Appendix C 

Mobile Source Precursor 

Discussion 

PM2.5 Precursors to Consider 

40 CFR 93.119(f)(7) through (10) identifies the pollutants for PM2.5 for which 

regional emissions analysis needs to be performed for transportation conformity 

purposes. These parts of the rule are listed below: 

§119(f)(7) – PM2.5 in PM2.5 areas; 

§119(f)(8) – Reentrained road dust in PM2.5 areas only if the EPA 

[Environmental Protection Agency] Regional Administrator or the director of 

the State air agency has made a finding that emissions from reentrained road 

dust within the area are a significant contributor to the PM2.5 nonattainment 

problem and has so notified the MPO and DOT [Department of 

Transportation]; 

§119(f)(9) – NOX [nitrogen oxides] in PM2.5 areas, unless the EPA Regional 

Administrator and the director of the State air agency have made a finding 

that emissions of NOX from within the area are not a significant contributor to 

the PM2.5 nonattainment problem and has so notified the MPO and DOT; and 

§119(f)(10) – VOC [volatile organic compounds], SO2 [sulfur dioxide] and/or 

ammonia in PM2.5 areas if the EPA Regional Administrator or the director of 

the State air agency has made a finding that any of such precursor emissions 

from within the area are a significant contributor to the PM2.5 nonattainment 

problem and has so notified the MPO and DOT. 

Primary, or direct, PM2.5 tailpipe emissions must be considered for the 

transportation conformity regional emissions analysis. The other precursor 

pollutants and re-entrained road dust only need to be considered if the State air 

agency and/or the EPA have deemed the pollutant as a significant contributor to the 

PM2.5 nonattainment problem. The following sections discuss the significance of the 

precursor pollutants and re-entrained road dust to the PM2.5 nonattainment 

problem. The discussion of the significance of these precursors for the attainment 

demonstration is located in Section 7.2. 

Nitrogen Oxides (NOX)

As EPA has not made a finding that emissions of NOX are not a significant 

contributor to the PM2.5 nonattainment problem in the Birmingham area, a mobile 

source budget will be established. Please see Section 7.2 for a discussion of NOX as a 

precursor in the development of the control strategy. 

Sulfur Oxides (SOX)

The PM2.5 precursor SO2 could not be deemed insignificant to the overall PM2.5

nonattainment problem since sulfate is such a large fraction of the PM2.5

composition. However, ADEM determined that SO2 emitted by the mobile source 

sector is insignificant. EPA in its Federal Register notice for PM2.5 does not address 

the mobile sector in its listing of significant sources of SO2 emissions. ADEM agrees 

with the following statements addressing SO2 from onroad mobile emissions as 

published in the May 6, 2005, Federal Register, 70 FR 24283: 
“While speciated air quality data show that sulfate is a relatively significant 

component (e.g., ranging from nine to 40 percent) of PM2.5 mass in all regions 

of the country, emissions inventory data and projections show that onroad 

emissions of SOX constitute a ‘‘de minimis’’ (i.e., extremely small) portion of 

total SOX emissions. Emissions inventory data for 1999 for the 372 potential 

PM2.5 nonattainment counties for PM2.5 (based on 1999–2001 air quality data) 

show that onroad sources were responsible for only two percent of total SOX

emissions. 

Furthermore, EPA has already adopted two regulations that will greatly 

reduce emissions of SOX from onroad sources by the time such regulations are 

both in full effect in 2009. First, in 2004 the low sulfur gasoline program 

began to be phased in and will be fully effective in 2007 (February 10, 2000, 

65 FR 6697). This regulation will reduce the sulfur content of gasoline by 

approximately 90 percent when fully effective. Second, in 2006 the low sulfur 

diesel program will begin to be phased in and will be fully effective by 2009 

(January 18, 2001, 66 FR 5001). This regulation will reduce the sulfur content 

of diesel fuel by approximately 97 percent nationally when fully effective. 

Projections of onroad emissions of SO2 in 2020 indicate that onroad sources 

will be responsible for less than one percent of the total SO2 emissions in 2020 

in the 372 potential PM2.5 nonattainment counties (based on 1999 – 2001 air 

quality data). These projections confirm that the implementation of the fuel 

regulations discussed above will ensure that as a general matter of SO2

emissions from onroad sources remain at insignificant levels in all areas.” 

Although SO2 is a significant component of the PM2.5 nonattainment problem in 

Birmingham, the majority of the SO2 emissions in 2009 came from the stationary 

point source sector. The mobile source sector only contributed 0.1% of the 2009 or 

2012 statewide SO2 emissions. This finding is consistent with what was stated 

above. 

Since the mobile source SO2 contribution is insignificant, ADEM is not establishing 

MVEBs for this precursor. An affirmative insignificance finding from EPA only 

relieves the transportation partners from a regional emissions analysis for SO2

emissions for these areas, but does not relieve them of the other transportation 

conformity requirements. The transportation partners will need to note the SO2

insignificance finding (if found adequate and approved by EPA) in future conformity 

determinations.

Volatile Organic Compounds (VOC) and Ammonia (NH3)

The two remaining species that comprise PM2.5, ammonia (NH3) and volatile organic 

carbons (VOCs), have been deemed as insignificant contributors to the total mass of 

PM2.5 by EPA. With regards to NH3, 72 FR 20586 states NH3 “is presumed not to be 

a PM2.5 attainment plan precursor, meaning that the State is not required to address 

[NH3] in its attainment plan or evaluate sources of [NH3] emissions for reduction 

measures.” Likewise, the final rule for VOCs states that “States are not required to 

address VOC in PM2.5 implementation plans and evaluate control measures for such 

pollutants unless the State or EPA makes a technical demonstration that emissions 
of VOCs from sources in the State significantly contribute to PM2.5 concentrations in 

a given NAA.” 

ADEM agrees with EPA’s presumptive exclusion of NH3 and VOCs with regards to 

this PM2.5 implementation plan. Therefore, ADEM did not set MVEBs for NH3 and 

VOCs in the Birmingham PM2.5 NAA. 

Road and Construction Dust

The majority of the roads in the Birmingham area are paved, so there is minimal 

road dust due to the paved roads. The factor used to calculate re-entrained road 

dust on paved roads is very small. What dust is generated, has been shown in the 

literature, Methodology to Estimate the Transportable Fraction (TF) of Fugitive Dust 

Emissions for Regional and Urban Scale Air Quality Analyses, US EPA, August 3, 

2005, to be inconsequential. This fact is affirmed by the small crustal component in 

the PM2.5 speciated data which measures only 7% at the North Birmingham 

monitoring site and only 6% at the Wylam monitoring site in 2002. 

Since re-entrained road dust is not a significant contributor to the PM2.5

nonattainment problem, ADEM did not establish MVEBs for this source category.