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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
AUG " 2 2007
OFFICE OF
AIR AND RADIATION
MEMORANDUM
SUBJECT:
FROM:
Policy Guidance on the Use of the November 1, 2006, Update to AP-42
for Re-entrained Road Dust for SIP Development and Transportation
Conformity
TO
Merrylin Zaw-Mon, Director
Transportation and Regional Programs Division
Office of Transportation and Air Quality
A \
Peter Tsirigotis, Director jr /%* ği S
Sector Policies and Programs Divisisjij
Office of Air Quality Planning and Standards
EPA Regional Air Directors, Regions I - X
On November 1, 2006, EPA's Office of Air Quality Planning and Standards
(OAQPS) released changes to the AP-42 methods used for calculating PM2.5 re-entrained
road dust emissions. These changes revise the approved version of AP-42 for PM^.s and
PMio road dust calculations, and will result in 40% less PMb.5 road dust from paved roads
and 33% less PM2.5 road dust from unpaved roads. Please note, however, that the PMio
estimates of re-entrained road dust remain unchanged.
The attached guidance provides additional details about the changes made to the
AP-42 methods for road dust and how and when to use them in PM2.s SIPs and
transportation plan and transportation improvement program (TIP) conformity
determinations. It also reaffirms that PMio road dust estimates are unchanged from
the previous version. The attached guidance supersedes the AP-42 portions of EPA's
February 24, 2004, guidance, which addressed both MOBILE6.2 and AP-42. The
portions of the February 2004 guidance addressing MOBILE6.2 remain in effect.
We hope you will find this document helpful for conveying information to your
states and transportation agencies that will be including road dust in their PM? 5 and PMio
SIPs and transportation conformity determinations. For additional information:
Internet Address (URL) http://www.epa.gov
Recycled/Recyclable (Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 50% Postconsumer content)
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Today's policy guidance, the 2004 policy memo, and the Federal Register notice
in which EPA approved MOBILE6.2 and the previous version of AP-42 can be
found at: http://www.epa.gov/otaq/stateresources/transconf/policy.htm.
The new AP-42 methods for calculating road dust can be found at:
http ://www. epa. go v/ttn/chief/ap42/ch 13/index. html.
General questions about this guidance can be directed to Laura Berry of EPA's
Office of Transportation and Air Quality (OTAQ) at email address:
berry.laura@epa.gov or phone number: (734) 214-4858, or Joe Pedelty of the
same office at: pedelty.ioe@epa.gov or phone number: (734) 214-4410.
Specific questions regarding the new AP-42 methods should be directed to John
Bosch of OAQPS at email address: bosch.john@epa.gov or phone number (919)
541-5583.
Attachment
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Attachment
Using the November 2006 AP-42 Update in SIPs and Transportation Conformity
Determinations in PM2.s and PMi0 Nonattainment and Maintenance Areas
1. What is AP-42?
AP-42 is EPA's compilation of data and methods for estimating average emission
rates from a variety of activities and sources from various sectors. This guidance is
concerned only with the sections of AP-42 that address emissions of re-entrained road
dust (also referred to as "road dust" in this document) from paved and unpaved roads:
Section 13.2.1, Paved Roads and Section 13.2.2, Unpaved Roads. Refer to EPA's
website http://www.epa.gov/ttn/chief/ap42/index.html for more information about AP-42
in general.
The AP-42 methods for road dust previously in effect were issued in December
2003. For the purposes of this guidance, the "November 2006 update" refers to the
update to these two sections released on November 1, 2006. The "December 2003
version" refers to the AP-42 methods previously in effect for these two sections, issued in
December 2003 and approved on May 19, 2004 (69 FR 28830) to replace PART 5
emissions factors and earlier AP-42 methods for road dust.
2. How is the November 2006 update different from the December 2003
version?
PMg_5: The only changes to the AP-42 methods are the factors for calculating the
portion of PMio road dust emissions that consist of PM2.5 from paved and unpaved roads,
which are contained in Sections 13.2.1 and 13.2.2 of AP-42.1 EPA's AP-42 methods
estimate the amount of PM2.5 road dust as a percentage of PMio road dust. The
November 2006 update changed the December 2003 version by decreasing the
percentage of PMio road dust that is thought to be PM2.5 road dust. As a result of this
change, calculated emissions for the PM2.5 fraction of re-entrained road dust from paved
and unpaved roads will be reduced by approximately 40% and 33%, respectively.
Nothing else in the methods has changed. For example, there are no changes to the basic
algorithm used for the calculations or the data required to perform the calculations for
road dust.
PMio: The November 2006 update does not change the calculations of PMio road
dust emissions, since the November 2006 update incorporates the previous version's
road dust methods.
1 These changes apply for all purposes. For example, if other sections of AP-42 reference Sections 13.2.1
and 13.2.2, the November 2006 update applies.
EPA420-B-07-005
August 2007
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3. Why did EPA make these changes?
EPA updated the PM2.s fractions of road dust from paved and unpaved roads in
AP-42 because a number of fugitive dust studies indicated that the December 2003
version overestimated PM2 5. Specifically, studies found the amount of PMi0 consisting
of PM2.5 measured by EPA Federal Reference Method (FRM) samplers was significantly
lower than predicted by the December 2003 version. A February 2006 study confirmed
that the method used to develop the December 2003 version results in more PM2.5 than
FRM samplers measure.2 Based on this information, EPA corrected the emission factors
in the November 2006 update.
4. When do PM2.s nonattainment or maintenance areas have to use the
November 2006 update for SIPs that are currently under development or will
be in the future?
In general, EPA believes that areas should begin to use the November 2006
update for their PM2.5 state implementation plans (SIPs) as soon as possible, unless EPA
has approved a locally-developed road dust methodology. The Clean Air Act requires
that SIP inventories and control measures be based on the most current information and
applicable models that are available when the SIP is developed.3 If you have questions
about which model should be used in your SIP, please consult your EPA Regional Office.
5. When do PM2.s areas have to use the November 2006 update for including re-
entrained road dust in transportation conformity determinations?
First, note that transportation conformity applies with respect to re-entrained road
dust in a PM2.5 area only if emissions from re-entrained road dust are determined to be a
significant contributor to the PM2.s nonattainment problem (40 CFR 93. 102(b)(3)):
Prior to a PM^ area having adequate or approved PM^ SIP budgets, re-entrained
road dust would only be included in regional emissions analyses and hot-spot
analyses if the EPA Regional Administrator or state air quality agency determines
that re-entrained road dust is a significant contributor to the PM2.s nonattainment
problem and has so notified the MPO and DOT (40 CFR 93. 102(b)(3) and
93. 1 19(f)(8)). In other words, PM2.5 areas can presume that re-entrained road dust
is not a significant contributor and therefore not include road dust in PM2.s
transportation conformity analyses before SIP budgets are adequate or approved,
unless EPA or the state finds road dust significant.
2 "Proposed Revisions to Fine Fraction Ratios Used for AP-42 Fugitive Dust Emission Factors", Midwest
Research Institute Project 110397, February 1, 2006. This study can be found on EPA's website at:
http://www.epa.gov/ttn/chief/ap42/chl3/bgdocs/bl3s02.pdf.
3 See Clean Air Act section 172(c)(3) and 40 CFR 51.112(a)(l).
EPA420-B-07-005
August 2007
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After a PMg_5 area has adequate or approved PMg_5 budgets, the area will need to
include re-entrained road dust in regional emissions analyses and hot-spot
analyses only if the PM2.5 SIP budgets include re-entrained road dust (40 CFR
93.102(b)(3)).
When a PM2.s area must consider re-entrained road dust in transportation
conformity and does not have an EPA-approved local method, the November 2006
update should be used for any new transportation plan or TIP conformity analyses begun
after the release of today's guidance.4 The transportation conformity rule requires that
the conformity determinations be based on the latest emission factor models that are
approved by EPA for SIP purposes (40 CFR 93.11 l(a)). While the transportation
conformity rule provides for a grace period of three to 24 months before a new model is
required to be used for transportation conformity determinations (40 CFR 93.11 l(b)), the
changes to the PM2.5 fractions in the November 2006 update do not constitute a new
model and therefore no grace period applies for its release.
6. When do PMio areas have to use the November 2006 update for including re-
entrained road dust in new SIPs and transportation conformity
determinations?
areas that are in the middle of developing a SIP or transportation plan or
TIP conformity determination using the December 2003 version of AP-42 can continue
using that version because there is no change in the PMio road dust emissions calculated
by the November 2006 update. Because the November 2006 update supersedes the
December 2003 version, PMio areas should use the November 2006 update of AP-42 for
any new SIPs or transportation plan or TIP conformity analyses begun after the date of
this memo simply for clarity and consistency. The November 2006 update is now found
on EPA's AP-42 website instead of the December 2003 version at
http://www.epa.gov/ttn/chief/ap42/chl3/index.html.
4 The release of the November 2006 AP-42 update does not trigger the need for quantitative PM25 or PM10
hot-spot modeling at this time. Qualitative analyses are currently required for certain projects in PM2 5 and
PM10 areas (40 CFR 93.123(b)).
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7. Can PM2.s and PMi0 areas use alternative approaches to the November 2006
update to determine re-entrained road dust emissions for SIPs and
transportation conformity determinations?
Yes, with EPA's approval. Some nonattainment or maintenance areas have
historically used alternative methods for estimating re-entrained road dust emissions that
may be more appropriate than the AP-42 methods given specific local conditions. Other
nonattainment or maintenance areas may develop alternatives to the AP-42 methods in
the future. EPA will evaluate such alternative approaches in the context of their planned
use to determine if they can be used instead of AP-42, and will approve alternative
approaches where appropriate for use in particular areas. Once EPA approves an
alternative approach, that approach would be the latest emissions model for road dust in
the area for which we approved it, based on 40 CFR 93.11 l(a). State and local agencies
should consult with EPA Regional Offices about the technical validity of proposed
alternative approaches and about how today's guidance would apply to those approaches
prior to using an alternative method to develop a SIP or conformity determination for re-
entrained road dust. EPA Regional Offices will evaluate alternatives to AP-42 in
consultation with EPA's Office of Air Quality Planning and Standards (OAQPS) and
Office of Transportation and Air Quality (OTAQ).
EPA also notes that the November 2006 update is the approved method only for
situations for which silt loading, mean vehicle weight, and mean vehicle speed on paved
roads fall within ranges given in AP-42, section 13.2.1.3 and with reasonably free
flowing traffic. For other conditions, areas may use an appropriate method approved by
EPA on a case-by-case basis, as described above.
8. Does this document supersede EPA's February 24, 2004, memo covering the
use of MOBILE6.2 and the December 2003 AP-42 version?
This document supersedes only the AP-42 portions of the February 24, 2004, EPA
memo entitled, "Policy Guidance on the Use of MOBILE6.2 and the December 2003 AP-
42 Method for Re-Entrained Road Dust for SIP Development and Transportation
Conformity."5 The portions of the February 2004 guidance addressing MOBILE6.2
remain in effect.
9. Does this document create any new requirements?
The discussion in this document is intended solely as guidance. The statutory
provisions and EPA regulations described in this document contain legally binding
requirements. This document is not a regulation itself, nor does it change or substitute
for those provisions and regulations. It merely explains how to apply the change
A copy of the February 24, 2004, memo can be downloaded from EPA's website at
http://www.epa.gOv/otaa/models/mobile6/mobil6.2 letter.pdf
EPA420-B-07-005
August 2007
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contained in the November 2006 update to AP-42 for calculating re-entrained road dust.
EPA retains the discretion to adopt approaches on a case-by-case basis that may differ
from this document, but still comply with the statute and regulations. Any decisions
regarding a particular SIP or transportation conformity determination will be made based
on the statute and regulations. This document may be revised periodically without public
notice.
EPA420-B-07-005
August 2007
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