1

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2000 TRAVERWOOD ROAD

ANN ARBOR, MI 48105

May 4, 2012

OFFICE OF AIR AND RADIATION

MEMORANDUM

SUBJECT: PM Emissions Impacts

FROM: Lauren Steele, Office of Transportation and Air Quality

TO: Docket EPA-HQ-OAR-2011-1032

Estimate of Particulate Matter Emissions from Regulated Emergency 

Vehicles 

The Heavy-Duty Highway Rule was projected to reduce direct particulate matter (PM)

exhaust emissions from the heavy-duty on-road fleet by 71,000 tons in 2020, and by 96,000 tons 

in 2030. In that rule’s final Regulatory Impact Analysis, EPA projected that in 2010 the 

controlled heavy-duty fleet would emit approximately 57,000 tons of direct exhaust PM 

emissions, decreasing to 15,000 tons in 2020 and 8,000 tons direct PM exhaust in 2030, as a 

result of that rule.1For purposes of this memo, we are considering that rule’s control case as the 

current fleet baseline scenario.

As noted in Section VII.B of the proposed rule preamble, class 3 through 8 fire apparatus 

and ambulances represent about 0.8 percent of annual new heavy-duty truck registrations.2If we 

assume these vehicles have the same emissions profile over time as other vehicles, we could 

estimate that their annual emissions collectively may also represent a comparable fraction of the 

fleet emissions in a given year. For the years projected in the RIA for the Heavy-Duty Highway

Rule, this would indicate emergency vehicles may have emitted on the order of 500 tons direct 

exhaust PM in 2010, may emit about 100 tons in 2020, and may emit about 60 tons in 2030.

However, we have information that these vehicles have longer than average useful lives, 

leading to a slower fleet turnover rate. We also have information that these vehicles have below 

average vehicle miles traveled, with a large fraction of their engine hours at low load or idle.

While these two factors would affect the emissions inventory in opposite directions, they are 

 

1

 Final Regulatory Impact Analysis for the “2007 Heavy-Duty Highway Rule,” EPA420-R-00-026, December 2000, 

Chapter II Health and Welfare Concerns and Emissions Benefits, Table II.B-5, available at 

http://www.epa.gov/otaq/highway-diesel/regs/ria-ii.pdf

2

ICCT, May 2009, “Heavy-Duty Vehicle Market Analysis: Vehicle Characteristics & Fuel Use, Manufacturer 

Market Shares.”
2

unlikely to cancel each other out. Because we do not have enough data to quantify how these 

factors affect the emission estimates, we will use the values presented above for purposes of the 

current analysis.

On a per-truck basis, the direct PM exhaust emissions from a certified engine would be 

0.01 gram per brake-horsepower-hour. As an illustration, we have estimated what that might 

represent in terms of emissions from a truck operating at 200 horsepower over a typical year with 

1,200 operating hours. This would be representative of a 500-hp pumper truck with an average 

engine operating load of 40 percent of capacity. This illustration would yield a hypothetical fire 

truck that would emit about 5.3 pounds of direct PM exhaust emissions in a year, or 0.002 tons 

per year, or 0.04 pounds in 8 hours.

These estimates are provided as a basis for comparison, as the following sections explore 

some different scenarios.

Estimate of Particulate Matter Emissions from Emergency Vehicles with 

Periodic Uncontrolled Emissions

We expect manufacturers who choose to develop optional auxiliary emission control 

devices (AECD’s) for emergency vehicles to employ strategies that prevent the occurrence of 

abnormal conditions with respect to the emission control system. Where preventive strategies 

alone are not demonstrated to be failsafe, EPA expects there may be instances where it is 

justified to provide engine exhaust backpressure relief, either mechanical or through other 

means. While we expect this will not be a widespread solution, there may be cases where a relief 

valve may be employed on a vehicle whose diesel particulate filter (DPF) became plugged

frequently, allowing temporary emission control bypass to occur as a last resort to prevent engine 

failure.

We expect manufacturers would consider reduced PM control as an option of last resort. 

Nonetheless, if it were to be employed with DPF’s on vehicles that frequently become plugged, 

leading to temporary emission control lapses, there would be some potential emissions impacts.

Some factors to consider when estimating potential emission impacts include how many vehicles 

employ this option, how frequently these vehicles are in a condition of reduced PM control, the 

duration of each reduced control event, and the uncontrolled engine-out PM emission rate.

Without speculating what fraction of the fleet may employ this option of last resort, we 

will focus on a per-truck estimate for this illustration. In this analysis, we are using a low 

estimate of engine-out PM at 0.06 g/bhp-hr.3 We are also estimating a high engine-out PM rate 

of 0.2 g/bhp-hr. We chose this as a high-end value based on the case of a certified engine with a 

DPF that has a 95 percent removal efficiency. The engine-out emission rate would backcalculate to be 0.2 g/bhp-hr in that case. We do not have specific information about an engine 

with higher engine-out PM than this, though it is possible such engines are on the market. We 

are now going to continue with our example of a truck with a 500-hp engine operating at an 

average load of 40%, for a working power of 200 hp and 1,200 operating hours in a year. If this 

 

3 NREL 2000, DESCE Final Report, test results interpolated for 15 ppm S fuel from results given at 3 and 350 ppm 

S fuel.
3

vehicle has a regeneration interval of 25 engine hours, this example assumes a worst case where 

the DPF plugs during every regeneration cycle. That is, we assume it is not able to be actively

regenerated due to engine operating conditions and reaches a backpressure sufficient to open a 

bypass every 25 engine hours. We also estimate the operating time under bypass before the 

plugged DPF is regenerated and normal conditions resume may range from one to four hours. 

Table 1 presents the results of this illustrative example.

Table 1: Per-Truck Temporary Bypass PM Emission Rates

Scenario Pounds per 

Year

Tons Per Year Pounds Over 

Worst 8 Hours

100% Controlled Status 5.3 0.003 0.04

1-hour bypass each cycle at low 

engine-out PM 6.3 0.003 0.06

1-hour bypass each cycle at high 

engine-out PM 9.3 0.005 0.12

4-hour bypass each cycle at low 

engine-out PM 9.5 0.005 0.12

4-hour bypass each cycle at high 

engine-out PM 21.4 0.011 0.37

As shown in Table 1, the effects of bypass lead to a quadrupling of annual per-truck 

emissions in the case of our high-end estimate. In the last column, the value presented indicates 

the 8-hour period includes a bypass of the length specified with the balance of the time in a 

controlled status. The final scenario indicates the 8-hour PM emissions would be less than half a 

pound if during four of those hours the engine emitted PM at 0.2 g/bhp-hr.

One of the AECD options discussed in the preamble is an engine recalibration. The 

possibility exists that some manufacturers may choose to employ engine recalibration or alter 

other engine combustion parameters to reduce engine-out PM emissions, reducing the soot load 

to DPF’s. This would work to reduce the occurrence of, or prevent the situation where a DPF 

could plug and enter into the reduced PM control conditions discussed here.

Estimates of Particulate Matter Emissions from Structural Fires

In the comment letters EPA received urging swift action providing relief for emergency

vehicles, comparisons were made regarding the magnitude of the pollution from a structural fire 

in relation to the tailpipe emissions of a fire truck. To provide some perspective on this, EPA is 

providing an example of one possible method of estimating emissions from structural fires, as 

described in a 2001 report sponsored by EPA as part of its Emission Inventory Improvement 

Program, with its state regulatory partners.

4

 

 

4 Report for the Area Sources Committee of the Emission Inventory Improvement Program (EIIP), Volume III, 

Chapter 18, Structure Fires, January 2001.
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Following the second alternative method for estimating emissions from structural fires

described in that report, some national average factors are employed: an average frequency

factor of the fires occurring over time in a population, a default fuel loading of tons burned per 

fire, and an emission factor of PM emitted per ton of fuel burned. That 2001 document 

presented a frequency factor of 2.3 fires per 1,000 residents based on 1994 data.5 It also included 

a PM emission factor of 10.8 lb per ton burned that was derived in 1972 and has been 

incorporated in both EPA and California methodologies.

6

Those factors are employed in this 

analysis, along with a 2012 U.S. population estimate of 313 million.7The estimate of fuel 

loading appears to be more variable, thus we present three values here. The 2001 EIIP report 

estimated a fuel loading of 1.15 tons burned per fire, while an updated CARB area source 

methodology guideline estimates fuel loading at 1.28 tons burned per fire.8 A similar guideline 

issued by the Bay Area AQMD estimates a fuel loading of 1.95 tons burned per fire.

9

The 

methodology for assessing this value yields variable results because it involves assumptions 

regarding the size and value of a home, its combustible contents, and the percent loss rate. Using 

these three different estimates of fuel loadings, EPA has estimated a range of likely emissions 

from structural fires in the U.S. The results are presented in Table 2.

Table 2: Structural Fire Emission Estimates

Source Pounds PM per fire Tons PM per Year

2001 EIIP 12.4 4,500

1999 CARB 13.8 5,000

1999 BAAQMD 21.1 7,600

From this analysis, EPA estimates that something on the order of 4,000 to 8,000 tons of 

PM may be emitted in the U.S. each year from structural fires, with each individual residential 

structure fire incident possibly emitting on the order of 10 to 20 pounds of PM.

As a comparison, the baseline heavy-duty fleet at the time of the Heavy-Duty Highway

Rule emitted far more direct exhaust PM than is emitted by structural fires in the U.S. Since that 

time, the PM emissions from the fleet have been substantially reduced; making it likely that the 

fleet now emits less PM than is emitted by such fires. Comparing the estimates from individual 

fire incidents with the 8-hour emission estimates of individual trucks, even using the high

engine-out PM estimate with 4-hour bypass, the 8-hour truck emissions would likely be two 

orders of magnitude less polluting than the fires.

 

5 Based on total fires reported for 1994 by FEMA with 1994 population data from the US Census Bureau; see EIIP 

Report at 18.5-2.

6

See 2001 EIIP; see also California ARB Area Source Methodology, Structural and Automobile Fires, March 1999, 

available at http://www.arb.ca.gov/ei/areasrc/ONEHTM/ONE7-14.HTM.

7 Estimate based on U.S. Census Bureau statistics, retrieved May 2012 from http://www.census.gov/

8

See Note 6 above.

9 Bay Area Air Quality Management District, August 2001, Area Source Methodology, Accidental Structural Fires, 

available at http://www.arb.ca.gov/ei/areasrc/districtmeth/BayArea/C750.pdf.