High Wind Exceptional Events 

And Control Measures for PM10 Areas 

Air Quality Division 

October 13, 2009
10-13-09 White Paper 

High Wind Exceptional Events & Control Measures for PM10 Areas 

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High Wind Exceptional Events and Control Measures for PM10 Areas

1.0 Background 

Under the authority of the Clean Air Act (CAA), the Environmental Protection Agency (EPA) develops 

National Ambient Air Quality Standards (NAAQS) for each of the criteria pollutants (carbon monoxide, 

lead, nitrogen oxides, ozone, particulate matter, and sulfur dioxide). For each of these pollutants, EPA 

tracks air concentrations based on actual measurements of pollutant concentrations in the ambient 

(outside) air at selected monitoring sites throughout the country. Once an air quality standard for a 

particular pollutant is established, monitors record when ambient concentrations of that pollutant exceed a 

level at which human health is no longer protected.1

 Should these exceedances continue or grow in 

intensity, the regulatory remedy is to declare the area in violation of the standard and designate the area 

nonattainment for that pollutant. The only time an exceedance would not count toward a violation of a 

standard is if that exceedance could be classified as an exceptional event. 

Exceptional Events impact air quality when their influence results in ambient air concentrations of 

particulate matter that fall outside the range of normal statistical fluctuations. For particulate matter, 

these events can be the result of large fires, high winds, man-made events such as explosions, or natural 

events such as volcanic eruptions. In the Western United States, high winds can accompany large storms 

(e.g., haboobs) that move across a regional or state-wide swath of land, or high winds can accompany 

“micro-bursts” with or without rain that descend upon a small, localized area. When a monitor within a 

planning area registers an exceedance of the PM10 NAAQS – or 150 micrograms per cubic meter (μg/m3

) 

over 24 hours – the exceedance is flagged in the EPA Air Quality System (AQS) database and an 

examination of the event begins to determine if the event could be classified as exceptional.2

 More than 

three exceedances at the same particulate matter monitor within three consecutive years add up to a 

violation of the NAAQS. A violation has a consequence to a planning area of either preventing an area 

from reaching attainment or returning an area to nonattainment that was previously redesignated to 

attainment. If an exceedance is flagged as an exceptional event, however, and the EPA’s Regional Office 

concurs, the exceedance is not counted toward a violation of the NAAQS. 

Control measures are established to bring a nonattainment area into attainment for a particular NAAQS; 

the control measures are to remain in place even after the area is redesignated to attainment. These 

control measures can include Best Available Control Measures (BACM) or Technology (BACT), 

Reasonable Control Measures (RACM) or Technology (RACT), Most Stringent Measures (MSM), and 

Lowest Achievable Emission Rate (LAER). Other control measures can be case-by-case control 

measures or practices that have been shown to be effective in reducing pollution from a particular source; 

for PM10 examples include Agricultural Best Management Practices (AgBMPs), stabilization of vacant 

lots, or paving unpaved roads or shoulders. A nonattainment area can be classified as Moderate, Serious, 

or Severe. The CAA requires areas designated as serious nonattainment for PM10 to implement BACM 

and BACT on all significant sources of PM10. 

 

1.1 PM10 Planning Areas in Arizona 

 

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 A primary NAAQS standard prevents damage to human health; a secondary standard prevents environmental and 

property damage. 

2

 Areas redesignated to attainment are subject to maintenance plans. An exceedance of the NAAQS is treated the 

same way as a nonattainment area; however, maintenance plan design values can be flagged as well but are not 

addressed by the federal Exceptional Events Rule. Maintenance plans are in place for 20 years. 
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Arizona has nine PM10 planning areas: 

 

Ajo Nonattainment; new plan under development 

Bullhead City Redesignated to attainment under a Limited Maintenance Plan 

Douglas-Paul Spur Nonattainment; new plan under development 

Hayden Nonattainment; plan to be revised3

Maricopa County Nonattainment; under Five Percent Plan4

Miami Nonattainment; Limited Maintenance Plan submitted to EPA 

Nogales Nonattainment; plan to be revised; also nonattainment for PM2.5 

Payson Redesignated to attainment under a Limited Maintenance Plan 

Rillito Nonattainment; Limited Maintenance Plan submitted to EPA 

Yuma Nonattainment; Maintenance Plan submitted to EPA 

Detailed information about the planning areas can be obtained by going to ADEQ’s Web site at 

http://www.azdeq.gov/environ/air/plan/index.html. 

 

1.2 High Wind Exceptional Events Versus Fire Events

As previously stated, exceptional events can include among other events, fire events and high wind 

events. For the purposes of this discussion, however, the focus is on high wind events as they are often 

the more prevalent cause of exceedances in areas throughout Arizona as well as other parts of the Western 

United States due to complex terrain and unique weather patterns. The characteristics of these high wind 

events are discussed further in a white paper titled, Impact of Exceptional Events ‘Unusual Winds’ on 

PM10 Concentrations. 

2.0 Control Measures 

The development of a state implementation plan (SIP) for an area designated as nonattainment for PM10

includes at the minimum: 

 a review of all relevant monitoring sites and ambient monitoring data; 

 development of an emissions inventory of all contributing sources of PM10; 

 application of emission factors to the ambient concentrations recorded in the emissions inventory; 

any controls already in place for PM10 emissions; 

 any controls in place in areas throughout the country, particularly the West, with similar 

characteristics to the planning area; 

 any current research on effective control measures; current level of enforcement for any control 

measures already in place; 

 and any modeling that shows the effectiveness of control measures on PM10 emissions. 

Fortunately, there is a moderate amount of information about control measures for PM10; unfortunately, 

much of the information is source and area-specific. Information on the relative effectiveness of control 

measures is not as abundant, nor is it easily applicable to the specific conditions in most of the PM10

planning areas in Arizona. Because the demonstrations required to obtain a concurrence on an 

exceptional event should account for the relative complexity of the emitting source mix, parsing out a 

specific source or source category along with the applicable control measure for a determination of 

relative effectiveness can be difficult and may even be counter-productive. This is compounded by the 

 

3

 The Hayden area was previously included with the Miami area; split into two planning areas later approved by 

EPA. 

4

 Maricopa County nonattainment area also include Apache Junction portion of Pinal County. 
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fact that PM10 cannot be characterized by a specific source category through routine examination of the 

particulates deposited on a monitor’s filter. Through the SIP development process, however, the overall 

make-up of the planning area is known and fairly reliable assumptions about source contribution and 

implemented control measure effectiveness can be made in most cases. 

2.1 Evaluating the Effectiveness of Control Measures; Implementation and Enforcement 

There are several ways to evaluate the effectiveness of a control measure. The most direct approach is 

actual field testing. Planners can also query representatives of other planning areas for examples of 

effective control measures, sharing testing data, or replicating specific field tests. Another approach is to 

research control measures that have been recognized as BACM by EPA, other states, source emitters, or 

in some cases by the courts. Regardless of how a control measure is chosen and its relative effectiveness 

evaluated, it is often subjected to a computer model to demonstrate whether the control measure or 

measures actually show a reduction in emissions or prevent an increase in the concentration of emissions 

(i.e., reasonable further progress toward attaining the NAAQS). 

Optimum effectiveness is measured by the degree of control efficiency; efficiency can be a set percentage 

or a range (i.e., the range can represent how the control measure is to be implemented over time) in 

relation to the level of compliance, or control efficiency multiplied by the compliance level. In the case 

where a control measure is a one-time, succinct action such as paving an unpaved road, the effectiveness 

should be high.5

 Other calculations of effectiveness can be less straightforward due to the variables in 

implementation and enforcement. Degree, range and timing of control measure implementation coupled 

with a variable level of enforcement make field inspections of vital importance to determine the true 

effectiveness of certain control measures. Without reliable or complete reporting of the implementation 

of control measures, the lower or lowest range of control efficiency is most likely reflective of actual field 

situations. It is difficult to calculate other types of control measure effectiveness when the 

implementation is more qualitative than quantitative in nature, even though the measure has proven 

effective in field tests (e.g., actions required only when special circumstances arise, or actions that rely on 

training and advanced communication). Field inspections, reporting, and a noticeable reduction in 

emissions provide verification of projected control efficiency for a particular control measure. A pattern 

of non-compliance would provide verification that the control efficiency has been compromised or the 

control measure itself was perhaps unenforceable as currently designed or implemented. 

Due to the varied nature of control measures, a planning approach that commits to a series of control 

measures rather than relying solely on only a few is often the best way to gain an overall high level of 

control effectiveness for a planning area. The commitment to implement these measures suggests that an 

optimal level of enforcement will be in place, often strengthened further by permit conditions, rules, and 

ordinances with specific consequences for non-compliance. Control measures that are voluntary in nature 

can also have a high degree of effectiveness but often rely on ongoing outreach and inspections. 

Contingency measures are designed to provide additional emission reductions should the committed 

control measures fail to provide sufficient reductions. These measures are often not modeled toward 

progress in reducing emissions of a particular pollutant, but they can be effective should sources not wish 

to move to the higher degree of control these measures can represent. 

By considering a control measure or suite of control measures in place around the time of an exceptional 

event, such as those triggered by high winds, it should be possible to determine the following: either (a) 

the control measure effectiveness was insufficient to control the background concentration of emissions 

thereby contributing to cumulative emissions sufficient to exceed the NAAQS when coupled with high 

 

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 In areas where a maintenance plan is in place, control measures that may appear permanent must be routinely 

revisited to assure that they are in good condition, or authority to enforce certain measures is still in place. 
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winds, or (b) the wind conditions were unique and severe enough to overwhelm the control measures 

regardless of control effectiveness. 

3.0 Effect of Exceptional Events (High Winds) on Control Measures; Overwhelmed Controls 

In parts of the Western United States high wind events can generally be classified as one of two types: 

mesoscale events or synoptic scale events. Mesoscale high winds associated with thunderstorm 

development are localized events that are often initially devoid of rain and cause a reduction in visibility 

due to lofted particles from disturbed arid land or certain soil types. Synoptic scale high wind events, on 

the other hand, have a greater tendency to transport particles over long distances. Both types of high wind 

events are capable of transporting particles into an area from distances outside of a specific planning area. 

This paper does not attempt to explain in detail the effect of these types of high winds, but in relation to 

planning area control measures it is important to note that at times these winds can overwhelm or greatly 

reduce the effectiveness of a control measure or suite of control measures. 

Unfortunately, information on the effects of high wind events on control measures, specifically measures 

for the control of particulate matter, is not readily available. Most of the information is anecdotal in 

nature, observations after the fact. There are studies on the effects of high winds on particulate matter 

itself – effects of particle size bombardment and particle size relative to transport – but the information 

does not always translate well to control measure applications or is too case-specific. More to the point, 

however, is an understanding that certain measured high wind speeds do cause lifting of certain soil types 

or increase background concentrations of already existing particulates. There is a fair amount of 

agreement and observations to support that high winds over a certain speed can overwhelm most controls 

regardless of the level of implementation or enforcement. The phenomenon is discussed further in a 

corresponding white paper titled, Impact of Exceptional Events ‘Unusual Winds’ on PM10 Concentrations. 

4.0 Tracking Relative Effectiveness of Implemented Control Measures During High Winds 

As stated previously, it is essential that the control measures for a planning area be routinely inspected for 

implementation and level of enforcement in order to have a meaningful sense of control measure 

effectiveness. Text of permit conditions needs to be periodically reviewed, dust complaints need to be 

properly logged and the subject of timely and appropriate response, inspections and any enforcement 

actions that resulted need to be reported, along with any other action necessary to determine the full 

implementation of a control measure. When comparing the control measures to the effects of a high wind 

event, it is helpful to know what the conditions were on the ground and in the air prior to the high wind 

event, the day of the high wind event, and after the high wind event. The time before the event can 

determine possible contributors to elevated PM10 concentrations while the time after the event could 

expose non-compliance patterns that would not otherwise be directly tied to the event. 

In order to obtain a better picture of what control measures are in place during the intervals of a high wind 

event, a simple reporting form has been constructed. This form, when reviewed by analysts of the high 

wind event, should provide a clearer picture of what could have contributed to the exceedance and could 

add to an argument that the event was indeed exceptional. The form requires several pieces of useful 

information. (1) Because some, but not all, planning areas have High Pollution Advisory (HPA) 

notification procedures, including high wind watches for particulate matter pollution, noting the existence 

of a HPA action on the day of or days prior to the event is important.6

 (2) Complaints are useful bits of 

 

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 For areas that do not have a HPA program in place, the National Weather Service issues storm watches and 

warnings to alert the public to possible heavy rain, flooding, gusty winds and blowing dust. Airports, both large and 

small regional air fields, also have data sources that register high winds and reduced visibility so aircraft warnings 

can be issued. 
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information as they often reveal insufficient implementation or enforcement of control measures, 

especially if the complaints are for the same source or group of sources; in the case of voluntary measures 

they can be the only available measure of effectiveness. (3) Inspections completed routinely or for 

special purposes (e.g., nighttime inspections) give specific information on what is happening in and 

around the planning area, particularly inspections required during a HPA. (4) Finally, any record of 

notices or actual enforcement actions on a source in the planning area tells a story of serious noncompliance with implemented control measures, or it tells of control measures that have been 

insufficiently implemented. 

A sample form is attached to this paper. Each party responsible (i.e., State, County, or any other relevant 

public officer/agency) for sources in the planning area would complete the form, which would then be 

reviewed by those preparing the analysis that demonstrates whether the high wind event is truly an 

exceptional event. For those events that are obviously of such severity that control measures would be 

overwhelmed, it is still useful to note the control measure actions for the planning area, especially an 

issuance of a HPA or pertinent weather forecast. The reverse side of the form contains the committed 

control measures for the planning area. Control measures can vary considerably for particular planning 

areas, but including them on the form reminds everyone that they need to be considered in the exceptional 

event analysis. 

5.0 Conclusion 

A demonstration that a high wind event is in fact an exceptional event requires a robust analysis of the 

wind conditions coupled with an examination of the control measures in place in and around the area in 

which the monitor registered an exceedance. This demonstration must not only pass the scrutiny of 

regulators but also the general public. Should the high wind event that caused the exceedance truly reflect 

an influence that resulted in ambient air concentrations of particulate matter that fell outside the range of 

normal statistical fluctuations, then EPA’s concurrence with the demonstration allows for a focus on PM10 

emissions that can be controlled.