Guidance on the Preparation of Exceptional
Events Demonstrations for Stratospheric Ozone
Intrusions
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\ UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
V RESEARCH TRIANGLE PARK, NC 27711
NOV - 8 2011
OFFICE Of
AIR QUALITY PLANNING
AND STANDARDS
MEMORANDUM
SUBJECT: Guidance on the Preparation of Exceptional Events Demonstrations for
Stratospheric Ozone Intrusions
FROM: Richard Wayland, Director
Air Quality Assessment Division
Anna Marie Wood, Director /
Air Quality Policy Division ^
TO: Regional Air Division Directors. Regions 1-10
The purpose of this memorandum is to distribute a non-binding guidance document titled.
"Guidance on the Preparation of Exceptional Events Demonstrations for Stratospheric Ozone
Intrusions."
The EPA Headquarters and EPA Regional offices collaborated in the development of this
guidance to assist air agencies with preparing exceptional events demonstrations for stratospheric
ozone intrusions that meet the requirements of Clean Air Act section 319(b) and the Exceptional
Events Rule signed on September 16, 2016. and posted on EPA's website at:
Please share this memorandum with appropriate contacts at state, local and tribal air
agencies. If you have questions concerning this document, please contact Ben Gibson at (919) 541 -
3277 or >»;,/>, >//,//»»/ <(* , .><,/ for further information.
Attachment
Internet Address (URL) • http://www.ftpa.90v
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EPA-457/B-18-001
November 2018
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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
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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
iv
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AQS
CAA
CFR
CO
DV
ENSO
EPA
FLEXPART
FR
FT
GOES
hPa
HYSPLIT
IDEA
IPV
K
km
LIDAR
NAAQS
NASA
NO A A
NO
NOx
NWS
OMI
PBL
PM
ppb
PT
RAQMS
RH
VOC
Z
Acronyms
Air Quality System
Clean Air Act
Code of Federal Regulations
Carbon monoxide, or Colorado
Design value
El Nino / Southern Oscillation
Environmental Protection Agency
FLEXible PARTicle dispersion model
Federal Register
Free troposphere
Geostationary Operational Environmental Satellite
Hectopascals
HYbrid Single-Particle Lagrangian Integrated Trajectory
Infusing Satellite Data into Environmental Applications
Isentropic potential vorticity
Kelvin
Kilometer
Light detection and ranging
National ambient air quality standard or standards
National Aeronautics and Space Administration
National Oceanic and Atmospheric Administration
Nitric oxide
Nitrogen oxides
National Weather Service
Ozone monitoring instrument
Planetary boundary layer
Particulate matter
Parts per billion
Potential temperature
Real-time Air Quality Modeling System
Relative humidity
Volatile organic compound or compounds
Zulu (coordinated universal time; same as Greenwich Mean Time)
v
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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 Rule2 to 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.sov/air-aualitv-analvsis/treatment-data-influenced-exceptional-events.
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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 event-
influenced 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.
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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 tropopause-
folding 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 Nino-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 stratospheric-
tropospheric 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.
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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 case-
by-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
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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.
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Figure 1: Flowchart of the EPA's recommended process* for preparing, submitting, and
reviewing exceptional events demonstrations for stratospheric ozone intrusion events.
1) Event-influenced exceedance or violation.
3) Air agency submits initial notification of potential exceptional event to its EPA Regional office.
8) Air agency prepares and submits a draft demonstration.
7) The EPA, in collaboration with the air agency, advises whether a Tier 1 or Tier 2 demonstration is
appropriate.
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.
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.
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.
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.
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.
* 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.
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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).
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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:
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• 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 event-
influenced exceedances were outside the bounds of generally expected ozone levels.
Figure 2 shows an example of a potential plot comparing historical concentrations from non-
event 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.
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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 period8 that
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 event-
related 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.
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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
Sample 03 time series at a high elevation site based on 10-year record
(wildfire smoke as brown triangles, stratospheric intrusions as red circles)
0.1 ,
2015 0.070 ppm NAAQS
F f • - V ^
>>•• A*
o
0 31 61 92 122 153 183 214 244 27S 305 336 366
Day of Year
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 stratospheri c i ntrusions 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 summer-
like meteorology in April).
11
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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.
12
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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 03
(ppb)
48.4
49.7
49.3
51.2
49.8
49.8
45.6
Max June Daily Max 8-hr 03
(ppb)
77
124
107
141
125
126
106
Standard Deviation of June Daily
Max 8-hr 03 (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.
13
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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.
14
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Figure 3a. Map of Peak Daily 8-Hour Ozone on April 22, 2017 in the Four Corners region11
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Colorado12
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11 Tliis sample map was developed via the Navigator tool on the AirNowTech website: https://www.aimowtech.org/.
This site lias 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-qualitv-data/air-data-concentration-plot.
This website lias a long archive of ozone data, back into the 1980's if the monitoring site has been operational that
long.
15
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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.
16
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Figure 4a. Map of surface pressures and frontal locations at 1800Z for sample case day13
1800Z SURFACE ANALYSIS
DATE: SAT APR 22 2017
ISSUED: 19302 SAT APR 22 2017
BY WPC ANALYST SANTORELLI
COLLABORATING CENTERS: WPC, NHC, OPC
Figure 4aa. Map of relative humidity values in western Colorado region at noon local time
for sample case day14
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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/arcliives/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.
17
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Figure 4b. Map of 300 hPa meteorological observations and isotachs for sample case day15
170422,"1200 300 MB UA OBS, ISOTACHS, STREAMLINES, DIVERGENCE
National Weather Service
Stcrrn Prediction Center
15 There are numerous sources of upper air synoptic meteorological analyses via the internet. This particular plot
was accessed from: http://wvyw.spc.noaa.gov/obswx/maps/. This site also has an archive of maps dating from the
present day to 1998.
18
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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 three-
dimensional 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.
19
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Figure 5a. Water vapor imagery from the GOES-West satellite from sample case day16
f
Figure 5b. Map of satellite-estimated total column ozone data from the day before the case
event17
OMI total ozone 21-04-2017 KNMI/NASA
Ozone density TDobson Units]
150 175 200 225 250 275 300 325 350 375 400 425 450 475 500
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://fti3.mivl.noaa. gov/GOES/color WW. 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/03total.html which has an archive going back to 2004.
20
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Figure 5c. Plan view (top) and vertical cross-section (bottom) of isentropic potential
vorticity (10~6 in 2 s"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:
littps ://www. unidata. ucar.edu/software/idv/.
21
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Figure 5d. Model-estimated tropospheric CO column from RAQMS on the case day19
iol/crtr-2)
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 "skew-
T, 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://raams-ops.ssec.wisc.edu/previous products/. Data products exist back to 2010.
22
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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
72476 GJTGrand Junction
72476 GJT Grand Junction
200
600
800
900
-10
-40 -30
00Z23 Apr 2017
-20
10 20 30
University of Wyoming
12Z22 Apr 2017
of Wyoming
• 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.iiwvo.edu/upperair/souiiding.html. There are other sources of this
information on the internet. Most have archives going back to 1948.
23
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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 three-
dimensional perspective of ozone (Yates et al., 2013), but these data are relatively
rare.
Figure 6c. Ozonesonde data from sample launch from Huntsville, AL21
Huntsville, Alabama
11 March 2017
Temperature (°C)
^00-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50
35
30
¦g 20
4-J
< 15
10
5
°0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Ozone Partial Pressure (millipascals)
Generated FSfli,CMP - 201? March i* 01 ,irri
Total Ozone - 302 DU
Ozone
Temperature
21 This plot accessed from: https://www.esrl.noaa.gov/gmd/ozwv/ozsondes/.
24
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Figure 6d. Time-height ozone cross-sections from May and June 2017 L1DAR
measurements near Las Vegas, NV22
17 MAY - 30 JUN 2017
100 120 140 160
,1 J
I
OZONE (ppbv
J 8000
i i (
-4000
fc n !•' ML*;
00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00
05/18 05/19 05/20 05/21 05/22 05/23 05/24 05/25 05/26 05/27 05/28 05/29 05/30 05/31 06/01
Tim® , PDT
rai
•s
I
| i
i k U
poi t I
w
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!
4000 (
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.0
00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00
06/02 06/03 06/04 06/05 06/06 06/07 06/08 06/09 06/10 06/11 06/12 06/13 06/14 06/15 06/16
Time , PDT
4000-
2000-
: J i |
ill a V 'J
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il
8000
-6000 -
!
-4000
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00:00 00:00 00-00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00
06/17 06/18 06/19 06/20 06/21 06/22 06/23 06/24 06/25 06/26 06/27 06/28 06/29 06/30 07/01
Time , PDT
• 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, especi ally
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 Intmsion Work Group
titled "A brief overview of FAST-LVOS: Fires, Asian and Stratospheric Transport - Las Vegas Ozone Study".
25
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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.
26
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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
NOAA HYSPLIT MODEL
Backward trajectory ending at 1800 UTC 22 Apr 17
HRRR Meteorological Data
-118
-116
i -iin
AO
2500
2000
1500
1000
500
300 ^
06
04/22
04/21
Job ID: 194175
Source 1 lat.: 37.15 Ion.: -107.75 height: 300 m AGL
/ Direction: Backward Duration: 48 hrs
lotion Calculation Method: Isentropic
Meteorology: 1800Z 22 Apr 2017 - HRRR
Job Start: Sat Jan 20 1826:49 UTC 2018
Trajectory
Vertical Mt
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://raqms-
ops.ssec.wisc.edu/.
25 This trajectory map was generated at: https://readv.arl.noaa.gov/HYSPLIT trai.php.
27
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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"7 and 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 I, 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.
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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 high-
elevation 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 7 a. Time series of ozone at sites in the Four Corners region on April 21-23, 201727
4 Corners 03, April 21-23,2017
100 .
A/23/2017 0:00 4/23/2017 12:00 4/24/2017 0:00 4/24/2017 12:00
Utel Ute 3 Na/ajoLefce Gothic
0
4/20/2017 12:00
4/21/2017 0:00 4/21/2017 12:00 4/22/2017 0:00
MesaVerdeNP Farmngton Substation
4/22/2017 12:00
-B bom field -
• 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.
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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.
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Figure 7b. RAQMS-estimated ozone near the surface at 1800Z on April 22, 201728
"gtunaer BasirlWY i
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W-aniTDU Springs 50
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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 concentrations). 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://raqms-
ops.ssec.wisc.edu/.
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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, [A ir Agency Name] has established that a
clear causal relationship exists between the stratospheric intrusion event(s), which
occurred on [dales] in [location], and the monitored ozone exceedance on [dates/time of
data requestedfor exclusion or reference to summary table in demonstration]. The clear
causal relationship evidence also demonstrates that the event affected air quality at the
monitor."
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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 cross-
sections
• Trajectory models
(several likely needed)
• Rawinsonde data (multiple
sites)
• LIDAR, tower, aircraft?
• Detailed met model cross-
sections (multiple variables)
• Online AQ model cross-
sections
• 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
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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)(l)(A)(iii), 40 CFR 50.l(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 '[njatural 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)(l)(A)(ii), 40 CFR 50.l(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).
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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.
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United States Office of Air Quality Planning and Standards Publication No. EPA-457/B-18-001
Environmental Protection Air Quality Policy Division/Air Quality Assessment Division November 2018
Agency Research Triangle Park, NC
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