EPA Comments on Utah's 

"Particulate Matter (PM) Exhaust Emissions Reductions from 

Diesel Inspection and Maintenance (liM) Programs in Utah 

(Davis, Salt Lake and Utah Counties)," 

January 20, 2005 

1. General Comments 

In order for Utah to claim 2% PM10 emission reduction credit for their diesel I/M 

program, the technical justification must meet the requirements of EPA's memorandum entitled, 

"Guidance on Incorporating Voluntary Mobile Source Emission Reduction Programs [(VMEP)] 

in State Implementation Plans (SIPs)," dated October 24, 1997. This policy requires the state to 

submit a SIP which 1) identifies and describes a VMEP; 2) contains projections of emission 

reductions attributable to the program, along with relevant technical support documentation; 3) 

commits to monitor, evaluate, and report the resulting emissions effect of the voluntary measure; 

and 4) commits to remedy in a timely manner any SIP credit shortfall if the VMEP program does 

not achieve projected emission reductions. In addition, as part of the SIP submittal, Utah must 

commit to monitor, evaluate, and report the resulting emissions effect of the voluntary measure. 

If there is an emissions reduction shortfall, Utah must pursue appropriate follow-up, or 

"backstop", actions in a timely fashion including, but not limited to: adjusting the voluntary 

measure, adopting a new measure, or revising the VMEP emission credits to reflect actual 

emission reductions. 

Under the VMEP policy, the total amount of emissions reductions from voluntary 

measures shall not exceed 3% of the emissions determined necessary to demonstrate progress 

toward, attainment of, or, maintenance of the relevant National Ambient Air Quality Standards 

(NAAQS). Utah must commit to conducting program evaluations within an appropriate timeframe and report the resulting information to EPA within an appropriate time-frame in order to 

document whether the program is being carried out, and emission reductions are being achieved. 

The SIP submittal must contain a good faith estimate of emission reductions, including 

technical support documentation for the conclusion that the measure will produce the anticipated 

emission reductions. The SIP must describe how the state plans to evaluate program 

implementation and report on program results in terms of actual emission reductions. The 

procedures designed should compare projected emission reductions with actual emission 

reduction achieved. Finally, Utah must provide timely post-evaluation reports to EPA. 

UDAQ used EPA's PARTS emissions model to develop their mobile source inventories. 

We recommend using our more recent vehicle emissions model, MOBILE6.2, officially released 

on May 19,2004 (69 FR 28830). MOBILE6.2 is a significant improvement to its predecessor, 

PARTS. Please note though, MOBILE6.2 does not include degrading (increasing) levels of 

emissions for heavy-duty diesel vehicles. That is, a vehicle is estimated to have the same 

emissions when it is brand new as it does when it is 1S years old. The goal of any liM program is 

aimed at limiting the increased vehicle emissions over time by assessing the emissions and taking 

corrective actions to reduce them. This means, UDAQ and every other state are already getting a 

diesel I/M emission credit because engine degradation (over time) is not included in mobile 
source inventories that are estimated with the MOBILE6.2 model. 

UDAQ's report references the "Heavy-Duty Vehicle Inspection and Maintenance Final 

Report," Radian Corporation, May 1988. Further review will be needed to determine if the 

correlation established between opacity and particulate emissions, used in this report, is va}id and 

can be used by UDAQ. We ask the state to perform a current literature search to determine if 

more recent studies, other than those cited by EPA, relating opacity and particulate emissions are 

available. As with any innovative approach, we strongly suggest using the latest, most defensible 

data possible. For example, has Radian's position changed or do they still support their 1988 

findings? EPA has cited more recent and possibly more reliable studies that find opacity is not a 

good predictor of PM emissions. In addition, it appears that the dTEMLs from the 1988 Radian 

study, are hard to identify in your report and may be mislabeled Please review this issue and also 

provide us with a copy of the full study from which UDAQ obtained its methodology and 

multipliers. 

The program evaluation section needs to analyze not only PM10 emission reductions, but 

any incidental increases in other pollutants, such as NOx (which is a secondary component of 

PM10). Opacity based liM programs seek to reduce the visible smoke exiting vehicle exhausts. 

Adjustments or repairs are then made to reduce the visible smoke. We are concerned with 

decreasing opacity leading to an increase in NOx. Utah currently acknowledges, in their 

approved PM10 SIP, that NOx is transformed into or is a precursor to PM. Any decreased PM 

from a diesel I/M program should consider the offsetting impact of increase in PM precursors of 

NOx. We note that this tradeoff is not considered in Utah's draft report and must be included. 

UDAQ's report applies to repaired vehicles only, or only failed vehicles. UDAQ will 

need to apply the findings of this report to the entire fleet of diesel vehicles, both light-duty and 

heavy-duty. We also suggest verifying the failure rates and repair rates used in the report. 

2. Specific Comments 

2.1 Program Description: 1st and 3rd bullets contradict one another, with the 1st bullet 

stating that all vehicles are tested annually, but the 3 rd bullet states that vehicles less than six 

years old are tested biennially. Does the biennial test apply to all counties, and not just Utah 

County? Should the last bullet include Salt Lake County? Please clarify that the OBD tests are 

only for 1996 and newer model year vehicles. Are the counties running OBD-only or are they 

performing opacity tests on vehicles with the MIL on? 

Calculation 2: Please verify the values for"% targeted" and "a" as 100%. The vehicle 

inspection is biennial for the first 6 years and the test is valid for 2 years ifthe vehicle passes. 

Should the 100% values be re-determined to reflect this? Also, please explain the value for "b" 

as 1 00%. Does this mean all of the diesel vehicles fail the inspection and need repairs? 

Tables 1 and 2: Inventory Change in Salt Lake County, UT and Inventory Change 

in Utah County, UT: The values for the Heavy-Duty Control Factor appear to be incorrect. For 

Salt Lake County, we calculated them to be: 84.66 for 1968- 1984, 86.87 for 1985-1987, and 

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97.18 for 1988-2002. For Utah County, we calculated them to be 82.43 for 1968- 1984, 81.52 

for 1985- 1987, and 95.65 for 1988-2002. Would there be a significant impact to these values 

ifUDAQ used more recent data from 2003 and 2004? We need an explanation for the paragraph 

under Table 1 as we cannot understand this. Does the Heavy-Duty Control Factor apply to the 

heavy-duty vehicles only? Are the values for the number of vehicles tested, and the number of 

vehicles failed for heavy-duty vehicles only? Should UDAQ subtract out the light-duty portion? 

Please provide a sample calculation for 0.612 tons-per-day (tpd) PM and 0.634 tpd PM for Salt 

Lake County and 0.220 tpd PM and 0.236 tpd PM for Utah County. 

Appendix: We ask UDAQ to clarify if the calculation for the weighted average should 

be the weighted average of the failed vehicles or all vehicles? Also, the sentence which states 

"The diesel inventory is both heavy-duty diesel vehicle and light-duty diesel vehicle emissions so 

the reduction must be calculated for the whole diesel inventory" is worded awkwardly and needs 

to be clarified. Finally, we are having difficulty understanding the calculation as presented 

Further clarification is necessary. 

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