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Review of Documentation on Sacramento, CA 2008 Fire Exceptional 

Events for Ozone 

I. Introduction

In September 2009, the California Air Resources Board (ARB) submitted documentation to 

EPA in support of a request that EPA flag as exceptional various 1-hour ozone 

concentrations at multiple sites in the Sacramento Regional Ozone Nonattainment Area. 

ARB and Sacramento Metropolitan Air Quality Management District submitted 

documentation that ozone concentrations were affected by smoke from wildfires 

throughout northern California during June and July of 2008 (see list of documentation 

reviewed at end of this document). The days covered by the documentation reviewed here 

are listed in the table below. This document provides EPA Region 9 comments on 

documents to clarify where additional analysis and/or documentation is needed to for EPA 

to exclude these data. It is not an EPA guidance document, nor a statement of EPA 

concurrence or nonconcurrence on the events. 

Monitor Day Hour O3 (ppb)

Folsom 6/23/2008 13 129

Folsom 6/23/2008 14 151

Folsom 6/23/2008 15 161

Folsom 6/23/2008 16 132

Folsom 6/27/2008 15 129

Folsom 7/7/2008 15 145

Folsom 7/7/2008 16 166

Sloughhouse 7/7/2008 16 137

Sloughhouse 7/7/2008 17 148

Cool 7/7/2008 15 130

Cool 7/7/2008 16 128

Placerville 7/7/2008 18 139

Placerville 7/7/2008 19 136

Placerville 7/7/2008 20 136

Roseville 7/7/2008 14 132

Roseville 7/7/2008 15 134

Folsom 7/10/2008 11 129

Folsom 7/10/2008 12 150

Folsom 7/10/2008 13 141

Cool 7/10/2008 13 129

Placerville 7/10/2008 17 137

Sloughhouse 7/25/2008 15 130
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II. Background

Overview of the Exceptional Event Rule requirements

EPA’s Exceptional Event Rule (“EER”, 72 FR 13569, March 22, 2007) allows for ambient 

monitoring measurements that exceed the National Ambient Air Quality Standards 

(NAAQS) to be excluded from NAAQS attainment/nonattainment decisions, if EPA 

determines they are caused by exceptional events (including natural events). Under the 

EER, a State submits documentation to EPA including a demonstration that the exceedance 

is due to an exceptional event. EPA has not finalized guidance to Regional Offices on 

evaluating the documentation. Determinations on exceptional events are made on a case by 

case basis by the EPA Regional Administrator, and indicated via the appropriate flag in the 

AQS database, generally accompanied by direct notification of the State.

Under 40 CFR 50.14 (c)(3)(iii), the State demonstration to justify data exclusion must 

provide evidence that:

A. The event satisfies the criteria set forth in 40 CFR 50.1(j) for the definition of an 

exceptional event, which are that the event:

affects air quality;

is not reasonably controllable or preventable, and

is an event caused by human activity that is unlikely to recur at a 

particular location or a natural event; 

B. There is a clear causal relationship between the measurement under 

consideration and the event that is claimed to have affected the air quality in the 

area;

C. The event is associated with a measured concentration in excess of normal 

historical fluctuations, including background; and

D. There would have been no exceedance or violation but for the event.

The demonstrations must fully meet all the above criteria to EPA’s satisfaction; failure to 

meet any one of the criteria will result in the non-concurrence of the event in question. The 

definition of an exceptional event provided in 40 CFR 50.1(j) also explicitly excludes 

stagnation of air masses or meteorological inversions, a meteorological event involving 

high temperatures or lack of precipitation, or pollution relating to source noncompliance. 

Exceptional events based on any of these conditions would not be eligible for exclusion 

under the EER. 

According to the EER preamble in the Federal Register (13569), generally the “affects air 

quality” definition criterion will be satisfied by demonstrations of the “clear causal 
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relationship” and the “in excess of normal historical fluctuations” criteria. For wildfires, it 

is obvious that their emissions affect air quality in a general sense, including particulate 

matter and in some circumstances ozone. However, the demonstration should show that 

fire event had a causal relationship with each monitored exceedance of ozone in the 

exclusion request and that each exceedance was in excess of normal historical fluctuations.

The nonrecurring human activity/natural event criterion will generally be satisfied by 

wildfires, since they are by definition (preamble 13566) natural events, or else by arson, 

etc. which would be the result of nonrecurring human activity.

Note that lack of precipitation is excluded an exceptional event. The rule preamble (13577, 

footnote 15) clarifies that for high wind events, demonstration of a truly “severe drought” 

could provide evidence for an exceptional event.

Due to the complexities of ozone chemistry and its interaction with meteorology, wildfire 

exceptional event demonstrations for ozone will generally be more difficult than for 

particulate matter. An exceptional events demonstration would therefore be strengthened 

if it includes a conceptual model of the exceptional event’s ozone impact. This is a 

qualitative description of how emissions, chemistry, and meteorology interacted to create 

elevated ozone concentrations; it is a narrative that helps tie together and is supported by 

the monitored evidence and analyses, but is also in itself a form of evidence for causal 

relationship. Further ideas for additional specific analyses to strengthen the evidence for 

meeting the rule criteria will be discussed below.

Also, per 50.14(c)(3)(i) and (iv), the documentation package must show that the public 

comment process was followed, and must include any public comments, or a statement that 

there were none received.

Finally, under 40 CFR §51.930, a State must take “reasonable actions” to mitigate the health 

impact of exceptional events. These include public notification when NAAQS exceedances 

are expected, education on actions individuals can take to reduce exposure, and 

“appropriate measures” to protect public health. Documentation of these is not required 

for EPA to make determinations on particular exceptional events, but can provide useful 

background information, and help assure EPA and the public that the mitigation 

requirements of the EER are met.

Overview of wildfire effect on ozone levels

The presence of smoke from a fire does not in itself prove that ozone increased; ozone 

could increase or decrease, and the effect could vary by time and place. Research has 

shown that fire can cause ozone, but an exceptional event demonstration should address 

whether it did so for the particular event under review.

The mix of pollutants emitted during a fire depends on physical and chemical details of the 

fuel material, and the details of the burning process. The chemical composition of a smoke 

plume depends on many factors, including post-emission atmospheric reactions, dilution 
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rates and chemistry of the downwind air that mixes with the plume, photolysis rates, 

position within the plume, altitude, and smoke temperature. All of these depend on time of 

day, fuel type and combustion stage. The downwind effect of a fire plume on ozone 

concentration depends also on the initial vertical extent of the plume, the meteorological 

and chemical conditions the plume encounters, and how far downwind the measurements 

are made. The USDA Forest Service has published a very useful overview of the effects of 

fire on air quality, including a discussion of fire plume chemistry (Sandberg et al., 2002). 

Fire emissions can increase ozone. Oxides of nitrogen (NOx) and volatile organic 

compounds (VOCs) emitted by fires can lead to ozone production in the plume itself, and 

can add to the ozone precursor load downwind. Fires emit oxygenated VOCs, an important 

source of the radicals that allow ozone levels to build up. Ozone formation in fire plumes is 

generally limited by the availability of NOx (Jacob et al., 1992; Cheng et al., 1998; Junquera 

et al., 2005); ozone increases can be greater once the plume reaches a NOx-rich urban area 

(Wotawa and Trainer, 2000; McKeen et al., 2002). Lower NOx emissions in rural areas 

(such as northern California) could result in little additional ozone formation from the fire. 

The amount and spatial distribution of emissions could thus be important for 

demonstrating increased ozone due to fires.

On the other hand, fire emissions could decrease ozone. Ozone may decrease just 

downwind of a fire: as for any combustion source, the NO emitted by fires consumes ozone 

to form NO2 ("titration"). The particulates emitted by fire absorb and scatter light; this 

decreases photolysis of NO2 and hydrocarbons, slowing ozone formation, and may occur 

over a large area or just nearby on the plume’s underside. The oxygenates emitted by fires 

can increase or decrease ozone production depending on initial NOx and CO concentrations 

relative to background (Mason et al., 2001). During plume travel, simple dilution, chemical 

loss, or deposition onto surfaces of ozone, NO2 (via HNO3), and hydrocarbons may decrease 

ozone and precursor concentrations to levels hard to distinguish from the background. A 

large influx of fire plume VOC radicals could "sink" NOx into HNO3 and unreactive organic 

nitrates, lessening the amount available to form ozone. Even if a fire plume does produce 

ozone, the air mass containing it may simply displace urban air containing still higher 

ozone concentrations, for a net decrease.

Given the variation and complexity within any smoke plume, demonstration of an ozone 

impact requires information and analysis on the source emissions, meteorological 

conditions, and ambient measurements, and should include a conceptual model of how 

these interacted to affect ozone concentrations during these specific events.

III. Review of Evidence Submitted for High Ozone in the Sacramento Regional

Nonattainment Area Due to Wildfires

A. Criteria in 40 CFR §50.1(j) definition of exceptional event

As discussed in section IIA above, these criteria are generally intrinsically met by lightninginduced wildfires, if criteria that (a) there was a “clear causal relationship” between the 
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measured exceedance and the wildfire emissions and (b) the measured ozone 

concentration was “in excess of normal historical fluctuations” are met.

B. Clear causal relationship

Conceptually, for showing causal relationship, a demonstration of a fire exceptional event 

should include evidence that (1) the fire existed, (2) its plume followed a transport path to 

the area in question and pollutants from the plume did in fact arrive in the area, and (3)

pollution in the plume increased ozone, consistent with the science of ozone formation and 

the conceptual model of the event, as discussed above in Section II.A. The causal part of a 

demonstration would be far more compelling if it evaluates and rules out alternative 

causes for the elevated ozone concentration, e.g. high temperature and stagnant winds. 

Without that analysis, correlation of smoke and ozone may be mistaken for causation.

The documentation does not have a conceptual model of these ozone episodes or of typical 

ozone formation. A conceptual model of ozone formation in the Sacramento Metropolitan 

area around the monitors included in the demonstration, showing the typical magnitude of 

1-hour values and the daily pattern for all the monitoring sites in the area should be 

included along with a conceptual model combined with supporting data from each day and 

place claimed to be affected by an exceptional event to explain how ozone values differ due 

to the presence of a fire plume. It should also address why only certain days have elevated 

ozone, despite the presence of a fire plume over multiple days. A conceptual model would 

help guide selection, organization, and evaluation of the available evidence in support of 

the demonstration.

1. Evidence that the Fire Event Occurred

The following information was provided to demonstrate that the fire event occurred:

FEMA CA Wildfire Incidences Map (specific fires or monitors not noted)

Description of the fire season in general and some of the more well noted public 

actions that ensued (e.g. the governor issuing States of Emergency).

The document should have clearer identification of the “event” that is claimed to be 

exceptional and to have caused the exceedances, e.g. which fire(s) contributed when. A 

table listing each fire suspected of influencing air quality along with the fire's date, size, 

locations and distance relative to the affected monitors would serve to help clarify which 

fires are the contributing event. It would also be helpful if maps included the relative 

locations of the source fire(s), county boundaries, and air pollution monitors. 

2. Evidence that the fire plume reached the monitor 

The following information was provided to demonstrate that the fire plume reached the 

monitor on the specific days:
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Meteorological discussion of days in question including days between fires and 

exceedances

NOAA Satellite Fire Detection Map for June 27, July 7, July 25(neither fire location 

nor monitors noted)

NASA Satellite Images for June 27, July 7 (box noting Sacramento Region moves, 

neither fire location nor monitors noted)

Time series of PM2.5 maximum and O3 1-hour maximum from June 20 – July 25 for 

Roseville and Folsom monitors. 

Table 2 shows the mean and maximum values for the highest 24-hour average PM2.5 

concentration measured during June and July at Roseville for the years 1999-2008 

(2008 mean value is not listed)

Included in attachments, although not discussed, are meteorological maps and 

tables of surface observations.

The provided information is useful, however, the items below are additional analyses 

that could strengthen the evidence for this criteria, or are requests for clarification or 

correction of the information that has already been submitted.

a. Other useful analyses could include forward and backward trajectories (e.g. from 

HYSPLIT), gridded wind fields, and/or a qualitative map with overall direction 

arrows supplemented with wind vectors at individual meteorological sites.

b. Maps should include the relative locations of the source fire(s), county boundaries, 

and air pollution monitors. Data from other monitoring sites between the fire and 

the site claiming the exceptional event exclusion could be included to show the 

progression of the fire plume to the monitor or area of monitors in question. Some 

of the techniques ARB has used in past triennial assessments of ozone transport 

between air basins could be useful for this (ARB, 2001).

c. Satellite photos of June 27, July 7 and July 25 show what seem to be obvious smoke 

plumes from individual fires in the general vicinity on these days. Please describe 

what all the NOAA and NASA satellite figures are showing (e.g. what wavelength, 

corresponding to what pollutant, column versus surface etc.) and how this might 

relate to the other days not shown. Also, there is no indication of the locations of 

monitors or specific fire names, so it is difficult to tell which fire(s) may be affecting 

the monitors on the dates included in this package. Distinguishing plumes from 

clouds and from terrain of similar color is difficult. However, when labeled with 

locations, and related to particular exceedance dates, the photos would be useful in 

making the case that fire plume(s) reached the monitors. Infrared imagery to show 

fire locations would also be a useful supplement. Satellite products such as fire 

detects and aerosol optical depth, and column ozone could also be useful. 

d. The last sentence on page 6 states “On this day, PM2.5 measurements showed...”; 

please clarify which day of the June 23-27 event this refers to.

e. The second full paragraph on page 7 states that lowered visibilities continued to 

bring smoke into the Sacramento Region. Please clarify sentence to indicate the 

intended causal relationship.
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f. Page 9 contains a reference to Auburn as one of the sites exceeding the standard on 

July 7. Further references in the document to the July 7 flags use “Cool” to denote the 

monitor that is exceeding. Please clarify whether these refer to the same monitor or 

this is a typographic error, and revise the report to have consistent terminology.

g. Please clarify what criterion is being supported by the PM2.5 information in Table 2

and explain why the mean value for 2008 is not included.

h. Levoglucosan measurements at stations throughout CA are available from ARB’s 

Monitoring and Laboratory Division (MLD). These data should be included where 

available as they would strengthen support for the causal connection with biomass 

combustion. Support for this would also be strengthened by inclusion of PM 

speciation data where available from Speciation Trends Network sites in California 

e.g. for evidence of increased organic carbon or increased ratio of organic to 

elemental carbon.

3. Evidence that the fire plume increased ozone at the monitor 

Much of the documentation focuses on a relationship between fires and smoke, or fires and 

PM2.5, rather than between fire and ozone, the object of the request. Overall, the 

impression given is that there was a lot of smoke around from various fires, which, based 

on the literature review, could have caused exceedances. There is no discussion of when 

fires do not increase ozone, or in what circumstances one would expect a 10 ppb increase 

rather than a much larger increase. Without explaining why one would expect the results 

from other studies to be relevant, the link to the cause of the exceedance in these events is 

not clear. In summary, this review documents that fire emissions can elevate ozone, 

however, without a discussion to say how this occurs or how it compares to the event days 

in question, it is not clear that fire emissions did elevate ozone on these days in these 

locations. Also, it is clear from the documentation that the event is the fire and its 

emissions, but it is not clear if the documentation is meant to demonstrate that the 

exceedance was caused by fire emissions alone or by a combination of fire emissions plus 

urban emissions.

As mentioned above, EPA strongly encourages the development of a conceptual model for 

typical exceedance-day ozone formation for comparison to the event days to support the 

causal connection argument. Using such a model for a framework could help tie together

the timing of fire, emissions, plume transport, and ozone chemistry, and explains how these 

exceedance days differ from typical high ozone days, or from neighboring days within the 

period when smoke was present and high ozone concentrations were not. Without this 

framework, it is significantly more challenging to demonstrate a causal relationship. 

Therefore, graphed information should be discussed more fully in the context of a 

conceptual model of the events to have any weight in terms of demonstrating a causal 

relationship, rather than correlation alone.

Using this conceptual model and other analyses (see attachment for suggestions), the

documentation for these events should discuss the following:
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how the exceedance days differ meteorologically from a typical day in general or 

from a typical day with elevated ozone,

meteorological factors that may partly explain the higher ozone, e.g. the day being 

warmer or more stagnant than typical,

the timing of the plume generation relative to the timing of elevated ozone levels at 

specific monitoring stations, and

how spatial and temporal patterns of ozone and its precursors differed between the 

exceedance days and typical high ozone days. There is some comparison to an 

average (not a typical) diurnal temporal profile (e.g. Graph 5), but no analysis of 

why the profile takes the shape it does, and why it is different than neighboring days 

that are also smoke-affected.

The submitted documentation for this exceptional event request includes the following to 

demonstrate that ozone increased at the monitor due to wildfire emissions:

graphical comparisons of the diurnal pattern of PM2.5 1-hour maximum and ozone 

1-hour maximum for June 20- July 25 for Roseville and Folsom monitors and

discussion of previous research.

Graphical comparisons of the diurnal pattern of ozone concentration during the 

exceptional event request days and the average of the top five ozone days from 2005-2007

(see graphs 8-12) were included in the discussion of “in excess of historical fluctuations.” 

EPA notes that similar plots may be more useful in supporting the clear causal relationship 

criteria. Additional suggestions about these plots are given in section C below. The 

following is a list of comments and clarification requests about the information presented 

to satisfy the criteria of the causal link between fire emissions and ozone exceedances at 

these monitors on these dates.

a. Pages 6-13 indicate the correlation of the flagged days between the presence of 

smoke plumes from the fire with elevated PM2.5 concentrations throughout the 

region and measured O3 exceedances at the monitors that were flagged. (e.g. page 

11, 12). To draw conclusions about causal relationship, comparisons could be made 

to data from days not claimed to be affected by fire, both historically and in the same 

year as the fire. Otherwise there is difficult to know whether the ozone diurnal 

pattern often looks like this, independent of fires (or conversely, whether PM2.5 

often looks like this, without being followed by ozone).

b. It is not clear which criterion the argument on page 7 regarding the rate of O3

concentration change is intended to support. In addition, arguments based on how 

fast ozone changes is not convincing without comparisons to the normal or largest 

change at that site historically, during the same season or meteorological conditions.

c. Please clarify what evidence was presented to support the statement on page 12 

that “Without the additional emissions from the wildfires, ozone concentrations 

would not have exceeded the 1-hour standard in the Sacramento Valley. “ Likewise, 

the causal argument for July 25 was supported by a statement on page 12 that 

meteorological conditions and PM2.5 concentrations indicate “that some wildfire 
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emissions were probably present” and that observations of haze at the airport 

indicates the presence of wildfire emissions. Both of these statements require 

greater substantiation.

d. Pages 11 & 13 indicate a contribution from urban (anthropogenic) emissions not 

addressed later. Please include discussions of the contribution from anthropogenic 

precursors in your conceptual model framework.

e. The last sentence on pg 13 states that “Each of these ozone violations can be 

attributed to ozone precursor emissions from the wildfires.” This statement has not 

been supported, since there is no discussion (even qualitative) about how fire 

emissions resulted in elevated ozone or violations.

f. Page 14 states that the design value for some of these sites had been decreasing in 

previous years and therefore would have been below the standard in 2008 if not for 

the fires. To make this argument more convincing, it should include a more 

thorough examination of why the trend was expected to continue. This would 

require an analysis that at a minimum includes an evaluation of the emissions 

inventory for ozone precursors and a comparison of meteorology for this year 

versus previous years. 

g. Pg 15 states that the impact of the wildfire is indicated by elevated PM2.5 levels. 

While the elevated PM2.5 levels (along with other evidence) may indicate the 

presence of the primary fire emissions (or elevated secondary aerosol formation 

from fire emissions), this is indicative of the fire impact on PM2.5 concentrations 

measured at those sites. Without further discussion this concept does not indicate 

anything about the impact of the fire on ozone (as compared to PM2.5) 

concentrations measured at those sites, as this criterion requires.

h. The Nicolov 2008 citation is a presentation at a workshop and not a widely available 

(or peer reviewed) document and thus should be used with caution or eliminated. 

Also, a statement that “biomass burning can significantly increase surface ozone 

concentrations” does not provide a convincing causal link between fire and ozone 

concentrations for any other event without a discussion of when one would expect 

the fire to increase ozone or not, and which case is more relevant to the cases under 

examination. 

i. Plots 1-2 do not sufficiently demonstrate a direct correlation between PM2.5 and O3, 

nor a causal link. Including a time lag between the two would potentially be more 

illustrative. This could be informed by the conceptual model of how ARB expects 

hourly PM2.5 maxima to relate to hourly O3 maxima, and why. Nevertheless, there 

are times with high PM2.5 without high ozone and high O3 without high PM2.5, 

making the correlation less than convincing. The presence of correlation does not 

indicate causation, particularly in the absence of a conceptual model of what is 

causing the correlation.

j. The discussion of high PM2.5 concentrations and related Table 2 on page 16 is not 

linked with ozone, so does not support a clear causal relationship.

k. Page 1 of the “Exceptional Event Justification” attachment states that “As a result, 

ozone precursors originating from these fires increased the Sacramento region’s 

ozone concentration causing violations of the 1-hour ozone NAAQS...” This 

statement is not supported, as there is no causality discussed. For example, are the 
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precursors higher on these days than any other, or do they exhibit different spatial 

or temporal patterns?

The demonstration should be comparing similar meteorological conditions, for example, 

times during which the air parcel is coming from the same locations, during strong, low 

inversions and passing through Sacramento to outlying monitor. Comparing and 

contrasting the exceedance days with similar days, typical high ozone days, and other nonexceeding nearby monitors may help strengthen the evidence that the fire as an 

exceptional cause of the ozone exceedance and other scenarios.

Comparisons are sometimes made between the exceedance day and the average of other 

days, which is less meaningful than comparison to typical ozone exceedance days or days 

with similar meteorology, or to the envelope of a range of other days. 

C. Concentrations in excess of normal historical fluctuations

The following information was submitted to demonstrate this criterion:

Bar graphs showing number of days in each 1-hour maximum ozone concentration

bin for the 2008 ozone seasons compared to a historical data set (2005-2007 ozone 

seasons) for each monitor. The event days in question are noted as is the 

concentration bin that is 3 above the mean of the historical data set.

Diurnal plots of 1-hour ozone concentrations for the exceptional event days and for 

the average of the top five days in 2005-2007.

A plot of 1-hour ozone maxima at Folsom and Sloughhouse from June 20 – July 25

Concentration maps of 1 hour ozone maxima throughout the Sacramento 

Metropolitan area on the day before and day of event for two events (6/23 and 7/7)

The following lists our concerns about whether this criterion was met, some requests for 

clarifications and suggestions for further analyses.

a. 40 CFR Part 51, Appendix W, 8.3.1.2 indicates 5 years of data is the minimum period 

needed to adequately represent the variability in ambient pollution concentrations. 

Therefore, a data set comprised of 5 years may strengthen the argument that it 

encompasses the expected range of variability. Also, comparisons to the historical 

data set may be more convincing if data reflected the same meteorological

conditions.

b. The text states that concentrations on the event days are greater than 3 standard 

deviations (above the mean of the historical data set, presumably). Since 

concentration data typically has a lognormal distribution, this could be translated 

into a statement about the likelihood of the observed concentrations, given the 

underlying historical distribution. Adding an analysis to include a discussion of this 

type of likelihood may add to the argument of why or how these concentrations are

“in excess of normal historical fluctuations”.

c. The 3σ line on graphs 3-7 (see pages 17-19) is at times lower than historical data 

that are not flagged, with no discussion of why those historical should not be 
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considered for this criterion: Folsom: July 27; Sloughhouse: July 9, 25; Placerville: 

June 26; Cool: June 23 and 27, July 1. This evidence would also be strengthened by 

this type of discussion for the other points either on the 3 line or in the same 

concentration “bin” as historical data.

d. Graphs 8-13 (diurnal plots of 1-hour ozone concentrations for the exceptional event 

days and for the average of the top five days in 2005-2007) are analyses that may be 

useful with some changes to supporting the clear causal relationship requirement. 

The evidence presented in those graphs could better satisfy EER criteria with the 

following additions/changes:

i. There should be a discussion of the diurnal pattern on a normal, non-fireaffected, high ozone day. 

ii. Comparing a single day to a five day average may be misleading since the 

average inherently smoothes out any variability in the variable of interest – in 

this case the diurnal shape of the ozone concentration and timing of the peak 

level of O3 at a certain location. It is possible that comparing it to the range of 

top 5 days would be more illustrative.

iii. If available, five years of data would be more representative of historical 

fluctuations.

iv. There should be a description of how is this is evaluated for days with 

incomplete data, e.g. 2008 data at Placerville.

e. Please explain the reason for incomplete data at Placerville on the days requested 

for exceptional event exclusion as seen in Graph 11.

f. Clarify the conclusion intended by the correlation of 1-hour maximum ozone 

concentration between Folsom and Sloughhouse (graph 13 and accompanying text 

on page 23). As presented this correlation does not clearly demonstrate that any of 

the requirements of the exceptional event rule are met.

g. Figures 6-7 (Concentration maps of 1 hour ozone maxima throughout the 

Sacramento Metropolitan area on the day before and day of event for two events 

(6/23 and 7/7)) and corresponding discussion does not support “in excess of 

normal historical fluctuations” without a discussion of how fast O3 concentrations or 

extent can/has built up in this valley before. If revised, this evidence may be more 

useful in demonstrating the “clear causal relationship” or the “but-for” criteria.

h. The conclusions stated in the first paragraph on page 24 that the ozone 

concentrations on June 23 and July 7 are atypical need more clarification (atypical in 

what way?) and justification (what evidence is being used to support what claim?). 

D. No exceedance but for the event

In order to demonstrate that there would have been no ozone exceedance but for the 

wildfire event, the District provided a regression analysis that empirically calculates the 

expected maximum 1-hour ozone concentration in a county on a given day. The regression 

model for each county contains different meteorological variables and account for 55 to 

79% of the variability in 1-hour ozone and is based on 1997-2003 data.

Other relevant information that may support this requirement include qualitative causal 

arguments, using the conceptual model framework, documenting how the event days were 
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expected to be different than observed. It would also be possible to apply a photochemical 

model to this event, as was done for 2007 California wildfires (Pfister, 2008). This work 

was cited in the documentation (page 5) as evidence that fire can produce ozone, but such 

modeling was not used to show that ozone was produced in the 2008 fires. Note that under 

the BlueSky modeling framework ( http://www.airfire.org/bluesky/ ), Sonoma Technology 

runs the CMAQ photochemical model twice daily for the entire U.S., including estimates of 

fire emissions partly derived from satellite data.

The following lists our recommendations for the information that was presented, as well as 

requests for clarifications.

a. Smoke contribution is estimated as the difference between the measured value and 

empirically calculated data from the regression model. Please address the 

assumption that smoke is the only variable that contributes to the difference 

between a forecasted day and the observed exceedance day.

b. There should be a discussion of the uncertainty or confidence limits in the model 

results. Please include a quantitative estimate of the uncertainty in the 

concentration calculated by the regression equations and confirm whether or not 

some of the higher estimates (particularly 118 ppb) are distinguishable from an 

exceedance. 

c. Please also discuss whether it is reasonable to still use these models derived from 

1997-2003 data and how well this model is predicting non-fire impacted days in 

2008. Does a regression model based on data from more recent years produce 

similar results? Are there reasons one would expect the regression results to 

change (or not) as the ozone formation regime changes in the area due to emission 

reductions or changes?

d. Please clarify if the study investigated the influence of artifacts of regression 

analysis, such as serial correlation, on the results. 

e. Please discuss the spatial resolution of estimates from regression models. The 

equations were formed using county-wide data. Please explain more clearly how 

you use this information to derive a concentration at a certain site.

f. Please discuss whether the actual observed meteorological variables from each day 

could be used as input for the predictive formula, instead of the four predictions of 

meteorological variables, and whether one would expect this to reduce the 

uncertainty of the results.

g. The July 9 day at Sloughhouse does not meet the criteria used for other days, but it 

is still argued that it satisfies the “but-for” argument. The estimated contribution 

from smoke ranges from -4.3 to 10.7 ppb, meaning that the concentration could 

have been from 117.3 to 132.3. This is not clearly below the NAAQS. The discussion 

continues and states that contributions calculated for nearby counties were larger –

therefore proving that the exceedance at this monitor was caused by the fire. This 

argument is unsubstantiated, especially given that these regression equations are 

done separately for each county and have different variables in them and therefore 

would not necessarily be comparable.

h. The argument that July 9, 2010 at Sloughhouse is one of the highest days, and 

therefore satisfies the “but-for” argument is not substantive. There are historical 
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days that are higher and not flagged, as well as ten other days above the NAAQS 

from 2005-2008. In addition, each criteria must be satisfied independently, 

acknowledging that they may interrelate.

i. The existing documentation does not contain enough information to allow a full 

understanding of the results presented. It is not clear how this derived quantity 

estimated smoke contribution (page 3 of Ex. Event Justification) is used in 

comparison to the contribution as calculated as the difference between the observed 

and predicted value. 

E. Evidence of Public Notice and Comment

Documentation included the public notice, including the proof of publication in six 

newspapers, which informed the public that documentation for this exceptional event 

request was available for review. The District included the two public comments they 

received and their response to comments.

IV. Other notes and clarifications

1. There should be clearer identification in figures and elsewhere of event-affected 

days and data as opposed to unaffected days, and in general labeling in figures 

should be improved. For example, Figure 1 and Figure 3 differ on where the box 

representing Sacramento is located. 

2. Please include a map of the monitoring stations in the area, both stations that are 

included in the exceptional event request and others nearby.

3. Page 25 refers to information submitted in the 8-hour O3 exceptional event package. 

This information was not reviewed. If this information is relevant to the 1-hour 

ozone exceptional event demonstration, please submit it as a part of that package so 

that it can be included in the administrative record for this review. 

4. Please refer to the discussion above to revise the outline of the rule requirements 

given on page 26. For example, the bulleted items in the paragraph that starts "A 

clear causal relationship was demonstrated..." are not logical subpoints to the first 

sentence. They are assertions about other parts of the EE criteria. Also 

"significantly influenced" is not an element of the EE criteria.

V. Documents reviewed

" Exceptional Events Demonstration for High Ozone in the Sacramento Regional 

Nonattainment Area Due to Wildfires” Sacramento Metropolitan Air Quality 

Management District and California Air Resources Board dated July 8, 2009 and associated 

cover letter from Karen Magliano (CARB) to Joseph Lapka (EPA) dated September 17, 2009. 

This documentation also included the following attachments:
Page 14 of 14

Attachment 1 to ARB submittal (9/17/2009)

Cover letter from Bridget Tollstrup, Program Coordination Division Manager, Sacramento 

Metropolitan Air Quality Management District, to Gayle Sweigert, California Air Resources 

Board, June 15, 2009 and attachments. The attachments are as follows:

Exceptional Event Justification

Appendnix A – Regression Tool Development Documentation

Appendix B – Table of Estimated Smoke Contributions

Appendix C – Media and Public Advisories, Health Statements 

Attachment 2 to ARB submittal (9/17/2009)

This attachment provides an in-depth discussion of the weather pattern for the three 

events: 1) June 23-27, 2008, 2) July 7, 9 and 10, 2008, and 3) July 25, 2008, The figures used 

to support the discussion include satellite images and aerosol optical depth, various 

meteorological surface observations and time series plots of ozone, PM2.5, temperature, 

wind speed and direction.

Also included in Attachment 2 was a cover letter from Bridget Tollstrup to Gayle Sweigert, 

dated September 9, 2009, that included public notices of the exceptional event package, 

proof of publication of these notices, written public comments and the District’s responses 

to the comments.