures and submission of demonstrations].” 

Under 40 CFR §50.14(c)(3)(iv),13 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 (see above); 

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”. 

The definition of an exceptional event provided in 40 CFR § 50.1(j) explicitly excludes 

“stagnation of air masses or meteorological inversions, a meteorological event involving high 

temperatures or lack of precipitation, or pollution relating to source noncompliance.”14 

Exceedances due to these events would not be eligible for exclusion under the EER. For 

 

11 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.” 

12 §50.14 (b)(2) and (b)(3) contain criteria relevant only to firework events and prescribed fire events. 

13 Prior to the publishing of the 2010 CFR the citation was §50.14(c)(3)(iii) 14 For further explanation see “Treatment of Data Influenced by Exceptional Events; Final Rule,” 72 FR at 13577, 

(March 22, 2007). 
Draft for Public Notice and Comment 

Revision Date: June 2012 7 

example, if sources out of compliance with fugitive dust or other rules contributed significantly 

to an exceedance, then the exceedance would not be excluded as due to an exceptional event. 

2.3 Mitigation Requirement 

40 CFR §51 Subpart Y includes mitigation requirements at 51.930. While the EER does not 

require that mitigation measures be submitted to the EPA as part of the demonstration package 

(or otherwise), it is nonetheless a requirement of Subpart Y that “[a] State requesting to exclude 

air quality data due to exceptional events must take appropriate and reasonable actions to protect 

public health from exceedances or violations of the national ambient air quality standards.” The 

mitigation requirement is addressed in Section 4 of this document. 

2.4 Process Requirements per EER 

In addition to technical demonstration requirements, the EER specifies the process an air agency 

must follow to request data exclusion: 

 "A State shall notify EPA of its intent to exclude one or more measured exceedances of an 

applicable ambient air quality standard as being due to an exceptional event by placing a 

flag in the appropriate field for the data record of concern which has been submitted to the 

AQS database.” 40 CFR § 50.14(c)(2)(i). 

 “The placement of the flags and the submittal of an initial event description must be done 

not later than July 1st of the calendar year following the year in which the flagged 

measurement occurred.” 40 CFR § 50.14(c)(2)(iii). 

 “A State that has flagged data as being due to an exceptional event and is requesting 

exclusion of the affected measurement data shall, after notice and opportunity for public 

comment, submit a demonstration to justify data exclusion to EPA not later than the lesser 

of, 3 years following the end of the calendar quarter in which the flagged concentration was 

recorded or, 12 months prior to the date that a regulatory decision must be made by EPA. A 

State must submit the public comments it received along with its demonstration to EPA.” 

40 CFR § (50.14(c)(3)(i)). 

 With the submission of the demonstration, the air agency “must document that the public 

comment process was followed.” 40 CFR § (50.14(c)(3)(iv)). 
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Revision Date: June 2012 8 

3. Evidence to be Included in a High Wind Dust Event Demonstration 

Package 

As discussed in Section 2.2, the EER identifies technical elements (i.e., criteria or evidence) that 

an air agency must address and demonstrate before the EPA can concur that an exceedance is 

due to an exceptional event. Table 1 shows the complete list of technical elements air agencies 

must submit and satisfy as part of a demonstration for high wind dust events. The EPA cannot 

concur on an air agency’s request to exclude data under the EER if the air agency has failed to 

satisfy any of the criteria. 

Table 1. EER Technical Demonstration Elements Required by the EER for High Wind 

Dust Events 

Element Abbreviation 

Section of this Document 

Containing Additional 

Explanation 

affects air quality* AAQ 3.4 

not reasonably controllable or preventable nRCP 3.1 

caused by human activity unlikely to recur at a 

particular location OR a natural event15* HAURL / 

Natural Event 3.5 

clear causal relationship between the measurement 

and the event CCR 3.3 

no exceedance or violation but for the event NEBF 3.6 

the event is associated with a measured 

concentration in excess of normal historical 

fluctuations, including background 

HF 3.2 

*These elements are typically met when the other elements have been satisfied. 

The EPA uses a “weight-of-evidence” approach in reviewing air agency requests for data 

exclusion under the EER. Evidence and narrative that constitute a strong demonstration for one 

element can also be part of the demonstration for another element, but cannot make up for the 

absence or failure to satisfy another element. Although a strong demonstration on one element 

should not compensate for a failure of another, the strength of the demonstration for one 

requirement could influence the persuasiveness of evidence used for another. 

In reviewing the supporting documentation in several high wind dust event demonstrations, the 

EPA has found the following EER elements play a significant role: nRCP, CCR, and NEBF. 

The criterion that the event be in excess of normal historical fluctuations (HF) is a technical 

element that the EPA expects to be satisfied by the submittal of data as outlined in Section 3.2. In 

addition to satisfying the HF criterion, these data are expected to inform the CCR and NEBF 

demonstrations. 

 

15 High wind dust events are considered natural events if sources are entirely natural or if contributing anthropogenic 

sources are reasonably controlled and therefore it is not relevant to consider whether the event was caused by human 

activity unlikely to recur. 
Draft for Public Notice and Comment 

Revision Date: June 2012 9 

The EPA has generally found that two elements identified by statute, AAQ16 and Natural Event, 

are necessarily also satisfied for a high wind dust event if the other elements are satisfied. Air 

agencies can typically meet these criteria by satisfying the requirements for nRCP, CCR, and 

HF. While not identified as a separate demonstration element in Table 1, wind data (e.g., wind 

speed and direction) is vital in informing the EPA’s decision regarding “not reasonably 

controllable or preventable” and “clear causal relationship.” 

Finally, the EPA recommends that air agencies begin their technical demonstration for a high 

wind dust event with a conceptual model of how the event occurred. An air agency’s conceptual 

model can use text and/or schematics to identify and describe the relationship between various 

phenomena (e.g., weather and dust emissions) that caused an exceedance. In its simplest form, 

the conceptual model could be a narrative description of how the event unfolded and resulted in 

the exceedance(s). The conceptual model will be similar to a report abstract and should help tie 

the various rule criteria together into a cohesive explanation of the event. 

Sections 3.1-3.6 of this document describe and clarify each element identified in Table 1. Section 

6 provides example analyses and a recommended structure for the preparation of demonstration 

packages for high wind dust events. 

In summary, the technical demonstration for a high wind dust event package should include the 

following technical elements: 

 Not Reasonably Controllable or Preventable - Analyses and descriptions should show that 

the event was not reasonably controllable or preventable. Required by EER. 

 Clear Causal Relationship - Analyses and descriptions should show that there was a clear 

causal relationship between the ambient concentration measurement under consideration 

and the event that is claimed to have affected the air quality in the area. Required by EER. 

 No Exceedance But For the Event - Analyses and descriptions should show that there 

would have been no exceedance or violation but for the event. Required by EER. 

 Affects Air Quality - Statutory technical element that is generally satisfied once the 

submitter provides historical fluctuations analyses (HF), establishes a clear causal 

relationship (CCR), and provides explicit information indicating satisfaction of requirement 

through clear causal and historical fluctuations showings. Required by EER. 

 Natural Event - Statutory technical element that is generally satisfied once the submitter 

shows the event to be not reasonably controllable or preventable (nRCP), establishes a 

clear causal relationship (CCR), and provides information demonstrating these 

requirements have been met. Required by EER. 

 

16 The preamble to the EER clarifies the AAQ criteria in section V.B. (p. 13569) by stating that the following criteria 

establish that the event affected air quality: “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” and “the event is associated 

with an unusual measured concentration beyond typical fluctuations including background.” On this basis AAQ is 

satisfied once CCR has been demonstrated and evidence for HF has been provided. 
Draft for Public Notice and Comment 

Revision Date: June 2012 10 

 Historical Fluctuations – Air agencies should provide analyses and descriptions in the 

format suggested in this document. The EPA will review this information in a weight-ofevidence showing. Required by EER. 

 Wind Data - Data on wind speed and direction support the technical elements required by 

the EER such as CCR and nRCP. 

 Conceptual Model - Narrative summary at the beginning of a demonstration package 

describing how the event unfolded to produce elevated PM at the monitor(s) that recorded 

the exceedance(s) and providing context for the supporting elements. 

3.1 Not Reasonably Controllable or Preventable (nRCP) 

Exceedances caused in whole or in part by anthropogenic dust sources within the air agency’s 

control are unlikely to be eligible for treatment as exceptional events under the EER, even under 

conditions of high winds, unless the air agency shows that the event (i.e., dust entrained by high 

winds) was not reasonably controllable or preventable. The 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.17 The factors and approach identified in this 

section should assist air agencies in developing adequate high wind dust exceptional event 

demonstration packages and promote consistency. The EPA will consider each package on a 

case-by-case basis per the EER. 

3.1.1 Controls on Natural versus Anthropogenic Sources

According to the definition of an exceptional event, the event must be “not reasonably 

controllable or preventable” (40 CFR § 50.1(j)). For natural sources of dust, a high wind dust 

event can generally be considered to be not reasonably controllable or preventable18, 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 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. 

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

Reasonable controls on anthropogenic sources (item 1 on the list above) are fundamental to the 

event being not reasonably controllable or preventable. An event with both anthropogenic and 

natural components can be considered a “natural” event if the anthropogenic component is 

reasonably controlled. Air agencies must also demonstrate that natural events are reasonably 

controlled by showing that no additional controls are reasonable for the event. Additionally, 

“reasonable controls” refers to a collection of reasonably controlled sources. The term “not 

 

17See SJV Attainment Affirmation, 73 FR73 14691, for a prior high wind dust event in which the EPA considered 

controls and wind speed, along with other factors. 

18 The EPA expects that in most cases it would not be reasonable to have controls on natural sources but this will be 

evaluated for each event. 
Draft for Public Notice and Comment 

Revision Date: June 2012 11 

reasonably controllable or preventable” refers to the event (i.e., dust entrained by high winds), 

rather than to any particular source. Further, in determining whether the event is not reasonably 

controllable or preventable, the EPA will consider whether the collection of anthropogenic 

sources has been reasonably controlled. For anthropogenic sources, it is the high wind 

overwhelming the collection of reasonable controls, that have been effectively implemented and 

enforced, that make the event not reasonably controllable or preventable. 

For purposes of evaluating high wind dust exceptional events in the West, the EPA will generally 

use the definitions of natural and anthropogenic windblown dust emissions that have been 

developed in the Western Regional Air Partnership (WRAP) Fugitive Dust Handbook.

19 

According to the WRAP Fugitive Dust Handbook, all mechanically suspended dust from human 

activities should be considered anthropogenic emissions, while windblown dust from lands not 

disturbed or altered by human activity should be considered natural emissions. Furthermore, 

windblown dust from surfaces that have been significantly disturbed or altered by humans should 

be categorized as anthropogenic emissions. Such surfaces may include: undeveloped lands20, 

construction and mining sites, material storage piles, landfills, vacant lots, agricultural lands, 

roadways, parking lots, artificially exposed beds of natural lakes and rivers, exposed beds of 

artificial water bodies, areas subject to off-road vehicle activity, and areas burned by prescribed 

or other human-ignited fires. Natural sources may include: naturally-dry river and lake beds; 

barren lands; sand dunes; exposed rock; sea spray from natural water bodies; non-agricultural 

grass, range, and forest lands; areas burned by wildfires; and glacial silt. 

The EPA generally considers dust entrained by high wind from undisturbed land (e.g., 

undisturbed desert) to be not reasonably controllable or preventable, because of the likely 

disturbance to natural ecosystems and the cost of treating large land areas. The EPA also 

generally considers that windblown dust from previously disturbed land that is being allowed to 

fully return to natural conditions by effective prevention of any new disturbance is also not 

reasonably controllable or preventable, provided that there are no reasonable active measures 

that could be taken to control dust during the transition back to natural conditions.21 While 

emissions from most other natural sources of windblown dust could be similarly not reasonably 

controllable, the EPA will consider those on a case-by-case basis. In areas where events recur, 

the EPA may request increased characterization of the natural sources (e.g., historical surface 

disturbance, water diversions, vegetation changes, etc.). 

While the EPA generally does not expect controls on emissions from natural sources (e.g., 

undisturbed land) for an event to be not reasonably controllable or preventable, the EPA does 

expect reasonable controls to be in place on the windblown anthropogenic contribution to the 

concentration measured during the event. Experience in several areas in the western United 

States has shown that it is practical and reasonable to apply dust-suppression controls to 

disturbed lands and other anthropogenic dust sources, and that these controls help limit ambient 

concentrations of PM during high wind dust events, up to certain wind speeds. For example, 

many areas in the west have successfully controlled dust with measures such as water or 

chemical stabilization of disturbed areas such as construction zones, or limiting disturbance 

 

19WRAP Fugitive Dust Handbook, Prepared for Western Governors’ Association, Countess Environmental (WGA 

Contract No. 30204-111), September 7, 2006. Available at http://www.wrapair.org/forums/dejf/fdh/index.html 

20 Undeveloped lands refer to those that are disturbed for purposes of development but not yet developed. 

21 An example of such a measure might be the restoration of all or part of natural surface water flows. 
Draft for Public Notice and Comment 

Revision Date: June 2012 12 

activities on windy days. If reasonable controls on windblown anthropogenic sources were in 

place, then the event would be considered “not reasonably controllable or preventable” and 

would satisfy the nRCP element of the definition of an exceptional event. That is, an air agency 

can generally meet the nRCP element for high wind events by identifying the contributing 

anthropogenic sources of windblown dust for a particular event and showing that reasonable 

controls were in place, effectively implemented, and enforced (as appropriate). The prior 

preparation of and EPA approval of a prospective controls analysis or high wind action plan (see 

Sections 3.7.1 and 3.7.2) addressing all the anthropogenic sources that might contribute during 

wind events can alleviate the need for this showing in each event case. For each event with 

windblown anthropogenic contributions, it will be important for an air agency to explicitly show 

that the exceedance occurred despite the implementation of those reasonable controls (i.e., to 

show that wind speeds were high enough to overwhelm the reasonable controls). The prior 

establishment of a high wind threshold (see Section 3.1.4 below) can make this showing less 

resource intensive per event. The EPA will evaluate the reasonableness of controls based on the 

controls that should have been in place given the information the air agency had when the event 

occurred (see Section 3.1.2 for factors that the EPA will consider in determining the 

reasonableness of the controls). The level of rigor required to demonstrate that reasonable 

controls were in place, implemented/enforced, and overwhelmed by high winds, will depend 

upon the wind speed of the event relative to the high wind threshold if one has been established 

(see Section 3.1.4 and 3.1.5). 

Typically, measured ambient air concentrations during an event will include some contribution 

from natural or anthropogenic sources whose emissions are not affected by high wind, for 

example transportation and industrial point sources: these are considered non-event sources. 

Non-event sources are not subject to the nRCP requirement of the EER, but an air agency may 

apply full-time or event-dependent controls on such sources as part of its attainment/maintenance 

SIP or as part of meeting the mitigation requirement under 40 CFR §51.930. 

3.1.2 Factors Considered in Determining the Reasonableness of Controls 

This section describes an approach for determining the reasonableness of the controls in place. 

Among other factors to consider, reasonableness needs to be judged in light of the technical 

information available to the air agency at the time the event occurred. The EPA would consider 

air agencies experiencing the following scenarios and/or with the following technical 

information to have had technical information indicating the need for high wind dust controls: 

 More than one expected exceedance per year from high wind dust 

 Exceedances due to windblown dust when the sustained wind speed is less than the high 

wind threshold (or default) for the area 

 Requirement for high wind dust BACM resulting from either nonattainment status or 

previous Natural Events Action Plan (NEAP)22

 

22 On May 30, 1996, Mary D. Nichols, Assistant Administrator for Air and Radiation issued a memorandum to EPA 

regional offices entitled, “Areas Affected by PM10 Natural Events.” The policy, known as the PM10 Natural Events 

Policy, or simply the Natural Events Policy, set forth procedures for protecting public health through the 

development of a Natural Events Action Plan, which implements Best Available Control Measures (BACM) for 

human-generated particulate emissions in areas that could violate the PM10 NAAQS due to natural events. 

Promulgation of the Exceptional Events Rule superseded the Natural Events Policy. 
Draft for Public Notice and Comment 

Revision Date: June 2012 13 

 Formal communication from the EPA indicating the need for high wind dust controls 

 Promulgation of new/revised Federal rules that would require controls on particular sources 

The EPA does not expect high wind dust controls in areas with no history of high wind dust 

exceedances. 

The set of control requirements mandated by the area’s designation status is the primary factor 

used in evaluating reasonableness (see Section 3.1.2.2 for additional detail). Table 2 shows 

example factors that the submitter and the EPA may consider when assessing the reasonableness 

of controls as part of the nRCP criterion. Table 2 is not intended to be all-inclusive or 

quantitative. 

 

Table 2. Example Factors Considered In Determining the Reasonableness of Controls. 

“Reasonableness” Factor Description of “Reasonableness” Factor 

1. Control requirements based on area 

attainment status 

The reasonableness of the controls depends upon 

historical concentrations and designation status. 

2. Frequency and severity of past 

exceedances 

More stringent controls may be reasonable if an area 

experiences frequent23 and/or severe24 exceptional event 

exceedances due to high winds than if the area has 

experienced only non-recurring25 and/or mild isolated 

exceedances.26

3. Ease and effectiveness of control 

implementation 

The EPA may consider cost-effective and readily 

deployable controls more reasonable. 

4. Use of measures that are in 

widespread use 

Controls that are considered “standard practices” and/or 

measures in widespread use for dust control in other 

areas would be considered more reasonable. 

5. Jurisdiction The EPA expects air agency demonstration submittals to 

address the status of control measures for interstate and 

international transport. However, the EPA also expects 

that air agency demonstrations can generally satisfy the 

“not reasonably controllable or preventable” criterion 

with less detailed characterization of sources in the 

upwind state or country than of sources in the same state 

as the affected monitor.27 

 

23 Frequent is enough exceedances from high wind dust events to cause of violation of the NAAQS. 24 A severe exceedance could be a 24-hour average PM10 concentration > 250 μg/m3

. 25 Non-recurring is fewer than one high wind dust event per year 

26 A mild isolated exceedance could be, for example, an exceedance at one site close to the standard. 27 Considering the sovereignty issues associated with interstate and international transport, the EPA believes that 

“reasonable control” showings can rely on the concept that it is not reasonable to expect the downwind state (i.e., the 

state submitting the demonstration) to have convinced the upwind country or state to have implemented controls on 

sources sufficient to limit event-related air concentrations in the downwind state. As with any demonstration 

submittal, the submitting (downwind) state should sufficiently identify all natural and anthropogenic contributing 

sources of emissions (both in-state and out-of-state) to show the causal connection between an event and the affected 

air concentration values. A submitting state may provide a less detailed characterization of sources in the upwind 

state or country than of sources within its jurisdiction. After completing the source characterization, the submitting 

state should assess whether emissions from sources within its jurisdiction (i.e., in-state sources) were not reasonably 

controllable or preventable. Although the submitting state should also provide available information on the status of 
Draft for Public Notice and Comment 

Revision Date: June 2012 14 

Table 2. Example Factors Considered In Determining the Reasonableness of Controls. 

“Reasonableness” Factor Description of “Reasonableness” Factor 

6. Controls on primary sources 

expected to have contributed to the 

event 

Were primary sources of anthropogenic windblown dust 

controlled during the event? 

7. Significant contribution of sources 

to the exceedance 

There is no defined de minimis emission rate or ambient 

contribution that limits which sources should be 

considered for control, and the EPA will review this on 

a case-by-case basis. However, as a starting point, we 

believe it is generally reasonable to consider source 

categories that may contribute 5 μg/m3 or more to an 

exceedance of the 150 μg 24-hour PM10 standard.28 In 

some cases (i.e., wind speeds above the high wind 

threshold) it may not be necessary to consider sources 

down to 5 μg, while in other situations it may be 

appropriate to consider sources below 5 μg. This starting 

point may be revisited should the PM10 NAAQS be 

revised. De minimis levels for PM2.5 have not been 

clearly established. 

8. Overall benefit of controls There may be benefits to controlling even small 

anthropogenic sources. Reducing ambient 

concentrations may have a public health benefit. 

3.1.2.2 Consideration of attainment status in judging reasonableness 

In order for the anthropogenic sources to be considered to be reasonably controlled, the EPA 

expects, at a minimum, that it is reasonable for an air agency to have the controls required for an 

area’s attainment status. Generally, the EPA does not expect areas classified as attainment, 

unclassifiable, or maintenance for a NAAQS to have the same level of controls as areas that are 

nonattainment for the same NAAQS. Also, if an area has been recently designated to 

nonattainment but has not yet been required to implement controls, the EPA will expect the level 

of controls that is appropriate for the planning stage. 

3.1.2.3 Consideration of BACM/RACM 

 control measures for emissions from out-of-state sources, the submitting state may determine based on available 

information that the implemented controls on out-of state sources constitute reasonable controls and that the “not 

reasonably controllable or preventable” criterion is satisfied. When assessing emissions transported from other states 

or countries, the submitting state can say that it characterized the out of state sources, determined that these sources 

contributed to the noted exceedance or violation, and determined, based on jurisdictional boundaries and other 

available information, that contributing emissions from the upwind state or country were not reasonably controllable 

or preventable. Submitting states are further required to submit evidence/statements supporting the other exceptional 

event criteria (i.e., clear causal relationship, but for, human activity unlikely to recur or a natural event, affects air 

quality, and historical fluctuations). The EPA refers the reader to Question 23 in the “Draft Exceptional Events Rule 

Frequently Asked Questions” for additional information on this topic. 

28 5μg is the “significant impact level” (SIL) used in NSR permitting to decide whether an individual source has a 

significant contribution to a 24-hr PM10 NAAQS violation, based on 40 CFR 51.165(b)(2), and so is used here for a 

similar use. 
Draft for Public Notice and Comment 

Revision Date: June 2012 15 

Although Reasonably Available Control Measures (RACM) and Best Available Control 

Measures (BACM) for windblown dust are not necessarily required to have been in place at the 

time of the event for all areas, they are measures that the EPA and affected agencies have 

identified as being reasonable. The CAA requires BACM for serious PM10 nonattainment areas 

and RACM in moderate PM10 nonattainment areas; therefore, for such areas, the EPA, at a 

minimum, will use the local list of BACM or RACM measures (as applicable) as a reference 

point to review the reasonableness of controls. The control measures evaluated must be related to 

windblown dust. Having BACM/RACM in place during the time of the event is an important 

consideration, but may not be sufficient on its own (e.g., the BACM/RACM measures may not 

be related to windblown dust, they may not be sufficient by themselves particularly if the SIP has 

not been recently reviewed or revised), or if it focuses on air quality issues during specific 

periods without high winds (such as winter stagnation events). Generally, the EPA will consider 

windblown dust BACM to constitute reasonably controls if they have been reviewed and 

approved in the context of a SIP revision for the emission source area within the past three years. 

In some cases, a lower level of control could be reasonable, while in other cases it could be 

reasonable to require controls more stringent than current BACM or RACM, for example, if a 

new significant source is identified. Other areas (i.e., attainment, maintenance, or unclassified 

areas) are not required to have put BACM in place and also may not have implemented RACM. 

In these cases, the EPA may use local RACM measures, where available, along with other 

RACM measures that may be appropriate for the location and source categories, as the reference 

point. RACM/BACM lists may be a reference point, but not the sole means, by which the 

EPA assesses the reasonableness of controls. If an air agency believes that RACM/BACM 

should not be used by the EPA as the reference point to judge the reasonableness of controls, the 

air agency should include this justification in the demonstration package. 

In addition to BACM/RACM, the U.S. Department of Agriculture’s Natural Resources 

Conservation Service develops best management practices (under various program titles), some 

of which are designed to effectively reduce fugitive dust air emissions and prevent loss of soil 

during high winds, which may also be informative and useful controls in particular situations. 

These best management practices are not intended to be a surrogate for BACM/RACM but could 

be included in the collection of control measures. 

3.1.3 Implementation and Enforcement of High Wind Dust Control Measures

As stated in Section 3.1.1, the second condition the EPA will consider in its determination of 

whether the event meets the nRCP criterion is implementation and enforcement (where 

appropriate) of control measures on contributing sources of dust. In their demonstration 

submittals, air agencies should submit available inspection reports and/or notices of violations 

(NOVs) in upwind areas as evidence that all reasonable controls were implemented and 

functioning properly at the time of the event.29 The EPA recognizes that records may not be 

 

29 The EPA recognizes that agencies have varied methods of permitting and enforcement and does not expect all

agencies to have these records for all events. The EPA does, however, expect agencies to make a general showing 

that controls are being enforced to a reasonable degree (not necessarily on the particular day of the event); if several 

categories of anthropogenic sources are identified as significant or likely contributors to an event, the means used to 

determine compliance with reasonable control requirements for each category should be described in the 

demonstration. 
Draft for Public Notice and Comment 

Revision Date: June 2012 16 

available for all events. Cases where relevant control measures were not being fully implemented 

or properly enforced, but reasonably could and should have been, are unlikely be eligible for data 

exclusion under the Exceptional Events Rule. 

3.1.4 Consideration of Wind Speed 

The third condition stated in Section 3.1.1 for the EPA to consider for the nRCP criteria for an 

event with anthropogenic sources is whether the wind speed was high enough to overcome 

reasonable controls. In all cases (i.e., those including natural and/or anthropogenic sources) wind 

speed informs the rigor of the nRCP analysis. It is important to note that the EPA is not setting a 

bright line as to what speed constitutes a high wind dust event or to categorically concur with all 

events with sustained winds above a given level. This section describes how the EPA will use 

wind speed in its evaluation of the nRCP criterion. 

Typically, undisturbed desert landscapes in the west have a natural crust that protects the surface 

and tends to prevent windblown dust emissions. Similarly, many reasonably-controlled 

anthropogenic sources (e.g., disturbed surfaces) employ techniques that stabilize surfaces to 

reduce or prevent emissions since disturbed surfaces are a primary source of anthropogenic dust. 

Numerous studies have been conducted to determine the minimum wind speed capable of 

overwhelming reasonable controls on anthropogenic sources or causing emissions from natural 

undisturbed areas. The speed at which this occurs varies by location, depending on 

characteristics of the local landscape (e.g., soil type) and controls (See Appendix A). The EPA 

recommends that agencies develop a high wind threshold for each area experiencing high wind 

dust events (see Appendix A3 for additional information on the development of a high wind 

threshold). Appropriate area-specific thresholds would consider local conditions and the 

variation in control strategies and specify a minimum wind speed above which these controls 

would be overwhelmed. If nonanthropogenic sources are a significant source of emissions for a 

particular area, a high wind threshold may also be based on the level of wind speed capable of 

causing emissions from those specific natural undisturbed areas. This approach is consistent with 

the Natural Events Policy where the EPA required the air agencies to define the conditions in 

which BACM level controls were overwhelmed. The area-specific high wind threshold should be 

representative of conditions (i.e., sustained wind speeds30) that are capable of overwhelming 

reasonable controls (whether BACM, RACM, or other) on anthropogenic sources and/or causing 

emissions from natural undisturbed areas. The threshold is not intended to represent the 

minimum wind speed at which any level of emissions could occur (e.g., aerodynamic 

entrainment), but rather when significant emissions begin due to reasonable controls or natural 

undisturbed areas becoming overwhelmed. The high wind threshold can be submitted in advance 

of submittal of the demonstration package, with a letter of intent, with a demonstration package, 

as part of a prospective controls analysis, or as part of a High Wind Action Plan. If an agency is 

unable to develop an area-specific high wind threshold, the EPA will accept a threshold of a 

sustained wind of 25 mph for areas in the west provided the agencies support this as the level at 

which they expect stable surfaces (i.e., controlled anthropogenic and undisturbed natural 

surfaces) to be overwhelmed.31 If the EPA has specific information based on relevant studies to 

choose an alternative high wind threshold, the EPA will notify the air agency once this 

 

30 See Section 6.2.2.2 for details on the calculation of sustained wind speed. 

31 The 25 mph threshold is based on studies conducted on natural surfaces. 
Draft for Public Notice and Comment 

Revision Date: June 2012 17 

information has been submitted. Throughout this document, the term “high wind threshold” will 

be used to define the minimum threshold wind speed capable of overwhelming reasonable 

controls on anthropogenic sources (i.e., significant emissions from controlled sources) or causing 

emissions from natural undisturbed areas. 

If a demonstration can show that the sustained wind speed was at or above the high wind 

threshold at or proximately upwind of the location of the exceedance, then two streamlined 

approaches are available to meet the nRCP criterion: 

1. Rely on an already-approved prospective controls analysis. A set of controls for an area 

could be approved with the high wind threshold in advance of submittal of a package for 

a specific event (see Section 3.7.1). Once the prospective controls analysis was approved, 

additional information on controls for specific packages would typically be limited to 

information on enforcement and implementation. 

2. Conduct a basic controls analysis. Minimal amounts of information on sources and 

controls would be required for each event (see Section 3.1.5 and 3.1.5.1). 

If the sustained wind speed for an event was below the high wind threshold, the EPA can still 

review the package and possibly concur provided the air agency submits additional controls 

information as described in the comprehensive controls analysis (See section 3.1.5 and 3.1.5.2). 

The EPA believes that streamlined information (i.e., a pre-approved prospective controls analysis 

or a basic controls analysis) is sufficient when wind speeds are at or above the high wind 

threshold because, in this situation, controls to prevent windblown dust are likely to become 

overwhelmed. Thus, the event is more likely to be not reasonably controllable or preventable. If 

most controls on wind-blown dust become overwhelmed at or above the high wind threshold, air 

agencies would likely find it difficult to identify additional reasonable controls that could be put 

into place to reduce windblown dust. 

In contrast, if the wind speeds associated with the event are below the threshold levels required 

to initiate dust emissions from natural or stable (i.e., reasonably-controlled) sources, the EPA is 

likely to require that air agencies submit more detailed information (i.e., a comprehensive 

controls analysis) to satisfy the nRCP requirement. The EPA believes air agencies should submit 

a comprehensive controls analysis when wind speeds are below the high wind threshold because 

events with wind speeds below this threshold should entrain very little dust from natural and 

reasonably-controlled disturbed surfaces. Further, the EPA expects that windblown emissions 

would include significant contributions from sources that are neither natural nor reasonablycontrolled. Thus, the event is less likely to be not reasonably controllable or preventable. In these 

cases it is important for air agencies to identify the various land areas contributing to the event, 

evaluate the controls in place on those land areas, and determine whether those controls were 

reasonable based on those factors identified in Section 3.1.2. 

3.1.5 Controls Analysis for Individual Events

Air agency demonstration submittals need a controls analysis for each specific high wind dust 

event. The extent of the controls analysis should primarily depend upon the level of the wind 

speed relative to that of the high wind threshold for the area. A basic controls analysis may be 
Draft for Public Notice and Comment 

Revision Date: June 2012 18 

sufficient for cases when the sustained wind speed at the source area32 is greater than or equal to 

the high wind threshold, while a comprehensive controls analysis may be necessary when 

sustained wind speeds are below the high wind threshold (Table 3). If an air agency has not 

prospectively determined the high wind threshold for the area, then this determination, or 

establishing that the default threshold of 25 mph applies, is the first step in the controls 

analysis.33 Next, the EPA recommends that air agencies develop their nRCP analysis to evaluate 

the sustained wind speed during the event. This process may indicate that only the streamlined 

basic controls analysis is needed. See Section 6.3.2.2 for wind speed considerations for nRCP. 

Table 3. Summary of Recommended Controls Analysis Elements for not Reasonably 

Controllable or Preventable Demonstration 

Control Analysis Elements

Basic Controls Analysis Comprehensive Controls 

Analysis

Description of anthropogenic sources 

within the area and existing controls X X

Description of natural sources within the 

area and existing controls X X

Statement regarding reasonableness of 

controls X X

Explanation that emissions occurred 

despite controls X X

Identification and implementation status of 

controls previously recommended by the 

EPA as reasonable, if applicable

X X

Evidence of effective implementation and 

enforcement of reasonable controls, if 

applicable

X X

Back trajectories of source area X

Source apportionment X 

Source-specific emissions inventories X

3.1.5.1 Basic controls analysis 

The most basic controls analysis would include a brief description of local/upwind sources that 

were suspected to significantly contribute to the event and a description of the controls on the 

anthropogenic sources in place at the time of the event (e.g., local BACM measures). The EPA 

also expects evidence that the controls determined to be reasonable, if any, were effectively 

implemented and appropriately enforced. For the sources identified, the analysis should explain 

how significant dust emissions occurred despite having reasonable controls in place (e.g., that 

controls were overwhelmed by high wind). In addition to identifying controls on anthropogenic 

sources, it is important that the analysis indicate whether the natural sources could have been 

 

32 Cases where dust was entrained by sustained winds at or above the high wind threshold upwind of the monitor 

and subsequently transported at lower wind speeds to the monitor could still qualify for the basic controls analysis 

category as long as the state shows that sustained winds were at or above the high wind threshold in the expected 

source area. Cases of long-range transport (e.g., >50 miles) could still qualify for a basic controls analysis but a 

robust trajectory analysis (and/or satellite plume imagery) would need to be included as part of the nRCP or CCR 

demonstration. 

33 See Appendix A3 for additional methods for establishing area-specific high wind thresholds. 
Draft for Public Notice and Comment 

Revision Date: June 2012 19 

reasonably-controlled. If the EPA recommended controls improvements as part of a previous 

high wind dust exceptional event review then the controls analysis should address the 

reasonableness and potential impact of these control improvements. See Section 6.3.2.3 for 

examples of a basic control analysis. 

Even if a prospective controls analysis has been approved and included as part of the basic 

controls analysis, the air agency should specifically identify likely sources in the upwind source 

area and discuss specific controls if these were not discussed in the prospective analysis. 

3.1.5.2 Comprehensive controls analysis 

When events occur under conditions with sustained wind speeds below the high wind threshold, 

the EPA and the air agency must further consider the appropriateness, implementation, and 

enforcement of controls. For example, exceedances can occur when reasonable controls are in 

place but not properly enforced. Or, new or additional uncontrolled sources may be contributing 

to the exceedance. In these cases, the demonstration would need to be more complex and 

compelling for the EPA to concur. Examples of more complex analysis include: back-trajectories 

of source area, source apportionment, day specific emissions inventories of specific sources in 

source area, and evidence of effective implementation and enforcement, where appropriate, of 

controls. In addition to identifying controls on anthropogenic sources, it is important that a 

submitting agency indicate whether any natural sources could have been reasonably-controlled. 

As with the basic controls analysis, if the EPA recommended controls improvements as part of a 

previous high wind dust exceptional event review, then the controls analysis should address how 

these controls improvements have been addressed. See Section 6.3.2.4 for an example of a 

comprehensive controls analysis. 

3.1.6 Consideration of Controls on Tribal Lands

When reviewing the “reasonableness of controls” element within tribal exceptional event 

demonstration submittals, the EPA will consider both controls on tribal sources and cultural 

factors for tribal lands. For example, the EPA could consider tribal cultural factors and 

subsequently identify “reasonable” controls. It might have been reasonable for the tribal 

government to encourage the use of certain practices, but not to have required them as a matter 

of tribal law. 

3.2 Historical Fluctuations (HF)

Air agencies should include data showing historical fluctuations of concentration in the area in 

their demonstration package and make a conclusion as to whether the agency considers the data 

to be outside the normal historical fluctuations. This information satisfies the HF criterion and 

serves as an important basis for the CCR, NEBF, and AAQ criteria (see Table 2). The more a 

concentration stands out from historical concentrations, the more plausible it is that the event 

was the cause of the exceedance. The objective of the HF analysis is to give a full and accurate 

portrayal of the historical context for the claimed event day. The EPA expects, at a minimum: 

1. comparison of concentrations on the claimed event day with past historical data (3-5 

years), with previous high wind dust events identified; 
Draft for Public Notice and Comment 

Revision Date: June 2012 20 

2. percentile of concentration relative to annual data with and without high wind dust 

events; and 

3. percentile of concentration relative to seasonal data with and without high wind dust 

events. 

4. comparison of concentrations on the claimed event day with a narrower set of similar 

days 

Because the methods of analyses influence the conclusions that can properly be drawn from the 

historical fluctuation statistics (e.g., percentile calculations are dependent on the number of data 

points included), the EPA recommends specific analyses, statistics, and calculations as described 

in Section 6.3.3 of this document. 

It is important to note, however, that there is no outcome of the “historical fluctuation” statistical 

comparison that, by itself, can guarantee that the CCR and NEBF elements will also be 

successfully demonstrated. The EPA will use a weight-of-evidence approach to assess each 

demonstration and comparison of the concentrations during event(s) in question with historical 

concentration data on a case-by-case basis. The EPA acknowledges that natural events, such as 

high wind dust events, can recur and still be eligible for exclusion under the EER. Therefore, 

events do not necessarily have to be rare to satisfy this element. 

3.3 Clear Causal Relationship (CCR) 

40 CFR §50.14(c)(3)(iv) requires demonstration of a clear causal relationship between the 

ambient concentration measurement under consideration and the event that is claimed to have 

affected the air quality in the area. The CCR demonstration must show that dust from high wind 

caused an exceedance of the NAAQS. The CCR demonstration is expected to establish causality 

between the event and a portion of the ambient concentration. Simply showing that high wind 

was coincident with high concentrations is unlikely to establish causality. A correlation between 

high wind and high concentrations is important, but does not independently demonstrate that 

windblown dust from the natural undisturbed and/or reasonably controlled anthropogenic sources 

caused the high concentrations. CCR demonstrations should include analyses showing that the 

event in fact occurred and that emissions were transported in the direction of the monitors 

recording the elevated concentration measurements. CCR analyses should support the conceptual 

model and address the concepts identified in Table 4. Examples of the quantitative analyses that 

could be performed are included in Section 6.3.4. 

Table 4. Example Evidence and Analyses for CCR Demonstration 

Example CCR Evidence Types of Analyses/Information to Support 

Evidence 

1. Occurrence and geographic extent of the 

event 

Special weather statements, advisories, news 

reports, nearby visibility readings, 

measurements from monitoring stations, 

satellite imagery 

2. Transport of emissions related to the event 

in the direction of the monitor(s) where 

measurements were recorded 

Wind direction data showing that emissions 

from sources identified as part of the nRCP 

demonstration were upwind of the monitor(s) 

in question, satellite imagery 
Draft for Public Notice and Comment 

Revision Date: June 2012 21 

Example CCR Evidence Types of Analyses/Information to Support 

Evidence 

3. Spatial relationship between the event, 

sources, transport of emissions, and recorded 

concentrations 

Map showing likely source area, wind speeds, 

wind direction, and PM concentrations for 

affected area during the time of the event, 

trajectory analyses 

4. Temporal relationship between the high 

wind and elevated PM concentrations at the 

monitor in question 

24-hour time series showing PM 

concentrations at the monitor in question in 

combination with sustained and maximum 

wind speed data at area where dust was 

entrained 

5. Chemical composition and/or size 

distribution of measured pollution that links 

the pollution at the monitor(s) with particular 

sources or phenomenon 

Chemical speciation data from the monitored 

exceedance(s) and sources; size distribution 

data 

6. Comparison of event-affected day(s) to 

specific non-event days 

Comparison of concentration and wind speed 

to days preceding and following the event;; 

comparison to high concentration days in the 

same season (if any) without high wind; 

comparison to other high wind days without 

elevated concentrations (if any); comparison of 

chemical speciation data 

7. Historical comparison of PM concentration 

and wind speed (e.g., 3-5 years) data 

 Identification of historical trends or 

relationships between wind speed and PM 

concentrations. 

A demonstration will be less compelling if there is evidence that is inconsistent with the 

conceptual model of how the event caused the exceedance. For example, if the agency describes 

the event as a regional dust storm, then the EPA would expect monitors within the same 

regional scale to be similarly affected by the dust storm. Comparison of concentrations and 

conditions at other monitors could thus be very important for the demonstration of a clear causal 

relationship. Alternatively, eliminating plausible non-event causes may also support a causal 

relationship to the high wind dust event. (See Section 6.3.4.7 for an example of eliminating 

alternative hypotheses.) 

3.4 Affects Air Quality (AAQ) 

The AAQ element is generally supported by historical fluctuations in concentration data (HF) 

and demonstrated as part of the clear causal relationship (CCR).34 Submitting agencies that 

 

34 In the definition of “exceptional event”, 40 CFR §50.1(j) begins: “Exceptional event means an event that affects 

air quality...” The preamble clarifies this in section V.B. What Does It Mean for an Event To “Affect Air Quality”?

(p. 13569) : 
Draft for Public Notice and Comment 

Revision Date: June 2012 22 

provide HF analyses and conclusions and that demonstrate the CCR element will generally, have 

also satisfied the “affects air quality” (AAQ) part of the definition of an exceptional event. The 

demonstration should nevertheless explicitly recognize this element, and describe how this 

requirement is met through meeting the HF and the CCR criteria. 

3.5 Caused by Human Activity Unlikely to Recur at a Particular Location (HAURL) or 

a Natural Event (Natural Event) 

According to both the regulatory and statutory definition, an exceptional event must be “an event 

caused by human activity that is unlikely to recur at a particular location or a natural event.” 

High wind dust events that meet the criteria established in this guidance document would be 

considered natural events, thus the HAURL does not apply and would not need to be considered 

for a high wind dust event. 

A natural event is defined as “an event in which human activity plays little or no direct causal 

role” (40 CFR §50.1(k)). An event involving windblown dust solely from undisturbed natural 

sources is clearly a natural event. However, many high wind dust events affecting the ambient 

monitoring network include significant contributions from anthropogenic sources of dust, and 

their treatment under the EER is more complicated. In these cases, a high wind dust event can be 

considered a natural event, even when a portion of the wind-driven emissions are anthropogenic, 

as long as those emissions were determined to be not reasonably controllable or preventable. 

Exceedances that include a significant contribution by anthropogenic sources of windblown dust 

that were not reasonably-controlled will not be considered as due to a natural high wind dust 

event. In addition, high dust concentrations outside the period of high wind (e.g., dust from rockcrushing or tilling that precedes the period of high wind) cannot be considered as due to a natural 

event and therefore could not be considered as part of a high wind dust event. In both of the 

above cases, it would be assumed that human activity played a large and direct causal role and 

therefore these exceptional events claims could only be considered under the criterion of “human 

activity unlikely to recur.”35

 Under the Final Rule, the demonstration to justify data exclusion must provide a justification that: (a) The 

event qualifies in accordance with section IV.D. and if applicable, with the EPA policies and guidance for 

certain events as described in section IV.E, (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 an unusual measured concentration beyond typical fluctuations including background, and 

(d) there would have been no exceedance or violation but for the event (discussed in section V.C). The 

second and third criteria establish that the event affected air quality. [emphasis added] 

In this passage, the second criterion is (b) “clear causal relationship”, and the third is (c) “concentration beyond 

typical fluctuations”. These are the same as the EER requirements for CCR (“clear causal relationship”) and HF (“in 

excess of normal historical fluctuations”) at 40 CFR §50.14(c)(3)(iii)(B) and (C). 

35 In theory, a high wind dust event for which anthropogenic sources were not reasonably-controlled could be 

considered an anthropogenic event if the event satisfies certain criteria, including being unlikely to recur. However, 

if the event (which includes the dust from both natural and anthropogenic sources) was not “not reasonably 

controllable or preventable” then the event does not meet the definition of an exceptional event. For this reason, the 

EPA does not believe it is useful to pursue a line of reasoning that would consider a high wind dust event to be an 

anthropogenic event. If the very unlikelihood of recurrence of similarly high winds means that controls in addition 

to those that were in place would not have been reasonable, the event can be treated as a natural event and must then 

meet the criteria laid forth in the EER and explained in this document.
Draft for Public Notice and Comment 

Revision Date: June 2012 23 

Since anthropogenic sources of windblown dust must be reasonably-controlled for the event to 

be considered a natural event under the EER, the air agency must demonstrate that the criterion 

for nRCP is met (see Section 3.1). Further, to satisfy the EER, air agencies must also 

demonstrate that the windblown dust generated by high wind has a clear causal relationship 

(CCR) to the measured exceedance. In summary, the EPA will generally consider a high wind 

dust event to be a natural event if the air agency demonstrates both the nRCP and CCR elements 

to the EPA’s satisfaction. 

3.6 No Exceedance or Violation But For the Event (NEBF) 

40 CFR 50.14(b)(1) directs the EPA to exclude data only where an air agency demonstrates that 

an event caused a concentration in excess of a NAAQS. This means that there was a 

concentration in excess of the NAAQS when the event occurred that would have been below the 

NAAQS if the event had not occurred. §50.14(c)(3)(iv)(D) requires the air agency to submit 

evidence that “[t]here would have been no exceedance or violation but for the event.”36 Figure 1 

depicts the NEBF concept. 

Figure 1. Depiction of the Concept of No Exceedance But For the Exceptional Event. 

This analysis generally does not need a single or precise approximation of the estimated air 

quality impact from the event. The EPA is not prescribing the type of analysis that air agencies 

need to perform to satisfy this regulatory requirement, but air agencies should show that the 

measured concentration would have been below the applicable NAAQS without the impact of 

the high wind dust event. For most cases, the EPA expects a quantitative NEBF analysis. For 

 

36 In addition, Section 319 of the Clean Air Act requires that a clear causal relationship must exist between the 

measured exceedances and the exceptional event, meaning that exceptional events dealt with in the EER must be 

exceedances 
Draft for Public Notice and Comment 

Revision Date: June 2012 24 

events where the typical concentrations on non-event days are well below the applicable 

NAAQS, the NEBF demonstration may be relatively straightforward and a qualitative NEBF 

demonstration may be acceptable. However, demonstrating NEBF becomes increasingly difficult 

if concentrations on non-event days during the same season exceed the standard and/or if the 

contribution of non-event pollution sources produce concentrations near the applicable NAAQS. 

For example, if days without high winds that neighbor the claimed event day were near the 

standard (e.g., 150 μg/m3 for PM10), the NEBF analysis would need to be very rigorous to show 

that the exceedance would not have happened but for the high wind dust event. Examples of how 

to conduct the NEBF analysis are provided in Section 6.3.7. 

The NEBF demonstration builds upon and will be informed by the rigor of the nRCP and CCR 

analyses and further supported by information provided for HF. To illustrate the NEBF and CCR 

relationship, if there is no CCR, then NEBF becomes moot since there is no portion of the 

exceedance that can clearly be attributed to the event. For these reasons, the EPA recommends 

that the air agency complete the NEBF analyses after it completes all other analyses. 

3.7 Optional Analyses for nRCP Requirement

3.7.1 Prospective Controls Analysis

As stated in Section 3.1.1, a high wind dust event that includes anthropogenic sources can meet 

the nRCP criterion if: 

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

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

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

A prospective controls analysis is an optional, generic37 review of the current windblown dust 

controls (item 1 above) and the high wind threshold (item 3 above) for an area. This optional step 

would occur in advance of the EPA’s review of any particular event. In the prospective controls 

analysis, the air agency would provide information on attainment status, identify natural and 

anthropogenic windblown dust sources and emissions, provide the status of SIP submittals (if 

applicable), and identify the wind speed up to which the collective windblown dust controls are 

expected to be effective (see Section 6.2 for details on how to prepare a prospective controls 

analysis). Air agencies could submit their prospective controls analysis in advance of an agency 

submittal and the EPA review of any specific demonstration submittal, with the letter of intent, 

or with their demonstration package submittal. Once the EPA approves a prospective controls 

analysis, air agencies could reference this pre-approved analysis in subsequent packages for 

events with winds above the established/approved high wind threshold to satisfy items 1 and 3 

above and provide information on implementation and enforcement of controls (item 2 above). 

The EPA review and approval of controls identified in the prospective controls analysis would 

typically be effective for a minimum of three years. After the three-year time period, the EPA 

will notify the agency if the EPA intends to re-review the controls. In some limited cases, the 

EPA may re-review the controls within the three-year timeframe if information on sources or 

 

37 “Generic” means a general review rather than a review specific to an identified event. 
Draft for Public Notice and Comment 

Revision Date: June 2012 25 

enforcement suggests that controls may be inadequate or not implemented/enforced. The EPA 

may also re-evaluate the controls identified in the prospective controls analysis when an area that 

does not typically have recurring high wind dust events experiences unexpected recurrent events. 

If the EPA has approved a SIP revision to windblown dust controls within the past three years of 

the event, then the submitting agency can use the SIP-approved controls to satisfy item 1 of a 

prospective controls analysis. 

3.7.2 High Wind Action Plan

3.7.2.1 Purpose 

Air agencies can develop High Wind Action Plans to document their plans to implement needed 

controls on newly-identified sources that could emit dust during subsequent high wind events 

(similar to the process used in a Natural Events Action Plan). A High Wind Action Plan is an 

optional mechanism to implement necessary controls more expeditiously than with the normal 

regulatory planning process. Preparation of such a plan and its approval by the EPA could 

promote a common understanding between the air agency and the EPA about whether 

subsequent high wind dust events are not reasonably controllable or preventable. In addition, the 

High Wind Action Plan could document current windblown dust controls and current and/or 

planned mitigation measures as part of §51.930. 

3.7.2.2 How Does the High Wind Action Plan Option Work? 

The EPA will judge the reasonableness of controls based on information that was available to the 

air agency at the time of the event (see Section 3.1.2). However, in the course of preparing or 

reviewing a high wind dust exceptional event demonstration submittal, the air agency or the EPA 

may identify previously unknown sources that should be subject to reasonable controls. The EPA 

and/or the air agency may determine that additional controls could minimize the likelihood or the 

health impact of future events. The EPA does not want to penalize air agencies for lack of 

controls on sources unknown to the air agency at the time of the event, but air agencies must 

recognize that EPA cannot consider the collection of sources and associated controls to meet the 

nRCP criterion for subsequent events once either party identifies a significant uncontrolled 

source. The High Wind Action Plan provides an optional mechanism that allows the EPA to 

concur on an air agency’s request to exclude data associated with the current and subsequent 

events while the air agency implements controls on previously unidentified sources. Preparation 

of a High Wind Action Plan and its approval by the EPA could promote a common 

understanding between the air agency and the EPA about whether subsequent high wind dust 

events are not reasonably controllable or preventable. Note that a High Wind Action Plan does 

not replace planning actions required under the Clean Air Act based on an area’s attainment 

status. For example, a PM10 serious nonattainment area is still required to implement BACM. In 

this type of case the High Wind Action Plan can address sources identified after BACM has been 

implemented. 
Draft for Public Notice and Comment 

Revision Date: June 2012 26 

An air agency can submit a High Wind Action Plan with the exceptional events demonstration 

package or as a separate submittal.38 Establishing a High Wind Action Plan to address additional 

reasonable controls consists of the following steps: 

1. Air agency development and submittal of the High Wind Action Plan after an opportunity 

for public comment 

2. The EPA approval of the High Wind Action Plan 

3. Air agency implementation of the identified and approved control measures 

Assuming all other exceptional event criteria are met, once the EPA approves the High Wind 

Action Plan, the EPA could then concur upon the event under current consideration. If an air 

agency chooses to develop a High Wind Action Plan, it will have six months from the time the 

EPA provides notification that newly-identified sources require reasonable controls to submit an 

adequate (i.e., approvable) plan. If the air agency meets this timeframe, then the EPA will 

consider anthropogenic sources contributing to high wind dust events that occur within the sixmonth timeframe to be reasonably-controlled, assuming the air agency appropriately 

implemented and enforced required controls and wind speeds are high enough to overwhelm 

those reasonable controls. 

If an agency does not meet the six-month timeframe and/or the High Wind Action Plan is not 

approvable, the EPA would treat any high wind exceptional events that occur within and beyond 

the six months as not meeting the nRCP criterion. Therefore, under most conditions, the EPA 

would not be able to concur on high wind exceptional events. Under this scenario, the EPA and 

the air agency would include associated event data in planning decisions. The High Wind Action 

Plan is unlikely to be an option for an area in the midst of a planning action since the planning 

timeframes are unlikely to accommodate a multi-month development of a High Wind Action 

Plan. 

Once approved, an air agency’s High Wind Action Plan is effective for three years and would 

establish that sources have reasonable controls for events where wind speeds exceed the high 

wind threshold. For future events that occur under an approved High Wind Action Plan, the EPA 

would consider whether controls were implemented effectively according to the High Wind 

Action Plan as part of the nRCP evaluation. While the High Wind Action Plan is not in itself 

enforceable, the EPA’s concurrence of exceptional events relies upon the implemented plan (i.e., 

the newly-identified sources would not be considered to meet the nRCP criterion if the High 

Wind Action Plan is violated). Therefore, if an air agency does not effectively implement its 

plan, the EPA may be unable to concur on future exceptional events. 

An approved High Wind Action Plan does not automatically mean that the EPA will find all 

subsequent events to meet the not reasonably controllable or preventable criterion. For example, 

 

38 If an air agency submits the High Wind Action Plan separately from the exceptional event demonstration package, 

the air agency should provide an opportunity for public comment as the High Wind Action Plan would be part of the 

basis for the EPA’s decision on subsequent events. 
Draft for Public Notice and Comment 

Revision Date: June 2012 27 

the EPA may not concur if the EPA finds that an air agency did not effectively implemented or 

enforced the controls in place.39

3.7.2.3 Content of a High Wind Action Plan

At a minimum, a High Wind Action Plan to address new sources should include the following: 

 identification of sources and proposed controls 

 an assessment of reasonableness 

 timeframes for implementation 

 a plan for enforcement, if appropriate. 

The High Wind Action Plan should also identify the current high wind threshold and whether 

this threshold should be revised as a result of the newly identified controls. As mentioned 

previously, a High Wind Action Plan could also document current controls, especially if these 

were not included in a previous demonstration package, and mitigation measures. 

3.7.2.4 Comparison of a Natural Events Action Plan and High Wind Action Plan

The concept of a High Wind Action Plan originated from the Natural Events Action Plan 

(NEAP). Table 5 presents a comparison of the two plans. 

Table 5. Comparison of Natural Events Action Plan and High Wind Action Plan. 

Element Natural Events Action Plan High Wind Action Plan 

Establish public notification 

and education programs 

Required component of plan. Optional component of plan to 

address mitigation 

requirement for EER (40 CFR 

51.930). 

Minimize public exposure to 

high levels of windblown dust 

Required component of plan. Optional component of plan to 

address mitigation 

requirement for EER (40 CFR 

51.930). 

Current reasonable controls on 

contributing controllable 

sources 

Required component of plan 

(BACM). 

Required component of plan 

(but not necessarily BACM). 

Plan for putting reasonable 

controls in place on 

contributing controllable 

sources 

Required component of plan. Required component of plan if 

additional controls are needed 

as condition for concurrence 

on current event(s). 

Re-evaluate the controls Every five years. ≥ 3 years or if high wind dust 

patterns change and suggest 

new sources. 

Timeframe for submittal Within 18 months of violation. Within 6 months from the 

 

39 Note that if and when the EPA takes a regulatory action that hinges on a decision to exclude data under the 

Exceptional Events Rule, the EPA may be required to consider and appropriately respond to public comments on 

whether the event was “not reasonably controllable or preventable.” 
Draft for Public Notice and Comment 

Revision Date: June 2012 28 

Element Natural Events Action Plan High Wind Action Plan 

EPA’s notification of newlyidentified uncontrolled 

source(s). 

Timing of EE concurrence Prior to NEAP submittal.40 After the EPA’s approval of 

High Wind Action Plan. 

High wind threshold Required part of Natural 

Events Policy41 but not 

specifically required to be 

included in NEAP. 

Required component of plan. 

3.7.2.5 Attainment status and the role of High Wind Action Plan 

The High Wind Action Plan can be a useful tool for any area that has newly identified sources 

requiring reasonable controls. However, depending on an area’s attainment status, air agencies 

may have other options as identified below. 

Attainment/unclassifiable/maintenance: As mentioned previously, the EPA agrees that controls 

are not expected for the first high wind dust exceptional event in a PM attainment area. However, 

the EPA is unlikely to consider a first high wind dust event, because the first event is unlikely to 

result in a violation of the NAAQS. If an area is violating the standard because of more than one 

high wind dust event in a three-year period, then the EPA will consider the area to “have a 

history” of high wind dust events. If wind speeds exceed a predetermined wind threshold, then 

the EPA may not consider the implementation of controls at the time of the event to be a 

condition for a determination that the event was not reasonably controllable. The EPA will 

determine this during the demonstration review process. The EPA does not expect to simply 

concur on events where sources that were not controlled caused a violation just because the area 

was previously in attainment. The EPA expects to look closely exceedances that occurred with 

wind speeds below the wind threshold to overwhelm reasonable controls or cause emissions 

from natural undisturbed areas. If an attainment area has violations below the high wind 

threshold and the EPA determines that the concentrations are attributable to sources that could be 

reasonably controlled going forward, the EPA will do one of the following: 

1. Request that the submitting agency develop and implement an adequate High Wind 

Action Plan and concur upon approval of the HWAP going forward if all other EER 

criteria have been met. 

2. Nonconcur. If the nonconcurrence results in an area’s designation as non-attainment, then 

this nonconcurrence would also trigger preparation of a SIP incorporating appropriate 

PM controls. 

 

40 Unless the NEAP was not adhered to (e.g., BACM never implemented for previous events). 

41 “Areas Affected by PM10 Natural Events” (the PM10 Natural Events Policy), memorandum from Mary D. 

Nichols, Assistant Administrator for Air and Radiation, to EPA Regional Offices, May 30, 1996, page 7, “The 

conditions that create high wind events vary from area to area with soil type, precipitation and the speed of wind 

gusts. Therefore, the State must determine the unusually high wind conditions that will overcome BACM in each 

region or subregion of the State.” 
Draft for Public Notice and Comment 

Revision Date: June 2012 29 

The EPA intends to work with the air agency to determine which approach listed above, or other 

approach, will get reasonable controls in place most effectively and efficiently. 

Nonattainment: A PM nonattainment area is expected to have reasonable controls in place, but 

there may be new sources or improved controls that are identified after the original 

implementation of the reasonable controls. Additionally, during high wind conditions, sources 

outside the designated nonattainment area may contribute to violations in the nonattainment area. 

The EPA will consider the wind speeds in the event(s) in question relative to the high wind 

threshold in determining if additional controls are reasonable. In cases where additional 

reasonable controls are needed, the EPA may be compelled to do one of the following: 

1. Request that the submitting agency develop and implement an adequate High Wind 

Action Plan, and concur on exceptional events upon approval of the HWAP if all other 

EER criteria have been met. Subsequent events with wind speeds above the high wind 

threshold could similarly be concurred upon. 

2. Nonconcur in the absence of additional actions beyond what is required by the SIP and its 

associated deadlines. In these cases, the EPA would not remove the events from 

regulatory decisions. As a result, the event-related concentrations may play a role in 

regulatory actions such as a clean data finding or a failure to attain decision. For example, 

this option may be most sensible to all parties early in a planning cycle when an agency is 

working on implementing controls that are expected to result in significant improvements 

in an area’s dust control. 

3. Nonconcur and conduct a SIP call. If the EPA identifies a major deficiency in the SIP 

controls, then the EPA may choose to nonconcur and issue a SIP call to expedite 

implementation of reasonable controls on particular sources. 

The EPA intends to work with the air agency to determine which approach listed above, or other 

approach, will get reasonable controls in place most effectively and efficiently. 

 
Draft for Public Notice and Comment 

Revision Date: June 2012 30 

4. Mitigation 

The EER was promulgated pursuant to Section 319 of the Clean Air Act which contains a 

provision that each air agency “must take necessary measures to safeguard public health 

regardless of the source of the air pollution...” This provision was the basis for the mitigation 

requirements in 40 CFR §51.930 and the requirement in the EER at 40 CFR §50.14(c)(1)(i) that 

all air agencies must “notify the public promptly whenever an event occurs or is reasonably 

anticipated to occur which may result in the exceedance of an applicable air quality standard.” 

The language at 40 CFR §51.930 requires that: 

“(a) A State requesting to exclude air quality data due to exceptional events must take 

appropriate and reasonable actions to protect public health from exceedances or violations of the 

national ambient air quality standards. At a minimum, the State must: 

(1) Provide for prompt public notification whenever air quality concentrations exceed or are 

expected to exceed an applicable ambient air quality standard; 

(2) Provide for public education concerning actions that individuals may take to reduce 

exposures to unhealthy levels of air quality during and following an exceptional event; 

and 

(3) Provide for the implementation of appropriate measures to protect public health from 

exceedances or violations of ambient air quality standards caused by exceptional 

events.” 

Although the language at 40 CFR §51.930 does not require the preparation or submittal of a 

mitigation plan, it does require that the air agency develop and implement processes and 

measures that could easily become the elements of a formal, written plan. The mitigation criteria 

focus on specific measures and actions to protect public health, rather than on measures that 

control or prevent emissions associated with a specific event. So, a mitigation plan may include 

measures that apply to emissions sources in general (e.g., dust suppression or covering 

techniques for mineral processing) rather than those measures or controls that might be discussed 

in the “not reasonably controllable or preventable” portion of an event demonstration (e.g., 

controls/measures X, Y, and Z were in place on sources A, B, and C during the time of the 

event). A mitigation plan may also include procedures and responsibilities for public alerts and 

sheltering advisories. Because having a mitigation plan in place will help air agencies meet the 

EER requirements at 40 CFR §50.14(c)(1)(i) related to public notification more systematically, 

the EPA encourages the development and submittal of a mitigation plan with the demonstration 

package if one has not already been adopted. 
Draft for Public Notice and Comment 

Revision Date: June 2012 31 

5. Process Issues for Exceptional Events Including High Wind Dust Events 

5.1 Demonstrations Package Submittal and Review 

The EPA encourages air agencies to engage in regular communication with the EPA to prepare 

complete demonstration packages that meet the requirements of the EER as interpreted by this 

document, but that also avoid unnecessary detail and the associated preparation effort. The EPA 

will make its decision based on information presented by the air agency, along with any 

information that the EPA may have or generate. Determinations on Exceptional Event 

demonstrations do not constitute final agency action until they are relied upon in a regulatory 

decision such as a finding of attainment or nonattainment which will be made through noticeand-comment rulemaking process. 

5.2 Timeframes 

The EPA recommends the following timeframes for exceptional events processes as outlined in 

Table 6 below. 

Table 6. Timeframe for Exceptional Event Processes. 

Exceptional Event 

Demonstration 

Action 

Timing Timing 

Specified by 

EER? 

Air agency submits a 

prospective controls 

analysis 

Any time in advance of the EPA’s review of a 

particular event or set of events. 

No 

1. Air agency places 

flags in AQS 

Flags and an initial event description should be 

placed in AQS in accordance with the schedules for 

submission of data to the AQS database (i.e., within 

90 days of the end of the previous quarter) but not 

later than July 1st of the calendar year following the 

event in which the flagged measurement occurred. 

Yes 

2. Air agency submits 

letter of intent to 

submit a package 

(optional)

To promote early communication, the EPA suggests 

that air agencies provide a letter of intent to submit a 

demonstration package for flagged data in AQS as 

soon as possible after the agency identifies the 

significance of the event, if possible within 12 

months from the event occurrence.42

This is an optional step that would alert the EPA of 

an air agency state’s intention to submit a package for 

a flag. See Appendix C for an example letter of 

intent. 

No 

 

42 The Letter of Intent is an optional step and the EPA recognizes that states may need additional time to prepare and 

submit demonstration packages particularly where the basis of the exclusion is violating an annual standard or a 3-

year design value. Similarly, a state could consider submitting an annual letter of intent if annual submittal makes 

sense for resource planning or for historically seasonal events. If a state decides to submit a letter of intent, the EPA 

recommends that it be submitted as expeditiously as possible following the state identifying the event or events as 

having significance. 
Draft for Public Notice and Comment 

Revision Date: June 2012 32 

Exceptional Event 

Demonstration 

Action 

Timing Timing 

Specified by 

EER? 

3. The EPA responds 

to an air agency’s 

notice of intent and 

informs the air agency 

of EPA’s review 

plans. 

Anticipated to be within 60 days of receipt of letter of 

intent to submit a package from air agency. The EPA 

response will provide the regional office’s best 

assessment of the priority that can be given to the 

submission once received and any case-specific 

advice the EPA may have to offer for the preparation 

of the demonstration. 

The EPA will generally give priority to exceptional 

event decisions that affect near-term regulatory 

decisions. 

No 

4. Air agency submits 

exceptional event 

package to the EPA 

The EER allows air agencies to submit packages up 

to 3 years following the end of the calendar quarter in 

which the event occurred, or 12 months prior to the 

date that a regulatory decision must be made by the 

EPA, whichever is sooner. Schedules are generally 

tailored in 40 CFR Part 50 when NAAQS revisions 

result in initial designations for new NAAQS. 

Yes 

5. Air agency submits 

High Wind Action 

Plan (optional) 

Submit with EE package or within 6 months from the 

EPA’s notification of newly-identified uncontrolled 

source(s). 

No 

6. The EPA completes 

initial review of 

exceptional event 

package & sends letter 

to air agency outlining 

preliminary 

assessment of 

completeness of 

package/need for 

additional 

information43

Anticipated within 120 days of receipt by the EPA. No 

7. Air agency provides 

supplemental 

information requested 

by the EPA, if needed 

Requested within timeframe identified by the EPA in 

the initial review letter (step 4). This will typically be 

60 days from receipt of the letter from the EPA. The 

EPA recognizes that air agencies may need more than 

60 days to prepare and submit some types of 

supplemental information. The EPA is willing to 

work with agencies on supplemental timeframes; 

however, the mandatory timing of EPA actions may 

limit the response time the EPA allows. 

No 

 

43 The EPA may also request additional information during later steps (e.g., as part of the final review (step 8)). 
Draft for Public Notice and Comment 

Revision Date: June 2012 33 

Exceptional Event 

Demonstration 

Action 

Timing Timing 

Specified by 

EER? 

8. The EPA final 

review of EE package 

For EE packages that impact a regulatory decision the 

EPA intends to make a decision regarding 

concurrence within 18 months of submittal of the 

complete package, or sooner if required by a 

regulatory action. 

No 

5.3 Public Comment 

If an air agency submits substantial supplemental information to the EPA after the air agency’s 

initial opportunity for public comment, the air agency may need to provide an additional 

opportunity for public comment. The EPA will make a case-by-case decision regarding 

supplemental opportunities for public comment during the demonstration preparation, submittal, 

and review process and will inform the air agency of this decision. As part of this decision, the 

EPA may consider potential impact and/or expressed public interest in the claimed event, data 

uncertainty, historical application of demonstration approach, etc. Additionally, certain 

regulatory actions that may rely on exceptional event data exclusions (e.g., proposed designation 

or redesignation classifications and attainment determinations) require the EPA to provide an 

opportunity for public comment prior to the EPA’s taking final Agency action. 

If an additional opportunity for public comment is needed, the air agency should submit the 

additional information to the EPA within the timeframe outlined in step 7 above and then make 

the information available for public comment. Once the opportunity for public comment has 

closed, the air agency should submit the public comments, if any, along with the air agency’s 

responses, to the EPA within 30 days of the close of the public comment period. If air agencies 

do not submit High Wind Action Plans as part of the exceptional event demonstration package, 

air agencies should also provide opportunity for public comment for the Plan. 
Draft for Public Notice and Comment 

Revision Date: June 2012 34 

6. Recommendations for Preparing High Wind Dust Exceptional Event 

Demonstrations 

Section 6 provides practical information on preparing and evaluating exceptional events 

demonstrations for high wind dust events based on the guidance in this document and the EPA’s 

experience from demonstrations that the EPA has reviewed since the promulgation of the EER. 

Section 6.1 provides the general, suggested framework to prepare a high wind dust event 

demonstration. Section 6.2 outlines the suggested steps to creating a prospective controls 

analysis. Section 6.3 provides details and examples for the technical elements. The EPA 

encourages air agencies to include a description of mitigation strategies as part of the 

demonstration although submission of a mitigation plan is not a regulatory requirement. 

6.1 Framework for Preparing Evidence to Support a High Wind Dust Exceptional 

Event 

While the technical elements outlined in the EER suggest that each element can be demonstrated 

independently, many of the elements are linked. The EPA suggests the following approach to a 

demonstration, as depicted in Figure 2. 

Optional Pre-Step. Provide a prospective controls analysis to EPA for approval, which can 

subsequently be used to supplement the evidence in step 2. 

Step 1. Develop a conceptual model of how the event unfolded and resulted in the exceedance(s). 

Step 2. Address not Reasonably Controllable or Preventable (nRCP). 

 Identify high wind threshold (see Appendix A) 

 Calculate sustained wind speed: wind speed will inform whether a basic or comprehensive 

controls analysis is needed. 

 Assess general wind direction and identify potential sources 

 Develop controls analysis 

Step 3. Present Historical Fluctuations analyses and a conclusion for EPA’s review of whether 

the event was in excess of normal historical fluctuations (HF). 

Step 4. Address Clear Causal Relationship (CCR). 

 Conduct CCR analyses 

 Consider whether CCR identified sources not addressed in nRCP. 

Step 5. Once sufficient HF analyses have been completed and CCR has been demonstrated, then 

Affects Air Quality (AAQ) will generally have also been satisfied. Provide information that 

AAQ has been met by providing HF analyses and demonstrating CCR. 

Step 6. Once nRCP and CCR have been satisfied, then the element for Natural Event will 

generally have also been satisfied. Provide information that Natural Event has been satisfied by 

demonstrating nRCP and CCR. 
Draft for Public Notice and Comment 

Revision Date: June 2012 35 

Step 7. Address No Exceedance But For the event (NEBF) only after all previous criteria have 

been satisfied. 

The EPA recommends reviewing and revising if necessary the conceptual model after the air 

agency completes each identified step. 

Figure 2. Suggested order for preparing technical elements for demonstration packages for 

high wind dust events. 

Step 1

Develop a Conceptual 

Model 

Step 2 

nRCP 

Basic Controls 

Analysis

 wspd ≥ high wind 

threshold 

Comprehensive 

Controls Analysis

 wspd < high wind 

threshold 

Step 4 

CCR 

Step 3 

HF 

Step 5 

NEBF 

Natural Event 

(derived from 

nRCP and CCR) 

AAQ 

(derived from HF 

and CCR)

Optional Pre-step 

Prospective Controls Analysis 
Draft for Public Notice and Comment 

Revision Date: June 2012 36 

6.2 Preparing a Prospective Controls Analysis (Optional) 

An air agency's request for a prospective determination of reasonable controls, independent of a 

specific exceptional event request, should include the following elements: 

1. Statement of which NAAQS (i.e., pollutant, date of relevant NAAQS, averaging time), area 

(e.g., Ventura County, California) and time period (e.g., January 1, 2012 – December 31, 

2015) are to be covered by the prospective determination. 

2. Statement of the area's attainment status for the pollutants (e.g., serious nonattainment with 

an extended attainment date for the 2006 24-hour PM2.5 standard) and resultant required 

CAA control level (e.g., nothing, RACT, RACM, BACT, BACM and/or Most Stringent 

Measures (MSM)). 

3. Current emissions inventory quantifying natural (if available) and anthropogenic sources of 

the pollutants and identifying all those the air agency considers significant. 

4. Information on whether natural sources are reasonably controlled. 

5. Chronology of agency SIP (and NEAP) submittals to address the required CAA control level 

and EPA actions on these submittals. This should identify whether the submittals relied on 

any commitment for subsequent action and whether the EPA actions were less than full 

approval. The air agency’s request for a prospective determination should summarize the 

status of SIP submittals and EPA action for each significant source category. The EPA will 

consider this in its review.

6. Summary of all exceptional event requests and other exceedances for the pollutants in the 

area over the last 10 years. This should include the number, location, cause (sources) and 

EPA action for all requests. 

7. Discussion of any and changes to presence/understanding/requirements for controls and/or 

emission sources since EPA’s approval in #5 including: 

a. Significant evolution in understanding of control levels in #2 (e.g., while 15 ppm 

was approved as BACM in 2001, 4 ppm is now widely accepted as BACM). 

b. Significant changes in the emissions inventory since #5 suggests possible 

additional significant sources (e.g., past planning efforts focused on wood-smoke, 

but recent information suggests significant wind-blown dust sources as well). 

c. Exceptional event requests and other exceedances discussed in #6 (e.g., newly 

recurring exceptional event requests might suggest a new source that lacks 

reasonable controls despite previous EPA approval of controls and an attainment 

demonstration). 

8. A wind speed threshold up to which controls are expected to be effective (i.e., high wind 

threshold (See Section 3.1.4 and Appendix A)). Should address controlled anthropogenic 

sources as well as undisturbed natural surfaces. 
Draft for Public Notice and Comment 

Revision Date: June 2012 37 

Upon review of the prospective controls analysis EPA will approve, disapprove, or request 

additional information. The EPA expects to approve prospective controls determinations when 

all of the following occur: 

 The air agency provides all the above items. 

 EPA has fully approved the required CAA control level in #5. 

 EPA has confirmed fulfillment of all underlying commitments in #5. 

 The air agency certifies there are no additional considerations in #7 and EPA agrees. 

 EPA agrees with the wind threshold (#8). 

EPA does not expect to approve prospective determinations if any of the following occur: 

 The air agency's request does not include all the items listed above. 

 The air agency has not demonstrated the required CAA control level in #5. 

 The air agency has not confirmed fulfillment of commitments in #5. 

 Considerations in #7 indicate that some significant sources are lacking reasonable 

controls and/or implementation of controls. 

 EPA disagrees with the wind threshold (#8). 

The EPA may subsequently approve an initially unapprovable prospective controls analysis, if 

the submitting air agency revises the demonstration to address EPA concerns (e.g., missing 

elements are provided, the high wind threshold is altered, or new information on existing 

controls is provided). If the EPA’s inability to approve the prospective controls analysis is based 

on the need for reasonable controls on additional sources, then EPA may not be able to approve 

the prospective controls analysis until the submitting agency develops, submits, and establishes a 

plan (e.g., High Wind Action Plan) to implement the needed controls. 

6.3 Recommended Methods for the Technical Elements of a High Wind Dust 

Exceptional Events Package 

This section contains recommendations for preparing and demonstrating the technical elements 

for high wind dust events. These recommendations and examples do not represent the full suite 

of analyses that could be conducted as part of a high wind dust exceptional events package, but 

are intended to show the kinds of analyses and descriptions that EPA expects. The examples 

were taken from EPA Region 9 analyses and the following high wind dust event demonstration 

packages that were submitted to EPA Region 9:44

 

44 Full exceptional event demonstration packages are available as follows: 

 Anaheim (SCAQMD, event date: October 13, 2008) at 

http://www.aqmd.gov/pub_edu/notice_exceptional_events_2009.html

 Assessment of Qualification for Treatment under the Federal Exceptional Events Rule: High Particulate (PM10) 

Concentration Event in the Phoenix Area on April 30, 2008. Technical report prepared by the Arizona 

Department of Environmental Quality, Air Quality Division. August 16, 2010. 

 State of Arizona Exceptional Event Documentation for the Events of July 2nd through July 8th 2011, for the 

Phoenix PM10 Nonattainment Area. Report prepared by Arizona Department of Environmental Quality 

(ADEQ), March 8. 2012. 
Draft for Public Notice and Comment 

Revision Date: June 2012 38 

 Anaheim: South Coast Air Quality Management District (SCAQMD) 

 Phoenix: Arizona Department of Environmental Quality (ADEQ) 

6.3.1 Step 1: Develop a Conceptual Model

A demonstration package for a high wind dust event should include a conceptual model of how 

the event occurred. A conceptual model is generally used to describe various concepts and their 

relationships. For exceptional events, the conceptual model should identify the various 

phenomena (e.g., high wind conditions and emissions) that occurred that resulted in the 

exceedance. In its simplest form, this could be a narrative description of how the event unfolded 

and resulted in the exceedance(s). The conceptual model will be similar to an abstract and should 

help tie the various rule criteria together into a cohesive explanation of the event. The following 

information is suggested to be included in the conceptual model: 

 Description of weather phenomena that resulted in high wind 

 Description of sources (land areas, industrial sources, other anthropogenic sources, 

natural sources, types of PM/dust) likely entrained by the high wind 

 Explanation of the path by which the dust reached the monitor(s) 

 Description of and map showing relevant monitors, topography, and other relevant 

geographic features that assist in understanding how the event developed and resulted in 

the exceedance. 

 Description of how the event day differs from non-event days 

 Description of concentration and wind patterns for the exceeding monitor(s) and for 

surrounding area 

The following is an example of the type of narrative EPA suggests for the conceptual model.45

Southern California’s South Coast Air Basin (Basin) consists of 10,743 square miles and 

consists of Orange County and the non-desert portions of Los Angeles, Riverside and San 

Bernardino Counties. The population of the Basin is approximately 16 million people, 

with approximately 11 million gasoline powered vehicles and 300,000 diesel vehicles. 

The coastal plain contains most of the population of the Basin, which is surrounded by 

tall mountains, including the San Gabriel Mountains to the north, the San Bernardino 

Mountains to the northeast, and the San Jacinto Mountains to the east. The coastal range 

of the Santa Ana Mountains separates the inland part of Orange County from Riverside 

County. The proximity of the Pacific Ocean to the west has a strong influence on the 

climate, weather patterns and air quality of the Basin. The mountains also have a 

significant impact on the wind patterns of the Basin. Offshore winds flow down slope and 

are warmed and dried by compressional heating, gaining momentum through the passes 

and canyons. Northeasterly winds, known as Santa Ana winds, typically account for the 

highest wind events in the Basin, occurring several times each year. Onshore high-wind 

 

45 Letter dated November 22, 2010 to Matthew Lakin, Manager Air Quality Analysis Office USEPA Region 9, from 

Karen Magliano, Chief Air Quality Data Branch California Air Resources Board, transmitting final report dated 

August 5, 2010 entitled, “Analysis of Exceptional Events Contributing to High PM10 Concentrations in the South 

Coast Air Basin on October 13, 2008.” 
Draft for Public Notice and Comment 

Revision Date: June 2012 39 

events also occur with the strongest winds typically occurring in the mountains and 

deserts. 

Violations of the PM10 NAAQS were recorded at the South Coast Air Basin Anaheim 

monitoring station on October 13, 2008, due to high winds. The 24-hour mass 

concentration at Anaheim was measured with a federal equivalent method (FEM) 

Tapered Element Oscillating Microbalance (TEOM) continuous monitor, with a 

midnight-to-midnight 24-hour average concentration of 199 g/m3. This was not a 

sampling day for the Federal Reference Method (FRM) filter measurements in the Basin. 

While no other PM10 measurements exceeded the federal standard level (150 g/m3), 

other stations in the Basin had elevated concentrations during the same period. 

A strong Santa Ana wind event developed on October 13th, causing very high northerly 

through easterly winds in the mountains and deserts, especially through and below the 

wind-favored passes and canyons in the Basin. National Weather Service (NWS) weather 

stations measured extremely high peak wind gusts throughout the day in areas upwind of 

the high SCAQMD PM10 stations, including: 87 mph by in [sic] the Santa Ana 

Mountains of Orange County (Freemont Canyon RAWS); 87 mph in the San Gabriel 

Mountains of Los Angeles County (Chilao RAWS); 79 mph in the Malibu Hills of Los 

Angeles County; 61 mph at Ontario International Airport in San Bernardino County; 55 

mph at Corona Airport in Riverside County; 51 mph at Chino Airport in San Bernardino 

County and 41 mph at the Santa Ana – John Wayne Airport in Orange County. 

Due to the widespread winds, sources of the windblown dust were both natural areas, 

particularly from the mountains and deserts, and BACM-controlled anthropogenic 

sources. The timing of this event is verified with the high wind observations and reports 

of reduced visibility and blowing sand and dust, in conjunction with the hourly TEOM 

and BAM PM10 measurement data from nearby monitors in the Basin, when available. 

The following maps support the conceptual model: 

 Map of the South Coast Air Basin Showing Air Monitoring Stations and Forecast 

Areas 

 Map of South Coast Air Basin with Selected Cities and Topography 

 Map of South Coast Air Basin PM10 Monitors 

6.3.2 Step 2: Address not Reasonably Controllable or Preventable (nRCP) 

The nRCP demonstration should identify the emissions that were expected to have contributed to 

the event, both natural and anthropogenic, and indicate how they were not reasonably 

controllable or preventable. Generally, the nRCP will include identification of natural sources 

and whether they are reasonably controllable, and identification of anthropogenic sources and the 

associated reasonable controls. 

6.3.2.1 Identify source areas and source categories expected to have contributed to the event 

EPA recommends that the first step of the nRCP demonstration is to identify the likely source 

area and source categories expected to have contributed to the event. The source areas and 

categories can be general, such as, “The area upwind of the monitor includes portions of the 
Draft for Public Notice and Comment 

Revision Date: June 2012 40 

Santa Ana Mountains to the NE of the station and extending down into the Basin. Sources of the 

windblown dust were both natural areas, particularly from the mountains and deserts, and 

BACM-controlled anthropogenic sources.”46 It is important to identify the geographic references 

on a map. 

6.3.2.2 Consideration of wind speed 

The demonstration should indicate what the expected high wind threshold is for the local area 

and whether the sustained wind speed exceeded this level (See Appendix A2 and A3 for 

information on developing a high wind threshold). The wind speed data do not necessarily have 

to be at the location of the exceedance, but they should represent the source area generating the 

emissions. Generally, the EPA will accept that high winds could be the cause of a high 24-hour 

average PM10 or PM2.5 concentration if there was at least one full hour in which the hourly 

average wind speed was above the area-specific high wind threshold. Potential issues arise when 

determining the hourly average wind speed if wind speeds are not recorded at specified intervals 

throughout each hour. While some sources of wind speed data use hourly averages, other data 

sources employ 1 - 5 minute (“short-period”) averages. When the available wind speed data 

consist of only the wind speed during a fixed short period of each hour (e.g., the first or last five 

minutes of each hour) or the wind speed during the variable short period when wind speed was at 

its maximum during the hour, the EPA will generally accept that the hourly average wind speed 

was above the threshold if the reported short-period wind speed was above the threshold. Where 

wind speed is recorded at specified intervals throughout each hour, agencies should use all 

recorded data to calculate the hourly average wind speed.47 EPA may, however, consider 

multiple occurrences of high wind measured at these shorter averaging times as part of the 

weight-of-evidence demonstration. At a minimum, demonstrations should include the maximum 

sustained wind speed for each hour of the event and also the number of periods above the high 

wind threshold. 

The EPA notes that The National Climate Center has started archiving the 2-minute winds for 

every 2-minute period of each hour for all ASOS stations in the country. Almost all sites have 

data since March 2005, with most archiving data since 2000. The EPA has further developed a 

preprocessor to AERMOD, called AERMINUTE, that takes short-period wind speed 

observations and calculates an hourly average wind that can be fed into AERMET, the 

AERMOD meteorological processor. The AERMINUTE output is user friendly. AERMET can 

also accept, process, and calculate hourly average wind speeds from sub-hourly data with a 

resolution equal or greater than 5-minutes from sources other than AERMINUTE. 

The EPA will consider shorter-term “snapshots” of wind data such as National Weather Service 

hourly summaries as part of the weight-of-evidence demonstration. 

 

46 Letter dated November 22, 2010 to Matthew Lakin, Manager Air Quality Analysis Office USEPA Region 9, from 

Karen Magliano, Chief Air Quality Data Branch California Air Resources Board, transmitting final report dated 

August 5, 2010 entitled, “Analysis of Exceptional Events Contributing to High PM10 Concentrations in the South 

Coast Air Basin on October 13, 2008.” 

47 While the National Weather Service defines a “sustained wind” as the wind speed determined by averaging 

observed values over a two-minute period, the EPA believes that it would take a longer period of high wind speeds 

to raise enough dust to significantly influence measured 24-hour average values of PM10 or PM2.5 
Draft for Public Notice and Comment 

Revision Date: June 2012 41 

Generally, the EPA recommends using NWS data or the National Climate Data Center. Where 

meteorological data are not available for a particular area and such data are critical for the 

demonstration, agencies may substitute modeled surface wind speeds for actual measured data. 

Models that agencies can use to develop estimates for actual measured wind speed surface 

measurements include MM5, WRF, and, possibly, NAM. Wind speed data from a multitude of 

sources and with different averaging times, including model outputs may be included in a high 

wind dust event demonstration. 

6.3.2.3 Basic controls analysis 

Generally, if the wind speed data is at or above the area-specific high wind threshold, the air 

agency can provide a basic controls analysis to show that the event was not reasonably 

controllable or preventable (see Section 3.1.5.1). A basic controls analysis should identify all 

contributing emission sources in upwind areas and provide evidence that those sources were 

reasonably controlled, whether anthropogenic or natural. 

A basic controls analysis would include a brief description of local/upwind sources contributing 

to the event and a description of the controls on the anthropogenic sources in place at the time of 

the event (e.g., local BACM measures). In general, EPA expects all upwind areas of disturbed 

soil to be considered potential contributing sources. For the sources identified, the submitter 

should explain that dust emissions occurred despite having reasonable controls in place (e.g., 

controls that overwhelmed by high wind). The EPA may request additional information on 

specific sources once the information has been reviewed. 

An example of a basic controls analysis for the anthropogenic sources in a nonattainment area 

is:48

This requirement is met by demonstrating that despite reasonable and appropriate 

measures in place, the October 13, 2008 wind event caused the NAAQS violation. During 

this event, there were no other unusual PM10-producing activities occurring in the Basin 

and anthropogenic emissions were approximately constant before, during and after the 

event. SCAQMD has implemented regulatory measures to control emissions from 

fugitive dust sources and open burning in the South Coast Air Basin. Implementation of 

Best Available Control Measures (BACM) in the Basin has been carried out through 

SCAQMD Rule 403 (Fugitive Dust), as well as source-specific rules. With its approvals 

of the South Coast PM10 Attainment Plans in the State Implementation Plan (SIP), EPA 

has concluded that this control strategy represents BACM and Most Stringent Measures 

(MSM) for each significant source category, and that the implementation schedule was as 

expeditious as practicable. 

 SCAQMD Rule 403 establishes best available fugitive dust control measures to 

reduce fugitive dust emissions associated with agricultural operations, 

construction/demolition activities (including grading, excavation, loading, 

crushing, cutting, planning, shaping or ground breaking), earth-moving activities, 

 

48 Letter dated November 22, 2010 to Matthew Lakin, Manager Air Quality Analysis Office USEPA Region 9, from 

Karen Magliano, Chief Air Quality Data Branch California Air Resources Board, transmitting final report dated 

August 5, 2010 entitled, “Analysis of Exceptional Events Contributing to High PM10 Concentrations in the South 

Coast Air Basin on October 13, 2008.” 
Draft for Public Notice and Comment 

Revision Date: June 2012 42 

track-out of bulk material onto public paved roadways, and open storage piles or 

disturbed surface areas. 

 SCAQMD Rule 1156, Further Reductions of Particulate Emissions from Cement 

Manufacturing Facilities, is a source-specific rule that applies to all operations, 

including material handling, storage and transport at cement manufacturing 

facilities. It restricts visible emissions from facility operations, open piles, 

roadways and unpaved areas and requires enclosed systems for loading, unloading 

and transfer of materials. Other operations must employ wind fencing and wet 

suppression systems or be enclosed with permitted control equipment. 

 SCAQMD Rule 1157, PM10 Emissions Reductions from Aggregate and Related 

Operations, is a source-specific rule applicable to all permanent and temporary 

aggregate and related operations that produce sand, gravel, crushed stone or 

quarried rocks. Like Rule 1156, this rule restricts the discharge of fugitive dust 

emissions into the atmosphere through plume opacity tests and limiting visible 

plume travel to within 100 feet of the operation. This rule requires: prompt 

removal of material spillage; stabilization of piles with dust suppressants; the 

control of loading, unloading, transferring, conveyors, and crushing or screening 

activities with dust suppressants or other control methods; stabilization of 

unpaved roads, parking and staging areas; sweeping of paved roads; and the use 

of track-out control systems. 

 SCAQMD Rule 1158, Storage, Handling, and Transport of Coke, Coal and 

Sulfur, is a source-specific rule that applies to any facility that produces, stores, 

handles, transports or uses these materials. This rule restricts visible emissions 

and requires that piles be maintained in enclosed storage and that unloading 

operations be conducted in enclosed structures with water spray systems or 

venting to permitted air pollution control equipment. It also has specific 

requirements to control emissions from roadways, other facility areas, and 

conveyors and the loading of materials. 

 SCAQMD Rule 1186, PM10 Emissions from Paved and Unpaved Roads and 

Livestock Operations, requires rapid removal of paved road dust accumulations 

and establishes a treatment schedule for unpaved roads, street sweeper 

procurement standards, and design standards for new road construction. 

SCAQMD Rule 1186.1, Less-Polluting Sweepers, requires procurement of 

alternative-fueled equipment when governmental agencies replace street 

sweepers. 

 SCAQMD Rule 444, Open Burning, ensures that open burning is conducted in a 

manner that minimizes emissions and impacts, and that smoke is managed to 

protect public health and safety. This rule requires authorization for agricultural 

and prescribed fire, limited to days that are predicted to be meteorologically 

conducive to smoke dispersion and that will not contribute to air quality that is 

unhealthy for sensitive groups or worse. It also restricts residential and waste 

burning. 

 SCAQMD Rule 445, Wood Burning Devices, reduces pollution from woodburning fireplaces and other devices through requirements for new construction, 

curtailment of wintertime wood burning in specified areas when poor air quality is 

forecast and restriction of the sale of unseasoned firewood. The SCAQMD 

Healthy Hearths program provides public education on how to reduce air 
Draft for Public Notice and Comment 

Revision Date: June 2012 43 

pollution from wood burning and encourages the conversion to natural gas 

burning fireplaces through an incentive program. 

October 13, 2008 was designated an agricultural and prescribed wildland “no-burn” day, 

in accordance with SCAQMD rule 444. The PM2.5 24-hour averages at all stations in the 

Basin, including Anaheim, were well below the 24-hour PM2.5 NAAQS and the PM10

was estimated to be composed of 87% PM-Coarse particles (PM10-2.5) and only 13 

percent PM2.5. This shows that mostly crustal material comprised the PM10 mass and not 

transported or locally generated urban pollution or combustion sources. 

A survey of the SCAQMD complaint records and inspection reports for Anaheim and all 

other areas of the Basin indicated no evidence of unusual particulate emissions on 

October 13, 2008 other than related to the strong winds. The complaints are summarized 

in Table 2-7 from the SCAQMD Clean Air Support System (CLASS) database for 

complaints and compliance actions. Due to the windy conditions, SCAQMD compliance 

staff responded to 17 complaints related to windblown dust on October 13. Most were in 

Riverside and San Bernardino County, but two were in Orange County with no further 

compliance action taken. No Notices of Violation or Notices to Comply were issued in 

the Basin for fugitive dust on this day. Several complaints were directly related to the 

strong winds and windblown dust that overwhelmed the strict fugitive dust controls that 

are enforced in the Basin. The control methods were generally effective throughout the 

Basin, but were apparently overwhelmed in several instances by the strong, gusty winds, 

causing windblown dust and sand to be entrained in the atmosphere. 

In addition to the information provided in the example above, the basic controls analysis should 

also include an explanation of why the control measures should be considered as reasonable. For 

example, an explanation could include statements similar to: “the source area is within the 

boundaries of a serious nonattainment area and therefore control measures required to reduce 

windblown dust as part of the area’s approved SIP should suffice as reasonable controls, as 

additional controls, beyond what is currently required, are economically and technically 

infeasible.” The basic controls analysis should also ensure that the controls discussed include 

controls for disturbed areas and materials open storage areas susceptible to winds, as well as 

controls on production sources such as materials loading and unloading. 

While the above example provided a basic controls analysis for anthropogenic sources in a nonattainment area, an area attaining the NAAQS can similarly present the current rules, if any, and 

how the identified rules are reasonable given the attainment status. 

In addition to identifying controls on anthropogenic sources, it is important to indicate whether 

the natural sources could have been reasonably-controlled. For example, the following statement 

could fulfill this need: “Wind speeds were high enough to entrain dust from natural areas 

including undisturbed mountain and desert areas upwind of the monitor. Emissions from these 

sources were not reasonably controllable due to the cost of applying controls over such a large 

land area and because of the detrimental effect on the natural ecosystem that could result.” 

Finally, if EPA recommended controls improvements (e.g., as part of a previous high wind dust 

exceptional event review) then the controls analysis should address how these controls 

improvements have been addressed. 
Draft for Public Notice and Comment 

Revision Date: June 2012 44 

6.3.2.4 Comprehensive controls analysis 

Generally, if the wind speed data is below the area-specific high wind threshold, the air agency 

will be expected to provide a comprehensive controls analysis to show that the event was not 

reasonably controllable or preventable (see Section 3.1.5.2). Significant emissions from 

reasonably-controlled anthropogenic or natural sources are not expected below the high wind 

threshold, and therefore the analysis should further consider whether all contributing sources are 

reasonably-controlled. Comprehensive controls analysis should include detailed analyses such 

as: back-trajectories indicating specific sources in the upwind area, a day specific inventory of 

the contribution for significant sources, detailed descriptions of controls and their effective 

implementation and enforcement (where appropriate), and also include a detailed explanation of 

why the control measures should be considered as reasonable. 

For comprehensive controls analysis, EPA will place significantly more weight on the wind 

speed data associated with high particulate matter concentrations. The demonstration is likely to 

be increasingly difficult as sustained wind speeds decrease below the applicable high wind 

threshold and many of these cases may not be concurrable. 

One type of analysis that an air agency could use when developing a comprehensive controls 

analysis is a source contribution analysis, similar to the analysis presented below, for multiple 

hours of the day. For most events, a single back trajectory may not account for wind direction 

fluctuations during the event and may not accurately capture all the sources that may be 

contributing to the exceedance, and where continuous PM measurements are available, 

trajectories for hours with the greatest PM concentrations are most critical. Also, when moderate 

winds are responsible for high levels of measured particulate matter, considerably more attention 

should also be placed on the hours of the day preceding the event to adequately assess the 

sources contributing to the exceedance that may have influenced particulate matter 

concentrations before the arrival of the claimed event. 

Following is an example of a methodology of a back-trajectories and inventory for a 

comprehensive controls analysis49: 

 Back-trajectories were plotted in 5-minute links based on 5-minute average wind speed 

and wind direction data recorded at the West 43rd Avenue station. The back-trajectory 

plot for April 30, 2008 is shown in the following figure. These back-trajectories revealed 

that winds accompanying peak PM10 concentrations typically blew from the westsouthwest to the West 43rd Avenue station, crossing a mosaic of agricultural, residential, 

industrial, and riverbed lands. GIS files were used to determine the zoned uses of all 

lands within 1⁄2 mile of each back-trajectory track over which wind parcels travelled 

during the two hours prior to delivering the peak PM10 concentration to the West 43rd

Avenue monitor. Lands under active construction on each exceedance day were identified 

from earthmoving permit records. Parcel areas were aggregated within seven general 

 

49 Assessment of Qualification for Treatment under the Federal Exceptional Events Rule: High Particulate (PM10) 

Concentration Event in the Phoenix Area on April 30, 2008. Technical report prepared by the Arizona Department 

of Environmental Quality, Air Quality Division. August 16, 2010. 
Draft for Public Notice and Comment 

Revision Date: June 2012 45 

categories for which limited emission factor data were available: vacant, agriculture, 

construction, open/restricted access, riverbed, sand and gravel/landfill, and other lands. 

The uses of these land categories are generally defined as follows: 

     Vacant – represents undeveloped land to which public access is not restricted; 

 Agriculture – represents lands under agricultural cultivation; 

 Construction – represents lands being developed for long term use that will 

include ground coverage elements such as pavement, structures, or landscaping 

that will prevent the generation of windblown dust; 

 Passive/restricted open space – represents undeveloped or partially developed 

lands to which public vehicular access is restricted (these lands include public 

parks, national forests, military posts, and Indian reservations); 

 Riverbed – represents riverbed channels of the Salt and Gila River branches; 

 Landfill/sand and gravel – represents lands being used for mineral extraction or 

waste deposit; 

 Other – represents developed lands that are protected from windblown dust 

generation by elements such as paving, structures, and landscaping. 

 PM10 emissions were calculated for each back-trajectory hour using emission factors 

derived from the Nickling and Gillies data, 5-minute wind speed averages recorded at the 

West 43rd Avenue monitoring station, and the land use acreage along each back-trajectory 

computed by MAG staff. The emission factor equations were used to compute PM10
Draft for Public Notice and Comment 

Revision Date: June 2012 46 

emissions for each 5-minute portion of each back-trajectory hour. For each 5-minute 

period, the measured average wind speed was compared to the threshold friction velocity 

calculated at a 10-meter height to determine whether the threshold wind speed necessary 

to the generation of windblown PM10 on each land use, undisturbed and disturbed, had 

been exceeded. If the threshold velocity was exceeded, the appropriate Nickling and 

Gillies emission factor equation was used to compute PM10 emissions in units of gm/cm2-

sec. Emissions for each 5-minute period within each hour and within each land use 

category were converted to units of lb/acre-hr and then summed to produce hourly 

average PM10 emission rates per land use category. The emission rates for the other land 

use categories and the 2nd hour were calculated using a similar methodology. The land 

use category emission rates were then multiplied by the acreages within each appropriate 

land use category to derive PM10 emissions for each back-trajectory hour by land use 

category. The PM10 emissions for each of the back-trajectory hours on each exceedance 

day were summed together to calculate total emissions over each exceedance day backtrajectory by land use category. These land use category emissions were then grouped by 

anthropogenic and nonanthropogenic categories to assess the relative contribution of 

nonanthropogenic sources to exceedances recorded at the West 43rd Avenue monitoring 

station during 2008. A summary of the results of these calculations for the April 30, 2008 

exceedance day is presented in the following table. 

The analysis should also include information on controls on anthropogenic sources were 

appropriately implemented and enforced during the time of the event. In addition to identifying 
Draft for Public Notice and Comment 

Revision Date: June 2012 47 

controls on anthropogenic sources, it is important to indicate whether the natural sources could 

have been reasonably-controlled. Available inspection reports and/or notices of violations 

(NOVs) in upwind areas should be submitted, if available. The EPA recognizes that agencies 

have varied methods of permitting and enforcement and does not expect all agencies to have 

these records for all events. The EPA does, however, expect agencies to make a general showing 

that the agency has a program in place to ensure controls are being enforced as appropriate (even 

if no specific evidence exists for the particular day of the event). The EPA will also consider the 

overall compliance rates for specific source categories in determining whether reasonable 

controls were in place. 

The controls analyses when wind speeds are below the high wind threshold should address 

whether control improvements were recommended by EPA (e.g., as part of a previous high wind 

dust exceptional event review). If controls improvement had been previously recommended then 

the controls analysis should address how these controls improvements have been implemented. 

6.3.3 Step 3: Present Historical Fluctuations (HF) Analyses

As described in Section 3.2, the historical fluctuations (HF) analyses and an air agency 

conclusion will aid EPA’s review and will also inform CCR, NEBF, and AAQ. Specific analyses 

expected to provide the historical context for the event include: 

1. A time series for concentration for the event area for the previous three years, or 

longer if available, with high wind dust events identified: Concentration data should 

be 24-hour concentrations for each day. Depending on the quantity of data, it may be 

appropriate to present monthly maximums (note that it is not appropriate to present 

monthly-averaged daily data or any other average of the daily data as this masks other 

high values). It is appropriate to identify information such as: seasonal or monthly 

24-hour means, other event days, and relevant standards. The following figure50 is an 

appropriate example of this type of analysis. 

 

 

50State of Arizona Exceptional Event Documentation for the Events of July 2nd through July 8th 2011, for the 

Phoenix PM10 Nonattainment Area. Report prepared by Arizona Department of Environmental Quality (ADEQ), 

March 8. 2012. 

 
Draft for Public Notice and Comment 

Revision Date: June 2012 48 

2. Percentile of concentration relative to annual data with and without all high wind dust 

events: The percentile of the 24-hour average PM concentration should be provided 

for the event day relative to all measurement days over the previous three years or 

longer. The EPA expects a minimum of 300 data points to be included in this 

calculation. If the sampling schedule is 1-in-6 day sampling then this percentile 

should include five years of data (60 sample days/year for five years provides 300 

data points). Higher frequency sampling can utilize fewer years of data but not fewer 

than three years. If three years is not available, consult with EPA. 

3. Percentile of concentration relative to seasonal data with and without all high wind 

dust events: The percentile of the 24-hour average PM concentration should be 

provided for the event day relative to all measurement days for the season (or 

appropriate alternative 3-month period) of the event over the previous three years or 

longer. It is appropriate to use the same time horizon as used for the percentile 

calculated relative to annual data. 
Draft for Public Notice and Comment 

Revision Date: June 2012 49 

6.3.4 Step 4: Address Clear Causal Relationship (CCR)

As described in Section 3.3, the following types of evidence can support the CCR demonstration: 

 Occurrence and geographic extent of the event 

 Transport of emissions related to the event in the direction of the monitor(s) where 

measurements were recorded 

 Spatial relationship between the event, sources, transport of emissions, and recorded 

concentrations 

 Temporal relationship between the high wind and elevated PM concentrations at the 

monitor in question 

 Chemical composition and/or size distribution of measured pollution that links the 

pollution at the monitor(s) with particular sources or phenomena 

 Comparison of event-affected day(s) to specific non-event days 

Each of these types of evidence is treated in detail below. Note that information generated in this 

portion of the demonstration submittal may result in revisions to the conceptual model and 

controls analysis. As the flow diagram (Figure 2) suggests, preparation of a high wind dust 

exceptional event package is not necessarily a step-wise process. 

6.3.4.1 Occurrence and geographic extent of the event 

Air agencies can provide the following information to help establish the occurrence and 

geographic extent of the event: special weather statements, advisories, news reports; nearby 

visibility readings; measurements from monitoring stations; MODIS and other satellite maps; 

and description of weather conditions that created the high wind. 

 Special weather statements, advisories, news reports on both predictions and occurrence 

of the event: 

SCAQMD provided the following information for an exceptional event showing for 

Anaheim (Note that Appendices from the SCAQMD demonstration submittal are 

referenced in the excerpt below, but they are not provided as part of this document or the 

example). 

The National Weather Service had predicted this first strong Santa Ana event of the season 

well in advance and Governor Schwarzenegger issued a press release on October 10 to 

prepare the state for Santa Ana winds and the associated wildfire potential (see Appendix 

A.7). 

The Appendix to this document (Sections A.2 through A.6) contains the forecast discussions, 

short-term forecasts (nowcasts), fire weather forecasts, warnings and significant wind 

reports, as available from the NWS Los Angeles/Oxnard and San Diego Forecast Offices, 

whose areas of responsibility cover the Basin and much of southern California. These show 

that the strong Santa Ana wind event was well predicted in advance, warning the public of 

potentially damaging winds and windblown dust and sand, along with reduced visibilities. 

NWS advisories and warnings for high winds (Appendix, Section A.5) were already in place 

on October 12, extending through Tuesday, October 14, or longer. A Wind Advisory is 
Draft for Public Notice and Comment 

Revision Date: June 2012 50 

issued by NWS when sustained winds of 30 to 39 mph are expected for 1 hour or longer. A 

High Wind Warning is issued when sustained winds of 40 mph or more are expected for 1 

hour or longer, or for wind gusts of 58 mph or more with no time limit. NWS Oxnard issued 

High Wind Warnings on October 12, extending through the period for the Los Angeles and 

Ventura County Mountains and Wind Advisories for the Santa Monica Mountains, the 

Ventura County coastal and interior valleys, the Santa Clarita Valley, the Los Angeles 

County San Fernando Valley, and the Ventura and Los Angeles County coasts, including 

Downtown Los Angeles. NWS San Diego issued High Wind Warnings for the San 

Bernardino and Riverside County valleys (Inland Empire) and the Santa Ana mountains and 

foothills and Wind Advisories for the San Bernardino County mountains, Orange County 

coastal areas, the Riverside County mountains, the San Diego County mountains, and the San 

Diego County valleys, In short, High Wind Advisories and Warnings were in place for most 

of the South Coast Air Basin and much of southern California to warn the public of this high 

wind event. Northeasterly winds with sustained speeds in the 35 to 45 mph range were 

predicted throughout the region, along with damaging gusts to 70 mph, especially in the 

mountains and below passes and canyons in the Inland Empire. Hazardous driving conditions 

were predicted, especially through and below canyons and passes, as well as blowing dust 

and sand with reduced visibility, broken tree limbs and downed power lines. 

The AQMD Meteorology Section predicted high winds for October 13 in the Coachella 

Valley for AQMD Rule 403.1, which requires specific actions in this area when wind gusts 

exceed 25 mph. While there are no other AQMD rule requirements to forecast winds in the 

Basin, the daily forecast discussion by AQMD issued on October 12 for Monday, October 13 

predicted the strong winds. A smoke advisory was already in effect in the morning of 

October 12 and the strong winds were prominent in the forecast discussion, as follows: 

 SMOKE ADVISORY for Sunday: Concentrations of fine particulates may reach 

Unhealthy for Sensitive Groups or higher in areas of Los Angeles County directly 

impacted by smoke from a wildfire in the Angeles National Forest north of Pacoima. 

 Monday will be mostly clear, windy and warmer as the offshore Santa Ana winds 

strengthen. Gusty winds through and below canyons and passes will cause elevated 

particulate concentrations due to windblown dust and possibly continued wildfire 

activity. 

PM10 predictions were increased throughout the Basin for October 13 and agricultural and 

prescribed burning was prohibited with a No-Burn declaration for the entire Basin. AQMD 

issued a Smoke and Windblown Dust Advisory in the morning of October 13, reproduced in 

the Appendix, Section A.10, that warned of the likelihood of strong Santa Ana winds causing 

high PM10 concentrations in several areas of the Basin, including Central Orange County 

(Forecast Area 17, including Anaheim), as follows: 

In addition, strong Santa Ana winds will likely cause PM10 concentrations to reach 

Unhealthy for Sensitive Groups concentrations or higher in areas throughout the Basin 

downwind of the winds areas. This includes any areas where windblown dust is visible, 

especially through and below passes and canyons, until the winds subside. Wind prone areas 
Draft for Public Notice and Comment 

Revision Date: June 2012 51 

are likely to include: the San Bernardino Valley (Areas 32, 33, 34, 35), Riverside County 

Valleys (Areas 22, 23, 24, 25, 26), Orange County (Areas 16, 17, 18, 19, 20) and the Los 

Angeles County northern and southern coastal areas (Areas 2 and 4). 

 Nearby visibility readings: 

SCAQMD supplied the visibility readings and ADEQ submitted the visibility pictures for 

nearby airports during Arizona events. 

 MODIS satellite maps: 

SCAQMD provided the following maps showing the spatial distribution of windblown 

dust. 

 Description of weather conditions that created the high wind: 

SCAQMD provided the following description of weather conditions around the time of 

the event: 
Draft for Public Notice and Comment 

Revision Date: June 2012 52 

An upper level trough of low pressure moved through California, between October 9 and 

11. The low pressure system did not create much rain in California during this period, but 

temperatures were cool throughout the state. By Sunday, October 12, the backside of the 

trough was over California, providing upper level support for a developing strong Santa 

Ana wind event. The strong pressure gradients that developed between the high and low 

pressure aloft created strong winds. The National Weather Service (NWS) 500 millibar 

(MB) analyses every 12 hours between 0400 PST on October 12 and 0400 PST on 

October 14 are shown in the Appendix, Section A.11. The winds over California at the 

500 MB pressure level started out northwesterly in the morning of October 12 with 

speeds to 81 mph (70 knots), then became more northerly by the morning of Monday, 

October 13 with speeds to 57 mph (50 knots). The strong northerly flows aloft, coupled 

with strong northeasterly surface pressure gradients, enhanced the offshore flows at the 

surface. 

The passage of the low pressure trough aloft brought the first strong cold front of the 

season at the surface. Section A.12 in the Appendix shows the NWS sea-level pressure 

analyses, every three hours between 1600 PST on October 12 and 0100 PST on October 

14. By 1600 PST October 12, the surface low and cold front was over the northeastern 

border of New Mexico and high pressure was building over northern Nevada, increasing 

the northerly gradients. By 0100 PST on October 13, the high pressure over Nevada had 

increased to 1033 MB, strengthening the gradient flows across California. By 0700 PST, 

the area of high pressure had expanded and peaked at 1037 MB. The strength of the high 

pressure remained nearly the same through the rest of the day, while the broad area of 

high pressure slowly moved to the east, causing the winds to shift from northerly to 

northeasterly, then easterly throughout the day. The strong pressure gradients caused 

strong winds, especially in southern California as the flow of cold air from the area of 

high pressure further enhanced the winds as it flowed across the mountains. Some gusty 

winds had already been observed on October 12, but they increased considerably in the 

early morning of October 13. 

This is the classic Santa Ana wind pattern that brings strong winds to southern California. 

High pressure builds over the Great Basin desert region of the western United States in 

the cold air behind the front with lower pressure off the southern California coast. This 

pressure gradient creates strong north through northeasterly winds, enhanced by thermal 

gradients due to denser cold air over the Great Basin. The relatively cool air from the 

Great Basin deserts flows over the southern California mountains, gaining momentum on 

the lee side. The downslope flow causes compressional warming and drying of the air in 

the South Coast Air Basin. This combination of strong wind, high temperatures and low 

relative humidities make these Santa Ana conditions highly conducive to wildfires in 

southern California. 

The AQMD Meteorology Section routinely analyzes sea-level pressure gradients in 

southern California to assess winds and air pollution potential. The Summation Pressure 

Gradient (SPG) is a good indicator of the strength of the flow and whether it is onshore 

(positive) or offshore (negative), where 
Draft for Public Notice and Comment 

Revision Date: June 2012 53 

SPG = (SAN-LAS)51 + (LGB-DAG)52 + (RIV-DAG)53

In the morning of October 12, the 0700 PST SPG was 5.5 MB, indicating moderate 

offshore flow. At the same time in the morning of October 13, the SPG strengthened to 

14.7 MB, indicating a stronger offshore gradient. The gradient was enhanced by the 

upper level pattern and thermal gradient as described above, to create a strong wind 

event, especially for several hours through the morning of October 13. 

 Measurements from monitoring stations: 

The following figures show the kind of analyses based on measurements from air 

monitoring and meteorological stations that could be used to show the occurrence and 

geographic extent of the event.54 The figures also show that only when the wind speeds 

became elevated did PM10 concentrations also become elevated, supporting the causal 

relationship. 

 

51 Sea Level Pressure difference between San Diego and Las Vegas 

52 Sea Level Pressure difference between Long Beach and Daggett 

53 Sea Level Pressure difference between Riverside and Daggett 

54 EPA Region 9. ArcGIS analysis using PM10, wind speed, and wind direction data from AQS. 
Draft for Public Notice and Comment 

Revision Date: June 2012 54 
Draft for Public Notice and Comment 

Revision Date: June 2012 55 

Further analysis may include more refined GIS data combined with surface measurements from 

various sources. The EPA acknowledges that there are few areas, except large metropolitan areas 

that have the number of monitors to document the sequential nature of an event. The following is 

an example 55of the type of analysis that can occur with these types of resources. 

 

55 ADEQ 
Draft for Public Notice and Comment 

Revision Date: June 2012 56 
Draft for Public Notice and Comment 

Revision Date: June 2012 57 

6.3.4.2 Transport of emissions related to the event in the direction of the monitor(s) where 

measurements were recorded 

Example information to support transport of event-related emissions in the direction of monitor 

includes wind direction data showing that emissions from sources identified as part of the nRCP 

demonstration were upwind of the monitor(s) in question. 

 Map showing local sources and wind direction56 

Note that the topography gives an indication of sources in this map. Ideally, the likely 

significant sources such as agriculture fields, desert areas, mountains, and industrial 

sources would be identified (see next example). 

 

56 EPA Region 9 
Draft for Public Notice and Comment 

Revision Date: June 2012 58 

 Back Trajectories: 

Even if extensive comprehensive controls analysis is not needed, a back-trajectory 

analysis as shown in Section 6.2.2.5 would be appropriate as part of the CCR 

demonstration. Note that HYSPLIT trajectories that cover hundreds of miles are of 

limited use if the sources of dust are local. The example57 below uses 1-hour back 

trajectories based on local surface wind measurements during periods of high PM10 and 

GIS data to identify contributing source categories as well as the geographic extent of the 

event. The total area between the green lines represents the range of hourly back 

trajectories during periods of high PM10, while the hatched green area represents the 

portion of the total area that is located with the Gila River Indian Community boundaries 

(signified by the solid red line). 

 

57 GRIC 

Expected Source Area 
Draft for Public Notice and Comment 

Revision Date: June 2012 59 

 Wind roses: 

A wind rose for periods of the event day showing wind speed and direction at or near the 

concentration monitor, coupled with a description of the area suggested by the wind rose, 

could provide evidence of where the dust was transported from. This approach may not 

suffice for situations where the sources of dust are not proximate to the monitor. 

6.3.4.3 Spatial relationship between the event, sources, transport of emissions, and recorded 

concentrations 

The type of information that would support this evidence could be a map showing likely source 

area, wind speeds, wind direction, and particulate matter concentrations for the affected area 

during the time of the event: see the example figure below.58 

 

58 EPA Region 9 
Draft for Public Notice and Comment 

Revision Date: June 2012 60 

6.3.4.4 Temporal relationship between the high wind and elevated PM concentrations at the 

monitor in question 

Evidence for establishing the temporal relationship can include 24-hour time series showing PM 

concentrations at the monitor in question in combination with sustained and maximum wind 

speed data at the area where emissions originated. As shown below, it is most informative to 

include the sustained wind speed data and the concentration data on the same figure. 
Draft for Public Notice and Comment 

Revision Date: June 2012 61 

6.3.4.5 Similarity of chemical composition of measured pollution with that expected from 

sources identified as upwind 

Information such as chemical speciation data from the monitored exceedance(s) and sources, or 

size distribution data, could be part of this type of evidence. These data are not always available, 

but should be included wherever possible. An example of this type of analysis will be 

incorporated in this document as one becomes available. 

6.3.4.6 Comparison of event-affected day(s) to specific non-event days: 

The following types of analyses could be part of this piece of evidence: 

 comparison of concentrations and wind speed in the area to days preceding and following 

the event 

 comparison of concentration data to specific days that are similar to the event day with 

respect to emissions and meteorology 

 comparison to other high wind days without elevated concentrations 

 comparison of chemical composition (if available) 

0

100

200

300

400

500

600

700

800

900

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Anaheim TEOM PM10 (μg/m3)

Wind Speed (mph)

Time (PST)

Anaheim Hourly TEOM PM10 Anaheim Hourly Wind Speed

Anaheim 1‐Minute Wind Speed Freemont Canyon RAWS Wind Speed

Freemont Canyon RAWS Gust
Draft for Public Notice and Comment 

Revision Date: June 2012 62 

The following figure is an example of a comparison of concentrations in the area to days 

preceding and following the event.59

6.3.4.7 Alternative Hypotheses 

Eliminating other possible non-event causes supports the claimed causal relationship to the high 

wind event, although conclusively proving the absence of all possible or plausible other causes is 

not required or expected. For example, SCAQMD provided the following: 

Three wildfires were reported in southern California on October 13, fanned by the strong, 

dry Santa Ana winds, two in the San Gabriel Mountains north of the San Fernando Valley 

and one at Camp Pendleton in the north coastal part of San Diego County. Only one of 

these, the Marek Fire, was active during the early morning hours when the hourly PM10

concentrations spiked at Anaheim. Also, the northeasterly wind flows throughout the 

period, make it unlikely the smoke or ash from the fires contributed significantly to the 

PM10 measured at Anaheim. Crustal material from windblown dust was the primary 

 

59Letter dated November 22, 2010 to Matthew Lakin, Manager Air Quality Analysis Office USEPA Region 9, from 

Karen Magliano, Chief Air Quality Data Branch California Air Resources Board, transmitting final report dated 

August 5, 2010 entitled “Analysis of Exceptional Events Contributing to High PM10 Concentrations in the South 

Coast Air Basin on October 13, 2008.” 

0

20

40

60

80

100

120

140

160

180

200

10/6 10/7 10/8 10/9 10/10 10/11 10/12 10/13 10/14 10/15 10/16 10/17 10/18 10/19 10/20

24-Hour Average Conc., μg/m3

Date

Anaheim Long Beach Los Angeles Glendora

Rubidoux Lake Elsinore Fontana San Bernardino

October 13
Draft for Public Notice and Comment 

Revision Date: June 2012 63 

component of the measured PM10, as confirmed by comparing with the PM2.5 measured 

on this day. Prescribed, agricultural or residential burning did not appear to have added 

any significant amount of PM10 to the concentrations measured in the Basin; these 

activities were not permitted on this day. The PM2.5 portion of PM10, which would 

indicate combustion sources, was very small throughout the Basin. PM10 was emitted 

from some BACM-controlled sources (mainly agricultural and construction activities) as 

BACM controls were locally overwhelmed by the high winds. Natural particulate sources 

areas also contributed to the measured PM10, particularly the upwind mountain and desert 

areas. 

6.3.5 Address Affects Air Quality (AAQ)

Once sufficient HF analyses have been provided and CCR has been demonstrated the event will 

generally have been considered to have affected air quality at the exceeding monitor, and thus 

the AAQ element will have been met. The demonstration should include a statement that AAQ 

has been met by providing HF analyses and demonstrating CCR. 

6.3.6 Address [HAURL] / Natural Event

Once both CCR and nRCP have been demonstrated, the event will generally be considered a 

natural event, thus satisfying the [HAURL]/Natural Event element. The demonstration should 

include a statement that the Natural Event criterion has been met by demonstrating nRCP and 

CCR. 

6.3.7 Step 5: Address No Exceedance But For the Event (NEBF)

The NEBF demonstration generally builds on information gathered to support other elements of 

an exceptional event demonstration. Further, if the exceptional events demonstration fails on a 

different element then the NEBF analysis becomes moot since there is no portion of the 

concentration than can be attributed to an exceptional event. For these reasons, EPA suggests 

that air agencies complete the NEBF demonstration last after addressing all other EER elements. 

6.3.7.1 Qualitative NEBF 

If non-event pollution levels are typically significantly below the NAAQS during the season of 

the event then a qualitative NEBF may be adequate. The following is provided as an example60: 

Activities that generate anthropogenic PM10 were approximately constant in the Basin 

immediately preceding, during and after the event. Activity levels in the Basin were 

typical for the time of year and PM10 emissions control programs were being 

implemented, not only for fugitive dust-generating activities, but also for agricultural 

burning in the Basin. Furthermore, due to the forecasts for high winds on October 13, the 

SCAQMD compliance teams were ready to act quickly to fugitive dust complaints to 

 

60Letter dated November 22, 2010 to Matthew Lakin, Manager Air Quality Analysis Office USEPA Region 9, from 

Karen Magliano, Chief Air Quality Data Branch California Air Resources Board, transmitting final report dated 

August 5, 2010 entitled “Analysis of Exceptional Events Contributing to High PM10 Concentrations in the South 

Coast Air Basin on October 13, 2008.” 
Draft for Public Notice and Comment 

Revision Date: June 2012 64 

minimize emissions and to enforce mitigation methods like watering and soil 

stabilization. 

Vehicular traffic, cooking and residential fires do not directly cause PM10 24-hour 

NAAQS violations in the Basin. Activity levels in the Basin were typical for the time of 

year and PM10 emissions control programs were being implemented, for fugitive dustgenerating activities, as well as open burning. With the unsettled conditions on October 

13, such emissions would not contribute significantly to the PM10 measured. There were 

reasonable and appropriate measures in place to control PM10 in the Basin on October 13, 

2008, including SCAQMD Rules 403, 444, 445, 1156, 1157, 1158 and 1186. 

Examining the make-up of the PM10 in the Basin on this day using PM2.5 data, the coarse 

particles (PM10-2.5), which are associated with windblown dust, represent well over 75% 

of the total PM10 mass collected in the Basin. The three wildfires that were burning in the 

Basin, one of which started on October 12 and two other after the high hourly PM10

concentrations started, were not the primary cause of the high PM10. PM2.5 remained 

relatively low throughout the Basin on this day with no exceedance of the 24-hour 

NAAQS. While there were no PM10 filters collected on this day for laboratory analyses 

for soluble potassium, an indicator of wood smoke, the predominance of coarse particles, 

the timing of the fires and the lack of supporting wind directions to bring smoke to 

Anaheim provide support the conclusion that while there could have been a minor 

contribution from the wildfires, it was relatively small portion of the PM10 measured. 

Based on the data provided in this report, SCAQMD concludes that there would not have 

been exceedances of the PM10 NAAQS in the Basin on October 13, 2008 if high winds 

were not present. Even if the extreme 99.5 percentile concentration for the Basin, 139.5 

g/m3, were used as the background concentration to compare to the measured PM10

concentrations, the particulate contribution from the high wind event clearly caused these 

exceedances. The causal connection of the measured PM10 and the strong winds in the 

Basin, and throughout southern California, along with the high contribution of fugitive 

dust to the PM10 mass indicate that but for the high wind event this NAAQS violation 

would not have occurred. 

6.3.7.2 Quantitative NEBF 

A quantitative NEBF will generally be expected if concentrations on days without events during 

the same season exceed the standard or are near the standard and/or if the contribution of nonevent pollution produces concentrations near the applicable NAAQS. An example of a 

quantitative NEBF analysis will be incorporated in this document as one becomes available. 

6.4 Prepare High Wind Action Plan (optional)

A High Wind Action Plan is primarily used to document controls on additional sources that need 

reasonable controls for future events to be considered not reasonably controllable or preventable. 

If an air agency discovers (an) uncontrolled source(s) of dust during the course of the event 

demonstration, the air agency may choose to submit a High Wind Action Plan, either separately 

or along with the demonstration package. Alternatively, EPA may identify a source previously 
Draft for Public Notice and Comment 

Revision Date: June 2012 65 

unidentified by the air agency that EPA considers to be reasonably controllable. In this case, an 

air agency could submit a High Wind Action Plan following the submission of the demonstration 

package. A High Wind Action Plan is developed to address sources that could reasonably be 

controlled to minimize the occurrence of future events. As such, the following information 

would be included: 

 Source(s) targeted for controls 

 Description of controls 

 Oversight/enforcement plan, including on/before event days 

 Implementation timeline 

 Documentation of effective implementation and enforcement 

 The high wind threshold for the collective set of high wind dust sources, including those 

previously subject to reasonable controls and those that are being proposed for 

reasonable controls in the High Wind Action Plan (refer to Appendix A3 for determining 

this threshold). 

EPA has not established a particular format for the High Wind Action Plan but notes that most of 

the information needed for an approvable High Wind Action Plan was required in a Natural 

Events Action Plan (see Section 3.7.2); therefore, a NEAP may provide a useful template. When 

the High Wind Action Plan is submitted with a demonstration package, EPA recommends 

including it as an appendix and referencing it in the nRCP section. As mentioned in Section 

3.7.2, the High Wind Action Plan can be submitted before, with, or after submittal of a 

demonstration package. 
Draft for Public Notice and Comment 

Revision Date: June 2012 66 

Appendix A1. Summary of Studies on Windblown Dust Emissions 

Windblown dust is often but not always a controllable and preventable form of PM10 pollution. 

To ensure effective implementation of the EER, it is useful to determine the wind speed at which 

windblown dust no longer becomes reasonably controllable. Agencies may develop a high wind 

threshold for each area experiencing high wind dust events. Appropriate area-specific thresholds 

would consider local conditions, sources, and controls and specify a speed above which these 

controls would be overwhelmed. This approach is consistent with the Natural Events Policy 

where the EPA required the states/local agencies to define the conditions in which BACM level 

controls were overwhelmed. If an agency is unable to develop an area-specific wind threshold, 

the EPA will accept a default threshold of 25 mph. Areas with local data supporting of an areaspecific high wind threshold should submit this information to EPA for review and approval. 

The default 25 mph high wind threshold is mainly based on extensive windblown dust emissions 

research performed by the University of Nevada, Las Vegas (UNLV). The Clark County 

Department of Air Quality and Environmental Management (DAQEM) contracted with the 

Department of Civil and Environmental Engineering, University of Nevada, Las Vegas (UNLV) 

to conduct field studies to generate refined wind-blown PM10 emissions factors for stable natural, 

and unstabilized, disturbed surfaces. 61 The latest study was performed in 2004 using a portable 

wind tunnel at 31 locations in the Las Vegas valley that represented nine different soil groups.62

All of the test sites were determined to be stable through the same methods as outlined in 

DAQEM’s fugitive dust rules for open areas and vacant lots and thus provide a consistent 

measure of “stable” conditions.63 The sites chosen for the wind tunnel tests were determined to 

be stable “as-is” (i.e. no physical stabilization was performed to alter the site conditions).These 

same test sites were then intentionally destabilized and subsequently retested using the same 

wind tunnel approach that had been used on the previously stabilized surfaces. A summary of the 

2004 field study results can be seen in Figure ES-1. The 2004 data show that non-linear increases 

in PM10 flux generally begin to occur at sustained 10 meter velocities exceeding 25 mph. Note 

that the Clark County study found small amounts of entrainment below 25 mph. The small PM10

fluxes observed at lower winds speeds could be attributed to aerodynamic entrainment, which 

occurs primarily when fine particles are lifted directly off the ground and remain elevated. While 

it is expected that small amounts of aerodynamic entrainment could occur when wind speeds are 

below 25 mph, these are not expected to result in exceedances in most western areas, particularly 

the desert areas such as in Clark County. 

 

61Refined PM10 Aeolian Emission Factors for Native Desert and Disturbed Vacant Land Areas. Final Report, June 

30, 2006, http://www.clarkcountynv.gov/Depts/daqem/Documents/Planning/SIP/PM10/App_E_-

Refined%20Emission%20Factors.pdf.

62 Sites were characterized in terms of Wind Erodibility Groups (WEGs).

63Clark County Department of Air Quality and Environmental Management Air Quality Regulations, Section 90 – 

Fugitive Dust from Open Areas and Vacant lots, Subsection 90.4. Test Methods, revised 12/17/2002.
Draft for Public Notice and Comment 

Revision Date: June 2012 67 

The EPA believes that for Clark County and areas similar to it, these results clearly differentiate 

emissions from stable and disturbed conditions and provide a reasonable baseline for establishing 

a high wind threshold for exceptional events purposes. 

 

Furthermore, studies conducted by the Desert Research Institute (DRI) in Clark County, NV 

have concluded that windblown desert dust contributes to approximately 20% of measured PM10

in urban areas and that only desert soils that have been disturbed by anthropogenic activities are 

large emitters under common high wind conditions.64 These studies also conclude that 

windblown PM10 from urban/disturbed surfaces are not seen until 10 meter hourly average wind 

speeds are greater than 7 m/s (16 mph), while nonurban desert show a significant increase in 

PM10 emissions only when hourly average wind speeds are greater than 11 m/s (25 mph). See 

Figure 3-1 for a graphical representation of these data. The authors note that these results refute 

the argument that most urban dust derives from natural surfaces. 

 

64 Watson, J.G. and Chow, J.C. 2000. Reconciling Urban Fugitive Dust Emissions Inventory and Ambient Source 

Contribution Estimates: Summary of Current Knowledge and Needed Research. DRI Document No. 6110.4F.
Draft for Public Notice and Comment 

Revision Date: June 2012 68 

These results are also consistent with results obtained from wind tunnel studies performed 

throughout the state of Arizona.65 These studies suggest that windblown dust emissions from 

scrub desert and dune flat areas occur when wind speeds are greater than 11.3 m/s (25 mph) and 

18.31 (41 mph), respectively. The same study revealed that surfaces that had been disturbed by 

anthropogenic activities began to produce emissions when wind speeds ranged from 5.11 m/s (11 

mph) to 8.11 m/s (18 mph). The effect of surface disturbance on threshold wind speeds was 

further examined for a number of natural desert soils by a number of researchers.66 The main 

conclusion was that disturbance of soils profoundly lowers the threshold friction velocity of 

desert soils. 

 

65 Nickling, W.G. and Gillies, J.A. 1989. Emission of Fine Grained Particulates From Desert Soils. In 

Paleoclimatology and Paleometeorology: Maodern and Past Patterns of Global Atmospheric Transport. Leinen, M. 

and Sarnthein, M., (Eds.) Kluwer Academic Publishers. 133-165.

66Gillette, D.A. 1980. Threshold Velocities for Input of Soil Particles into the Air by Desert Soils. Journal of 

Geophysical Research. 85: 5621-5630; Gillette, D.A. 1982. Threshold Friction Velocities and Rupture Moduli for 

Crusted Desert Soils for the Input of Soil Particles into the Air. Journal of Geophysical Research. 87: 9003-9015; 

Belnap, J. 2007. Wind Erodibility of Soils at Fort Irwin, California (Mojave Desert), USA, Before and After 

Trampling Disturbance: Implications for Land Management. Earth Surface Processes and Landforms. 32: 74-84; 

Belnap, J. 1998. Vulnerability of Desert Biological Soil Crusts to Wind Erosion: The Influences of Crust 

Development, Soil Texture, and Disturbance. Journal of Arid Environments. 39: 133-142. 
Draft for Public Notice and Comment 

Revision Date: June 2012 69 

In EPA’s weight-of-evidence analysis of high wind dust events, the EPA will assumed that 

sustained wind speeds above the applicable high wind threshold (area specific or 25 mph default) 

are capable of overwhelming reasonable controls on anthropogenic sources or causing emissions 

from natural undisturbed areas in arid, semi-arid, or seasonally dry regions, such as in Clark 

County, NV. The EPA will further assume that wind speeds below this threshold will entrain 

more dust emissions per acre or square mile from disturbed anthropogenic sources that have not 

been reasonably-controlled than from natural surfaces and stabilized disturbed surfaces. 

. 
Draft for Public Notice and Comment 

Revision Date: June 2012 70 

Appendix A2. Summary of Available Relevant Literature Related to 

Establishing Area-Specific High Wind Thresholds 

Alfaro, S. 2004. Estimation of PM20 Emissions by Wind Erosion: Main Sources of 

Uncertainties. Geomorphology, 59: 63-74. 

Alfaro, S. 2001 Modeling Mineral Aerosol Production by Wind Erosion: Emission Intensities 

and Aerosol Size Distributions in Source Areas. Journal of Geophysical Research, 106: 

NO. D16, 18075-18084. 

Benlap, J. and Gillette, D. 1998. Vulnerability of Desert Biological Soil Crusts to Wind Erosion: 

the Influences of Crust Development, Soil Texture, and Disturbance. Journal of Arid 

Environments. 39: 133-142. 

Brazel, A.J. and Nickling, W.G. 1986. The Relationship of Weather Types to Dust Storm 

Generation in Arizona (1965-1980). Journal of Climatology. 6: 255-275 

Brazel, A.J. and Nickling, W.G. 1984. Temporal and Spatial Characteristics of Arizona Dust 

Storms (1965-1980). Journal of Climatology. 4: 645-660. 

Breed, C.S., and Reheis, M.C. (Eds.) 1999. Desert Winds: Monitoring Wind-Related Surface 

Processes in Arizona, New Mexico, and California. U.S. Geological Survey Professional 

Paper 1598. 

Chow, J.C., Pace, T.G., and Watson, J.G. 2000. Fugitive Dust Emissions. In Air Pollution 

Engineering Manual, Davis, W.T., (Ed.) Van Nostrand Reinhold, New York, NY. 

Countess Environmental. 2006. WRAP Fugitive Dust Handbook. Prepared for Western 

Governors’ Association by Countess Environmental, Westlake Village, California. 

September 7. 

Cowherd, C. 1995. Wind Tunnel Studies of BACM Effectiveness. Final Test Report. Prepared 

for South Coast Air Quality Management District by Midwest Research Institute (MRI). 

Contract No. 95017. MRI Project No. 3826. 

Cowherd, C. 2000. Wind Erodibility Assessment of the Stabilized Soils in the Antelope Valley. 

Test Report. Prepared for Southern California Edison and the City of Los Angeles 

Department of Airports by Midwest Research Institute (MRI). Contract No. U1109008. 

MRI Project No. 110028 

ENVIRON. 2004. Determining Fugitive Dust Emissions from Wind Erosion. Final Report. 

Prepared for Western Governors’ Association by ENVIRON International Corporation, 

Novato, California; ERG, Inc., Sacramento, California; Desert Research Institute, Reno, 

Nevada, MACTEC Engineering & Consulting, Gainsville, Florida and University of 

California Riverside, Riverside, California. March 12. 
Draft for Public Notice and Comment 

Revision Date: June 2012 71 

ENVIRON. 2006. Fugitive Windblown Dust Emissions and Model Performance Evaluation 

Phase II Final Report. Prepared for Western Governors’ Association by ENVIRON 

International Corporation, Novato, California and University of California Riverside, 

Riverside, California. May 5. 

Environmental Quality Management and MRI. 2006. An Inventory of Vacant Land Soil Stability 

and Private Unpaved Roads in the Las Vegas Valley Using Remote Sensing Imagery. 

Prepared for Clark County Department of Finance Purchasing and Contracts by 

Environmental Quality Management, Inc. and Midwest Research Institute. PN 

70204.001. 

Gillette, D.A. 1978. Tests with a Portable Wind Tunnel for Determining Wind Erosion 

Threshold Velocities. Atmospheric Environment. 12: 2309-2313. 

Gillette, D.A., Adams, J., Endo, A., and Smith, D. 1980. Threshold Velocities for Input of Soil 

Particles into the Air by Desert Soils. Journal of Geophysical Research. 85: 5621-5630 

Gillete, D.A., Adams, J., Muhs, D., and Kihl, R. 1982 Threshold Friction Velocities and Rupture 

Moduli for Crusted Desert Soils for the Input of Soil Particles into the Air. Journal of 

Geophysical Research. 87: 9003-9015. 

Gillette, D.A and Stockton, P.H. 1989. The Effect of Nonerodible Particles On Wind Erosion of 

Erodible Surfaces. Journal of Geophysical Research. 94: 12855-12893. 

Gillette, D.A. 1999. A Qualitative Geophysical Explanation for “Hot Spot’ Dust Emitting Source 

Regions. Contr. Atomos. Phys. 72: 67-77. 

Gillies, J.A. and Berkofsky, L. 2004. Eolian Suspension Above the Saltation Layer, the 

Concentration Profile. Journal of Sedimentary Research. 72: 176-183. 

Goudie, A.S. and Middleton, N.J. 1992. The Changing Frequency of Dust Storms Through Time. 

Climatic Change. 20: 197-225 

Houser, C.A. and Nickling, W.G. 2007. The Emission and Vertical Flux of Particulate Matter 

<10 μg from a Disturbed Clay Crusted Surface. Sedimentology. 48: 255-267. 

Fitz, D.R. 1996. Field Study to Determine Limits of Best Available Control Methods for Fugitive 

Dust Under High Wind Conditions. Final Report. Prepared for South Coast Air Quality 

Management District by University of California Riverside, CA. Contract R-C94159. 

Kjelgaard, J. 2004. PM10 Emission from Agricultural Soils on the Columbia Plateau: 

Comparision of Dynamic and Time-Integrated Field-Scale Measurements and 

Entrainment Mechanisms. Agriculture and Forest Meteorology. 125: 259-277. 

Mansell, G.E., Lau, S., Russell, J. and Omary, M. 2006. Fugitive Wind Blown Dust Emissions 

and Model Performance Evaluation Phase II. Prepared for Western Governors’ 
Draft for Public Notice and Comment 

Revision Date: June 2012 72 

Association by ENVIRON International Corporation and the University of California at 

Riverside College of Engineering. 

Macpherson, T., Nickling, W.G., Gillies, J.A., and Etyemezian, V. 2008. Dust Emissions from 

Undisturbed and Disturbed Supply-Limited Desert Surfaces. Journal of Geophysical 

Research. doi: 10.1029/2007JF000800. 

Neuman, C.M., Boulton, J.W., and Sanderson, S. 2009. Wind Tunnel Simulation of 

Environmental Controls on Fugitive Dust Emissions from Mine Tailings. Atmospheric 

Environment. 43: 520-529. 

Nickling, W.G. and Gillies, J.A. 1989. Emission of Fine Grained Particulates From Desert Soils. 

In Paleoclimatology and Paleometeorology: Maodern and Past Patterns of Global 

Atmospheric Transport. Leinen, M. and Sarnthein, M., (Eds.) Kluwer Academic 

Publishers. 133-165. 

Ono, D. 2006 Application of the Gillette Model for Windblown Dust at Owens Lake, CA. 

Atmospheric Environment. 40: 3011-3021. 

Roney, R.A. and White, B.R. 2004. Definition and Measurement of Dust Aeolian Thresholds. 

Journal of Geophysical Research. doi: 10.1029/2003JF000061. 

Shao, Y., 2000a. Physics and Modelling of Wind Erosion. Kluwer Academic Publishers, 

Dordrecht, 393 pp.

Sharratt, B., Feng, G, and Wending, L. 2007. Loss of Soil and PM10 From Agricultural Fields 

Associated with High Winds on the Columbia Plateau. Earth Surface Processes and 

Landforms. 32: 621-630. 

Sierra Research. 2003. Final BACM Technological and Economic Feasibility Analysis. Prepared 

for San Joaquin Valley Unified Air Pollution Control District by Sierra Research, Inc, 

Sacramento, California. March 21. 

U.S. EPA 1992. Fugitive Dust Background Document and Technical Information Document for 

Best Available Control Measures. EPA-450/2-92-004. 

Wancaser, R., James D., Jeong, H., and Pulurgurtha, S. 2006. Refined PM10 Aeolian Emission 

Factors for Native Desert and Disturbed Vacant Land Area. Final Report, June 30,2006. 

Watson, J.G. and Chow, J.C. 2000. Reconciling Urban Fugitive Dust Emissions Inventory and 

Ambient Source Contribution Estimates: Summary of Current Knowledge and Needed 

Research. DRI Document No. 6110.4F.
Draft for Public Notice and Comment 

Revision Date: June 2012 73 

Appendix A3. Methods for Establishing Area-Specific High Wind Thresholds 

As explained in Appendix A1, the EPA primarily based the 25 mph threshold on extensive 

windblown dust emissions research performed by the University of Nevada, Las Vegas (UNLV). 

During UNLV’s studies, researchers used a wind tunnel to quantify emissions from undisturbed 

areas meeting the definition of “stable” surfaces within Clark County’s (Nevada) BACM level 

fugitive dust regulations and mechanically disturbed open areas. The research performed by 

UNLV is one of the few field studies that clearly relate BACM level control of windblown dust 

from open areas and PM10 emissions. The EPA believes that the study results clearly 

differentiate emissions from these two types of conditions and provide a reasonable baseline for 

establishing a high wind threshold for exceptional events purposes for such areas. 

While the UNLV study stands out as the most definitive source of information concerning wind 

speeds capable of overwhelming BACM for open area windblown dust sources and/or causing 

emissions from natural undisturbed areas, EPA believes that other sources of information can be 

used to develop an area-specific high wind threshold. 

First, the EPA encourages state, local, and tribal agencies to evaluate the existing windblown 

dust literature identified in Appendix A2 when developing an area-specific threshold and 

determine if any of the preexisting information is applicable to their area. 

Secondly, while full-scale windblown dust emissions field studies are not always feasible, 

agencies may deploy temporary monitoring stations or use existing monitoring data to evaluate 

the effects of wind speed on different source categories. For example, as explained in Appendix 

A1, DRI used existing monitoring sites in Clark County to evaluate the relationship between 

urban/construction and non-urban/desert conditions.67 While this data was independent of the 

detailed wind tunnel emissions studies performed by UNLV in the same area, the results were 

similar: nonurban desert show a significant increase in PM10 emissions only when wind speeds 

are greater than 11 m/s (25 mph). The EPA believes that this is valid method for determining an 

area-specific threshold, but the use of existing monitoring sites (or temporary sites) to establish a 

wind speed/PM relationship for different source categories should be carefully evaluated for 

representativeness. For example, sites used to evaluate emissions from natural undisturbed desert 

areas should not be located downwind of any potential anthropogenic sources, as the influence 

from such sources would lower the expected high wind threshold. Also, simply correlating PM to 

wind speed without assessing representativeness of the monitoring site locations does not 

provide useful information for exceptional events purposes. 

Finally, area and/or source specific research may be performed, if needed. Specific information 

on the techniques used to assess windblown dust emissions can be found within the literature 

listed in Appendix A2. 

Regardless of the method used, an area-specific high wind threshold must consistent with the 

requirements of the EER, specifically nRCP, and representative of wind speeds capable of 

 

67 Watson, J.G. and Chow, J.C. 2000. Reconciling Urban Fugitive Dust Emissions Inventory and Ambient Source 

Contribution Estimates: Summary of Current Knowledge and Needed Research. DRI Document No. 6110.4F. 
Draft for Public Notice and Comment 

Revision Date: June 2012 74 

overwhelming reasonable controls or causing emissions from natural undisturbed areas. The 

EPA will not approve the use of an area-specific threshold if these basic principles are not 

upheld. The EPA encourages the state, local, and tribal agencies responsible for developing an 

area-specific threshold to consult with their respective regional office during the development 

process. 
Draft for Public Notice and Comment 

Revision Date: June 2012 75 

Appendix B1. Checklist for High Wind Exceptional Events Demonstration 

Submission 

Completeness Checklist for High Wind Dust Exceptional Events. 

Instructions: This checklist is provided as a guideline to help submitting agencies identify the 

types of information and analyses to include in an exceptional events demonstration package. In 

some cases (e.g., wind speeds above the identified high wind threshold), agencies will not need 

to include all parameters under each criterion. The EPA encourages agencies to include a 

completed checklist with their submitted exceptional events demonstration package. Note that 

completion of this checklist does not indicate that the event in question is concurrable nor does it 

guarantee a “complete” package. The EPA may ask for clarification or additional information to 

support a specific criterion. 

Site Name/AQS ID: ____________________________________________________________ 

Pollutant: ____________________________________________________________________ 

Date(s): ______________________________________________________________________ 

Procedural Criteria EPA Use 

Did an exceedance of the NAAQS occur? [Y/N] 

Were data flagged by July 1st of following year? [Y/N] 

Was there a 30-day public comment period? 

Is documentation for the comment period included? 

[Y/N] 

[Y/N] 

If public comments were received, are the public comments and responses 

included? 

[Y/N] 

Was the package 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? 

[Y/N] 

(over) 

 
Draft for Public Notice and Comment 

Revision Date: June 2012 76 

Evidence Information Included Page(s) EPA Use

Conceptual Model 

-description of weather phenomena resulting in 

high wind 

[Y/N] [page #] 

-description of what sources were likely 

entrained by the high wind 

[Y/N] [page #] 

-explanation of the path by which the dust 

reached the monitor(s) 

[Y/N] [page #] 

-map showing relevant monitors, topography, 

other relevant geographic features 

[Y/N] [page #] 

-description of how the event day differs from 

non-event days 

[Y/N] [page #] 

-description of concentration and wind patterns 

for the exceeding monitor(s) and surrounding 

area 

[Y/N] [page #] 

 

Wind Statistics 

-max sustained wind (Hourly avg) [X mph] [page #] 

- max sustained wind (1-5 min avg) [X mph] [page #] 

-max gust (1 min avg) [X mph] [page #] 

-wind trajectories included? [Y/N] [page #] 

 

-other: [list other wind 

analyses] 

[page #] 

 

nRCP 

-Area-specific high wind threshold (default = 

25mph) 

[25 mph] [page #] 

-sources contributing to event identified, 

including anthropogenic vs. natural? 

[Y/N] [page #] 

-controls identified for anthropogenic sources? 

(note: level of control analysis depends on wind 

speed) 

[Y/N] [page #] 

-are natural sources not reasonably controllable? [Y/N] [page #] 

-was a High Wind Action Plan included? [Y/N] [page #] 

 

HF 

-were time-series analyses for concentration and 

wind data included? 

[Y/N] [page #] 

-annual comparison to historical data (wind and 

concentrations) 

[%ile] [page #] 

-seasonal comparison to historical data (wind 

and concentrations) 

[%ile] [page #] 

 

CCR (=> AAQ & / Natural Event) 
Draft for Public Notice and Comment 

Revision Date: June 2012 77 

-were spatial analyses included, establishing a 

spatial relationship between the event, sources, 

transport of emissions, and recorded 

concentrations? 

[Y/N] [page #] 

-were temporal analyses included, establishing a 

temporal relationship between the high wind and 

elevated PM concentrations at the monitor? 

[Y/N] [page #] 

-comparison of event-affected day(s) to specific 

non-event days? 

[Y/N] [page #] 

-was the dust shown to be from the sources 

discussed in the nRCP section? 

[Y/N] [page #] 

-were alternative hypotheses discussed? [Y/N] [page #] 

-was a causal (not just correlational) relationship 

established? 

[Y/N] [page #] 

 

NEBF 

-was a but-for analysis included? [Y/N] [page #] 

Draft for Public Notice and Comment 

Revision Date: June 2012 79 

Appendix C. Sample Letter of Intent 

September 16, 2011 

Matthew Lakin 

U. S. Environmental Protection Agency, Region 9 

75 Hawthorne Street 

Mail Code: AIR-7 

San Francisco, CA 94105 

SUBJECT: Exceptional Event Documentation 

District: San Luis Obispo APCD 

Event Type: PM2.5 - Wildfire/Smoke Impact 

Event Date: August 14, 2009

Site AQS No POC Pollutant/Monitor Concentration 

Atascadero 060798001 3 PM2.5 FEM BAM 51.6 g/m3

 

Dear Mr. Lakin: 

The San Luis Obispo County Air Pollution Control District (APCD) submitted Exceptional Event Documentation on 

July 22, 2010 to the U. S. Environmental Protection Agency and the California Air Resources Board (CARB) that 

addresses wildfire emission impacts on PM2.5 concentrations at the Atascadero monitoring station on August 14, 

2009. This data has been appropriately flagged in the AQS data base. The EPA was notified of the intent to submit 

this documentation via a CARB email on June 21, 2010. CARB provided comments to the APCD and the APCD 

revised and resubmitted the report to CARB on May 25, 2011. On June 15, 2011, CARB provided additional 

comments to the APCD illustrating how the documentation should be specifically modified to ensure acceptance by 

EPA. 

Before the APCD proceeds with further modifications to the documentation based on CARB comments, the APCD 

respectfully requests that EPA provide feedback as to whether EPA will act on this Exceptional Event 

Documentation package. In addition, please indicate whether the 2009 data year will be used for future San Luis 

Obispo County PM2.5 attainment demonstrations. 

The Exceptional Event Documentation dated July 22, 2010 is located on the APCD website: 

http://www.slocleanair.org/air/pdf/2010/ExceptionalEventAug_14_2009_AtascaderoPM2.pdf

The revised working draft dated May 25, 2011 that was submitted to CARB for comment is located on a not-public 

location on the APCD website: 

http://www.slocleanair.org/air/epa.php

If you have need for additional materials, please contact me at (805) 781 5743 or garcemont@co.clo.ca.us. 

Sincerely, 

Gary Arcemont 

Air Quality Specialist 

GJA/arr 

cc: Karen Magliano, ARB