1. Road Paving as an Option for Generating PM10 Emission
Reduction Credits
Temporary Paper ID # 69538
Joel Reisman
Greystone Environmental Consultants, Inc., 10470 Old Placerville Road, Suite 110, Sacramento,
California 95827
ABSTRACT
When applying for air permits for major sources in areas designated as state- or federal
nonattainment areas, in addition to using lowest achievable emission rate (LAER) controls, it is
also necessary to “offset” potential project emissions increases of the nonattainment criteria
pollutant or pollutants by finding and retiring emissions somewhere in the same air basin.
Virtually all of the Air Basins in California are classified as nonattainment relative to the state
Ambient Air Quality Standards for particulate PM10. Furthermore, the scarcity of PM10
Emission Reduction Credits (ERCs) is causing great difficulties in the permitting of large
sources, especially power plants. In certain desert southwest areas, fugitive dust from vehicular
traffic on unpaved roads has been identified as a significant contributor to the regional PM10
nonattainment status; hence, road paving is a legitimate option for reducing PM10 emissions and
generating valid ERCs. This paper presents the methodology and procedures for generating PM10
ERCs by paving unpaved roads. The steps required in obtaining an ERC certificate, including
data necessary to establish and justify existing baseline PM10 emissions that will qualify as
ERCs, calculational procedures, and other application requirements, are discussed.
INTRODUCTION
When applying for an air permit to construct a new major source project or major modification to
an existing source in a nonattainment area for a given regulated pollutant, New Source Review
(NSR) regulations require that, in addition to using lowest achievable emission rate (LAER)
controls, the applicant must also “offset” planned emissions increases of the nonattainment
pollutant (or any of its precursors) by reducing or removing emissions elsewhere in the
nonattainment area. The emission reductions must meet certain requirements set by the
regulatory agency before they can be certified as “Emission Reduction Credits” (ERCs), and
“banked” in the agency’s registry. Needless to say, it is becoming very difficult and expensive to
either purchase banked ERCs (if available and for sale), or generate qualified ERCs by locating
existing sources of emissions, negotiating with their owners, and installing controls in return for
the ERCs.
This paper focuses on the issues associated with obtaining PM10 (particulate matter smaller than
10 microns in aerodynamic diameter) ERCs. This paper discusses the current costs of ERCs, the
requirements of qualifying as an ERC, potential sources of ERCs, how to use paving of unpaved
roads to generate ERCs, and the regulatory process of applying for and creating ERCs.
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2. PM10 ERCs
PM10 ERCs are scarce and expensive. For the purposes of estimating the cost of offsetting the
PM10 emissions, we will assume a typical 520 MW combined-cycle, natural gas-fired power
plant. Table 1 presents the range of the variables that would affect the costs of ERCs.
Table 1. Estimate of Costs of PM10 ERCs Necessary to Offset a Typical 520 MW Natural Gas
Power Plant.
Variable Range of Variables
Low High
PM10 Emission Rate 50 tpy* 150 tpy
Unit Cost of PM10 ERCs $5,000 per ton $20,000 per ton
(market price)
Offset Ratio Required 1.0 2.0
Total Cost $250,000 $6,000,000
* tpy = tons per year
Hence, for this type of project, the cost of offsetting PM10 emissions by purchasing ERCs
(assuming they are available and the owner(s) is willing to sell them) is estimated to be
somewhere between $250,000 and $6,000,000.
ERC Eligibility
Any source may reduce or eliminate PM10 emissions; however, there are a number of conditions
that must be satisfied before any reduction of PM10 emissions will qualify for certification by an
air quality regulatory agency as banked ERCs. Specifically, to qualify as ERCs, emission
reductions must be:
• Real
• Permanent
• Quantifiable
• Surplus
• Enforceable
• Timely
A brief explanation of each qualification is provided:
Real
Emissions must actually be or have been occurring, implemented and not artificially devised.
For example, all of a source’s permitted emissions do not automatically qualify as ERCs,
because the source may not have actually been operating or emitting at the permitted rate. Also,
reducing emissions by virtue of changing an emission factor would not qualify.
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3. Permanent
In the case of a shutdown, the permit must be surrendered. If the equipment is to be modified or
a limit taken, such as on hours of operation, the permit must be modified or surrendered and a
new permit issued.
Quantifiable
The applicant must provide some method of quantifying the actual historical emissions. For
example, by records of hours of operation, emission factors, and/or other data, the applicant must
be able to calculate what the actual historic emissions have been within a recent time period,
such as the most recent two years. Sometimes, another operating period may be negotiated, if,
for example, the last two years can be shown not to be representative of normal operations.
Oftentimes a source test is necessary to establish the actual emission factor.
Surplus
Emission reductions must be in excess of what are otherwise required by Federal, State, or local
law, rule, order, permit or regulation. For example, only emission reductions over and above any
existing regulations identifying Reasonably Available Control Technology (RACT) or
Reasonably Available Control Measure (RACM), for the equipment or process being modified
or shutdown in the area where the modification will occur, will be eligible.
Enforceable
The reduction in emissions must be verifiable and legally binding.
Timely
In most situations, the administering agency requires that ERCs be applied for in a reasonable
amount of time after a change is made, typically six months.
Potential Sources of PM10 ERCs
Potential sources of eligible PM10 ERCs and their limitations are presented in Table 2.
Table 2. Potential Methods for Reducing PM10 Emissions.
Method Limitations
Shutdown, replace, or install Cost. Quantity of emissions that would be
controls on combustion sources. eligible as ERCs may be limited.
Change fuel type, e.g., from Cost of retrofit and availability of fuel must
diesel to natural gas. be considered. Source test would likely be
required.
Install controls on material Most methods are already required as part of
handling sources of fugitive permitting requirements.
dust.
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4. Limit/control certain agricultural Difficult to quantify, justify, enforce.
activities (e.g., burning).
Install controls on mobile Usually only valid for a limited time.
sources.
Take enforceable permit limits Must still demonstrate that resulting
(e.g., limit hours of operation). reductions are real; i.e., lower than historical
emissions.
Upgrade woodstoves. Cost, limited quantity of PM10. Only
available in certain locations and time of
year.
Pave unpaved roads Need sufficient traffic to be cost-effective.
Not viable in areas that already require this
as part of an air quality attainment plan.
ROAD PAVING OPTION
In those areas where there are no agency rules that specifically require paving of modified roads,
and if sufficient traffic is present to make the project cost-effective, the applicant must be able to
quantify and justify the amount of ERCs applied for, and to know and follow the application
process.
Calculating Potential PM10 ERCs
The maximum potential quantity of PM10 ERCs that can be banked for a road-paving project is
the difference between the present unpaved road emissions and the paved road emissions that
would occur after the project is complete.
The Unpaved Road Emission Factor, Eext, as defined in AP-42, section 13.2.21, is:
Eext = k(s/12)a (W/3)b [(365-p)/365] [lb/VMT] [1]
(Mdry/0.2)c
Where,
Eext = annual size-specific emission factor extrapolated for natural mitigation
k = particle size multiplier for particle size range and units of interest (k = 2.6
lb/VMT for PM10)
s = surface material silt content (%) (applicable range 1.2 – 35%)
W = mean vehicle weight (tons) (applicable range 1.5 – 290)
Mdry = surface material moisture content under dry, uncontrolled conditions
(default value 0.2%)
p = number of days per year with at least 0.254 mm (0.01 in.) of precipitation
VMT = Vehicle miles traveled
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5. Equation [1] assumes the vehicle speed exceeds 15 mph. If vehicle speeds on the roadway are
less than 15 mph, actual PM10 emissions would be significantly lower than predicted and the
equation is not valid. Furthermore, AP-421 makes a point of noting that the vehicle-related
source conditions refer to the average weight and number of vehicles traveling the road. For
example, if 98 percent of the traffic on the road is 2-ton cars and trucks and the remaining 2
percent is 20-ton trucks, the mean weight is 2.4 tons. Equation [1] is not intended to be used to
calculate a separate emission factor for each vehicle class within a mix of traffic on a given
unpaved road (i.e., it is not appropriate to determine one factor for the 2-ton vehicles and a
second factor for the 20-ton trucks).
The Paved Road Emission Factor as defined in AP-42, section 13.2.12, is:
E = k(sL/2)0.65 x (W/3)1.5 [lb/VMT] [2]
Where,
E = particulate emission factor (having units matching the units of k)
k = particle size multiplier for particle size range and units of interest (k =
0.016 lb/VMT for PM10)
sL = road surface silt loading (grams per square meter) (g/m2)
W = average weight (tons) of the vehicles traveling the road
Hence, the amount of potential PM10 ERCs is:
PM10 ERCs = ∑ [Eext x VMTunpaved x hrs/yr] – ∑ [E x VMTpaved x hrs/yr] (lb/yr) [3]
The summation sign indicates that there may be several terms due to differences in VMT for
different vehicle mixes, variations in traffic patterns by month or season, etc. In most cases, the
VMTunpaved is equal to the VMTpaved; however, there may be some situations where the existence
of a paved road may result in increased traffic.
Data Needs
In order to calculate the unpaved road and paved road emission factors in equations [1] and [2],
the following data are needed:
• Traffic, VMT
• Mean vehicle weight, W
• Silt content, s
• p, k, a, b, c
• Moisture content, Mdry
• Silt loading, sL
Traffic and mean vehicle weight data are often the most difficult to establish. This subject is
discussed in the next section.
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6. Silt content, s, is readily determined by obtaining a soil sample of the road and analyzing it using
test method ASTM-C-136, or equivalent. “p” may be interpolated from isopleth data presented
in Figure 13.2.2-1 of AP-42 for geographic areas in the United States. For unpaved roads, the
constants k, a, b, and c are presented in Table 13.2.2-2, and are equal to 2.6, 0.8, 0.4, and 0.3,
respectively, for PM10 (other constants are also available for PM2.5 and PM30, if needed, in the
same table). For paved roads, k is presented in Table 13.2.1-1 as 0.016 for PM10, with units of
lb/VMT.
Fortunately, default values are available in AP-42 for Mdry and sL. In the absence of site-specific
data, Mdry should be set to 0.2 percent. The recommended default silt loading value for public
paved roads under worst-case conditions is 3 g/m2 for “Low” ADT (average daily traffic) roads,
which are most likely the type being considered in this type of situation. See AP-422 for detailed
discussion.
Traffic Data
Actual traffic data measured on the roadway over a sufficient amount of time is preferred to
establish the typical condition. The amount of time for data collection will vary according to the
variability in traffic. For example, for an access road to a single destination with a constant
amount of employees, several weeks may be sufficient to confirm the traffic pattern. However,
for multiple destinations or a situation where activities are seasonal, several months may be
required to characterize the traffic flow. If time does not permit waiting for high-season activity
to begin again, it may be necessary to make estimates using direct or indirect data. For example,
for harvesting activity, the commute traffic could be based upon the number of workers
employed historically and a reasonable assumption of employees per vehicle. Similarly, truck
traffic could be estimated based on historical data of the quantities of products shipped, and type
of trucks used. In some cases, the use of indirect data may be needed to provide a traffic ratio
between low and high season activity. For example, in the case of an access road to a seasonal
use subdivision of residences, traffic data could be collected during a low season period and
monthly sales data from a local store, or other “marker” data could be used to predict traffic
variations over the year.
Example
As an example, using equations [1], [2], and [3], recommended coefficients for PM10, and the
following assumptions:
s = 20 %
W = 2 tons (default for autos/pickup trucks)
Mdry = 0.2%
p = 20
sL = 3 g/m2 (paved road)
Road length = 1 mile
No. of VMT = 100 one-way trips per day, 5 days per week, 52 weeks per year
With this scenario, unpaved road PM10 emissions are calculated at 40.88 tpy; paved road
emissions are 0.01 tpy, for a net reduction of 40.87 tpy. If the total number of VMT remains the
same at 100, but the vehicle mix is changed to 90 one-way trips of autos/pickup trucks, five of
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7. empty tractor-trailers (12 tons), and five of fully loaded tractor-trailers (40 tons), the net
reduction becomes 56.04 – 0.04 = 56.00 tpy, a significant increase.
ERC PROCESS
Once a potential road-paving project is identified and preliminary data are collected to confirm
the cost effectiveness of the project is satisfactory, it is important to arrange for a meeting with
the administering air agency to present the proposed plan. The purpose of the meeting should be
to present the proposal to the agency, answer questions, confirm there are no “fatal flaws” that
would stop the project, and ensure that the agency is in agreement with the project. Some
potential stumbling blocks could be existing or planned Best Available Control Measure
(BACM) requirements, seasonal offsetting requirements, or similar previously proposed projects
that have been rejected for other reasons, such as negative comments from EPA or interveners.
Assuming the agency agrees with the proposal, data can then be collected, and an application for
ERCs may then be prepared. After agency review and comments are submitted, the owner of the
road should be contracted to ensure that it would surrender the ERC certificate once it is issued,
in return for the paving benefit. The agency will not issue an ERC certificate until the project is
completed, and then, only to the owner of the road, municipality, or agency responsible for
ensuring the road will be maintained for at least the life of the plant.
SUMMARY
Due to the current costs and availability of existing banked PM10 ERCs, applicants must become
more creative in generating them to ensure their projects will be permittable. There are a number
of qualifications that must be met to qualify as an ERC, and a solid argument of verification must
be built to gain concurrence from regulatory agencies. Under favorable circumstances, paving
unpaved roads may be a cost-effective method to generate sufficient PM10 ERCs to permit a
large project.
REFERENCES
1. U. S. EPA. Compilation of Air Pollutant Emission Factors, AP-42 Fifth edition, Supplement
E, Volume I: Stationary Point and Area Sources, Chapter 13.2.2, Unpaved Roads,
http://www.epa.gov/ttn/chief/ap42/, United States Environmental Protection Agency, Office
of Air Quality Planning and Standards, September 1998.
2. U. S. EPA. Compilation of Air pollutant Emission Factors, AP-42 Fifth edition, Update
2002, Volume I: Stationary Point and Area Sources, Chapter 13.2.1, Paved Roads,
http://www.epa.gov/ttn/chief/ap42/, United States Environmental Protection Agency, Office
of Air Quality Planning and Standards, October 2002.
KEY WORDS
Emission Reduction Credits
ERC
Fugitive Dust Emissions
PM10 emissions
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