MEMORANDUM 

TO: FILE 

FROM: Erik Snyder, Lead Regional Air Quality Modeler, Region 6 

DATE: August 2010 

SUBJECT: New Mexico Intereference with Maintenance for 1997 NAAQS Technical 

Support Document 

 

This TSD is EPA’s Technical Support Document that summarizes EPA’s technical 

review of the State Implementation Plan (SIP) revision submitted by the State of New 

Mexico for the purpose of addressing the “good neighbor” provisions of Clean Air Act 

(CAA) section 110(a)(2)(D)(i) for the 1997 8-hour ozone National Ambient Air Quality 

Standards (NAAQS or standards) and the 1997 PM2.5 NAAQS. This SIP revision 

addresses the requirement that the State of New Mexico’s SIP have adequate provisions 

to prohibit air emissions from adversely affecting another state’s air quality through 

interstate transport. In this action, EPA is proposing to approve the New Mexico 

Interstate Transport SIP provisions that address the requirement of section 

110(a)(2)(D)(i)(I) that emissions from New Mexico sources do not interfere with 

maintenance of the 1997 8-hour ozone NAAQS and the 1997 PM2.5 NAAQS in any other 

state. 
New Mexico’s submittal focused primarily on whether emissions in New Mexico 

contribute to nonattainment of the 1997 PM2.5 or 1997 8-hour ozone NAAQS in other 

states. As discussed previously, EPA has found that the emissions from the State of New 

Mexico do not contribute to nonattainment in a neighboring state (75 FR 33174). 

Consistent with our guidance and EPA’s approach to the Clean Air Interstate Rule 

(CAIR) (70 FR 25174), New Mexico did not separately evaluate whether emissions from 

New Mexico interfere with maintenance of the standards in another state. CAIR 

however, was remanded to EPA, in part, because EPA did not specifically address 

whether emissions from one state interfered with maintenance of the standard in another 

state. 

 On July 6, 2010, the EPA Administrator signed a proposed replacement to 

CAIR, the proposed Federal Implementation Plans to Reduce Interstate Transport of 

Particulate Matter and Ozone (proposed Transport Rule).1 The proposed Transport Rule 

proposes actions that States must take in order to eliminate transport of emissions that 

significantly contribute to nonattainment or interfere with maintenance of the 1997 8-

hour ozone and PM2.5 National Ambient Air Quality Standards (NAAQS) {1997 Annual 

and 2006 24-hour}. Air quality modeling was completed in support of EPA’s Proposed 

Transport Rule and is documented in the “Air Quality Modeling Technical Support 

Document: Transport Rule Proposal” (hereafter referred to as the “PTR TSD”). The 

document can be found at http://www.epa.gov/airquality/transport/tech.html. Since New 

Mexico did not focus primarily on maintenance sites in other states, EPA is utilizing 

modeling results that were generated as part of the overall modeling that was performed 

 

1

 U.S. EPA, 2010, “Federal Implementation Plans to Reduce Interstate Transport of Fine Particulate Matter 

and Ozone,” 75 FR 45210 (August 2, 2010). In this action the expression “proposed Transport Rule” 

refers to this proposed rule. 
for the proposed Transport Rule and other technical analyses to help support the EPA’s 

proposed finding that New Mexico does not contribute to maintenance sites in other 

states. 

The proposed Transport Rule modeling TSD contains detailed information on the 

model setup, emissions, meteorology, and model results for the Transport Rule 

modeling2. We are also relying upon modeling data that was not directly utilized in the 

proposed Transport Rule, but has been documented in a memorandum from Brian Timin 

of OAQPS on August 23, 2010.3 The air quality model used for the proposed Transport 

Rule was the CAMx photochemical model (http://camx.com/). Air quality modeling was 

performed for a 2005 base year and a 2012 “no CAIR” base case (no additional emission 

reductions due to the proposed Transport Rule). The modeling for 2005 was used as the 

base year for projecting air quality to the future. The 2012 base case modeling was used 

to identify future nonattainment and maintenance locations and to quantify the 

contributions of emissions in upwind states to concentrations of PM2.5, and 8-hour ozone 

at downwind receptors.4, 5 

In the proposed Transport Rule and the BT Memo, EPA projects future 

concentrations of ozone and PM2.5 to identify locations that are expected to be NAAQS 

nonattainment or maintenance areas in 2012. The nonattainment and maintenance 

locations are based on projections of future air quality at existing ozone and PM2.5

 

2

 U.S. EPA, 2010, “Technical Support Document (TSD) for the Transport Rule Docket ID No. EPA-HQOAR-2009-0491 Air Quality Modeling U.S. Environmental Protection Agency Office of Air Quality 

Planning and Standards Air Quality Assessment Division, June 2010, In this document the expression 

“PTR TSD” refers to the this proposed Transport Rule Technical Support Document. 3

 August 23, 2010 Memorandum, “Documentation of Future Year Ozone and Annual PM2.5 Design Values 

for monitors in Western States”, Brian Timin, EPA Office of Air Quality Planning and Standards. In 

this document this document will be referred to as the BT Memo. 4

 PTR TSD 

5

 BT Memo. 
monitoring sites. In the proposed Transport Rule EPA has proposed an approach to 

determine whether emissions from one state are interfering with maintenance in another 

state. EPA is using this proposed Transport Rule approach to defining a maintenance site 

as the approach we are using in this notice to determine if New Mexico’s emissions 

interfere with maintenance sites in another state. EPA is also relying upon the proposed 

Transport Rule modeling analyses and information in the BT Memo to indicate which 

areas are maintenance sites for this action.6,7

For this analysis and in the Transport Rule we are using the air quality modeling 

results in a “relative” sense to project future concentrations of ozone and PM2.5. Rather 

than use the absolute model predicted future year ozone and PM2.5 concentrations, the 

base year and future year predictions are used to calculate a (relative) percent change in 

ozone and PM2.5 concentrations. In this approach, the ratio of future year model 

predictions to base year model predictions are used to adjust ambient measured data up or 

down depending on the relative (percent) change in model predictions for each location. 

The use of ambient data as part of the calculation helps to constrain the future year 

predictions, even if the absolute model concentrations are over-predicted or underpredicted. As described below and in the BT Memo and PTR TSD, the procedures for 

projecting annual and 24-hour PM2.5 and 8-hour ozone conform to the methodologies 

contained in the final guidance for attainment demonstration modeling.8

 

6

 PTR TSD. 

7

 BT Memo. 

8

 U.S. EPA, 2007: Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of 

Air Quality Goals for Ozone, PM2.5, and Regional Haze; Office of Air Quality Planning and Standards, 

Research Triangle Park, NC. http://www.epa.gov/ttn/scram/guidance/guide/final-03-pm-rhguidance.pdf. In this notice this document will be referred to as the the modeling guidance. 
In the Transport Rule and this analysis we use measurements of ambient ozone 

and PM2.5 data from several State and Federal monitoring networks. The ambient data 

used in this analysis were obtained from EPA’s Air Quality System (AQS). 

In order to use the ambient data, the raw measurements must be processed into a 

form pertinent for useful interpretations. The ozone data were processed consistent with 

the formats associated with the 1997 8-hour NAAQS for ozone. The level of the 1997 8-

hour O3 NAAQS is 0.08 ppm. The 8-hour ozone standard is not met if the 3-year 

average (consecutive years) of the annual 4th highest daily maximum 8-hour O3 

concentration is greater than 0.08 ppm (greater than or equal to 0.085 ppm when rounded 

up). This 3-year average is referred to as the design value. 

The PM2.5 ambient data were processed consistent with the formats associated 

with the NAAQS for PM2.5. For PM2.5, we evaluated concentrations of both the annual 

PM2.5 NAAQS and the 24-hour PM2.5 NAAQS. The annual PM2.5 standard is not met if 

the 3-year average of the annual mean concentration is greater than 15.0 μg/m3 (15.05 

μg/m3 when rounded up). The 3-year average annual mean concentration is computed at 

each site by averaging the daily Federal Reference Method (FRM) samples by quarter, 

averaging these quarterly averages to obtain an annual average, and then averaging the 

three annual averages. The 3-year average annual mean concentration is referred to as 

the annual design value. 

The 1997 PM2.5 24-hour standard is not met when the 3-year average of the 

annual 98th percentile PM2.5 concentration is greater than 65 μg/m3 (65.5 μg/m3 when 

rounded up). The 3-year average mean 98th percentile concentration is computed at each 

site by averaging the 3 individual consecutive annual 98th percentile values at each site. 
The 3-year average 98th percentile concentration is referred to as the 24-hour design 

value. 

The modeling guidance recommends using the average of the three design value 

periods centered on the year of the base year emissions. Since 2005 was the base 

emissions year for the TR modeling, we used the design value for 2003-2005, 2004-2006, 

and 2005-2007 to represent the base period ozone and PM2.5 concentrations. Specifically, 

we used ambient ozone and annual and 24-hour PM2.5 design values for the periods 2003-

2005, 2004-2006, and 2005-2007 as the starting point for our projections of future design 

values for TR modeling for the 2012 future year without additional reductions due to the 

TR. The 2003–2005, 2004-2006, and 2005-2007 design values are accessible at 

www.epa.gov/airtrends/values.html. Both the average of the three design values and the 

maximum of the three design values are included in Appendix B of the PTR TSD or in 

the BT Memo.9

Ambient design values from monitoring sites were included in our analysis from 

the BT Memo or the proposed TR if the site had at least one complete design value in the 

2003-2007 period. As described in the 2006 modeling guidance and the PTR TSD, 

design values of PM2.5 in 2012 were estimated by applying the 2005 to 2012 relative 

change in model-predicted PM2.5 species concentrations to the measured (2003-2007) 

PM2.5 species concentrations. The PM2.5 species include sulfate, nitrate, ammonium, 

particle bound water, elemental carbon, salt, other primary PM2.5, and organic aerosol 

mass (by difference). 

For each FRM PM2.5 monitoring site, all valid design values (up to 3) from this 

period were averaged together. Since 2005 is included in all three design value periods, 

 

9

 BT Memo. 
this has the effect of creating a 5-year weighted average, where the middle year is 

weighted 3 times, the 2nd and 4th years are weighted twice, and the 1st and 5th years are 

weighted once. We refer to this as the 5-year weighted average design value 

concentration. The 5-year weighted average design values were used to project 

concentrations for the 2012 scenario in order to determine which monitoring sites are 

expected to be nonattainment for these future year scenarios. For the 2012 base case we 

also projected design values for each of the individual 3-year design value periods (i.e., 

2003-2005, 2004-2006, and 2003-2007). The projection of design values for these 

individual periods was used in the interstate contribution analysis to determine sites 

expected to have maintenance problems, as described below. 

The details for calculating the Base design value and Future design values are included in 

the PTR TSD, BT Memo and the modeling guidance. Following is a summary of the 

final parts of the method used to project future year annual PM2.5 design values. In the 

PTR TSD and BT Memo, EPA calculated the maximum future design value by 

processing each of the three base design value periods (2003-2005, 2004-2006, and 2005-

2007) separately. The highest of the three future values is the maximum design value. 

The maximum design values are used to determine future year maintenance sites in the 

proposed Transport Rule and in this action. The calculations for determining future year 

PM2.5 concentrations were applied for each FRM site. The calculated annual PM2.5 

design values are truncated after the second decimal place. This is consistent with the 

truncation and rounding procedures for the annual PM2.5 NAAQS. Any value that is 

greater than or equal to 15.05 μg/m3 is rounded to 15.1 μg/m3 and is considered to be 

violating the NAAQS. Thus, sites with future year annual PM2.5 design values of 15.05 
μg/m3 or greater, based on the projection of 5-year weighted average concentrations, are 

predicted to be nonattainment sites. Sites with future year maximum design values of 

15.05 μg/m3 or greater are predicted to be maintenance sites. Note that nonattainment 

sites are also maintenance sites because the maximum design value at a site is always 

greater than or equal to the 5-year weighted average at that site. For ease of reference we 

use the term “nonattainment sites” to refer to those sites that are projected to exceed the 

NAAQS based on both the average and maximum design values. Those sites that are 

projected to be attainment based on the average design value but exceed the NAAQS 

based on the maximum design value are referred to as “maintenance sites”. The 

monitoring sites that we project to be nonattainment and/or maintenance for the annual 

PM2.5 NAAQS in the 2012 base case, using information from either the Transport 

Proposal TSD or the BT Memo, are used as the nonattainment/maintenance receptors for 

assessing the contribution of emissions in upwind states to downwind nonattainment and 

maintenance of the annual PM2.5 NAAQS. The 2003-2007 base period and projected 

annual PM2.5 design values for each site for the 2012 scenario are provided in Appendix 

B of the Transport Proposal TSD or in the BT Memo. 

The level of the1997 8-hour ozone NAAQS is 0.08 ppm. The 8-hour ozone 

standard is not met if the 3-year average of the annual 4th highest daily maximum 8-hour 

ozone concentration is greater than 0.08 ppm (greater or equal to 0.085 ppm when 

rounded up). This 3-year average is referred to as the design value. The details for 

calculating the Base design value and Future design values are included in the PTR TSD, 

BT Memo and the modeling guidance. Following is a summary of the method used to 
project future year 8-hour ozone design values in the PTR TSD and in the BT memo. To 

project 8-hour ozone design values we also used the proposed Transport Rule 2005 base 

year, and 2012 future base (2012 emission levels prior to any emission reductions from 

the Proposed Transport Rule) model-predicted ozone concentrations to calculate relative 

response factors (RRFs). The methodology we followed is consistent with the attainment 

demonstration procedures included in EPA’s modeling guidance. The RRFs were 

applied to the 2003-2007 ozone design values through the following steps: 

Step 1: For each monitoring site calculate the mean of the modeled 8-hour daily 

maximum predictions across all days with 8-hour daily maximum predictions 

greater than or equal to 85 ppb (for some monitors a lower threshold may have 

been used in order to have enough days for the analysis). The mean 

concentrations are calculated using the predictions in the nine grid cells that 

include or surround the location of the monitoring site. The RRF for a site is the 

ratio of the mean prediction in the future year to the mean prediction in the 2005 

base year. The RRFs were calculated on a site-by-site basis. 

Step 2: The RRF for each site is then multiplied by the 2003-2007 5-year 

weighted average ambient design value for that site, yielding an estimate of the 

future year (2012) design value at that particular monitoring location. 

Step 3: We calculate the maximum future design value by projecting design 

values for each of the three base periods (2003-2005, 2004-2006, and 2005-2007) 

separately. The highest of the three future values is the maximum design value. 

This maximum value is used to identify the 8-hour ozone maintenance receptors. 

The preceding procedures for determining future year 8-hour ozone design values 
were applied for each ozone monitoring site with available data. The future year design 

values are truncated to integers in units of ppb. This approach is consistent with the 

truncation and rounding procedures for the 8-hour ozone NAAQS. Future year design 

values that are greater than or equal to 85 ppb are considered to be violating the NAAQS. 

Sites with future year 5 year weighted average design values of 85 ppb or greater are 

predicted to be nonattainment. Sites with future year maximum design values of 85 ppb 

or greater are predicted to be future year maintenance sites. Note that, as described 

previously for the annual and 24-hour PM2.5 NAAQS, nonattainment sites for the ozone 

NAAQS are also maintenance sites because the maximum design value is always greater 

than or equal to the 5-year weighted average. For ease of reference we use the term 

“nonattainment sites” to refer to those sites that are projected to exceed the NAAQS 

based on both the average and maximum design values. Those sites that are projected to 

be attainment based on the average design value but exceed the NAAQS based on the 

maximum design value are referred to as maintenance sites. The monitoring sites that we 

project to be maintenance for the ozone NAAQS in the 2012 base case in the Transport 

Proposal TSD are the maintenance receptors used for assessing the contribution of 

emissions in upwind states to downwind nonattainment and maintenance of ozone 

NAAQS as part of this proposal. The 2003-2007 base period and projected 8-hour ozone 

design values for all sites for the 2012 scenario used for determining the maintenance 

receptors/sites are provided in Appendix B of the Transport Proposal TSD or in the BT 

Memo. 

In summary, the 5-year weighted average concentrations were used to project 

concentrations in 2012 to determine which monitoring sites are expected to be 
nonattainment in this future year. EPA also projected 2012 design values based on each 

of the 3 year periods (i.e., 2003-2005, 2004-2006, and 2003-2007). The highest 

projection is referred to as the maximum design value and gives an indication of potential 

variability in future projections due to differences in actual meteorology and emissions 

from what was modeled. Those sites that are projected to be attainment based on the 

average design value but exceed the NAAQS based on the maximum design value are 

referred to as maintenance sites because they will have more difficulty in maintaining the 

standard if there are adverse variations in meteorology or emissions. These projected 

maintenance sites are the ones we have evaluated for determining if New Mexico’s 

emissions potentially interfere with maintenance in other States. 

OZONE and PM2.5 Maintenance Sites

In the Transport Rule Proposal, we used a new technique for creating a threshold test to 

determine if states should be considered for potential impacts on other states. This 

section documents the analysis of modeling results for maintenance sites and also the 

procedures used to quantify the impacts (i.e., contributions) of emissions in individual 

States on annual PM2.5, 24-hour PM2.5, and/or 8-hour ozone concentrations at projected 

2012 nonattainment and maintenance sites. The purpose of this assessment is to 

determine if New Mexico contributes to 1997 8-hour ozone, and/or 1997 annual PM2.5, 

and/or 1997 24-hour PM2.5 maintenance problems in other States in amounts at or above 

thresholds based on 1 percent of the corresponding NAAQS. The Transport Rule 

proposal included this 1 percent threshold based on the average amount of contribution to 

all modeled exceedances and we are proposing to use this same threshold for our basis of 
this action. The air quality contributions were quantified using source apportionment 

techniques in CAMx. In general, these techniques track the formation and 

transport of ozone and particulate matter from specific emissions sources and calculate 

the contribution of sources and precursors to ozone and PM2.5 at individual receptor 

locations. The strength of the photochemical model source apportionment technique is 

that all modeled ozone and/or PM2.5 mass at a given receptor location in the modeling 

domain can be tracked back to specific sources of emissions and boundary conditions in 

order to fully characterize culpable sources. This type of source apportionment is useful 

for understanding both the types of sources or regions and the magnitude of the 

contributions that are associated with ozone and PM2.5, as estimated by the model. More 

details on the implementation of source apportionment in CAMx can be found in the 

CAMx user’s guide.10

From the modeling analyses conducted for the proposed Transport Rule, the 

following sites have been identified as maintenance sites for the 8-hour ozone NAAQS: 

several sites in the Dallas-Ft.Worth (DFW) area, several sites in the 

Houston/Galveston/Brazoria (Houston) area, and other more distant sites in Georgia, 

Pennsylvania, New York and Connecticut11. The BT Memo documents (information 

not included in the Transport Rule docket) analysis of ozone levels in the 36 km grid of 

the Transport Rule modeling, including the 5-year weighted Future DV and the 

maximum 3-year Future DV. In evaluating this information we have concluded that 

several sites in southern and central California would be considered maintenance sites 

 

10 Comprehensive Air Quality Model with Extensions Version 5 User’s Guide. Environ International 

Corporation. Novato, CA. March 2009. Available at 

http://www.camx.com/files/CAMxUsersGuide_v5.0.pdf 11 Appendix B of PTR TSD and Appendices A and B of BT 2010. 
using our proposed definition of a maintenance site.12 It should be noted that no sites in 

the states, that border New Mexico (Arizona, Colorado, Utah, and Oklahoma) other than 

Texas were identified as maintenance sites based on the modeling analyses in the 

Transport Proposal or BT Memo.13 The BT Memo indicates that the highest Future Year 

design values (5-year center weighted) for monitors in the Denver area and Salt Lake 

City Areas respectively are 77 ppb and 74 ppb and the maximum of the three design 

values tested against the maintenance threshold are 80 ppb and 78 ppb respectively. 

Therefore we conclude that no monitors in Colorado or Utah would be categorized as 

maintenance monitors using the modeling results included in the BT memo. 

For the annual PM2.5 NAAQS, the following sites have been identified in the 

proposed Transport Rule as maintenance sites: a site in Cook County in Chicago, Illinois, 

monitor in Madison county, IL (East St. Louis area), a monitor in Indianapolis, IN and a 

site in Harris county of the Houston/Galveston/Brazoria area. The BT Memo documents 

analysis of annual PM2.5 levels in the 36 km grid of the Transport Rule modeling, 

including the 5-year weighted Future DV and the maximum 3-year Future DV. In 

evaluating this information we have concluded that two sites in southern California 

would be considered maintenance sites using our proposed definition of a maintenance 

site.14 

No areas were identified as maintenance for the 1997 24 hour PM2.5 NAAQS by 

the proposed Transport Rule. With regard to the 1997 PM2.5 24 hour standard of 65 

μg/m3, the PTR TSD and BT Memo did not include any 2012 projections outside the 12 

 

12 BT Memo, Appendix B. Monitors in the following counties: Merced, Placer, Riverside, and 

Sacramento. 13 Appendix B of BT 2010 

14 BT Memo, Appendix A. Monitors in the following counties: Los Angeles and Orange. 
km modeling domain that could be used to identify the sites that would be classified as 

maintenance sites in this notice for the 1997 24-hour PM2.5 NAAQS. For the western 

part of the continental U.S, including California, that was not in the 12 km modeling 

domain, we are relying on recent monitoring trends data to identify if any areas are close 

to the 1997 PM2.5 NAAQS. This approach does not have a clear cut point, but is more of 

a weight of evidence analysis that if no areas are monitoring values close to the standard, 

they would not be expected to be near the standard in the future given the general trend 

downward in emissions of PM2.5 pre-cursors in most areas. It should be noted that based 

on recent monitoring data (2007-2009 DVs that are under final review) that the highest 

24-hour PM2.5 DV in 47 states of the continental U.S (not including California) is 50 

μg/m3, which is well below the 1997 24-hour PM2.5 NAAQS of 65 μg/m3.

15 The only 

potential areas of the country that have current DVs (2007-2009) near or above the 1997 

24-hour PM2.5 standard are sites located in California16. The sites that are monitoring 

values either near or above the 1997 24-hour PM2.5 standard, are the only areas that could 

reasonable be anticipated to be potentially maintenance areas in the future. 

EPA has evaluated available analyses and conducted additional analyses for each 

of these identified maintenance sites that may be potentially impacted by emissions in 

New Mexico and determined that emissions in New Mexico do not have a significant 

impact on any of these identified maintenance sites. 

Ozone Maintenance Site Evaluation 

 

15 Data undergoing review from EPA’s Air Quality System which is EPA’s repository of ambient air 

quality data . (http://www.epa.gov/ttn/airs/airsaqs/) 16 Id. 
Specifically for the 8-hour ozone identified maintenance sites in the Houston and DFW 

areas, EPA has conducted a modeling estimate of impacts from New Mexico’s emissions 

using the Central Regional Air Planning Association (CENRAP) modeling of 2002 

emissions and meteorology that is a little older than the more recent modeling included in 

the Transport Proposal. The CENRAP modeling that EPA utilized was an earlier version 

of the CENRAP modeling that was submitted by a number of states as part of their 

Regional Haze State Implementation Plan submittals and is currently being reviewed by 

EPA. While the CENRAP modeling that EPA utilized is not the final modeling analysis 

submitted it is considered acceptable to use with the understanding that the final 

modeling may give slightly different answers, but in this case the expected change would 

not change the conclusions made. EPA analyzed source apportionment modeling with 

the 2002 based CENRAP modeling and concluded that maximum impacts from New 

Mexico’s emissions would be 0.2% and 0.4% respectively, which are less than the one 

percent threshold of 0.8 ppb included in the Transport Proposal that we are using in this 

notice. The Transport Proposal’s threshold of one percent is based on the average impact 

of a State on a downwind site. Comparing the maximum impacts shown by the CENRAP 

modeling to the Transport Proposal threshold is a conservative approach, since the 

average impact over all exceedance days at sites in DFW or Houston would be lower. 

Furthermore the CENRAP modeling analysis is conservative since it relies on 2002 

emission levels and additional reductions have occurred due to fleet turnover and other 

measures to reduce air pollution between 2002 and 2010 throughout the modeling domain 

that would result in lower overall pollution levels if taken into account. Since we did not 

have source apportionment modeling for New Mexico’s impacts from the proposed 
Transport Rule modeling, the 2002 CENRAP based modeling is considered to be 

acceptable in lieu of having source apportionment from the proposed Transport Rule. 

In addition, EPA has reviewed other documents that evaluate the cause of higher ozone 

concentration levels in the DFW and Houston areas for a more qualitative support in 

addition to the conservative CENRAP based modeling. Specifically, note Figure 1, 

which is from Figure 3-2 of TCEQ’s DFW Conceptual Model Report17 shows back 

trajectory density plots for high and low eight-hour ozone days from 2001 to 2004. In 

layman terms, a back trajectory is an approximation of where the centerline of the air 

mass came from that ended up at the endpoint of the back trajectory, which is a location 

in Dallas in this case. Each trajectory is 36 hours long, centered at Dallas Hinton St., and 

has a start time of 21 GMT (3:00 PM LST). The top plot shows back trajectory densities 

for all days, and the bottom plot shows the back trajectories for days when the ozone was 

greater than 85 ppb. Figure 1 shows that on days when eight-hour ozone concentrations 

are at least 85 ppb, back trajectories tend to have a more easterly and southeasterly 

component. Trajectory lengths also tend to be shorter, implying that wind speeds are 

slower; slow wind speeds are conducive to higher ozone production. Trajectories for all 

days are longer, implying higher wind speeds. Also, wind directions on all days tend to 

be more varied, with only a slight westerly component. In neither of these cases does it 

demonstrate much of an influence from the west that would be the most direct direction 

for transport of emissions from New Mexico. Figure 2, which is Figure 3-1 from the 

DFW Conceptual Model Report, includes back trajectories for all days on a seasonal 

basis for air masses that ended in the DFW area. As can be seen, the back trajectories in 

 

17 From Texas Commission on Environmental Quality’s 2007 DFW 1997 8-hour Attainment 

Demonstration SIP, Appendix G “DFW Nonattainment Area Ozone Conceptual Model” 
the summer months of June-September (when the DFW and Houston areas typically have 

their highest ozone levels) the back trajectories very rarely trace back to areas of New 

Mexico. 

From a study conducted by the Texas State Climatologist, John Nielsen-Gammon, on 

meteorological patterns that influence high ozone in DFW, Houston, and throughout East 

Texas the following language is included in the summary. 18 

“As an easterly or northeasterly wind becomes more and more frequent later in 

the summer, background and total ozone in eastern Texas begin rising. This takes 

place first in northern regions of eastern Texas where southeasterly winds bring 

dirty air, then in more southern regions later in August as large-scale easterlies 

and northeasterlies develop. Winds are also less steady than in the middle of 

summer, so continental transport becomes increasingly frequent.” 

As is further described in this report, elevated ozone levels in DFW and Houston are most 

commonly associated with light winds locally preceded by transport the previous day(s) 

from the East and/or Northeast. 

Therefore, these conceptual model analyses and back trajectories for the 1997 8-hour 

ozone standard provide further support to the determination from the 2002 CENRAP 

based modeling19 that when these two areas experience elevated ozone levels, the 

meteorological patterns only rarely trace the previous origins of these air masses to the 

New Mexico area. Therefore we are concluding that the overall analyses indicates that 

 

18 Nielsen-Gammon, John W., James Tobin, and Andrew McNeel. “A Conceptual Model for Eight-Hour 

Ozone Exceedances in Houston, Texas. Part I: Background Ozone Levels in Eastern Texas.” Center for 

Atmospheric Chemistry and the Environment, Texas A&M University. January 29, 2005. 

19 Further details are included in the Modeling TSD Memorandum for this notice. 
emissions from New Mexico would not significantly impact these areas of Texas when 

they were having ozone levels near or above the 1997 8-hour ozone NAAQS. 

The other more distant sites in Georgia, Pennsylvania, New York and Connecticut are 

much further away than Texas areas that are also East of New Mexico and the longer 

transport distance from New Mexico to the eastern U.S. areas would further dilute the 

impacts below impact levels observed for areas in Texas. The only other identified 

maintenance sites for 1997 8-hour ozone that might be close enough to be impacted were 

in southern and central California. As further discussed below in the discussion for 24-

hour PM2.5 in this TSD for this notice, the meteorological patterns do not transport 

emissions from New Mexico to California when California has elevated ozone or 

particulate levels.20 

PM2.5 maintenance site evaluation

For the annual PM2.5 NAAQS site located in the Chicago area, previous EPA 

modeling relied upon in the 2004 CAIR proposal indicated that impacts from New 

Mexico’s emissions were 0.02 μg/m3

),21 which is well below the one percent (0.15 

μg/m3) threshold that has been proposed as a threshold impact level in the Transport 

Proposal and that we are using as a threshold impact level in this action. The CAIR’s 

modeling also indicated a maximum impact on sites in St. Louis from New Mexico’s 

emissions were 0.02 μg/m3

, that is well below the 0.15 μg/m3 threshold. With a were 

 

20 “Central California Ozone Study (CCOS) Volume I Field Study Plan Version 3”, Eric Fujita, Robert 

Keislar, William Stockwell, Hans Moosmuller, David DuBois, Darko Koracin and Barbara Zielinska of 

Desert Research Institute and Saffet Tanrikulu and Andrew Ranzieri of the Planning and Technical 

Support Division of California Air Resources Board Division of Atmospheric Science, November 24, 

1999. 

21 EPA’s “Technical Support Document for the Interstate Air Quality Rule Air Quality Modeling Analyses 

Appendix H PM2.5 Contributions to Downwind Nonattainment Counties in 2010”, January 2004. 
0.02 μg/m3

 impact in the St. Louis area, this CAIR proposal modeling also confirms that 

New Mexico’s emissions would not significantly impact the maintenance site/monitor in 

Madison county, IL (East St. Louis area). The other maintenance site/monitor identified 

in this geographic area was a monitor in Indianapolis, IN. Indianapolis is between Cook 

County and Madison County but further East than both of these counties, therefore we 

conclude that impacts of 0.02 μg/m3 are a conservative value but also indicate that New 

Mexico’s emissions would not impact monitors in the Indianapolis, IN area above the 

threshold for consideration. 

EPA did not separately calculate the impact of New Mexico’s emissions on the 

Houston area as part of the CAIR modeling or in the Transport Rule Proposal modeling, 

but EPA believes that one can infer from New Mexico’s extremely small impact on other 

areas that New Mexico’s impact on the Houston area would also be less than one percent 

of the NAAQS. The only modeling available that provided source apportionment for 

annual PM2.5 values is the CAIR proposal modeling. The CAIR source apportionment 

results that would be expected to most closely match Houston from a transport 

phenomena perspective are the results for potential impacts on St. Louis. St. Louis is 

helpful for comparison because, while not the same direction from New Mexico it is the 

closest area evaluated in the CAIR proposal modeling that is to the east (in the same 

general transport direction) and at a similar distance from New Mexico as Houston. 

For the St. Louis area, the CAIR proposal modeling indicated that a maximum 

impact from New Mexico’s emissions of 0.02 μg/m3

, which is well below the 1% of the 
NAAQS screening threshold. If we consider that most of the PM2.5 emission sources 

(primary and secondary) are either population based (mobile, non-road, and area source 

emissions) or point and some area sources that are spread throughout the state but several 

of the larger sources are in the northwest quadrant of New Mexico. With this 

understanding of spatial allocation of emission sources in New Mexico, we can assume 

that the centroid of PM2.5 emissions (primary and pre-cursor) is in the Albuquerque area. 

Since emissions are diluted as they are transported, the distance of transport from the 

centroid of New Mexico’s emissions to the areas of concern can provide a ‘measuring 

stick’ for comparing potential impacts. The majority of the emissions of PM2.5 and 

PM2.5 precursors from New Mexico sources emanate from either the Albuquerque area or 

from points father west. The distance from Albuquerque to St. Louis, Missouri is 

approximately 920 miles and the distance from Albuquerque to Houston is about 750 

miles, which is about 81.5 % of the distance from Albuquerque to St. Louis.22 Even if 

one conservatively assumed that New Mexico emissions had twice the impact on 

Houston that EPA determined they do on St Louis due to the shorter transport distances, 

New Mexico’s impact on Houston would still be significantly below the 1% of the 

NAAQS threshold in the Transport Proposal. Given that the difference in distances is 

only 18.5 %, this is a conservative analysis that would indicate no significant impacts 

would be expected from New Mexico on sites in Houston, Texas. 

Using an even more conservative and slightly unrealistic approach of comparing 

the distances from the closest New Mexico border to the city centers of Houston and St. 

 

22 TSD. EPA believes that such a comparison is instructive because the majority of relevant New Mexico 

emissions occur from sources or activities located in the Albuquerque metropolitan area, or in areas 

further to the west. Even if measured from the New Mexico state border to St. Louis and Houston, 

however, the proportional impact would presumably be comparable. 
Louis, the distances are 480 miles and 700 miles respectively and Houston is 

approximately 68% of the distance that St. Louis is from the nearest New Mexico 

boundary. This is not realistic from the standpoint that the distance from the emission 

sources is one of the main variables that impacts the amount of dispersion/dilution that 

can occur but even at these distances it is conservative to assume a doubling of impacts as 

discussed above. 

Also, the relative amounts of emissions in New Mexico, when compared to the 

emissions in Texas support the conclusion that New Mexico emissions do not interfere 

with maintenance in areas in Texas. Using databases developed in connection with the 

Regional Haze (RH) program and submitted with the RH SIPs, the Texas emissions of 

SO2 are approximately 18 times larger than New Mexico’s, the NOx emissions are 

approximately 5 times greater than New Mexico’s, the fine particulate matter emissions 

are 12 times greater than New Mexico’s and the coarse particulate matter is 5.6 times 

greater than New Mexico’s emissions. The Transport Rule Proposal modeling 

information also includes emission summaries that indicate that Texas’s emissions are 

1,338,429 tpy of NOx and 639,505 tpy of SO2 whereas New Mexico’s emissions are 

240,892 tpy of NOx and 24,930 tpy of SO2. Both sets of data indicate that New 

Mexico’s emissions are much lower than Texas’s emissions of PM2.5 precursors. 

Moreover, most of the sources of PM2.5 precursor emissions in Texas are much closer to 

the maintenance receptor in Houston, and therefore less dispersion of the pollutants 

occurs, so Texas’s own PM2.5 and PM2.5 precursors will have a much larger impact on 

PM2.5 levels in Houston. 
In addition, the Texas Commission on Environmental Quality (TCEQ) and 

several researchers have conducted research into the likely causes of elevated annual and 

24-hour PM2.5 monitoring values in the Houston area. TCEQ and the researcher’s 

analyses do not indicate that emissions from New Mexico impact Houston to a degree to 

raise a concern for purposes of the 1997 PM2.5 NAAQS. These analyses indicate that 

meteorological patterns are transporting air masses from other directions (i.e., not from 

the direction of New Mexico) when Houston area sites are monitoring elevated PM2.5 

levels that have the greatest impact on the annual DV.23 While the concern in Houston is 

on the annual PM standard, detailed analyses by the state and researchers show that the 

highest values are heavily influenced by local sources and the measures that TCEQ has 

worked to get in place have been effective in reducing emissions. TCEQ and 

researcher’s analyses also indicate that distance and meteorological patterns during these 

times of elevated pollution levels in Houston also support the conclusion that New 

Mexico’s emissions do not have a significant impact on these areas’ elevated pollution 

levels. In summary, considering the available evidence, EPA concludes that New 

Mexico’s emissions do not interfere with maintenance of the 1997 PM2.5 NAAQS in 

Houston area. 

EPA also reviewed the potential for emissions from New Mexico sources to 

impact other areas with monitors suitable for comparison for interference with 

maintenance purposes. The other such areas are located in California. With regard to 

the 1997 PM2.5 24 hour standard of 65 μg/m3

, the only potential areas of the country that 

 

23 “Source Apportionment for PM2.5at Houston Clinton Drive”, David W. Sullivan The University of 

Texas at Austin Center for Energy & Environmental Research and Richard Tropp, University of 

Nevada Reno Desert Research Institute, TEXAQS II Workshop May 29, 2007 and TCEQ Fact Sheet 

“Harris County/Clinton Drive 1997 Annual Fine Particulate Matter (PM2.5)” November 2009. 
have levels near the 1997 standard currently are identified sites that have current DVs 

(2007-2009) are located in California.24 EPA has evaluated conceptual model 

documents and field study reports that indicate that transport patterns when elevated 

PM2.5 occurs in areas of California, the meteorological patterns are not such that transport 

of emissions from New Mexico to California is occurring to a degree to raise a concern.25 

A review of documents prepared for the San Joaquin Valley in California indicates that 

most ozone, PM-10 and PM2.5 episodes are due to limited vertical mixing and horizontal 

dispersion building up to higher pollutant loadings.26, 27 The dominant transport pattern 

is for air masses from the Eastern Pacific to enter California and a period of high stability 

(low vertical mixing) and lighter winds to occur that results in pollutions exceedances. 

During these events the meteorology is such that general flows are from west to east. 

Therefore it is very rare for meteorological patterns to occur that would transport 

emissions from New Mexico such that they impact California’s pollution levels in 

addition to the long distance that emissions would have to transport and the intervening 

mountainous topography between New Mexico and the areas of concern in California 

further support that transport of emissions from New Mexico is unlikely. Therefore, 

available information indicates that emissions from sources in New Mexico do not impact 

 

24 AQS data. 

25 “Historical Meteorological Analysis in Support of the 2003 San Joaquin Valley PM10 State 

Implementation Plan”, Shawn R. Ferreria Air Quality Meteorologist / Atmospheric Scientist And Evan 

M. Shipp Supervising Air Quality Meteorologist San Joaquin Valley Air Pollution Control District 

January 24, 2005. 26 “Historical Meteorological Analysis in Support of the 2003 San Joaquin Valley PM10 State 

Implementation Plan”, Shawn R. Ferreria Air Quality Meteorologist / Atmospheric Scientist And Evan 

M. Shipp Supervising Air Quality Meteorologist San Joaquin Valley Air Pollution Control District 

January 24, 2005. 27 “Central California Ozone Study (CCOS) Volume I Field Study Plan Version 3”, Eric Fujita, Robert 

Keislar, William Stockwell, Hans Moosmuller, David DuBois, Darko Koracin and Barbara Zielinska of 

Desert Research Institute and Saffet Tanrikulu and Andrew Ranzieri of the Planning and Technical 

Support Division of California Air Resources Board Division of Atmospheric Science, November 24, 

1999. 
areas of concern in California to the degree that would interfere with maintenance of the 

1997 NAAQS in those areas. 
FIGURE 1 
FIGURE 2 
Other References: 

Draxler, R.R., and G.D. Hess. Description of the HYSPLIT_4 Modeling System. 

National Oceanic and Atmospheric Administration Technical Memorandum. ERL-ARL22, Dec.1997. 

Nielsen-Gammon, John W., James Tobin, and Andrew McNeel. “A Conceptual Model 

for Eight-Hour Ozone Exceedances in Houston, Texas. Part I: Background Ozone Levels 

in Eastern Texas.” Center for Atmospheric Chemistry and the Environment, Texas A&M 

University.