Guidance on the Preparation of Exceptional 

Events Demonstrations for Stratospheric Ozone 

Intrusions
ii

EPA-457/B-18-001

November 2018
iii

Guidance on the Preparation of Exceptional Events Demonstrations for Stratospheric Ozone 

Intrusions

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Air Quality Policy Division

Air Quality Assessment Division

Research Triangle Park, NC
iv

Table of Contents

Acronyms

1. Overview

1.1. Purpose of this Document

1.2. Statutory and Regulatory Requirements

1.3. Stratospheric Ozone Intrusions

1.4. Weight-of-Evidence and Tiering Approaches for Demonstrations

1.5. Recommended Process for Developing, Submitting, and Submitting an Exceptional 

Events Demonstration for Stratospheric Ozone Intrusions

2. Conceptual Model

2.1. Rule Provisions related to Conceptual Models

2.2. Elements of a Conceptual Model

3. Clear Causal Relationship between the Specific Event and the Monitored 

Concentration

3.1. Rule Provisions Related to the Clear Causal Relationship

3.2. Determining the Appropriate Tier for the Event

3.3. Comparisons Against Historical Concentrations

3.4. Analyses to Establish a Clear Causal Relationship

3.4.1. Event overview

3.4.2. Analyses showing stratospheric-tropospheric exchange

3.4.3. Analyses showing stratospheric air reached the surface

3.4.4. Air quality analyses showing the impacts of the intrusion at the surface

3.5. Differing Levels of Analyses Within Tier 1 and Tier 2 Demonstrations

3.6. Example Conclusion Statement for the Clear Causal Relationship Criterion

4. Other Required Elements of the Exceptional Events Rule

4.1. Caused by Human Activity that is Unlikely to Recur at a Particular Location or a Natural 

Event

4.2. Not Reasonably Controllable or Preventable

4.3. Public Comment Process

References
v

Acronyms

AQS Air Quality System

CAA Clean Air Act

CFR Code of Federal Regulations

CO Carbon monoxide, or Colorado

DV Design value

ENSO El Nino / Southern Oscillation

EPA Environmental Protection Agency

FLEXPART FLEXible PARTicle dispersion model

FR Federal Register

FT Free troposphere

GOES Geostationary Operational Environmental Satellite

hPa Hectopascals

HYSPLIT HYbrid Single-Particle Lagrangian Integrated Trajectory

IDEA Infusing Satellite Data into Environmental Applications

IPV Isentropic potential vorticity

K Kelvin

km Kilometer

LIDAR Light detection and ranging 

NAAQS National ambient air quality standard or standards

NASA National Aeronautics and Space Administration

NOAA National Oceanic and Atmospheric Administration

NO Nitric oxide

NOX Nitrogen oxides

NWS National Weather Service

OMI Ozone monitoring instrument

PBL Planetary boundary layer

PM Particulate matter

ppb Parts per billion

PT Potential temperature

RAQMS Real-time Air Quality Modeling System

RH Relative humidity

VOC Volatile organic compound or compounds

Z Zulu (coordinated universal time; same as Greenwich Mean Time)
1

1. Overview

1.1 Purpose of this Document

This document is intended to assist air agencies in preparing demonstrations for stratospheric 

ozone intrusions that meet the requirements of the Exceptional Events Rule1. This guidance 

provides example language and sample analyses that air agencies may use to address the 

elements identified in Section 1.2 in demonstrations for stratospheric ozone intrusions. Because 

this guidance identifies analyses and language to include within an exceptional events 

demonstration and promotes a common understanding of these elements between the submitting 

air agency and the reviewing Environmental Protection Agency (EPA) Regional Office, the EPA 

anticipates expedited review of demonstrations prepared according to this guidance. Air agencies 

may also use well-documented, appropriately applied and technically sound analyses not 

identified in this guidance. This guidance does not impose any new requirements and shall not be 

considered binding on any party.

As appropriate under a weight-of-evidence approach, one purpose of this document is to help air 

agencies determine the appropriate kind of information and analyses to include in a 

demonstration, which will vary on a case-by-case basis depending on the nature and severity of 

the event. To ensure a “right-size” approach to demonstrations, this guidance identifies two tiers 

of analyses for developing evidence for exceptional events demonstrations for stratospheric 

ozone intrusions. Tier 1 analyses are intended for events that occur when conditions for 

photochemical production of ozone are clearly unfavorable and yet surface ozone concentrations 

are much higher than normal observations with the synoptic meteorological pattern suggesting a 

stratospheric intrusion may be the cause. These events will require less supporting 

documentation. Tier 2 analyses are appropriate for events where local photochemical ozone 

production may exist simultaneously with stratospheric ozone contributions, or for events where 

the observed ozone is in the range of normal seasonal values at that location. Tier 2

demonstrations involve more supporting analytical documentation than Tier 1 demonstrations. A 

similar tiering process is recommended in EPA’s guidance on wildfire events that may influence 

ozone concentrations (EPA, 2016). Ultimately, the goal of the EPA in collaboration with air 

agencies is to ensure that exceptional events demonstrations satisfy the rule criteria and support 

the regulatory determination(s) for which they are significant.

1.2 Statutory and Regulatory Requirements

Clean Air Act (CAA) section 319(b) allows the governor of a state to petition the EPA 

Administrator to exclude air quality monitoring data that is directly due to exceptional events 

from use in determinations by the Administrator with respect to exceedances or violations of the 

national ambient air quality standards (NAAQS). In 2016, the EPA promulgated an update to the 

Exceptional Events Rule2to address certain key concerns raised by state, local and tribal co-

 

1

“Treatment of Data Influenced by Exceptional Events; Final Rule,” 81 FR 68216, October 3, 2016.

2 The EPA has prepared this draft guidance to align with the Exceptional Events Rule revisions signed on 

September 16, 2016 (81 FR 68216), and available on the EPA’s exceptional events website at 

http://www.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events.
2

regulators and other stakeholders and to increase the administrative efficiency of the Exceptional 

Events Rule implementation process. 

The revisions to the EPA regulations at 40 CFR 50.14(c)(3)(iv) and (v) identify the following 

required elements and technical criteria that air agencies3 must include in their exceptional 

events demonstrations:

• A narrative conceptual model (emphasis added) that describes the event(s) causing the 

exceedance or violation and a discussion of how emissions or transport from the event(s) 

led to the exceedance or violation at the affected monitor(s);

• A demonstration that the event affected air quality in such a way that there exists a clear 

causal relationship (emphasis added) between the specific event and the monitored 

exceedance or violation, supported by analyses that compare the claimed eventinfluenced concentration(s) to concentrations at the same monitoring site at other times 

unaffected by events;

• A demonstration that the event was both not reasonably controllable and not reasonably 

preventable (emphasis added);

• A demonstration that the event was a human activity that is unlikely to recur at a 

particular location or was a natural event (emphasis added); and

• Documentation that the submitting air agency conducted a public comment process

(emphasis added).

As identified in 40 CFR 50.14(c)(2), air agencies should also contact their EPA Regional Office 

soon after identifying event-influenced data that potentially influence a regulatory decision 

and/or when an agency wants the EPA’s input on whether or not to prepare a demonstration.

1.3 Stratospheric Ozone Intrusions

The Exceptional Events Rule at 40 CFR 50.14(b)(6) and its preamble identify stratospheric 

ozone intrusions as natural events that could qualify as exceptional events under the CAA and 

Exceptional Events Rule criteria. This section of the guidance provides a brief scientific 

overview of stratospheric ozone and the exchange processes that enable potential contributions to 

surface ozone concentrations.

The characteristics and composition of the atmosphere vary with height. When considering the 

potential impacts of stratospheric ozone at the surface it is instructive to consider three specific 

atmospheric layers (from highest to lowest): the stratosphere, the free troposphere (FT)4, and the 

planetary boundary layer (PBL). The depths of each of these layers are dynamic and can depend 

on the time of year, the time of day, location, and meteorological conditions. The stratosphere 

generally extends from 10-15 kilometer (km) above the surface up to an altitude of 

approximately 50 km (Seinfeld and Pandis, 2006). Temperatures increase with height in the 

 

3 The term “air agencies” is used throughout this document to include state, local, and tribal air agencies responsible 

for implementing the Exceptional Events Rule. In the context of flagging data and preparing demonstrations, the 

roles and options available to air agencies may also include federal land managers of Class I areas and other federal 

agencies that either operate monitors affected by an event or that manage federal land.

4 For the purposes of this guidance document, the free troposphere is defined as the part of the troposphere above the 

planetary boundary layer.
3

stratosphere. When temperatures increase with height (i.e., a “temperature inversion”), vertical 

mixing of atmospheric material is limited. As such, the stratosphere is typically a distinct and 

highly stable layer that interacts minimally with atmospheric layers above and below. The 

stratosphere also features a large reservoir of natural ozone resulting from the photochemical 

reaction between ultraviolet light and molecular oxygen (O2). Ozone concentrations in the 

stratosphere can be orders of magnitude larger than what are observed at the surface (i.e., > 5000 

ppb). Below the stratosphere is the troposphere, a layer which extends from the surface to 10-15 

km. For the purpose of considering stratospheric-tropospheric exchange, it is instructive to 

subdivide this atmospheric layer into two separate ones (from higher to lower): the FT and the 

PBL. Both the FT and the PBL are generally well-mixed layers sometimes separated by a 

temperature inversion (or inversions) that limits transport of material between layers. The depth 

of the PBL depends on local meteorological conditions but can range from as low as 25 meter 

(m) on cold winter nights, to as high as 5-6 km on warm and dry summer days. While actual 

atmospheric conditions are typically more complicated than the simple 3-layer structure outlined 

here, any demonstrations of the causal impacts of stratospheric ozone should describe: 1) how 

material was transported from the lower stratosphere to the FT, and then 2) how the material was 

transported from the FT to the PBL.

As discussed above, the temperature inversion that separates the FT from the stratosphere 

typically limits the transport of stratospheric air into the troposphere. However, in some cases, 

“ribbons” or “filaments” or “streamers” of ozone-rich air from the stratosphere can be displaced 

into the FT via a process known as tropopause folding5(Holton et al., 1995). These tropopausefolding events frequently occur in conjunction with deepening upper-atmospheric low-pressure

disturbances (Danielsen, 1968) and can result in stratospheric air descending deep into the FT.

These “intrusions” of stratospheric air have been found to be associated with extratropical 

cyclones (Wernli and Bourqui, 2002) and, as such, occur more commonly in the winter/spring 

seasons than the summer/autumn seasons over the United States (U.S.). From a spatial 

perspective, suspected stratospheric intrusions are more common along the west coast of the 

U.S., although they can occur elsewhere (Langford et al., 2012). There can be year-to-year 

variability in the number of tropopause folding events that influence the U.S. depending on 

global climate features like the El Niño-Southern Oscillation (ENSO) (Lin et al., 2015) and this 

variability can affect ozone trends (Verstraeten, et al., 2016). Additionally, intrusion events can 

vary in magnitude and spatial extent. Exceptions exist, but they generally range from 200-1000 

km in length, 100-300 km in width, and 1-4 km in depth (Wimmers et al., 2003;). Stratospheric 

ozone can also be assimilated into the FT via other stratospheric-tropospheric exchange 

processes, such as deep convection (Tang et al., 2011).

Ozone transported into the troposphere by tropopause folding or any other stratospherictropospheric exchange process, may remain wholly within the FT or it may be mixed down to 

the surface. There have been numerous analyses that have shown stratospheric intrusions 

influencing high surface ozone concentrations at U.S. locations (Langford et al., 2009; Lin et al.,

2012; Yates et al., 2013; Zhang et al., 2014; Langford et al., 2015; Knowland et al., 2017). 

Stratospheric ozone intrusions are more likely to influence surface concentrations at high 

elevation sites where less downward movement is needed to affect a surface monitoring site. At 

these high elevation sites, stratospheric ozone intrusions have been estimated to contribute about 

 

5 The tropopause is defined as the boundary between the stratosphere and the free troposphere.
4

20-25 percent of the total tropospheric ozone budget and can cause relatively short-term (i.e., 

ranging from several hours to 2-3 days in duration) increases of surface ozone of 10-180 parts 

per billion (ppb) above normal background levels (EPA, 2013). Along with high elevation sites, 

days with very deep PBLs are also more likely to experience stratospheric impacts at the surface 

as greater amounts of stratospheric-influenced ozone can be captured within the PBL and 

thermally mixed to the surface. 

Because ozone has the same chemical structure whether produced naturally in the stratosphere or 

troposphere, the source of surface-level, monitored ozone can be difficult to identify. 

Stratospheric air does, however, have some properties that can be used to distinguish it from 

tropospheric air. While the troposphere contains varying amounts of ozone, carbon monoxide 

(CO), nitrogen oxides (NOX), particulate matter (PM) and water vapor, the stratosphere contains 

large amounts of naturally-produced ozone, as noted previously, and has low concentrations of 

CO, NOX, PM and water vapor (indicated by low relative humidity). These features can help 

distinguish intrusions from episodes with substantial transport of international pollution. The 

concurrent impacts on CO and relative humidity (RH), however, can be subtle when 

stratospheric air has mixed with tropospheric air as the mixing process can dilute the ozone 

enhancement and increase CO and water vapor concentrations relative to stratospheric 

conditions. In addition to the chemical and physical identifiers discussed above, isentropic 

potential vorticity (IPV) and potential temperature (PT) can also be used to help identify the 

“intrusion” of stratospheric air into the troposphere, as can certain beryllium and lead isotopes

(e.g., Be-10, Be-7, and Pb-210). IPV, for stratospheric air, is much higher than for tropospheric 

air and does not change as it mixes to the surface during intrusions. As a result, the IPV for 

stratospheric air can be up to two orders of magnitude (100 times) greater than the IPV of 

tropospheric air. Because IPV can vary by season and latitude, PT, which is also higher in the 

stratosphere than in the troposphere, can serve with IPV as an indicator of stratospheric air at the 

surface.

In summary, exceptional events demonstrations should contain analyses that demonstrate the 

processes by which air of stratospheric origin has been transported from the stratosphere into the 

PBL. Data or graphics showing correlations between elevated ozone and markers of stratospheric 

ozone (e.g., low CO, low RH, elevated IPV, higher PT) will be valuable elements of the weight 

of evidence showing for a stratospheric ozone intrusion exceptional event. We discuss these 

analyses and potential tools for developing these analyses, as well as our proposed tiering 

approach (discussed below) for developing demonstrations in the subsequent sections of this 

guidance document.

1.4 Weight-of-Evidence and Tiering Approaches for Demonstrations

The EPA reviews all exceptional events demonstrations with regulatory significance on a caseby-case basis using a weight-of-evidence approach. This means that the EPA considers all 

relevant evidence submitted with a demonstration or otherwise known to the EPA and 

qualitatively “weighs” this evidence based on its relevance to the Exceptional Events Rule 

criterion being addressed, the degree of certainty, its persuasiveness, and other considerations 

appropriate to the individual pollutant and the nature and type of event before acting to approve 

or disapprove an air agency’s request to exclude data under the Exceptional Events Rule. Each 

event eligible for consideration under the Exceptional Events Rule will likely have unique 
5

characteristics. Therefore, the documentation and analyses that air agencies should include in 

their demonstrations will vary depending on the nature and severity of the event, the 

characteristics of the typical ozone concentrations at the affected monitor, and the complexity of 

the airshed. 

As part of EPA’s strategy to ensure air agencies can “right size” demonstrations and manage 

resources associated with preparing demonstrations, the EPA intends to use a two-tiered

approach to evaluate demonstrations for stratospheric ozone intrusion events. The two tiers are 

delineated based on an event’s potential for influencing ozone concentrations at a given monitor 

and the history of non-event ozone concentrations at the affected monitor(s). This approach

recognizes that some intrusion events may clearly stand out from normally occurring ozone 

concentrations and, thus, may need less supporting evidence to satisfy the rule requirements, 

particularly for the clear causal relationship element. Within these two tiers of demonstrations, 

Tier 1 demonstrations are the simplest and least resource-intensive, and may be sufficient for 

stratospheric intrusion events that cause obvious ozone impacts during periods in which ozone 

concentrations are typically low and meteorological patterns are suggestive of potential transport 

from the stratosphere. Tier 2 demonstrations should be used when the relationship between the 

subject intrusion and the influenced ozone concentrations is complex and not fully elucidated 

with the simpler Tier 1 demonstrations. Subsequent sections of this guidance discuss the types of 

analyses that could be included within each tier.

1.5 Recommended Process for Developing, Submitting, and Reviewing an Exceptional 

Events Demonstration for Stratospheric Ozone Intrusions

Figure 1 provides an overview of the recommended process for preparing, submitting, and 

reviewing exceptional events demonstrations for stratospheric ozone intrusion events.

6 As 

indicated in 40 CFR 50.14(c)(2), the “Initial Notification of Potential Exceptional Event,” the 

EPA expects to discuss potential event-influenced exceedances with an affected air agency prior 

to the air agency preparing and submitting a demonstration. For stratospheric ozone intrusions, 

this “initial notification” will, in part, focus on observed ozone concentrations and how the 

subject event differs from non-event exceedances. As a result of this notification, the EPA and 

the air agency will begin discussions regarding the appropriate tier (Tier 1 or 2) for a

demonstration.

This guidance document is organized by Exceptional Events Rule-required elements in the 

recommended order for inclusion within an exceptional events demonstration. Section 2 covers 

the narrative conceptual model. Section 3 describes the recommended approach for tiering 

stratospheric intrusion events and provides guidance for establishing a clear causal relationship 

between the event and the ozone violations in question. Table 3 in Section 3.6 provides a 

summary of the different kinds of analyses that could be included in a demonstration to support a 

clear causal relationship for Tier 1 and Tier 2. Sections 4 and 5 discuss the additional required 

elements of an exceptional events demonstration, which are straightforward for stratospheric 

intrusions, as well as the public comment process.

 

6 The exact process order can vary depending on the specific situation.
6

Figure 1: Flowchart of the EPA’s recommended process* for preparing, submitting, and 

reviewing exceptional events demonstrations for stratospheric ozone intrusion events.

* Note: This flowchart is illustrative of a typical exceptional events demonstration process, but 

the order of some steps may vary based on case-specific circumstances. Please consult with your 

EPA Regional office at the beginning of the process to establish expectations. 40 CFR 50.14 

identifies the required components for the exceptional events demonstration process.

1) Event-influenced exceedance or violation.

2) Air agency flags data of interest in Air Quality System (AQS). The EPA encourages air agencies 

to use “I” series flags (informational) when they believe data may have been influenced by an event, 

but do not yet know if they will request exclusion of the data in an exceptional events demonstration.

11) The EPA reviews and acts on the submitted demonstration:

• The EPA intends to send an “on hold” (aka deferral) letter within 60 days of receipt of a

demonstration that does not have regulatory significance.

• For complete demonstrations that have regulatory significance, the EPA intends to reach a 

decision regarding concurrence/nonconcurrence as expeditiously as necessary if required for a 

near-term regulatory determination, but no later than 12 months following submittal.

3) Air agency submits initial notification of potential exceptional event to its EPA Regional office.

4) The EPA acknowledges receipt of initial notification and communicates findings regarding 

regulatory significance (intended response within 60 days of initial notification).

5) The EPA and air agency work collaboratively to determine appropriate scope of demonstration 

(days and monitors) based on regulatory significance and approvability considerations.

6) After agreement on scope (days and monitors) of demonstration, air agency revisits AQS to update 

flagged data accordingly, which may include changing “I” series flags to “R” series flags (request 

exclusion) and adding an associated event description.

7) The EPA, in collaboration with the air agency, advises whether a Tier 1 or Tier 2 demonstration is 

appropriate.

10) Air agency refines demonstration if necessary, conducts a 30-day public comment process, and 

submits final demonstration to the EPA with substantive public comments addressed.

8) Air agency prepares and submits a draft demonstration.

9) The EPA intends to conduct initial review of a demonstration that has regulatory significance 

within 120 days of receipt of draft demonstration, at which point the EPA will respond to the 

submitting air agency with a completeness determination and/or a request for additional information.
7

2. Conceptual Model of Event

2.1 Rule Provisions Related to Conceptual Models

The Exceptional Events Rule at 40 CFR 50.14(c)(3)(iv)(A) requires that demonstrations include 

a conceptual model, or narrative, that describes the event causing the exceedance, discusses how 

emissions (or transport) from the event led to the exceedance at the affected monitor(s), and 

identifies the regulatory decision affected by the exceptional event. Because this narrative should 

appear at or near the beginning of a demonstration, it will help readers and the reviewing EPA 

Regional Office understand the event formation and the event’s influence on monitored pollutant 

concentrations before the reader reaches the portion of the demonstration that contains the 

technical evidence to support the requested data exclusion. The EPA expects that the air agency 

could include in the conceptual model much of the information that the air agency provided to,

or discussed with, the EPA during the initial notification process. 

2.2 Elements of a Conceptual Model

A conceptual model is intended to frame the “state of the knowledge” regarding the influence of 

emissions, meteorology, transport, and/or other relevant atmospheric processes on air quality in 

an area (McMurray et al., 2004). A well-constructed conceptual model of ozone formation in the 

area can assist in the determination of a stratospheric ozone exceptional event by highlighting the 

contrast between typical, non-event, high ozone days and the event-influenced days in question. 

The conceptual model should provide a context for the more detailed clear causal analyses 

described in Section 3. To promote a shared understanding and interpretation of this information, 

the EPA recommends that the submitting air agency tie the presented evidence and analyses to 

the narrative conceptual model, which should contain all the following elements:

• Provide a map of the existing ozone monitors in the area and a description of the sites

(e.g., site ID, current design value (DV) over the last 3 complete years, elevation, recent 

ozone trends), and any other relevant information.

• Note the monitor(s) and days for which the air agency is requesting data exclusion.

• Briefly summarize the processes that lead to high ozone concentrations at the monitor on 

non-event days. The contents of this summary will vary by area, but could include: 

o the months in which high ozone days usually occur, 

o the diurnal evolution of a typical 8-hour ozone exceedance in the area, 

o typical spatial patterns of ozone on exceedance days, and/or

o the meteorological conditions often associated with typical high ozone days.

• Introduce the meteorology that caused the stratospheric ozone intrusion and provide a 

brief narrative for how stratospheric material was transported into the FT and ultimately 

mixed down through the PBL to the surface monitor.

• Describe the key differences between the observed event-related concentration(s) and a 

typical, local, non-event ozone exceedance.

• Summarize the affected area’s NAAQS attainment and classification information.

• Describe the regulatory determination influenced by the event-related data exclusion. 

Include a table of the monitor data requested for exclusion (e.g., date, hours, monitor 

values, and DV calculations with and without the exceptional event).
8

3. Clear Causal Relationship Between the Specific Event and the 

Monitored Concentration

3.1 Rule Provisions Related to the Clear Causal Relationship

The Exceptional Events Rule at 40 CFR 50.14(c)(3)(iv)(B) and (C) requires that an air agency’s 

demonstration to justify data exclusion must include a demonstration that “the event affected air 

quality in such a way that there exists a clear causal relationship between the specific event and 

the monitored exceedance or violation” including support from analyses comparing the claimed 

event-influenced concentration(s) to concentrations at the same monitoring site at other times. In 

addition to providing the historical context for the event-influenced data, an air agency should 

also support the clear causal relationship with evidence showing that ozone from the 

stratospheric intrusion was transported to the monitor.

3.2 Determining the Appropriate Tier for the Event 

As introduced in Section 1, the EPA recognizes that the “clear causal relationship” between 

certain ozone exceedances and associated stratospheric intrusions are more evident than others.

In some cases, the event-caused exceedance occurs outside the normal period in which high 

ozone is typically observed. In other cases, exceedances caused by stratospheric intrusions occur 

during times of day, or during meteorological conditions, that are not typically favorable to high 

ozone (e.g., nighttime, cooler conditions). In yet other cases, the stratospheric intrusion results in 

an anomalous spike in ozone concentrations at the monitor that cannot be explained by usual 

ozone formation processes in the area. When the clear causal relationship is readily apparent, 

EPA believes that the causality can be demonstrated with a smaller set of analyses than may be 

needed in other cases where the stratospheric contribution is mixed with other sources that may 

also be contributing to the exceedance.

As discussed in Section 1, the EPA expects to discuss potential event-influenced exceedances 

with an affected air agency prior to the air agency preparing and submitting a demonstration. As 

a result of this discussion, the EPA and the air agency will jointly identify the appropriate tier 

(Tier 1 or 2) for the event demonstration. While each stratospheric intrusion exceptional events

demonstration will involve a unique set of conditions, the general criteria listed below would 

suggest a Tier 1 demonstration to be appropriate:

• Meteorological analyses suggest intrusion was recent, nearby and expansive, e.g., associated 

with a frontal passage and with elevated ozone observed across a large region.

• Resulted in ozone values clearly distinguishable from usual conditions.

• Occurred outside the period in which high ozone from local and/or regional production is 

typically observed.

• Occurred when and where local photochemical production was minimal, e.g., at night, or 

associated with cold air advection, high wind speeds and/or strong dispersion conditions.

More complex situations, defined by the characteristics below, would suggest the need for a 

more detailed Tier 2 analysis:
9

• Resulted from long-distance, multi-day transport requiring detailed analyses.

• The event-influenced concentration was in the range of typical exceedances (i.e., close to the 

area’s design value).

• Occurred in season when ozone exceedances are historically common.

• Occurred in association with other processes and sources of ozone, or on days where 

meteorological conditions were conducive to local ozone formation.

Table 3 in Section 3.6 provides a more detailed summary of potential analyses for each tier.

3.3 Comparisons Against Historical Concentrations

The first component of establishing a clear causal relationship between the event and the 

monitored ozone exceedance is to prepare an analysis showing how the observed event 

concentration compares to the distribution or time series of historical concentrations measured at 

the same monitor and/or at other monitors in the area. Air agencies can show the relationship 

between the event-related concentration(s) and historical concentrations in a variety of ways. 

Table 1 provides a list of sample analyses that could be completed to show that the eventinfluenced exceedances were outside the bounds of generally expected ozone levels.

Figure 2 shows an example of a potential plot comparing historical concentrations from nonevent days versus days influenced by events7

, including the event days in the demonstration. This 

sample analysis illustrates 9 years of daily peak 8-hour ozone at a single location over all days of 

the year. The green circles are those days determined to be uninfluenced by exceptional events or 

unusual occurrences. The brown triangles depict days where wildfire smoke was expected to 

have influenced ozone concentrations. The red circles depict days where stratospheric ozone was 

expected to have contributed substantially to the observed ozone. In this hypothetical illustration, 

the black arrows point to the 2 days in April that are the exceptional events in question.

 

7 This can include events that were never officially determined to be exceptional events.
10

Table 1. Possible Analyses for Comparing Historical and Event-Related Ozone

Historical Concentration 

Evidence Types of Analyses/Supporting Information

1. Emissions trends

Provide assurance that the area has not experienced significant 

recent changes in emissions totals that could invalidate this 

comparison (e.g., large growth in a local sector of emissions).

2. Ozone data

Plot the maximum daily 8-hour (or 1-hour) ozone concentration 

at the affected monitor(s) for the most recent 5-year period8that 

includes the event(s). Can also supplement with a table that 

briefly describes percentile ranks of event-influenced days and 

comparisons against historical means and maxima.

3. Identify event influences

Distinguish any high ozone concentrations associated with 

concurred exceptional events, suspected exceptional events, or 

other unusual occurrences from high pollution days due to 

normal emissions (provide evidence when possible).

4. Diurnal ozone patterns

If a Tier 1 selection was based on the criteria that the eventrelated exceedance was measured at an unusual time of day, 

then show how the diurnal pattern differs due to the event.

5. Meteorological analogs

Utilize meteorological output (forecast models and real-time 

data) to compare the potential stratospheric event to a known 

stratospheric intrusion event.

 

8 Section 8.4.2.e of appendix W recommends using 5 years of adequately representative meteorology data from the 

National Weather Service (NWS) to ensure that worst-case meteorological conditions are represented. Similarly, for 

exceptional events purposes, the EPA believes that 5 years of ambient air data better represent the range of “normal” 

air quality than do shorter periods.
11

Figure 2. Sample historical comparison analyses showing how previous non-event days 

compare against event-influenced days. Data is from U.S. Air Force Academy site from 

2008 to 2017

When discussing this type of time series plot, describe how the seasonality of the event-related 

exceedance differs from the typical photochemical ozone season and how other exceedances, if 

any, during the time of year of the intrusion-related exceedance are not attributable to normal 

emissions and photochemistry or are clearly lower in magnitude than the intrusion-related 

concentrations. As part of this discussion, air agencies may also want to prepare similar time 

series plots for all monitors in the area.

As demonstrated by Figure 2, this example site experiences most of its photochemical ozone 

exceedances (i.e., not influenced by events) from mid-May through August, with the most 

frequent exceedances occurring in July. In late May through early June, ozone exceedances can 

occur with stratospheric intrusions or more typical conditions. However, the rare ozone 

exceedances observed in April, including those that are the subject of this sample demonstration, 

are more likely to be influenced by stratospheric impacts and are distinguishable from usual 

April conditions at the site. In this example, the historical comparison would also benefit from 

some explanation regarding how the two events in question differ from the one case where an 

exceedance was measured in April (e.g., perhaps that single day featured abnormally summerlike meteorology in April).
12

As a separate example, Table 2 is a concise summary of the ozone at sample sites on days of a

presumed exceptional event and how those data compare to historical values at these locations.

This sample table highlights that these particular locations experienced ozone values at the upper 

end of the historical distribution. As appropriate, a table like this could also include nearby sites 

and days preceding and following the event if that helps inform the conclusion that something 

differentiates the event-influenced days from typical observations.
13

Table 2. Example tabular summary of event-influenced ozone data in parts per billion (ppb) relative to historical concentrations.

9

Exceedances of the 2015 NAAQS in bold.

Statistic Fruitland, UT Myton, UT Whiterocks, UT Ouray, UT Redwash, UT Dinosaur, NM Rangely, CO

Data Years 2011-2015 2011, 

2013-2015

2011, 

2013-2015 2009-2015 2009-2015 2007-2010,2011-2015 2010-2015

Number of Samples 1,716 1,316 1,254 2,313 2,279 2,447 1,934

June 8 Max 8-hr Ozone (ppb) 66 71 73 71 74 74 70

June 8 Rank 34 of 1,716 58 of 1,316 18 of 1,254 138 of 2,313 95 of 2,279 51 of 2,447 21 of 1,934

June 8 Percentile 97.9th 95.6th 98.6th 94.0th 95.8th 97.9th 98.9th

June 9 Max 8-hr Ozone (ppb) 77 72 73 71 72 72 70

June 9 Rank 1 of 1,716 55 of 1,316 19 of 1,254 139 of 2,313 104 of 2,279 54 of 2,447 22 of 1,934

June 9 Percentile 99.9th 95.8th 98.5th 94.0th 93.9th 97.8th 98.9th

Mean June Daily Max 8-hr O3 

(ppb) 48.4 49.7 49.3 51.2 49.8 49.8 45.6

Max June Daily Max 8-hr O3 

(ppb) 77 124 107 141 125 126 106

Standard Deviation of June Daily 

Max 8-hr O3 (ppb) 8.8 12.6 10.1 15.2 12.8 11.9 10.7

 

9 Adapted from “Technical Support Documentation Ozone NAAQS Exceedances Occurring June 8 and 9, 2015

Uinta Basin of Utah”. Prepared by: Ute Indian Tribe of the Uinta and Ouray Reservation, U. S. EPA Region 8, Utah State University Bingham Energy Center, and the 

Utah Division of Air Quality; August 30, 2016.
14

3.4 Analyses to Establish a Clear Causal Relationship

The second element in establishing a clear causal relationship between the event and the 

monitored ozone exceedance is to develop any analyses needed to describe how ozone was 

transported from the stratosphere to the monitor in sufficient quantities to cause the exceedance.

Again, air agencies can describe the mechanics of the stratospheric impact in a variety of ways. 

Based on what is known regarding stratospheric intrusions, it is recommended that a 

demonstration establish the linkage between the intrusion event and the ozone exceedances in 

four parts:

• provide a concise overview of the surface ozone and meteorological patterns associated with 

the event;

• describe which specific meteorological processes resulted in the displacement of

stratospheric air into free-troposphere;

• further describe which specific meteorological processes enabled the stratospheric material to 

reach the surface (Section 3.3.2.3); and 

• demonstrate the simultaneous arrival of the stratospheric air with impacts on surface ozone 

concentrations.

3.4.1 Event overview

A brief overview of the measured ozone data and the synoptic meteorological pattern that 

governed the suspected event should be provided near the beginning of a clear causal

demonstration. Figure 3a provides an example of a possible graphic that could describe the 

observed air quality during an event day. Summaries of ozone data (graphical or tabular) on the 

days immediately preceding and following the event would also be appropriate. For the case10

depicted in Figure 3a, ozone exceedances were observed over high-elevation portions and 

generally rural portions of Wyoming, Colorado, and New Mexico. In total, nine sites exceeded 

the 2015 ozone NAAQS of 70 ppb on this day. The highest recorded value was 82 ppb at the 

Gothic site in Colorado at an elevation of 2926 m above mean sea level. These exceedances were 

generally surrounded by lower ozone concentrations in the 50-65 ppb range over the rest of the 

intermountain western U.S. The high ozone episode was relatively short-lived as there was only 

one exceedance on the preceding day and no exceedances on the following day in this region.

Figure 3b shows an annual time series of daily peak 8-hour ozone at the Gothic site which also 

depicts the drop off on subsequent days.

 

10 For consistency purposes, all the plots in Section 3.4.1 and 3.4.2 of the guidance focus on a particular case (i.e., 

Saturday, April 22, 2017, over the Four Corners region of the U.S). While this case is valuable for describing which 

analyses will be most useful in establishing a clear causal relationship between a stratospheric intrusion event and 

high observed ozone concentrations, in this guidance the EPA is making no judgement on whether there is a clear 

causal relationship between the stratospheric ozone intrusion and any monitored exceedance or violation nor 

whether these specific case data were impacted by an exceptional event. This episode was chosen because many 

analyses were readily-available with this relatively recent event. The EPA will develop and maintain a link of 

potential resources on the EPA exceptional events web page to help air agency staff develop demonstrations.
15

Figure 3a. Map of Peak Daily 8-Hour Ozone on April 22, 2017 in the Four Corners region11

Figure 3b. Time Series of Peak Daily 8-Hour Ozone in 2017 (through October) at Gothic,

Colorado12

 

11 This sample map was developed via the Navigator tool on the AirNowTech website: https://www.airnowtech.org/. 

This site has ozone data archived for periods dating back to the mid-1990’s.

12 Plot was generated at the EPA website: https://www.epa.gov/outdoor-air-quality-data/air-data-concentration-plot. 

This website has a long archive of ozone data, back into the 1980’s if the monitoring site has been operational that 

long. 
16

After providing a description of the observed ozone data, the event overview should briefly 

describe the key meteorological features that led to the displacement of stratospheric air into the 

FT. Each intrusion event will be unique, but the following graphics and associated descriptive 

text would be useful in establishing the basic meteorological context of the event:

• Maps of surface pressure and fronts at a 12-hr frequency (or finer) for the period 

encompassing the event (i.e., from initiation of the suspected intrusion through the hours in 

which event-influenced ozone was observed at the surface). In many cases, the patterns 

associated with a tropopause fold will include a surface cold front passing through the area 

with cooler dry air advection after the frontal passage. Figure 4a provides an example plot 

and depicts a case where high pressure had advected into the Four Corners region behind a 

cold front that moved from north to south through the region on the day before the 

exceedance. Surface relative humidity values ranged generally between 20-30% on the 

exceedance day, indicating very dry air had moved into the region. A surface temperature of 

53 degrees F and a dewpoint temperature of 10 degrees F, as is shown in southwestern 

Colorado in Figure 4a, indicates a relative humidity of 17.5%. Regional relative humidity 

levels on the afternoon of the 22nd are depicted in Figure 4aa.

• Maps of upper air meteorological conditions at a 12-hr frequency for the period 

encompassing the event at three different pressure levels: 700 hPa, 500 hPa, and 300 hPa. 

There are several suitable formats for these types of plots, but in many cases the primary 

objective would be to show that a substantial trough of low pressure, with associated

features, such as a jet streak, cold front, or well-developed cyclone was sufficiently close to 

the site in question to promote a mechanism of stratosphere-troposphere exchange. Figure 4b

provides an example plot and shows streamlines at 300 hPa which indicate a neutrally-tilted, 

but relatively broad trough exists just to the east of the Four Corners region. Higher jet 

stream winds are measured at the base of the trough. This pattern is favorable for the 

development of a fold in the tropopause to the west of the trough (i.e., over western 

Wyoming and western Colorado).

Wherever possible, these figures should be supported by text that describes the meteorological 

context and emphasizes the difference between this particular pattern and the weather patterns 

that are associated with non-event ozone exceedances, per the conceptual model.
17

Figure 4a. Map of surface pressures and frontal locations at 1800Z for sample case day13

Figure 4aa. Map of relative humidity values in western Colorado region at noon local time 

for sample case day14

 

13 There are numerous sources of surface synoptic meteorological analyses via the internet. This particular plot was 

accessed from: http://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive.php. At the time this document 

was written, this site also has an archive of maps dating from the present day to 2005.

14 There are numerous sources of surface relative humidity data via the internet. This particular plot was accessed 

from the AirNowTech Navigator tool. http://www.airnowtech.org. 
18

Figure 4b. Map of 300 hPa meteorological observations and isotachs for sample case day15

 

15 There are numerous sources of upper air synoptic meteorological analyses via the internet. This particular plot 

was accessed from: http://www.spc.noaa.gov/obswx/maps/. This site also has an archive of maps dating from the 

present day to 1998.
19

3.4.2 Analyses showing stratospheric-tropospheric exchange

Once a broad overview of the meteorological pattern associated with the event is established, the 

demonstration can begin to describe the three stages of the intrusion. 

• Water vapor imagery. As described in Section 1.3, one of the defining features of the 

stratosphere is the relative lack of water vapor. Therefore, a stratospheric intrusion will result 

in deep layers of the FT being drier than usual. Satellite instruments can detect the amount of 

total column water vapor above the earth’s surface. In many stratospheric ozone events, 

satellite images will show a large expanse of dry air on the back side of the low-pressure 

trough. This can be broadly symptomatic of a stratospheric intrusion. Figure 5a depicts a 

scenario in which the aforementioned trough of low pressure has moved east of the Four 

Corners region. However, the dynamics associated with this system have resulted in a threedimensional expanse of dry air (as exhibited by darker colors) from western Montana 

through southern Colorado and into Oklahoma. Note even drier air is located further to the 

southwest (marked by orange colors). This is likely unrelated to the stratospheric intrusion

and is instead due to tropospheric processes.

• Satellite detection of total column ozone data. During a stratospheric ozone intrusion event, 

the total column of air above the surface will be comprised of a larger-than-normal fraction 

of stratospheric air relative to tropospheric air. As a result, satellite instruments which detect 

total column ozone amounts will often exhibit higher-than-average quantities in association 

with an intrusion. In particular, a fold can be identified where gradients in total column ozone 

are large (Olsen et al., 2000; Ott et al., 2016; Knowland et al., 2017). Figure 5b shows the 

total column ozone data from the Ozone Monitoring Instrument (OMI) on April 21, 2017.

The plot shows a fetch of higher total column ozone (425-450 Dobson units) stretching from 

north to south into the Four Corners region. It is important to establish that these total column 

ozone values are higher than the climatological normal in this region and, therefore suggest 

that an ozone intrusion has at least made its way from the stratosphere to a portion of the FT.

• Meteorological analyses from a prognostic meteorological or air quality model. Numerous 

prognostic meteorological model simulations are conducted over the U.S. every day for 

weather or air quality forecasting purposes. These three-dimensional replications of the 

atmosphere often contain useful information about the physical state of the air column above 

the event site that may not have been observed by ambient instruments. Generally, it is better 

to use model-estimated fields from the initialization state or from a time step near the model 

initialization (e.g., < 24 hours) to minimize potential for model artifacts. There are a variety 

of possible products that can help demonstrate that the first stage of an intrusion (stratosphere 

to FT) occurred. Potentially valuable parameters include: isentropic potential vorticity (e.g., 

Figure 5c) or potential temperature, relative humidity, tropopause heights or pressures, and/or 

column estimates of specific stratospheric tracers (CO, RH, nitrous oxide (N2O), etc.). Figure 

5d shows the model-estimated CO column concentrations in the RAQMS model on April 22, 

2017. In this example, note the very low concentrations of this stratospheric tracer in the 

columns above Wyoming, Colorado, and into northern New Mexico associated with the back 

side of the 300 hPa trough. Again, this is complementary evidence that dry, ozone-laden but 

low CO air has been transported through the tropopause into (at least) parts of the FT.
20

Figure 5a. Water vapor imagery from the GOES-West satellite from sample case day16

Figure 5b. Map of satellite-estimated total column ozone data from the day before the case 

event17

 

16 There are numerous sources of water vapor satellite imagery available via the internet, though archived images 

can be harder to find. Figure 5a was accessed from: ftp://ftp.nnvl.noaa.gov/GOES/color_WV/. Real-time images can 

be accessed from: http://www.goes.noaa.gov/goes-w/goes-weus-wv.html.

17 Again, there are numerous potential sources for total column data or products. This particular sample was 

accessed from: http://www.temis.nl/protocols/O3total.html which has an archive going back to 2004. 
21

Figure 5c. Plan view (top) and vertical cross-section (bottom) of isentropic potential 

vorticity (10-6 m-2s

-1 K kg-1

) based on reanalysis data for 1200 UTC on April 22, 2017.

18

Horizontal line (red) shows the location of the W-E cross-section. Values of 1 or less are 

representative of tropospheric air. Higher values are suggestive of stratospheric influence.

 

18 Plots courtesy of Scott Landes, Colorado Department of Public Health and Environment. Plots were derived from 

the 0.5-degree Global Forecast System (GFS) model reanalysis fields using the Integrated Data Viewer available at: 

https://www.unidata.ucar.edu/software/idv/. 
22

Figure 5d. Model-estimated tropospheric CO column from RAQMS on the case day19

3.4.3 Analyses showing stratospheric air reached the surface

Once it has been established that an intrusion has occurred, the demonstration should show that 

the stratospheric air was able to penetrate to the lowest levels of the atmosphere (i.e., into the 

PBL) making surface impacts possible. Many stratospheric intrusions influence the FT but are 

prevented from reaching the surface due to, for example, stable conditions promoted by 

subsidence inversions or nocturnal boundary layers. Establishing a surface impact typically 

requires some three-dimensional perspective of the meteorological or chemical state of the 

atmosphere.

There are a variety of ways in which the vertical composition of the atmosphere can be assessed.

The specific analyses best-suited to each individual demonstration will vary depending upon the 

intrusion event itself and what products are available at a given location.

• A good starting point is to analyze the vertical profiles of temperature and dew point 

temperatures collected by the twice-daily rawinsonde network. Figures 6a and 6b are “skewT, log-P” diagrams for two times during the April 22, 2017, event at Grand Junction, CO.

The figure depicts a large reservoir of dry air above this location at a height of about 5 km 

above mean sea level, or approximately 3.5 km above the surface at this location. At 0000Z, 

about 24 hours before the exceedances occurred, there appeared to be a temperature inversion 

 

19 There are multiple air quality models that can be used to inform exceptional event demonstrations. The EPA 

maintains a resource page on its exceptional events webpage to help point air agencies to available products. This 

particular plot was retrieved from the Real-time Air Quality Modeling System (RAQMS) archive at:

http://raqms-ops.ssec.wisc.edu/previous_products/. Data products exist back to 2010.
23

that may have been separating this dry air from lower parts of the troposphere. The same plot 

from 12 hours later (1200Z) shows that the dry air layer has extended even lower into

troposphere by another 500m, such that the dry air is within 3 km of the surface. The 

temperature lapse rate within the PBL was approximately dry-adiabatic during the afternoon 

plot (0000Z) which signals that the lower part of the atmosphere was well-mixed during this 

episode. While these plots are rarely conclusive by themselves, they can provide a “first 

look” as to the vertical extent of the intrusion. Where available, ceilometers can also help 

estimate the depth of the surface mixing.

Figures 6a and 6b. Skew-T diagrams for 1200Z (left) April 22, 2017 and 0000Z (right)

April 23, 2017, at Grand Junction, CO20

• Actual vertical measurements of ozone are the best way to determine whether ozone that 

originated in the stratosphere has impacted the surface. Unfortunately, the networks that

provide these data are relatively sparse and are not often available for the time and location 

of a suspected stratospheric event. There are multiple possible observational platforms that 

can be valuable, including: ozonesondes, LIDARs, towers, and instrument-equipped aircraft.

o Ozonesondes are released into the atmosphere on an infrequent but routine basis at 

certain locations across the U.S. Figure 6c shows data collected from an ozonesonde 

launch from Huntsville, AL on March 11, 2017 and is a good example of an elevated 

ozone layer that does not impact the surface. The plot shows a layer of ozone between 

1-2 km of approximately 60-65 ppb (after unit conversion) with a sharp drop off in 

ozone closer to the surface. Peak 8-hour ozone in Huntsville on this day was 35 ppb, 

consistent with the ozonesonde data.

o LIDARs provide highly-resolved ozone data through the lowest layers of the 

atmosphere (along with other relevant data). Figure 6d shows an example of irregular 

LIDAR data over a 45-day period near Las Vegas, NV. The period is marked by 

differing patterns of ozone with some days indicative of transport of ozone from the 

FT into the PBL and others where the ozone formed within the PBL appears to be 

 

20 Plots were generated at: http://weather.uwyo.edu/upperair/sounding.html. There are other sources of this 

information on the internet. Most have archives going back to 1948.
24

separated from free tropospheric influence (Langford et al., 2017). June 2nd provides a 

good case study as the LIDAR data suggests that an existing layer of higher ozone at 

6-8 km above sea level descends to approximately 4 km above sea level (asl) where it 

then appears to be mixed down into the PBL and mixes with ozone formed at the 

surface on both June 2 and 3. A similar situation occurs on May 21.

o High-elevation towers equipped with ozone instruments at multiple heights and 

instrument-equipped aircraft traversing the PBL and FT can also inform a threedimensional perspective of ozone (Yates et al., 2013), but these data are relatively 

rare.

Figure 6c. Ozonesonde data from sample launch from Huntsville, AL21

 

21 This plot accessed from: https://www.esrl.noaa.gov/gmd/ozwv/ozsondes/. 
25

Figure 6d. Time-height ozone cross-sections from May and June 2017 LIDAR

measurements near Las Vegas, NV22

• As in Section 3.4.2, there can also be considerable value in accessing outputs from any 

available prognostic meteorological or air quality modeling to help demonstrate vertical 

transport of ozone. For this stage of the determination, the focus should be on how 

stratospheric ozone, or tracers of stratospheric air, are transported from the FT to the PBL.

Latitudinal or longitudinal cross-sections of model fields can be informative, especially 

showing how these fields evolve with time. The same meteorological variables discussed 

earlier can be used to show material may have been exchanged into the PBL (e.g., IPV, 

potential temperature, water vapor). Increasingly, prognostic air quality model simulations 

are now archived and available for retrospective analyses of potential exceptional events. Not 

only can these models provide temporal cross-sections of stratospheric proxies like areas of 

abnormally low CO concentrations, they can also provide estimates of ozone itself. Any 

demonstrations that use modeled representations of air quality to show stratospheric transport 

into the PBL should provide some evidence that the model is well fit for making that 

determination: those models that are systematically evaluated daily (and demonstrate 

relatively low levels of bias and error) are preferable, as are those that assimilate actual air 

quality data into the simulations. Figure 6e shows a cross-sectional representation of the 

RAQMS modeled ozone at a latitude of 40 degrees N. The model simulation for 0000Z on 

April 23 indicates that 12 hours into the simulation, a lobe of higher ozone has become 

detached from the stratosphere into the FT, with apparent further mixing down to the surface 

 

22 Taken from an October 17, 2017 presentation by Andrew O. Langford to the Stratospheric Intrusion Work Group 

titled “A brief overview of FAST-LVOS: Fires, Asian, and Stratospheric Transport – Las Vegas Ozone Study”.
26

at specific longitudes (e.g., high elevation at 105 degrees W and 112 degrees W). (Note: for 

this event, a cross-sectional analysis slightly further south and closer to the surface 

exceedances would have been more informative.)

• Another potential tool available for demonstrating downward exchange of air masses and 

source-receptor relationships are trajectory models like HYSPLIT.

23 These models use 

archived meteorological model initialization data fields to determine how air parcels moved 

horizontally or vertically to (or from) a given location. Backward trajectories, like the one 

shown in Figure 6f can help demonstrate possible stratospheric influence into the PBL when 

they show descending parcels of air originating in the FT but eventually lowering to heights 

near the surface. Figure 6f suggests that 48 hours prior to the ozone exceedances near 

Durango, CO on April 22, 2017, the air mass which eventually settled over Durango was

over southern ID at a height of approximately 2.5 km above the ground layer. This air mass 

descended quickly on the 21st into the PBL before being transported into the Four Corners 

region on the 22nd. There are several important choices involved in configuring a meaningful 

HYSPLIT analyses: choice of meteorological model (generally finer-resolution models are 

better), what surface height to choose (generally best to investigate back trajectories to 100-

500 m), what vertical method to use (all three options are worth investigating), and how 

many hours to simulate (uncertainty increases in analyses longer than 48-72 hours). When 

using trajectory models to demonstrate potential transport of stratospheric air into the PBL, it 

is best if multiple model configurations can be tested. Any configurations that are evaluated 

should be discussed in the demonstration text. Conclusions that are not strongly dependent on 

model configuration are given greater weight.

 

23 There are other trajectory models which can also be used, such as FLEXPART. Also, the IDEA tool is available 

which computes forward trajectories for locations with high satellite observed ozone.
27

Figure 6e. Model cross-section of RAQMS-estimated ozone at 0000Z on April 23, 201724

Figure 6f. 48-hour back trajectory from Durango, CO on 1800Z April 22, 201725

 

24 Again, there are multiple air quality models that can be used to inform exceptional events demonstrations. This 

particular plot was retrieved from the Real-time Air Quality Modeling System (RAQMS) archive at: http://raqmsops.ssec.wisc.edu/. 

25 This trajectory map was generated at: https://ready.arl.noaa.gov/HYSPLIT_traj.php.
28

3.4.4 Air quality analyses showing the impacts of the intrusion at the surface

Finally, when it has been established that there was an intrusion that over time was able to 

transport ozone-laden air from the stratosphere into the PBL, the demonstration should then 

show that the resultant impacts on ozone concentrations measured at the surface caused the 

exceedance. In some cases, there will be evidence of stratospheric contributions to surface 

exceedances in conjunction with significant coincident impacts from non-stratospheric sources.

These events will be the most challenging to verify and the demonstration should clearly 

describe what differentiates this exceedance from others with similar meteorological, seasonal, 

or emissions patterns. Ideally, this section of the analysis will be where all the individual 

elements of the demonstration will be tied together to produce a compelling narrative of a 

stratospheric ozone exceptional event that falls outside the usual scope of ozone exceedances in 

the area.

As with the other stages of the analysis, there are a variety of ways in which the causality of the 

stratospheric intrusion can be gauged. Again, the specific analyses best-suited to each individual 

demonstration will vary depending upon the intrusion event itself and what products (e.g., data, 

graphics) are available at a given location.

• Evidence that the ozone increases were coincidental with ground-based increases in 

stratospheric tracers such as, low water vapor, low CO, and/or high concentrations of certain 

isotopes is the most direct way of showing stratospheric impacts at the surface. Most ozone 

monitors should have co-located meteorological measurements. Additionally, a few rural 

high-altitude monitoring sites have both ozone and CO monitors.26 However, while surface 

CO measurements may be available, the typical CO monitors used for ambient air monitoring 

have operational ranges of 500 - 50,000 ppb (0.5 - 50 ppm) and are often not sufficiently 

sensitive to reliably measure the very low CO levels found in stratospheric air (50 - 150 ppb). 

The EPA urges air agencies to provide concurrent readings of ozone and CO and/or relative 

humidity in their exceptional events demonstrations if they have these data. As discussed in 

Section 1.3, there are two potential beryllium tracers of stratospheric air, specifically: 

beryllium isotopes Be-7and Be-10. These elements are produced primarily in the stratosphere 

by cosmic ray collisions with atmospheric gas atoms and can confirm the presence of 

stratospheric ozone in surface air (Cristofanelli, 2006). These measurements are, however, 

rare, expensive and, consequently, not normally available. Where available, vertical profiles 

of these tracer species measurements may more clearly indicate the presence of stratospheric 

air at the earth’s surface than some of the analyses previously discussed. If any of these 

data/analyses are readily available, the EPA encourages their inclusion in a demonstration.

• Time series of ozone data can be strong indicators of causal ozone impacts when the rates of 

hourly ozone increases are synchronized with meteorological evidence from Section 3.4.3 of 

a stratospheric intrusion into the PBL. This type of analysis can take many forms, but usually 

starts with a time series plot of hourly ozone across the network as per the example in Figure 

7a. Based on the analyses discussed above, there is evidence that a tropopause fold occurred 

 

26 A recent review of AQS data revealed 216 sites in the United States with collocated ozone and CO monitors in 

operation after January 1, 2014. Most of these sites are located in either urban or suburban locations. In these 

settings, local emissions would likely hide the stratospheric CO suppression.
29

in association with a mid-latitudinal trough that traversed the western U.S. from April 20 

through April 22 and was likely able to penetrate the lowest layers of the atmosphere over 

Utah, Wyoming, Colorado, and New Mexico late in the day on the 21st, or perhaps early on 

the morning of the 22nd. This narrative is supported by the ozone time series at the highelevation Gothic, Colorado site which shows an increase in ozone from 60 ppb around 0600 

local time to about 80 ppb by 1200 local time. This early morning rate of rise is not 

symptomatic of local photochemical ozone production and precedes similar ozone increases 

at nearby lower-elevation sites by a few hours. When this time series information is coupled 

with surface meteorological and/or air quality information that suggests ozone is rising 

despite a post-frontal transition to a less-photochemically conducive airmass (e.g., cool 

temperatures, gusty winds, low humidity, low concentrations of CO, PM, or NOX, etc.), this 

can be compelling evidence of a causal relationship between the intrusion and the 

exceedance.

Figure 7a. Time series of ozone at sites in the Four Corners region on April 21-23, 201727

• Comparisons between ozone concentrations on meteorologically similar days with and 

without stratospheric intrusion impacts could support a clear causal relationship between the 

 

27 This plot was generated internally within the EPA based on data from the 

https://www.airnowtech.org/data/index.cfm, although in most cases states will have direct access to the data.
30

subject and the monitored ozone concentration. Ozone formation and transport are highly 

dependent upon meteorology, therefore a comparison between ozone on meteorologically 

similar days with and without stratospheric intrusion impacts could provide additional 

support for event causality. Both ozone concentrations and diurnal behaviors on days with

similar meteorological conditions can be useful to compare with days believed to have been 

impacted by the intrusion. Since similar meteorological days are likely to have similar ozone 

concentrations, significant differences in ozone concentrations among days with similar 

meteorology may indicate influences from non-typical sources. Meteorological variables to 

include in a “matching day” analysis should be based on the parameters that are known to 

strongly affect ozone concentrations in the vicinity of the monitor location (i.e., from the 

conceptual model). These variables could include: daily high temperature, hourly 

temperature, surface wind speed and direction, upper air temperature [such as at the 850 or 

500 hPa level], relative or absolute humidity, atmospheric stability, cloud cover, solar 

irradiance, and/or others as appropriate (Eder et al., 1993; Eder et al., 1994; Camalier et al., 

2007). Air agencies should match these parameters within an appropriate tolerance. Since 

high ozone days may be relatively rare, air agencies should examine several years of data for 

similar meteorology versus restricting the analysis to high ozone days only. The complete 

range of normal expected ozone on similar meteorology days will have value in the 

demonstration. A similar day analysis of this type, when combined with a comparison of the 

qualitative description of the synoptic scale weather pattern (e.g., cold front location, high 

pressure system location), can help show that the intrusion potentially caused the elevated 

ozone concentrations. Air agencies may also want to consider non-meteorological factors 

such as choosing days with similar, non-event emissions (possibly avoiding holidays and 

special public events, weekend versus weekday mismatches, and any other days with unusual 

emissions).

• Prognostic or retrospective air quality models may be used to provide evidence that 

stratospheric material reached the surface, but these analyses must be accompanied by robust 

model performance evaluations that support their use for this purpose and the horizontal and 

vertical grid resolution of the model should be appropriate for capturing an event. There are 

many different potential uses of modeling to demonstrate stratospheric influence at the 

surface from linking model estimates to meteorological features, to more sophisticated 

approaches like tracer modeling or source apportionment. Figure 7b shows the lowest layer 

ozone outputs from the RAQMS model around noon local time on April 22. In the context of 

other material presented earlier, the coincident nature of the high model ozone plume from 

Wyoming to Colorado and into New Mexico with meteorological conditions suggestive of a 

descending streamer of stratospheric air provides additional evidence of a clear causal 

relationship.
31

Figure 7b. RAQMS-estimated ozone near the surface at 1800Z on April 22, 201728

3.5 Differing Levels of Analyses within Tier 1 and Tier 2 Demonstrations

More complex relationships between the subject stratospheric intrusion and the influenced ozone 

concentrations will typically require additional detail to satisfy the clear causal relationship 

element (i.e., a Tier 2 demonstration). This additional evidence can either show the relative 

contribution estimates to the exceedance from local and transported anthropogenic pollutants 

compared to the intrusion contribution (i.e., quantification and apportionment) or show that 

meteorological conditions were not conducive to local photochemical production of ozone and 

that the demonstrated intrusion best explains the elevated ozone concentration(s). The EPA 

anticipates that Tier 2 demonstrations would build upon the analyses prepared for Tier 1 

demonstrations with the potential approaches described in this section. The EPA does not expect 

an air agency to prepare all identified analyses, but only those that contribute to understanding 

the relationship between the event and the measured exceedance. As with all intended 

exceptional events demonstrations, the submitting air agency and the EPA Regional Office 

should discuss the appropriate level of evidence during the Initial Notification process.

 

28 Again, there are multiple air quality models that can be used to inform exceptional events demonstrations. This 

particular plot was retrieved from the Real-time Air Quality Modeling System (RAQMS) archive at: http://raqmsops.ssec.wisc.edu/.
32

There is no rigid set of rules as to which specific analytical elements will be needed to 

adequately demonstrate an exceptional stratospheric ozone event, as each case is unique. Table 3 

provides a checklist of possible analyses that could support the demonstration of a stratospheric 

event. Other assessments not specifically mentioned in this guidance can also be shown to be 

valuable. The final rubric for an approvable demonstration is one that builds a consistent 

analytical narrative that shows stratospheric air entered the FT, was advected down to the 

surface, and subsequently caused an ozone exceedance at the surface.

3.6 Example Conclusion Statement for the Clear Causal Relationship Criterion

With the aim of ‘right-sizing’ exceptional events demonstrations, the EPA encourages air 

agencies to provide a case-appropriate combination of the kinds of evidence and analyses 

identified in Sections 2 and 3 of this guidance and construct a descriptive narrative that supports 

the existence of a clear causal relationship between the stratospheric intrusion event and the 

monitored ozone exceedance. This portion of the demonstration should conclude with a 

statement similar to the language below:

“Based on the evidence, including comparisons and analyses, provided in [reference the 

clear causal section] of this demonstration, [Air Agency Name] has established that a 

clear causal relationship exists between the stratospheric intrusion event(s), which 

occurred on [dates] in [location], and the monitored ozone exceedance on [dates/time of 

data requested for exclusion or reference to summary table in demonstration]. The clear 

causal relationship evidence also demonstrates that the event affected air quality at the 

monitor.” 
33

Table 3. Potential demonstrative analyses for stratospheric ozone clear causal relationship

Type of Analysis Tier 1 Tier 2 

Conceptual Model What conditions generally lead to 

high ozone in the area? Same as Tier 1

Historical Comparisons

5 years (or more) of peak daily 

ozone data with other high event 

days flagged.

Table with percentile ranks of 

days

Same as Tier 1, plus:

Historical diurnal profile 

comparison

Event Overview

Spatial and temporal depictions of 

ozone during the event.

Description of surface and upper 

air meteorological conditions 

during the event.

Same as Tier 1, plus:

Begin to establish the complex 

relationship between the intrusion 

and eventual impact at surface.

Establish stratospheric 

intrusion

(1 of following is likely

sufficient)

• Water vapor imagery

• Total column ozone

• Simple met model evidence

(several of following are likely 

needed)

• Water vapor imagery

• Total column ozone

• Rigorous met model evidence

Establish stratospheric air 

reached surface

(1-2 of following is likely 

sufficient)

• Rawinsonde data

• Met model cross-sections

• Online AQ model crosssections

• Trajectory models

(several likely needed)

• Rawinsonde data (multiple 

sites)

• LIDAR, tower, aircraft?

• Detailed met model crosssections (multiple variables)

• Online AQ model crosssections

• Trajectory models (multiple)

Impacts at the surface

(1 of following is likely 

sufficient)

• Coincidence between high 

ozone and meteorological/AQ 

conditions characteristic of 

stratospheric intrusions

• Summary narrative

(several likely needed)

• Coincidence between high 

ozone and meteorological/AQ 

conditions characteristic of 

stratospheric intrusions

• Matching day analyses

• Model evidence of impacts

• Summary narrative
34

4. Other Required Demonstration Elements of the Exceptional Event Rule 

4.1. Caused by Human Activity that is Unlikely to Recur at a Particular 

Location or a Natural Event

According to the CAA and the Exceptional Events Rule, an exceptional event must be “an event 

caused by human activity that is unlikely to recur at a particular location or a natural event.” 42 

U.S.C. 7619(b)(1)(A)(iii), 40 CFR 50.1(j) & 50.14(c)(3)(iv)(E) (emphasis added). As noted in 

the preamble to the Exceptional Events Rule, “EPA generally considers wildfires,

stratospheric ozone intrusions, [...] to be natural events.”

29 And, as defined in the Exceptional 

Events Rule, a natural event means “an event and its resulting emissions, which may recur, in 

which human activity plays little or no direct causal role.” 40 CFR 50.1(k) (emphasis added).

Thus, treating (recurring) stratospheric intrusions as natural events is consistent with the CAA 

and the Exceptional Events Rule, and minimal documentation is needed to meet this element. Air 

agencies should address the “human activity that is unlikely to recur at a particular location or a 

natural event” element with a statement similar to the following: 

“The Exceptional Events Rule states that a ‘[n]atural event, which may recur, is one in 

which human activity plays little or no direct causal role.’ Therefore, stratospheric 

intrusions that cause monitored ambient ozone exceedances or violations are considered 

to be natural exceptional events. [Air Agency Name] has shown through the analyses 

provided in [reference the clear causal section] of this demonstration that the subject 

stratospheric intrusion caused each of the identified exceedances. Through these analyses 

and the fact that stratospheric intrusions are purely natural, the [Air Agency Name] has 

satisfied the ‘human activity that is unlikely to recur at a particular location or a natural 

event’ element of 40 CFR 50.14(c)(3).”

4.2 Not Reasonably Controllable or Preventable

According to the CAA and the Exceptional Events Rule, an exceptional event must be “not 

reasonably controllable or preventable.” 42 U.S.C. 7619(b)(1)(A)(ii), 40 CFR 50.1(j) & 

50.14(c)(3)(iv)(D). The preamble to the Exceptional Events Rule clarifies that the EPA interprets 

this requirement to contain two factors: the event must be both not reasonably controllable and

not reasonably preventable at the time the event occurred. 81 FR at 86235-6. This requirement 

applies to both natural events and events caused by human activities; however, it is 

presumptively assumed that stratospheric intrusions are natural events of a character that cannot 

be prevented or controlled. Thus, such events satisfy both factors of the “not reasonably 

controllable or preventable” element. 40 CFR 50.14(b)(6). Air agencies should address the “not 

reasonably controllable or preventable” element with a statement similar to the following: 

“The documentation provided in [reference the clear causal section] of this 

demonstration shows that the subject stratospheric intrusion caused each of the identified 

exceedances. Through these analyses and the fact that stratospheric intrusions are purely 

natural events that cannot be prevented or controlled, [Air Agency Name] has satisfied the 

‘not reasonably controllable or preventable’ criterion.”

 

29 “Treatment of Data Influenced by Exceptional Events; Final Rule” (81 FR 68216, October 3, 2016).
35

4.3. Public Comment Process

In addition to providing a conceptual model and evidence to support the Exceptional Events Rule 

elements, air agencies “must document [in their exceptional events demonstration] that the 

public comment process was followed” according to 40 CFR §50.14(c)(3)(v). Air agencies 

should include in their exceptional events demonstration the details of the public comment 

process including newspaper listings, website postings, and/or places (library, agency office) 

where a hardcopy was available. The agency should also include in the demonstration any 

comments received and the agency’s responses to those public comments. 
36

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39

United States

Environmental Protection

Agency

Office of Air Quality Planning and Standards

Air Quality Policy Division/Air Quality Assessment Division

Research Triangle Park, NC

Publication No. EPA-457/B-18-001

November 2018