Attachment 

Using the November 2006 AP-42 Update in SIPs and Transportation Conformity 

Determinations in PM2.5 and PM10 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? 

PM2.5: The only changes to the AP-42 methods are the factors for calculating the 

portion of PM10 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 PM10 road dust. The 

November 2006 update changed the December 2003 version by decreasing the 

percentage of PM10 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. 

PM10: The November 2006 update does not change the calculations of PM10 road 

dust emissions, since the November 2006 update incorporates the previous version’s 

PM10 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. 

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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/ch13/bgdocs/b13s02.pdf . 

3

 See Clean Air Act section 172(c)(3) and 40 CFR 51.112(a)(1).

3. Why did EPA make these changes? 

EPA updated the PM2.5 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 PM10 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 PM2.5 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 PM2.5 areas have to use the November 2006 update for including reentrained 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 PM2.5 nonattainment problem (40 CFR 93.102(b)(3)): 

• Prior to a PM2.5 area having adequate or approved PM2.5 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 PM2.5 nonattainment 

problem and has so notified the MPO and DOT (40 CFR 93.102(b)(3) and 

93.119(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 PM2.5

transportation conformity analyses before SIP budgets are adequate or approved, 

unless EPA or the state finds road dust significant. 

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4 The release of the November 2006 AP-42 update does not trigger the need for quantitative PM2.5 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)). 

• After a PM2.5 area has adequate or approved PM2.5 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 PM2.5 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 93.111(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 93.111(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 PM10 areas have to use the November 2006 update for including reentrained road dust in new SIPs and transportation conformity 

determinations? 

 PM10 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 PM10 road dust emissions calculated 

by the November 2006 update. Because the November 2006 update supersedes the 

December 2003 version, PM10 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/ch13/index.html. 

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5 A copy of the February 24, 2004, memo can be downloaded from EPA’s website at 

http://www.epa.gov/otaq/models/mobile6/mobil6.2_letter.pdf

7. Can PM2.5 and PM10 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 93.111(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 reentrained 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 AP42 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 

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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. 

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