Summary	of	Interagency	Working	Comments	on	Draft	Language	under

EO12866/13563	Interagency	Review.		Subject	to	Further	Policy	Review.

Environmental Protection Agency (EPA): Draft Guidance 

Supplemental Exceptional Events Implementation Guidance 

RIN 2060-ZA21 

General Comments 

1) The draft guidance addresses wildfire events. Has EPA guidance for prescribed fire 

events that describes the technical process for such a demonstration under the 

Exceptional Events Rule? Given the similarities in how an assessment would likely be 

done, it might best be contained in this document instead of elsewhere. 

Response: The preamble to the proposed Exceptional Events Rule revisions explains that 

the Draft Wildfire Ozone Guidance document is limited to wildfire event types, while 

also recognizing the technical section of the guidance document (Section 3) may be 

appropriate for application to prescribed fire events. The Notice of Proposed Rulemaking 

(NPRM) invites comment on whether it is necessary to expand the scope of the final 

guidance, and if so, whether the wildfire ozone guidance document should be expanded 

to cover other fire types or if a separate document should be developed. Given that the 

prescribed fire-related elements of the rule revisions are at the proposal stage, we believe 

that it is appropriate to cover only wildfire event types with this proposed guidance. We 

will consider this interagency comment supporting the expansion of the scope of this 

guidance document in combination with any comments received during the public 

comment period following the Notice of Availability for this guidance when determining 

whether (and in what format) to issue prescribed fire guidance. 

2) Reviewer recommends clarifying how an air agency would know what Tier their EER 

demonstration would fall into, specifically whether Tier 2 or Tier 3 evidence should 

apply. It is unclear where the line between Tier 2 and Tier 3 is. Thus, this could result in 

situations where states must submit under different tiers although the facts may be similar 

and potentially creates significant regulatory uncertainty. 

Response: The tiered approach proposed in the guidance document is intended to help air 

agencies determine the level of evidence that may be appropriate to meet the Exceptional 

Events Rule’s regulatory requirements for data exclusion requests. The individual event 

characteristics, event/monitor relationships, and the typical O3 concentrations in the area 

of focus will help determine the appropriate demonstration tier. Additionally, the initial 

discussions between the submitting air agency and the reviewing EPA regional office will 

help the air agency determine the appropriate tier for the demonstration. (The Exceptional 

Events Rule revisions NPRM proposes that these conversations take place before the 

affected air agency begins developing any demonstration.) Specifically, the guidance 

proposes that events that do not meet the Tier 1 (unique O3 concentration) or Tier 2 (Q/D 

and 99th percentile or higher O3 concentration) factors, should pursue a Tier 3 

demonstration. However, the appropriate evidence for a Tier 3 demonstration will be 

determined on a case-by-case basis. Discussions between the air agency and the EPA 

regional office will be help determine the evidence to support the Tier 3 demonstration. 
Summary	of	Interagency	Working	Comments	on	Draft	Language	under

EO12866/13563	Interagency	Review.		Subject	to	Further	Policy	Review.

2 

We have modified the language in section 3.6 (page 22) to include a reminder of the 

importance of air agency/EPA regional office discussions: 

”As discussed in Section 3.3, air agencies may need additional detail to satisfy the clear 

causal relationship element for events not meeting the Tier 1 or Tier 2 key factors. This 

section describes the additional evidence that may be appropriate for inclusion in for 

these Tier 3 event demonstrations. The appropriate level of evidence should be discussed 

between the submitting air agency and the EPA regional office during the Initial 

Notification discussions.” 

Specific Comments 

1) Page 8-9 Bulleted section: Emergency monitoring data and operational smoke forecasts 

which include model output for the surface are a common part of the Wildland Fire Air 

Quality Response Program and reviewer recommends including this here. 

Response: We have added the operational smoke forecasts as a potential piece of 

evidence on the list on page 8-9: 

“Smoke forecasts based on meteorology and burn conditions; these are often provided as 

part of the Wildland Air Quality Response Program.” 

We have also added text to page 19 of the guidance document (showing fire impacts 

reach the monitor) to explain that non-regulatory monitoring data may be used to support 

the clear causal relationship element of the demonstration: 

“While fires generate emissions of CO, NO, NO2, VOCs, PM10, and PM2.5, anthropogenic 

sources, such as from industrial and vehicular combustion, also emit these pollutants. 

Therefore, the air agency should distinguish the difference in the non-event pollutant 

behavior (e.g., concentration, timing, ratios, and/or spatial patterns) from the behavior 

during the event impact to more clearly show that the emissions from the fire(s) affected 

the monitor(s). Evidence from regulatory and non-regulatory (e.g., special purpose, 

emergency) monitors may be used to support these analyses.” 

2) Page 13: General Comment on 3.4.2 Trajectory Analysis section: Given the prevalence 

and ease of use of the HYSPLIT system, reviewer recommends that more detailed 

guidance on it use be offered in Appendix A.3. 

a. As part of that guidance, reviewer recommends an enhanced discussion on 

selection of meteorological database that is used for construction of back 

trajectories. If trajectories are used in areas of highly complex terrain and sourcereceptor relationships are relatively close (10’s – 100km), the resolution of the 

routinely used meteorological databases for HYSPLIT may not adequately 

capture the meteorological conditions which governed source-receptor 

relationships for a particular event. Careful consideration should be used when 
Summary	of	Interagency	Working	Comments	on	Draft	Language	under

EO12866/13563	Interagency	Review.		Subject	to	Further	Policy	Review.

3 

selecting meteorological databases, as these will largely determine the accuracy of 

the trajectory for a given event. 

Response: We have added the following text to page 51 of the guidance document 

(Appendix 3) to further address the importance of meteorological database choice: 

“If trajectories are used in areas of highly complex terrain and source-receptor 

relationships are relatively close (10’s – 100 km), the resolution of some of the routinely 

used meteorological databases for HYSPLIT may not adequately capture the 

meteorological conditions that govern source-receptor relationships for a particular event. 

Careful consideration should be used when selecting meteorological databases, as these 

will largely determine the accuracy of the trajectory for a given event. More information 

on meteorological databases and their applicability to HYSPLIT can be found at 

https://ready.arl.noaa.gov/archives.php.” 

3) Page 14 and page 21: On sentence: Air agencies could use other trajectory models, such 

as FLEXPART to demonstrate expected transport. Appendix A3 contains additional 

information on trajectory analyses. FLEXPART is not a trajectory model, but rather is a 

full Lagrangian Particle Dispersion Model (LPDM). The companion trajectory model, 

FLEXTRA, has not been updated in years and is not compatible with any modern source 

of meteorological data. However, FLEXPART itself is a viable tool for establishing an 

understanding of source-receptor relationships (SRR). Reviewer recommends that 

Appendix A.3 be updated to reflect use of inverse or adjoint LPDM methods (FLEXPAR 

or STILT models). Adjoint methods are more suitable when multiple monitors have been 

influenced by a single event. 

a. References include the following: 

i. M. Maione et al.: Estimates of European emissions of methyl chloroform. 

Atmos. Chem. Phys., 14, 9755–9770, 2014 

ii. X.L. Pan, et al.: Using Bayesian optimization and FLEXPART tracer 

model to evaluate CO emission in East China in springtime. Env. Sci. and 

Poll. Res., 21, 3873 - 3879, 201 

Response: We have removed the explicit mention of the FLEXPART model in this 

version of the guidance document because, as a result of this comment, we are now aware 

of potential inconsistencies in our description of these analytical tools. Prior to finalizing 

this guidance we will research the appropriate description and use of FLEXPART or 

STILT dispersion models for investigating source-receptor relationships and will 

consider adding information, as appropriate, to the final guidance. In the draft guidance, 

we have revised the language on page 14 (and again on page 21) to read: 

“Air agencies could use other trajectory models, such as FLEXPART, to demonstrate 

expected transport. Exceptional events demonstrations using other trajectory models 

should contain enough background information and detail supporting model application 

to allow reviewers to thoroughly understand the model and to reproduce the results if 

necessary.” 
Summary	of	Interagency	Working	Comments	on	Draft	Language	under

EO12866/13563	Interagency	Review.		Subject	to	Further	Policy	Review.

4 

4) Page 14 on 3.5: Key Factors of and Suggested Evidence to Include in Tier 2 

Demonstrations. Another analytical technique overlooked here which could be used to 

demonstrate the presence of smoke on a particular date at multiple monitors is the use of 

receptor modeling techniques such as PMF. EPA Region 7 in 2004 demonstrated that 

using strength of PMF biomass burning profiles from either STN data or IMPROVE on 

dates that coincide with O3 exceedances, coupled with use of inverse LPDM techniques 

can clearly indicate the presence of smoke influence on ozone events. Reviewer 

recommends that this technique also be included as a possible Tier II analytical 

technique. EPA Region 7 published a report in 2004 on the analysis of O3 exceedances 

in the KC metro area in April 2003 and the possibility of influence from the Flint Hills 

fires contribution to these events. These procedures are documented there. 

Response: We thank the reviewer for bringing the EPA Region 7 report to our attention. 

The authors of this guidance document were not aware of the report and agree that PMF 

analyses might be useful in establishing clear causal relationships between fire events and 

monitored O3 concentrations. Prior to finalizing this guidance, we will investigate the use 

of PMF analysis for fire-related O3 exceedances and, if appropriate, we will add language 

to the final guidance document. 

5) Page 15 on 3.5.1: Reviewers support the notion of simplified tool development for Tier I 

and Tier II demonstrations, but have general concerns about the usefulness of Q/d as tool 

for this purpose. As EPA states, both the appropriate ratio of NOx to rVOC and sunlight 

are necessary for ozone formation. Simple addition of NOx and rVOC emissions is not 

sufficient to describe Q because the ratio of NOx to rVOC determines whether there is 

ozone formation. Near the fire, ozone may be consumed rather than generated, and 

smoke plumes may impede photochemistry. Even though Q/d is used in conjunction with 

two other indictors within tier 2, the reviewer does not believe that Q/d is a useful 

surrogate and supports discussion on another approach.1 

Response: We thank the reviewer for supporting the tier-based approach and for 

recommending an alternative tool. We agree that the Q(NOx+VOC)/D tool is a simplified 

representation of very complex chemistry, but we believe that it can be used, in 

combination with high O3 concentrations, to indicate conditions where O3 impacts from 

fires are likely to occur. Our investigation of other screening tools is ongoing. If we 

receive additional information about alternate approaches during the upcoming public 

comment period, we will consider changes to the screening tool/key factor. Additionally, 

air agencies can use results from tools not described in the guidance document to support 

their demonstrations. 

6) Page 23 on 3.6.4: Reviewer recommends providing better references for useful 

techniques that can use climatology for ozone events include: 

a. Eder, Brian K., Jerry M. Davis, and Peter Bloomfield (1993): A Characterization 

of the Spatiotemporal Variability of Non-Urban Ozone Concentrations Over the 

Eastern United States. Atmospheric Environment 27A, 2645-2668. 

 

1 http://wrapfets.org/deasco3.cfm 
Summary	of	Interagency	Working	Comments	on	Draft	Language	under

EO12866/13563	Interagency	Review.		Subject	to	Further	Policy	Review.

5 

b. Brian K. Eder, Jerry M. Davis, and Peter Bloomfield, 1994: An Automated 

Classification Scheme Designed to Better Elucidate the Dependence of Ozone on 

Meteorology. J. Appl. Meteor., 33, 1182–1199. 

c. B. Anderson, and Davis, M. 2004: Analysis of April 12-13, 2003 Kansas City 

Ozone Exceedances. EPA Region 7 Technical Report. 

Response: We thank the reviewer for identifying these references, which we have added 

as citations in section 3.6.4. 

7) Page 27: Reviewer recommends modifying the sentence to state: 

a. Note: if a prescribed fire was declared to be a wildfire, then the air agency should 

supplement the language above with additional detail as to the conditions of the 

prescribed fire that led to the fire manager’s decision that the fire should be 

treated as a wildfire, for example if the prescribed fire escaped secure 

containment lines and required suppression along all or part of its boundary or if 

the prescribed fire escaped as a result of quickly changing weather and no longer 

mets meets the resource objectives (e.g., smoke impact, flame height). 

Response: We have corrected this error. 

8) Page 28 on 5.2: Reviewer recommends clarifying or removing the second paragraph as 

unnecessary in a demonstration. Whether a wildfire originated under the jurisdiction of 

an air agency or a state or federal land management agency is not relevant as the EER 

defines wildfire as not reasonably controlled or preventable. 

Response: We have incorporated the reviewer’s suggested edits.