Evidence for Flagging Exceptional Events

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What Evidence is Required to Flag an Exceptional Event? (EE)

The Exceptional Events Rule requires states that flag data to satisfy the requirements of 40 CFR 50.14 (c)(3)(iii) to provide evidence that:

1. The sample is a likely exceedance
2. The event satisfies the criteria that it was not reasonably controllable or preventable
3. There would have been no exceedances or violation but for the event.
4. The event is associated with a measured value in excess of historical values.
5. There is a clear casual relationship between the measured value and the event

1. Is there a likely exceedance?

Exceptional events flags are appropriate for samples that are likely to cause an exceedance. A site is in noncompliance if the 98 percentile of the PM2.5 concentration over a three year period is over 35 ug/m3. However, a site is in compliance if the number of PM2.5 > 35ug/m3 samples is less than 2 percent.

PM2.5 Exceedance Screening Tool

The PM2.5 samples that are potential contributors to non-compliance can be determined visually by the PM2.5 Exceedance Screening Tool. The data browser tool has a map view and a time series view. The map view shows the PM2.5 concentration as colored circles for each station for a specific date. The time view of the browser shows the concentration time series for a selected site.

The coloring of the PM2.5 concentration values (circles) is adjusted such that the concentrations above 35ug/m3 are shown in red. This provides an easy and obvious (to everyone except Neil) way to identify the candidate samples for noncompliance.

Further

2. Not Reasonably Controllable or Preventable

This line of evidence needs to establish that the event is 'not reasonably controllable or preventable'. The EE Rule identifes different categories of uncontrollable events:

• Exceedances Due to Transported Pollution (Transported African, Asian Dust; Smoke from Mexican fires; Smoke & Dust from Mining, Agricultural Emissions)
• Natural Events (Nat. Disasters.; High Wind Events; Wildland Fires; Stratospheric Ozone; Prescribed Fires)
• Chemical Spills and Industrial Accidents; Structural Fires; Terrorist Attack

3. The Exceedance or Violation would not Occur, But For the Exceptional Event

According to the EE Rule, observationas can be EE-flagged if the vioaltion is caused by the exceptional event.

• The leftmost figure shows a case when the 'exceptional' concetration raises the level above the standard. A valid EE to be flagged.
• In the next case, the concentration from controllable sources is sufficient to cause the exceedance. This is not a 'but for' case and should not be flagged.
• In the third case, there is no exceedance. Hence, there is no justification for an EE flag.

Illustration Exceptional Events, EE and non-EE events by the EE Rule.

4. The Event is in excess of the Historical Values

5. Clear Causal Relationship between the Data and the Event

There are multiple lines of evidence that can support the relationship between observations and the event. In the Exploratory Study the following lines of evidence were suggested:

• Chemical Signature
• Observed Pollutant Source and Transport
• Spatial Pattern of Pollutant
• Temporal Pattern of Pollutant

However, additional lines of evidence should be considered and illusterated. Community participation would be most desirable.