[Federal Register Volume 70, Number 176 (Tuesday, September 13, 2005)]
[Notices]
[Pages 54046-54051]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 05-18015]


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ENVIRONMENTAL PROTECTION AGENCY

[OAR-2003-0032; FRL-7965-4]


Interim Guidance on Control of Volatile Organic Compounds in 
Ozone State Implementation Plans

AGENCY: Environmental Protection Agency (EPA).

ACTION: Notice of interim guidance on SIP development.

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SUMMARY: The Environmental Protection Agency (EPA) encourages States to 
consider recent scientific information on the photochemical reactivity 
of volatile organic compounds (VOC) in the development of State 
implementation plans (SIPs) designed to meet the national ambient air 
quality standard (NAAQS) for ozone. This interim guidance summarizes 
recent scientific findings, provides examples of innovative 
applications of reactivity information in the development of VOC 
control measures, and clarifies the relationship between innovative 
reactivity-based policies and EPA's current definition of VOC. This 
interim guidance does not change any existing rules.

DATES: This interim guidance is effective on September 13, 2005.

ADDRESSES: EPA has established a docket for this action under Docket ID 
No. OAR-2003-0032. All documents in the docket are listed in the 
EDOCKET index at http://www.epa.gov/edocket. Although listed in the 
index, some information may not be publicly available, i.e., 
Confidential Business Information (CBI) or other information whose 
disclosure is restricted by statute. Certain other material, such as 
copyrighted material, is not placed on the Internet and will be 
publicly available only in hard copy form. Publicly available docket 
materials are available either electronically in EDOCKET or in hard 
copy at the Docket ID No. OAR-2003-0032, EPA/DC, EPA West, Room B102, 
1301 Constitution Ave., NW., Washington, DC. The Public Reading Room is 
open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding 
legal holidays. The telephone number for the Public Reading Room is 
(202) 566-1744, and the telephone number for the Air Docket is (202) 
566-1742.

FOR FURTHER INFORMATION CONTACT: William L. Johnson, Office of Air 
Quality Planning and Standards, Air Quality Strategies and Standards 
Division, Mail code C539-02, Research Triangle Park, NC 27711, 
telephone (919) 541-5245.; fax number: (919) 541-

[[Page 54047]]

0824; e-mail address: [email protected]v.

SUPPLEMENTARY INFORMATION:

Outline

I. General Information
II. Introduction
III. Short History of VOC Reactivity Policy and Science
IV. Use of VOC Reactivity in Developing SIPs
V. Relationship to Existing VOC Exemption Policy
VI. Summary

I. General Information

Does This Action Apply to Me?

    You may be an entity affected by this interim guidance if you are a 
State or local air pollution control agency that has, or is currently 
developing, an ozone SIP containing programs to control VOC emissions. 
Additionally, you may be impacted if you use or emit VOCs in 
commercial/industrial/manufacturing operations, as well as other 
consumer/commercial activities. If you have questions regarding the 
applicability of this action to a particular entity, consult the person 
listed in the preceding FOR FURTHER INFORMATION CONTACT section.
    This action does not impose any new mandates on States or industry, 
but rather provides information about options for meeting Clean Air Act 
mandates that are likely to be more effective, and more cost-effective, 
than the measures currently employed in most parts of the country.

II. Introduction

    Ground level ozone, one of the principal components of ``smog,'' is 
a serious air pollutant that harms human health and the environment. In 
April 2004, EPA designated 126 areas of the country as 
``nonattainment'' for the 8-hour ozone national ambient air quality 
standards (NAAQS). States and tribes are currently revising State 
Implementation Plans (SIPs) in order to bring air quality into 
compliance with the 8-hour ozone standard. The Agency has proposed that 
these SIP revisions must be submitted to EPA by June 15, 2007. Certain 
areas will need to submit separate reasonably available control 
technology (RACT) SIP revisions. There is final action pending that the 
Agency anticipates will require these revisions to be submitted by 
September 15, 2006. Some of the areas designated as nonattainment under 
the 8-hour standard have persistent air quality problems and will need 
to employ as many cost-effective controls as possible to achieve the 8-
hour ozone NAAQS as expeditiously as practicable but no later than by 
their attainment date.
    The development of measures to reduce ozone concentrations is 
complicated by the fact that ozone is not directly emitted. It is 
formed in the air by chemical reactions of nitrogen oxides 
(NOX) and VOCs in the presence of heat and sunlight. 
Therefore, ozone SIPs must address emissions of these ozone precursors.
    There are thousands of individual chemical species of VOCs that can 
react to form ozone. It is generally understood that not all VOCs 
contribute equally to ozone formation and accumulation. Some VOCs react 
slowly and changes in their emissions have limited effects on local or 
regional ozone pollution episodes. Some VOCs form ozone more quickly, 
or they may degrade through a series of reactions that generates more 
ozone than the reaction pathways of other VOCs. Others not only form 
ozone themselves, but also enhance ozone formation from other VOCs. The 
photochemical reactivity of a compound is a measure of its potential to 
form ozone. By distinguishing between more reactive and less reactive 
VOCs, it should be possible to decrease ozone concentrations further or 
more efficiently than by controlling all VOCs equally.
    Discriminating between VOCs on the basis of their contributions to 
ozone formation, or reactivities, is not straightforward. Reactivity is 
not simply a property of the compound itself; it is a property of both 
the compound and the environment in which the compound is found. The 
absolute reactivity of a single compound varies with VOC-NOX 
ratios, meteorological conditions, the mix of other VOCs in the 
atmosphere, and the time interval of interest. On the other hand, there 
are several scientifically valid methods that can be used to develop 
reactivity ``scales'' or weighting approaches based on the relative 
reactivity of different VOCs, and there is a high correlation between 
these different methods.
    The promise of a more efficient VOC control strategy has led the 
California Air Resources Board (CARB), EPA, and other organizations to 
invest in reactivity research. This research has produced improved 
methods for discriminating between VOCs on the basis of reactivity 
under a variety of conditions. Applying some of the lessons of this 
research, California and Texas have developed innovative regulations 
that use VOC reactivity information to improve the efficiency or 
effectiveness of VOC controls for specific source categories. As States 
develop their 8-hour ozone SIPs, EPA encourages them to consider how 
they may incorporate VOC reactivity information to make their future 
VOC control measures more effective and efficient.

III. Short History of VOC Reactivity Policy and Research

    The issue of VOC reactivity was first recognized by EPA in its 
initial guidance to States on the preparation of ozone SIPs in 1971. In 
this initial guidance, EPA emphasized the need to reduce the total mass 
of organic emissions, but also noted that ``substitution of one 
compound for another might be useful where it would result in a clearly 
evident decrease in reactivity and thus tend to reduce photochemical 
oxidant formation.'' EPA encouraged States to promulgate SIPs with 
organic emission control provisions similar to those outlined in Los 
Angeles District's Rule 66, which allowed many VOC species thought to 
have minimal adverse effects to be exempted from control.
    The Rule 66 exempt status for many of these organic emission 
species was questioned a few years later when research results from 
field studies conducted between 1971-1974 revealed that pollutant 
transport conditions were capable of enhancing ozone formation such 
that these ``exempt'' compounds were now considered significant ozone 
producers. Thus, in 1977, the EPA issued the ``Recommended Policy on 
Control of Volatile Organic Compounds,'' offering its own, much more 
limited list of ``negligibly reactive'' compounds to be exempted (42 FR 
35314, July 8, 1977). As new information about the reactivity of 
different compounds has become available, EPA has continued to add to 
the list of negligibly reactive compounds following the logic of the 
1977 policy. In 1992, this list of negligibly reactive compounds was 
explicitly excluded from the definition of VOC when it was codified in 
40 CFR 51.100(s) (57 FR 3941, February 3, 1992). Since 1977, EPA has 
designated approximately 50 compounds or classes of compounds as 
negligibly reactive and has excluded these compounds from the 
regulatory definition of VOC.
    Beginning in the early 1990s, CARB has pursued the development of 
regulatory approaches that more fully discriminate VOCs on the basis of 
reactivity. In 1991, CARB incorporated a reactivity scale for weighting 
the emissions of individual VOC species in their low emitting vehicle 
and clean fuels regulation. The scale was designed to account for the 
differences in the ozone-forming potential of exhausts from gasoline 
engines and alternative fueled vehicles. The scale adopted by

[[Page 54048]]

CARB was the Maximum Incremental Reactivity (MIR) scale, derived using 
a series of box model simulations with varying VOC composition and VOC-
NOX ratios.\1\ The MIR scale is commonly expressed in units 
of grams of ozone produced per gram of VOC emitted.
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    \1\ Carter, William P. L. ``Development of Ozone Reactivity 
Scales for Volatile Organic Compounds.'' Journal of the Air and 
Waste Management Association 44 (1994): 881-99.
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    Over the course of the 1990s, CARB continued to invest in the 
development of reactivity scales and to explore their potential 
regulatory applications. In June 2000, CARB adopted an aerosol coatings 
regulation that incorporates an updated MIR scale. This regulation is 
described in more detail below. Currently, CARB is exploring the use of 
reactivity scales in other programs, including regulations for 
architectural coatings.
    In 1998, EPA participated in the formation of the Reactivity 
Research Working Group (RRWG), which was organized to help develop an 
improved scientific basis for reactivity-related regulatory 
policies.\2\ All interested parties were invited to participate. Since 
that time, representatives from EPA, CARB, Environment Canada, States, 
academia, and industry have met in public RRWG meetings to discuss and 
coordinate research that would support this goal. The RRWG has 
organized a series of research projects that have addressed issues such 
as:
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    \2\ See http://www.cgenv.com/Narsto/reactinfo.html.
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     The sensitivity of ozone to VOC mass reductions and 
changes in VOC composition;
     The derivation and evaluation of reactivity scales using 
photochemical airshed models;
     The development of emissions inventory processing tools 
for exploring reactivity-based strategies; and
     The fate of VOC emissions and their availability for 
atmospheric reactions.
    This research has led to a number of findings that increase our 
confidence in the ability to develop approaches that discriminate 
between VOCs on the basis of reactivity. These findings include:
     State of the art chamber studies at low VOC-NOX 
ratios demonstrate that current atmospheric chemistry models generally 
perform as well under ``real world'' conditions as under the high 
concentration scenarios used in their development.\3\
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    \3\ Carter, William P.L., D.R. Crocker, III, D.R. Fitz, L.L. 
Malkina, K. Bumiller, C.G. Sauer, J.T. Pisano, C. Bufalino, and C. 
Song. ``A New Environmental Chamber for Evaluation of Gas-Phase 
Chemical Mechanisms and Secondary Aerosol Formation.'' Atmospheric 
Environment accepted for publication, July 15, 2005 (in press).
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     Substituting emissions of low reactivity compounds for 
emissions of high reactivity compounds can be effective in reducing 1-
hour and 8-hour ozone concentrations. Substitutions based on equal 
mass, equal carbon, or equal molar concentrations will achieve 
different levels of ozone reduction depending on the chemicals being 
substituted. Similar to decreases in mass of VOC emissions, reactivity-
based VOC substitution seems to reduce higher concentrations of ozone 
more than lower concentrations of ozone.\4\
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    \4\ Arunachalam, S., R. Mathur, A. Holland, M.R. Lee, D. Olerud, 
and H. Jeffries. ``Investigation of VOC Reactivity Assessment with 
Comprehensive Air Quality Modeling.'' Report to the U.S. 
Environmental Protection Agency, 2003; and Carter, William P.L., 
Gail S. Tonnesen, and G. Yarwood. ``Investigation of VOC Reactivity 
Effects Using Existing Regional Air Quality Models.'' Report to the 
American Chemistry Council, Contract SC-20.0-UCR-VOC-RRWG, 2003.
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     There are several scientifically valid methods that can be 
used to calculate reactivity scales, each with different strengths and 
weaknesses. Although there is a high correlation between the different 
methods (even the simplest ones), important differences exist in their 
geographical representativeness and in the amount of spread between low 
reactivity and high reactivity compounds.\5\
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    \5\ Carter, William P.L., Gail S. Tonnesen, and G. Yarwood. 
``Investigation of VOC Reactivity Effects Using Existing Regional 
Air Quality Models.'' Report to the American Chemistry Council, 
Contract SC-20.0-UCR-VOC-RRWG, 2003.
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     Using available reactivity scales, it is possible to 
construct a VOC substitution scenario that will achieve approximately 
the same ozone reductions as reducing the overall mass of VOC 
emissions. However, when applied, the substitution scenario may 
increase ozone in some areas and decrease ozone in others depending on 
the robustness of the reactivity scale used.\6\
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    \6\ Ibid.
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     Several reactivity metrics derived with airshed models 
(such as the Maximum Ozone Incremental Reactivity to Maximum 
Incremental Reactivity (MOIR-MIR) and Least Squares Relative Reactivity 
(LS-RR)) appear to be robust over different regions of the country, 
meteorological episodes, year of analysis, averaging times, and 
models.\7\
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    \7\ Hakami, A., M.S. Bergin, and A.G. Russell. ``Ozone Formation 
Potential of Organic Compounds in the Eastern United States: A 
Comparison of Episodes, Inventories, and Domains.'' Environmental 
Science and Technology 38 (2004): 6748-59; Hakami, A., M. Arhami, 
and A.G. Russell. ``Further Analysis of VOC Reactivity Metrics and 
Scales.'' Report to the U.S. Environmental Protection Agency, 2004; 
and Derwent, R.G. ``Evaluation and Characterization of Reactivity 
Metrics.'' Report to the U.S. Environmental Protection Agency, 2004.
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    EPA encourages all interested parties to continue working through 
the RRWG to improve the scientific foundation of VOC reactivity-based 
regulations. EPA will continue to update its guidance to States as new 
information becomes available. In the meantime, EPA encourages States 
to take advantage of the information that is now available in designing 
future VOC control strategies.

IV. Use of VOC Reactivity in Developing SIPs

    Although the traditional approach to VOC control focused on 
reducing the overall mass of emissions may be adequate in some areas of 
the country, an approach that discriminates between VOCs based on 
reactivity is likely to be more effective and efficient. In particular, 
reactivity-based approaches are likely to be important in areas for 
which VOC control is a key strategy for reducing ozone concentrations. 
Such areas include:
     Areas with persistent ozone nonattainment problems;
     Urbanized or other NOX-rich areas where ozone 
formation is particularly sensitive to changes in VOC emissions;
     Areas that have already implemented VOC RACT measures and 
need additional VOC emission reductions.
    In these areas, there are a variety of ways of addressing VOC 
reactivity in the SIP development process, including:
     Developing accurate, speciated VOC emissions inventories. 
EPA encourages States--and particularly States with persistent ozone 
problems--to develop emissions inventories that include emission 
estimates for individual VOC species, as opposed to only estimating 
total VOC mass. This type of information may be especially useful for 
identifying emissions of the most reactive VOCs in the most VOC-
sensitive areas. Currently, most States collect information on the mass 
of total VOC emissions. For air quality modeling purposes, this mass is 
apportioned to individual chemical species using EPA-provided profiles 
for each source category. Many industries, however, calculate their 
reported total VOC emissions from detailed speciated information that 
they routinely gather for other reasons. Where appropriate, States may 
want to gather such detailed speciated information and compare it to 
the national default speciation profiles.
    States should also consider emerging research on the actual 
availability of VOCs for atmospheric reaction. In estimating VOC 
emissions, especially

[[Page 54049]]

from coatings, solvents, and consumer products, it is often assumed 
that the entire volatile fraction is emitted and available for 
photochemical reaction, unless captured by specific control equipment. 
In some situations, however, otherwise volatile compounds may be 
trapped in liquid or solid phases or adhere to surfaces such that they 
are not actually released to the atmosphere. Once emitted into the 
atmosphere, VOCs may also be scavenged by rain, form particles, or 
deposit on surfaces.\8\ Taking this behavior into account should lead 
to more accurate VOC emissions inventories and photochemical modeling. 
It may also allow States to consider volatility thresholds or other 
approaches designed to reflect atmospheric availability in certain 
types of regulatory programs.
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    \8\ Reactivity Research Working Group. ``Final Proceedings of 
Workshop on Combining Environmental Fate and Air Quality Modeling.'' 
Research Triangle Park, NC, 2000.
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     Prioritizing control measures using reactivity metrics. 
Most States prioritize control measures for implementation based on the 
cost effectiveness of controlling the total mass of VOCs (i.e., $/ton). 
Using reactivity metrics and speciated VOC emission information, it is 
possible to calculate cost effectiveness on the basis of relative ozone 
formation (i.e., $/ozone decreased). By controlling the most reactive 
source categories first, a State may be able to decrease the total cost 
of reaching attainment. For example, Russell, et al.\9\ found that in 
Los Angles, selecting VOC controls on the basis of reactivity would 
decrease the cost of achieving any given level of ozone reduction as 
compared to a mass-based strategy up to a certain level of reduction. 
As more controls are required, the cost of strategies optimized on a 
reactivity basis converge with the cost of mass-based strategies as all 
the available controls are applied in both cases.
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    \9\ Russell, A.G., J.B. Milford, M.S. Bergin, S. McBride, L. 
McNair, Y. Yang, W.R. Stockwell, and B. Croes. ``Urban Ozone Control 
and Atmospheric Reactivity of Organic Gases.'' Science 269 (1995): 
491-95.
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     Targeting emissions of highly-reactive VOC compounds with 
specific control measures. With speciated emissions information, a 
State may develop control measures that specifically target sources of 
the most highly reactive VOCs. In the Houston-Galveston area, a 
comprehensive field study revealed that fugitive or episodic releases 
of several highly reactive compounds (e.g., ethylene, propylene, 1,3-
butadiene, and butenes) from petroleum refining and petrochemical 
facilities have contributed significantly to exceedances of the ozone 
NAAQS. In 2002, after consultation with the local industry, the Texas 
Commission on Environmental Quality (TCEQ) issued rules targeting 
emissions of these highly reactive VOCs from four processes: fugitive 
releases, flares, process vents, and cooling towers. These first rules 
emphasized additional monitoring, record keeping, and enforcement 
rather than establishing individual unit emission limits. In 2004, TCEQ 
adopted a cap-and-trade program for ethylene and propylene emissions 
from flares, vents, and cooling towers in Houston. Under this program, 
each site is assigned a daily and yearly emissions cap. Non-highly 
reactive VOC emissions may be used to offset highly reactive VOC 
emissions up to a limit of 5% of the facility's initial cap. The non-
highly reactive VOC emission offsets are discounted based on the ratio 
of the reactivity of the offsets to the reactivity of propylene. EPA 
has proposed approval of some facets of the Texas rules for the control 
and monitoring of highly reactive VOCs (70 FR 17640), and the Agency 
expects to propose action on other program elements, such as the cap-
and-trade program, in the near future. Although EPA has not completed 
its review of the SIP revisions provided by Texas for the Houston-
Galveston area, it does seem clear that targeting these highly reactive 
compounds for additional control will achieve substantial ozone benefit 
and is more cost effective than a rule targeting all VOCs.
     Encouraging VOC substitution and composition changes using 
reactivity-weighted emission limits. For some VOC source categories, 
such as paints, coatings, adhesives, and other formulated products, 
manufacturers may have the flexibility to change product formulations 
so as to change the composition as well as the mass of the VOC 
emissions. In some cases, changing the composition of the VOC emissions 
may be less costly and allow for better product performance than 
decreasing the mass of VOC emissions, while also providing greater 
benefits for ozone control. In 2000, CARB found that manufacturers were 
having difficulty meeting California's stringent mass-based VOC 
emission limits for aerosol coatings.\10\ After extensive study and 
consultations with stakeholders, CARB replaced the mass-based emission 
limits for aerosol coatings with reactivity-weighted emissions limits, 
using a version of the MIR scale. CARB gathered VOC composition and 
sales information from manufacturers to create VOC emission profiles 
for different categories of aerosol coatings products. Using this 
composition information, CARB calculated the MIR-weighted emission 
limits that would achieve the same ozone reduction as would have been 
achieved by the existing mass-based emission limits. To determine 
compliance with the reactivity-weighted limits, the weight percent of 
each individual VOC in the product is multiplied by its corresponding 
MIR value and then summed for all VOCs in the product. All VOCs with 
MIR values, including those that are considered ``negligibly reactive'' 
under the national policy, are included in the calculation. For complex 
mixtures, such as mineral spirits, CARB performed analyses to assign 
appropriate MIR values for different mixtures. CARB intends to review 
and, as appropriate, update the reactivity scale used in the rule to 
incorporate the latest scientific information. EPA has proposed 
approval of this rule for inclusion in California's SIP (70 FR 1640, 
January 7, 2005) and expects to finalize this approval in the near 
future. EPA and CARB view this rule as an important opportunity to 
gather additional information about the effectiveness and practical 
implementation issues associated with a reactivity-based program.
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    \10\ California Air Resources Board. ``Initial Statement of 
Reasons for the Proposed Amendments to the Regulation for Reducing 
Volatile Organic Compound Emissions from Aerosol Coating Products.'' 
2000.
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V. Relationship to Existing VOC Exemption Policy

    Although a continuous reactivity scale is likely to be the most 
effective approach for regulating VOCs in many areas of the country, 
such an approach is more difficult to develop and implement than 
traditional mass-based approaches because reactivity-based programs 
carry the extra burden of characterizing and tracking the full chemical 
composition of VOC emissions. In addition, although most existing VOC 
control programs do not discriminate between individual VOCs based on 
reactivity, they continue to provide significant ozone reduction 
benefits and will remain in place unless and until they are replaced by 
programs that achieve the same or greater benefits.
    Under virtually all existing programs, EPA and States exclude 
certain negligibly reactive compounds from the regulatory definition of 
VOC and thus exempt them from regulation as ozone precursors. This 
exemption policy serves two important purposes:
    (1) Because EPA does not give VOC reduction credit for programs 
that

[[Page 54050]]

reduce emissions of negligibly reactive compounds, control efforts are 
focused on emissions that contribute significantly to the formation and 
accumulation of ozone. The Agency continues to believe that it is not 
appropriate, and would be misleading, to give VOC reduction credit to 
States or industries for reducing emissions of compounds that have 
little or no effect on ozone concentrations.
    (2) Because negligibly reactive compounds are not subject to 
regulation as VOCs, industry has an incentive to use negligibly 
reactive compounds in place of higher reactivity compounds. The 
exemption approach also creates a strong incentive for industry to 
invest in the development of negligibly reactive compounds and low 
reactivity formulations. The Agency continues to believe that the 
substitution of ``VOC-exempt'' compounds for regulated VOCs is an 
effective ozone control strategy, even though it is not as effective or 
efficient as the use of a continuous reactivity scale to encourage 
optimal substitutions in terms of ozone control.
    Because the current exemption approach continues to serve these 
purposes, EPA will continue its efforts to identify negligibly reactive 
compounds and exclude them from the federal regulatory definition of 
VOC. The Agency expects that such compounds will also be exempt from 
state VOC control programs, with exceptions made for specific 
reactivity-based rules such as the CARB aerosol coatings rule.
    Since 1977, EPA has used the reactivity of ethane as the threshold 
of negligible reactivity. Compounds that are less reactive than or 
equally reactive to ethane have been deemed negligibly reactive. 
Compounds that are more reactive than ethane continue to be considered 
reactive VOCs and subject to control requirements. The selection of 
ethane is based on a series of smog-chamber experiments that underlies 
the 1977 policy. In these experiments, various compounds were injected 
into a smog chamber at a molar concentration that was typical of the 
total molar concentration of VOCs in Los Angeles ambient air at the 
time (4 ppmv). NOX was injected into the chamber at a 
concentration of 0.2 ppm, and as the chamber was exposed to sunlight, 
the maximum ozone formed in the chamber was measured. The maximum ozone 
formed in the chamber was compared to the level of the NAAQS, which at 
the time was 0.08 ppm of oxidants. Propane was the most reactive 
compound tested that did not cause a maximum ozone concentration 
greater than 0.08 ppm. Ethane was somewhat less reactive than propane. 
Based on these experiments, the Agency determined that ethane should be 
used as the benchmark for identifying compounds whose potential 
contribution to ozone formation was below regulatory concern.
    A more recent modeling study conducted under the auspices of the 
RRWG replicated the essence of the 1970s smog chamber experiments using 
a state-of-the-art airshed model of the eastern United States. In this 
study, Carter et al. replaced all anthropogenic VOC emissions with 
ethane and found that ozone formation decreased almost as much as when 
all anthropogenic emissions of VOC were removed. When anthropogenic 
emissions were removed or when they were replaced with ethane, the 
model still predicted ozone concentrations greater than the level of 
the NAAQS due to emissions of NOX and biogenic VOCs.\11\
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    \11\ Carter, William P. L., Gail S. Tonnesen, and G. Yarwood. 
``Investigation of VOC Reactivity Effects Using Existing Regional 
Air Quality Models.'' Report to the American Chemistry Council, 
Contract SC-20.0-UCR-VOC-RRWG, 2003.
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    The metric used to compare the reactivity of a specific compound to 
that of ethane has varied over time. The primary metric for comparison 
has been kOH, the molar rate constant for reactions between 
the subject compound and the hydroxyl radical (OH). In several cases, 
EPA has also looked at comparisons of MIR values expressed on both a 
molar and a mass basis. Comparing MIR values on a molar basis versus a 
mass basis can lead to different conclusions about whether a compound 
is less reactive or more reactive than ethane. In two cases, acetone 
(60 FR 31633, June 16, 1995) and tertiary butyl acetate (69 FR 69293, 
November 29, 2004), EPA has exempted compounds based on the finding 
that the compounds are less reactive than ethane when compared using 
incremental reactivity values expressed on a mass basis, even though 
they were more reactive on a molar basis.
    The molar comparison is more consistent with the original smog 
chamber experiments, which compared equal molar concentrations of 
individual VOCs, that underlie the selection of ethane as the 
threshold. The mass-based comparison is consistent with how MIR values 
and other reactivity metrics are applied in reactivity-based emission 
limits. The mass-based comparison is slightly less restrictive than the 
molar-based comparison in that a few more compounds qualify as 
negligibly reactive.
    Given the two goals of the exemption policy articulated above, the 
Agency believes that ethane continues to be an appropriate threshold 
for defining negligible reactivity. Furthermore, in light of the second 
goal of encouraging environmentally beneficial substitutions, EPA 
believes that a comparison to ethane on a mass basis strikes the right 
balance between a threshold that is low enough to capture compounds 
that significantly affect ozone concentrations and a threshold that is 
high enough to exempt some compounds that may usefully substitute for 
more highly reactive compounds.
    When reviewing compounds that have been suggested for VOC-exempt 
status, EPA will continue to compare them to ethane using 
kOH expressed on a molar basis and MIR values expressed on a 
mass basis. Consistent with past practice, the Agency will consider a 
compound to be negligibly reactive as long as it is equal to or less 
reactive than ethane based on either one of these metrics. The Agency 
will also consider other reactivity metrics that are provided with 
adequate technical justification, such as metrics based on airshed 
modeling. States may also wish to identify VOC exemptions in their SIPs 
in order to encourage VOC substitutions that would reduce ozone 
formation.
    In the past, concerns have sometimes been raised about the 
potential impact of a VOC exemption on environmental endpoints other 
than ozone concentrations, including fine particle formation, air 
toxics exposures, stratospheric ozone depletion, and climate change. 
EPA has recognized, however, that there are existing regulatory and 
non-regulatory programs that are specifically designed to address these 
issues, and the Agency continues to believe that the impacts of VOC 
exemptions on environmental endpoints other than ozone formation will 
be adequately addressed by these programs. The VOC exemption policy is 
intended to facilitate attainment of the ozone NAAQS, and questions 
have been raised as to whether the Agency has authority to use its VOC 
policy to address concerns that are unrelated to ground-level ozone. 
Thus, in general, VOC exemption decisions will continue to be based 
solely on consideration of a compound's contribution to ozone 
formation. However, if the Agency determines that a particular VOC 
exemption is likely to result in a significant increase in the use of a 
compound and that the increased use would pose a significant risk to 
human health or the environment that would not be addressed adequately 
by existing programs or policies, EPA reserves the

[[Page 54051]]

right to exercise its judgment in deciding whether to grant an 
exemption.
    In all but one of the past exemption decisions, EPA has exempted 
negligibly reactive VOCs from recordkeeping and reporting requirements 
as well as control requirements. Concerns have been raised that even 
negligibly reactive compounds, if present in sufficient quantities, can 
contribute significantly to ozone formation over large spatial scales. 
Without recordkeeping and reporting requirements, States and EPA have 
no regular mechanism for maintaining adequate emissions inventories of 
negligibly reactive compounds or tracking their collective contribution 
to ozone concentrations. One approach for addressing this issue would 
be to require recordkeeping and reporting of all negligibly reactive 
VOC emissions. The Agency recognizes, however, that efforts to develop 
State and local inventories of such emissions are a relatively low 
priority compared to other activities that are likely to be more 
important for reducing ozone concentrations. In particular, as noted 
above, efforts to develop speciated emissions inventories should be 
focused on highly reactive compounds because programs targeted at 
controlling emissions of these compounds are likely to be more 
effective than simply regulating all VOCs equally.
    Another approach that would allow policymakers to track potential 
increases in emissions of negligibly reactive compounds would be to ask 
manufacturers who are responsible for VOC exemption petitions to 
provide EPA with periodic estimates of the magnitude and distribution 
of emissions of the exempted compound. Although such an approach would 
not provide detailed information about the location of such emissions, 
this type of spatial definition is relatively unimportant for compounds 
with negligible reactivity. The Agency believes that parties submitting 
VOC exemption requests may be able to provide emissions estimates that 
are sufficient for purposes of tracking the potential effects of VOC-
exempt compound emissions on regional air quality. The Agency may 
consider such an approach in the future.

VI. Summary

    EPA encourages States, and particularly those with persistent ozone 
nonattainment problems, to consider recent scientific information on 
VOC reactivity and how it may be incorporated into the development of 
ozone control measures. Using reactivity information, States may be 
able to improve the effectiveness and efficiency of their VOC control 
policies. EPA encourages all interested parties to continue to work 
through the RRWG to improve the scientific foundation for reactivity-
based regulatory approaches. Although most existing VOC control 
programs do not discriminate between individual VOCs based on 
reactivity, they continue to provide significant ozone reduction 
benefits and will remain in place unless and until they are replaced by 
programs that achieve the same or greater benefits. Therefore EPA will 
continue its policy of granting VOC exemptions for compounds that are 
negligibly reactive. EPA will continue to evaluate new scientific 
information regarding VOC reactivity and will update this interim 
guidance as appropriate. This interim guidance does not change any 
existing rules.

List of Subjects in 40 CFR Part 51

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Nitrogen dioxide, 
Ozone, Particulate matter, Reporting and recordkeeping requirements, 
Volatile organic compounds.

    Dated: August 25, 2005.
Jeffrey R. Holmstead,
Assistant Administrator for Air and Radiation.
[FR Doc. 05-18015 Filed 9-12-05; 8:45 am]
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