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Attachment 3 

Request for Comments on the Draft Guidance Documents on the 

Implementation of the Exceptional Events Rule 

1. The EPA has developed draft exceptional event implementation guidance with the goal of 

establishing clear expectations to enable affected agencies to better manage resources as 

they prepare the documentation required under the EER. These draft guidance documents 

identify mechanisms (e.g., demonstration prioritization, review time lines, High Wind 

Action Plans) to streamline the demonstration development, submittal, and review process. 

The EPA seeks comment regarding other specific, broadly applicable, streamlining 

mechanisms that the EPA could incorporate into the exceptional event implementation 

process. 

2. The EPA has modified the exceptional events website at 

http://www.epa.gov/ttn/analysis/exevents.htm to include additional links to tools, such as 

the DataFed website, that submitting agencies may use in the development of their 

demonstration submittals. The EPA has also posted exceptional event demonstrations that 

have already been reviewed and acted upon by the EPA. The EPA solicits feedback 

regarding other web-based information, links, tools, or methodologies that we can similarly 

post on our website. 

3. In the draft exceptional events guidance documents, the EPA defines the high wind 

threshold as the minimum threshold wind speed capable of overwhelming reasonable 

controls on anthropogenic sources (i.e., capable of causing significant dust emissions from 

controlled sources) or causing emissions from natural undisturbed areas. The EPA further 

notes that this area-specific threshold, along with the submitter’s analysis of implemented 

reasonable controls and other factors, helps inform the analysis of the “not reasonably 

controllable or preventable” criterion. The EPA intends to allow air agencies to use wind 

data from a multitude of sources in the development of high wind thresholds. The EPA has 

identified several sources of local wind speed data including the National Weather Service, 

the National Climate Center, and local air monitoring stations. In addition, states may use 

models such as MM5, WRF, and NAM, to develop local wind speed data. The EPA solicits 

feedback on additional available sources of wind data and their applicability in informing 

local high wind analyses. 

4. As mentioned in Question 3, demonstrations for high wind dust events necessarily include 

wind speed analyses. 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 
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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. AERMINUTE, a preprocessor to AERMOD that takes 

short-period wind speed observations and calculates an hourly average wind, can assist in 

this calculation. AERMINUTE data, or other sub-hourly data with a resolution equal or 

greater than five-minutes, can be fed into AERMET, the AERMOD meteorological 

processor to get a user-friendly output. The EPA solicits additional feedback and tools to 

convert 1 - 5 minute wind speed data to hourly averages. 

5. Within the EPA’s Air Quality System (AQS), monitoring agencies can use two types of 

data validation, or data qualifier, codes: the Request Exclusion flags (“R”) and the 

Informational Only flags (“I”). Agencies should use the “I” series flags when identifying 

informational data and the “R” series flags to identify data points for which the agency 

intends to request an exceptional event exclusion and the EPA’s concurrence. Given that the 

EPA can act/concur only on “R” flags, some agencies have questioned the utility of “I” 

flags. Do AQS users find “I” flags in AQS useful? If so, how do users employ these flags? 

6. In response to comments received and in an effort to streamline the development of high 

wind demonstrations, the EPA has added an optional “Prospective Controls Analysis” 

process by which states, local agencies, and tribes can voluntarily 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. This 

optional analysis can facilitate agreement between states/local agencies/tribes and the EPA 

as to what constitutes “reasonable” controls in advance of an actual event. The EPA has 

also added an optional “High Wind Action Plan” that states/local agencies/tribes can use to 

document current in-place controls, document controls on new sources that need reasonable 

controls for future events, and/or document current and/or planned mitigation measures. 

Both of these approaches are described in more detail in the revised draft High Winds 

Guidance document. The EPA anticipates that air agencies would submit the prospective 

controls analysis in advance of or with a demonstration package and similarly expects that 

air agencies would submit the High Wind Action Plan following the EPA’s initial review of 

a demonstration package. The EPA recognizes that the information contained in the 

prospective controls analysis and the High Wind Action Plan is likely to overlap. The EPA 

solicits feedback on the anticipated use and functionality of these plans. Specifically, the 

EPA requests that commenters identify: (1) specific elements in the prospective controls 

analysis and High Wind Action Plan that are useful, (2) whether these concepts should be 

combined or kept separate, and (3) whether the flexibility to implement needed dust 

controls provided by the High Wind Action Plan as a voluntary alternative to the traditional 

regulatory nonattainment designation process is helpful. 
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7. In Table 3 of the revised draft High Winds Guidance document, the EPA identifies example 

technical analyses that air agencies should consider when preparing their high wind dust 

event controls analysis to demonstrate the not reasonably controllable or preventable 

criterion. The EPA solicits comment on the identified analyses and any additional technical 

analyses that air agencies could use to demonstrate that the wind exceeded an identified 

high wind threshold and that the exceedance was caused by emissions that were not 

reasonably controllable. 

8. The EPA acknowledges that certain extreme exceptional event cases may require more 

limited demonstration packages. Whether a particular event should be considered “extreme” 

for this purpose depends on the type and severity of the event, pollutant concentration, 

spatial extent, temporal extent, and proximity of the event to the violating monitor. Several 

meteorological phenomena that could be considered extreme events include hurricanes, 

tornadoes, haboobs, and catastrophic volcanic eruptions. The EPA addresses “extreme” 

high wind dust events in the draft Q&A document, but solicits comment on whether and 

how specific events of various types should be considered to be “extreme.”