                       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
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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.  For 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.  If 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 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.  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 back-calculate 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 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.  
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.  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.  Those factors are employed in this analysis, along with a 2012 U.S. population estimate of 313 million.  The 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.  A similar guideline issued by the Bay Area AQMD estimates a fuel loading of 1.95 tons burned per fire.  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.

