Background PM2.5 Analysis for 

the Eastern Gulf of Mexico 

Outer Continental Shelf 

Prepared by:

ENVIRON International Corporation

Lynnwood, Washington

Baton Rouge, Louisiana

Date:

April 2013
Contents i ENVIRON

Contents

Page

1 Introduction 2

2 Background 2

3 Meteorological Conditions in the EGOM 3

4 Air Monitoring Sites 4

4.1 PM2.5 Design Values 5

4.2 Trends 7

4.3 Monitoring Data Conclusions Regarding PM2.5 SILs and SMC 8

5 PM2.5 Precursors and Chemical Transformations 9

5.1 Summary 10

List of Figures

Figure 1: Windrose Plots from Various ASOS Stations and Buoys, with Active Lease Blocks

Figure 2: PM2.5 Background Assessment Sites

Figure 3: 2000-2011 EPA Design Value Trend for 24-Hour PM2.5

Figure 4: 2000-2011 EPA Design Value Trend for Annual PM2.5

Figure 5: SEARCH Monitor Locations [Blanchard 2013b]

List of Tables

Table 1: EPA Recommended Approaches for Assessing Primary and Secondary PM2.5 Impacts 

by Assessment Case

Table 2: EPA Reported Design Values and Averages

Table 3: IMPROVE Concentrations
Background PM2.5 Analysis, EGOM 2 ENVIRON

1 Introduction

Air emission sources complying with the federal Prevention of Significant Deterioration (PSD) 

rules are required to assess the influence of not only primary particulate matter less than 2.5 

micrometers in aerodynamic diameter (PM2.5), but also impacts of potential formation of 

secondary PM2.5. 

Precursors of PM2.5 include nitrogen oxides (NOx), sulfur oxides (SOx), and volatile organic 

compounds (VOCs). These precursors, under specific atmospheric conditions and in the 

presence of other chemicals integral to the photochemical processes that form secondary 

PM2.5, may coagulate on existing particles or nucleate into new particles. Understanding the 

state of atmospheric conditions and chemical composition near a PSD source is essential to 

assessing the influence that the PSD source could have on secondary PM2.5 formation. 

The U.S. Environmental Protection Agency (USEPA) has provided draft guidance that allows for 

a qualitative assessment of secondary PM2.5 formation for PSD sources. The intent is for the 

assessment to provide sufficient qualitative evidence that the potential for the formation of 

secondary PM2.5 would not result in overall PM2.5 levels that exceed the NAAQS, and that 

ultimately a quantitative assessment will not be required to evaluate the formation of secondary 

PM2.5. 

This memo has been prepared in response to the USEPA draft guidance regarding secondary

PM2.5 formation. Included is a review of atmospheric conditions and chemical composition 

within the Eastern Gulf of Mexico (EGOM) Outer Continental Shelf (OCS) region, which may be 

used to qualitatively predict the potential impact of precursors on the secondary formation of 

PM2.5.

This memo also addresses the use of the PM2.5 Significant Impact Levels (SILs) and significant 

Monitoring Concentration (SMC), which were vacated by the United States Court of Appeals for 

the District of Columbia Circuit on January 22, 2013.

This memo is targeted at oil exploration activities requiring PSD permits in the EGOM OCS.

2 Background

On March 4, 2013, the USEPA issued draft guidance1 for PM2.5 air quality impact assessments 

under the auspices of the federal PSD rules (Section 165(a)(3) of the Clean Air Act (CAA) and 

40 CFR sections 51.166(k) and 52.21(k)).

The USEPA PM2.5 guidance provides a summary of required information to support the 

quantitative assessment of secondary formation of PM2.5. This guidance delineates four (4) 

possible scenarios where quantitative impact assessments will ultimately be required. Each 

scenario is associated with direct emissions of PM2.5 and of secondary precursors (i.e.,NOx and 

SO2) in relation to their Significant Emission Rates (SER). Table 1 describes the various 

USEPA assessment recommendations as a function of the pollutant emissions.

 1 Available at http://www.epa.gov/ttn/scram/guidance/guide/Draft_Guidance_for_PM25_Permit_Modeling.pdf
Background PM2.5 Analysis, EGOM 3 ENVIRON

Table 1: EPA Recommended Approaches for Assessing Primary and Secondary PM2.5 Impacts by 

Assessment Case

Assessment 

Case

Description of Assessment Case Direct PM2.5 

Impact Approach

Secondary PM2.5

Impact Approach

Case 1: 

No Air Quality 

Analysis

Direct PM2.5 emissions < 10 tpy 

NOx and SO2 emissions < 40 tpy

Not Applicable Not Applicable

Case 2:

Primary Air 

Quality 

Impacts Only

Direct PM2.5 emissions > 10 tpy 

NOx and SO2 emissions < 40 tpy

Appendix W 

preferred or 

approved alternative 

dispersion model

Not Applicable

Case 3:

Primary and 

Secondary 

Air Quality 

Impacts

Direct PM2.5 emissions > 10 tpy 

NOx and/or SO2 emissions > 40 tpy

Appendix W 

preferred or 

approved alternative 

dispersion model

• Qualitative

• Hybrid qualitative /

quantitative

• Full quantitative

photochemical

grid modeling

Case 4:

Secondary Air 

Quality 

Impacts Only

Direct PM2.5 emissions < 10 tpy 

NOx and/or SO2 emissions > 40 tpy

Not Applicable • Qualitative

• Hybrid qualitative /

quantitative

• Full quantitative

photochemical

grid modeling

Note that although the PM2.5 guidance contains four cases, there are only two cases for primary 

PM2.5 (either less than or greater than the SER) and two cases for secondary PM2.5 (either less 

than or greater than the NOx/SO2 SER). Note also that the approach for addressing secondary 

PM2.5 is independent of how primary PM2.5 is addressed. If NOx/SO2 emissions are greater 

than their SER, a qualitative (or quantitative) approach to secondary PM2.5 is required, 

regardless of whether primary PM2.5 is greater than its SER or not.

The PM2.5 guidance also addresses how to use ambient monitoring data to (a) show that the 

PM2.5 SILs may still be applied as a screening tool, and (b) demonstrate compliance with the 

preconstruction monitoring requirements [Section 51.166(m) or 52.21(m)].

3 Meteorological Conditions in the EGOM

Windrose plots for a range of ASOS stations and environmental buoys in the Gulf of Mexico are 

shown in Figure 1. These data were extracted from the NCDC’s TD-3505 dataset. Also shown 

are the active lease blocks in the GOM, and the line dividing the EGOM from the Western GOM.

The generally stronger wind speed conditions found at sea can be seen, with more winds in 

excess of 10 m/s over the water. Near the South of the plot, the winds blow more often from the 

East or ESE. Closer to shore, the winds become more evenly distributed, though there is still a 

tendency for winds to blow from the East to Southeast. The land-sea breeze effect can clearly 

be seen in the on-shore windroses, which have a tendency to be aligned with winds 

perpendicular to the shoreline. However, most sites still exhibit a wide range of wind directions, 

consistent with weak topographic forcing. 
Background PM2.5 Analysis, EGOM 4 ENVIRON

Figure 1. Windrose Plots from Various ASOS Stations and Buoys, with Active Lease Blocks

Emissions of PM2.5 precursors from lease blocks in the EGOM would, according to the 

windroses from buoys located between the majority of EGOM lease blocks and shore, be nearly 

equally likely to be blown in any direction, with a slight preference for blowing toward the 

Louisiana shore. 

4 Air Monitoring Sites

Currently, there are only a handful of PSD permits and pending applications for sources in the 

EGOM. There are no environmental monitors in the vicinity of the existing lease blocks where 

oil exploration activities might take place. The PM2.5 concentration in the OCS is expected to be 

lower than the concentration on shore, due to the scarcity of sources in the EGOM, and the 

abundance of sources of PM2.5 on land (vehicles, industry, etc.). Using shore-based monitors is 

therefore conservative, in that it over-estimates the existing PM2.5 concentrations in the EGOM.

PM2.5 design values and trends were reviewed for sites bordering the Eastern Gulf of Mexico. 

The USEPA Air Quality System (AQS) federal reference monitors (FRM) and Interagency 

Monitoring of Protected Visual Environments (IMPROVE) monitors, selected as representative 

samples for the Eastern Gulf region, are depicted in Figure 2.
Background PM2.5 Analysis, EGOM 5 ENVIRON

4.1 PM2.5 Design Values

The USEPA publishes design values for all AQS monitors, which are used for determination of 

attainment with air quality standards. The 24-hour design values are defined as 3 year average 

98th percentile 24-hour PM2.5 concentrations, while the annual design values are the simple 3-

year average PM2.5 concentrations. The most recent design values for the sites shown as red 

diamonds in Figure 2 are provided in Table 2.

As illustrated in Figure 2 by the yellow circles, there are three (3) IMPROVE monitors in the 

EGOM region, located “near” or in the Class I areas. The 98th percentile 24-hr concentrations for 

each year (and the three-year average) for these sites are reported in Table 3, for the years 

2009 through 2011. These data were reviewed to ensure Class I areas are below the design 

values provided by the USEPA AQS monitors. 

Figure 2. PM2.5 Background Assessment Sites
Background PM2.5 Analysis, EGOM 6 ENVIRON

 Table 2: EPA Reported Design Values and Averages

EPA Design Values (DV) of PM2.5

for 2009-2011 (ug/m3)

Site Site Code 24-Hr DV Annual DV

Fairhope, AL 01-003-0010 21 9.7

Chickasaw, AL 01-097-0003 19 9.8

Theordore, AL 01-097-2005 20 9.2

Gainesville, FL 12-001-0023 20 7.7

Gainesville, FL 12-001-0024 20 7.5

Panama City, FL 12-005-1004 21 8.9

Crystal River, FL 12-017-0005 16 7.3

Ferry Pass, FL 12-033-0004 20 9

Tampa, FL 12-057-0030 16 9

Valrico, FL 12-057-3002 17 7.8

Tallahassee, FL 12-073-0012 25 9.9

St. Petersburg, FL 12-103-0018 16 7.7

Clearwater, FL 12-103-1009 15 7.4

Sarasota, FL 12-115-0013 15 6.8

Valdosta, GA 13-185-0003 28 10.4

Waveland, MS 28-045-0003 21 9.8

Gulfport, MS 28-047-0008 18 9.6

Pascagoula, MS 28-059-0006 21 9.5

Average 19.39 8.72

Minimum 15 6.8

Maximum 28 10.4

NAAQS 35 12

Table 3: IMPROVE Concentrations

IMPROVE Design Values (DV) of PM2.5 (ug/m3)

Site Site 

Code

24-Hr 

DV

Annual 

DV

Breton Island BRIS1 17.50 8.30

Chassahowitzka CHAS1 14.36 7.13

St. Marks SAMA1 17.81 7.73

Average 16.56 7.72
Background PM2.5 Analysis, EGOM 7 ENVIRON

4.2 Trends

The 3-year monitoring design values for the AQS and IMPROVE sites illustrated in Figure 2

were reviewed from 2000-2011 to establish a regional trend. 

Figure 3 illustrates the average 24-hour PM2.5 3-Year design value trends, the average design 

value plus the significant impact level (SIL) for 24-hour PM2.5 (1.2 μg/m3

), the 2007 NAAQS for 

24-hour PM2.5 (35 μg/m3

), and the gap between the design value plus SIL to the NAAQS. The 

trend shows an overall improvement in PM2.5 since the 3-year period ending in 2006. These 

decreases in PM2.5 design values may be a function of improved control technologies and the 

introduction of new regulatory measures. 

Figure 4 illustrates the average annual PM2.5 design value trends, the average design value 

plus the significant impact level (SIL) for annual PM2.5 (0.3 μg/m3

), the 2013 NAAQS for annual 

PM2.5 (12 μg/m3

), and the gap between the design value plus SIL to the NAAQS. The trend 

shows an overall improvement in PM2.5 since the 3-year period ending in 2006. These 

decreases in PM2.5 design values may be a function of improved control technologies and the 

introduction of new regulatory measures. 

Figure 3. 2000-2011 EPA Design Value Trend for 24-Hour PM2.5
Background PM2.5 Analysis, EGOM 8 ENVIRON

In the context of sources of PM2.5, the growing gap between DV+SIL (the design value plus the 

SIL) and the NAAQS corresponds with a decreased likelihood of needing to perform a 

quantitative NAAQS analysis. 

Figure 4: 2000-2011 EPA Design Value Trend for Annual PM2.5

4.3 Monitoring Data Conclusions Regarding PM2.5 SILs and SMC

Because the monitoring sites shown in Figure 2 are representative (or conservative) of the 

EGOM OCS region, the data shown in Tables 2 and 3 may be used in place of on-site preconstruction monitoring to satisfy 40 CFR 51.166(m) and 52.21(m) of the EPA’s regulation.

The PM2.5 24-hour NAAQS value is 35 μg/m3

, and the Class II SIL is 1.2 μg/m3. The annual 

NAAQS value is 12 μg/m3

, and the Class II SIL is 0.3 μg/m3. 

Because the maximum 24-hour design value shown in Tables 2 and 3 is less than 33.8 μg/m3

(the NAAQS minus the SIL), the 24-hour PM2.5 Class II SILs is sufficiently protective of the 

NAAQS, and may be used as a screening tool when modeling direct emissions of PM2.5. 

Similarly, because the maximum annual design value shown in Tables 2 and 3 is less than 

11.7 μg/m3 (the NAAQS minus the SIL), the use of the annual PM2.5 Class II SIL is sufficiently 

protective of the NAAQS and may be used as a screening tool when modeling direct emissions 

of PM2.5. 
Background PM2.5 Analysis, EGOM 9 ENVIRON

The Class I PM2.5 SILs were vacated/remanded along with the Class II SILs by the DC District 

Court. Class I SILs are normally used as a screening tool to assess whether a full Class I 

Increment analysis is required, and if so, whether a project significantly contributes to Increment 

consumption. The PM2.5 Increments became effective relatively recently (October 20, 2011), 

and only a few PSD projects have been proposed in the EGOM since the major source baseline 

date. Emissions from PSD projects outside of the EGOM would not affect a Class I area at the 

same time as emissions originating from the EGOM, due to the very different meteorological 

conditions. When the wind blows from the EGOM toward a Class I area, it blows emissions 

from land-based sources further in-land, away from the relevant Class I areas. Therefore, the 

Class I SILs may still be treated as a “target value,” below which it is unlikely the PM2.5 Class I 

Increments will be consumed. 

5 PM2.5 Precursors and Chemical Transformations

Sources within the EGOM contribute to increases in particulate concentrations through direct 

emission of particulates and emission of particulate precursors. As indicated in the Draft PM2.5

Guidance, precursors can be addressed either by a qualitative assessment of the state of 

atmospheric chemistry for a region, or by use of photochemical modeling. However, if a 

qualitative assessment provides ample evidence that precursors will not cause primary and 

secondary PM2.5 emissions to exceed or approach the NAAQS, then the application of 

photochemical models may not be required. 

Investigations of the Southeastern Aerosol Research and Characterization (SEARCH) 

monitoring network [Blanchard 2013a, Blanchard 2013b] have provided detailed information on 

the trends and speciation of the regional pollutants. The SEARCH network encompasses the 

eastern Gulf of Mexico and has operated since mid-1998 assessing numerous pollutants 

relevant to the development of secondary PM2.5. The SEARCH network includes monitoring 

locations at the sites illustrated in Figure 5.

Recently published studies of the SEARCH network have shown that concentrations of PM2.5

have trended downward from 1999 – 2010. Secondary formation of PM2.5 is due largely to 

reactions of SO2 and NOx with NH3 in the atmosphere, resulting in increased particle 

composition of sulfate and nitrate respectively, a notion supported by the SEARCH network 

analyses. Over the SEARCH network sampling period, the mean annual sulfate and nitrate 

concentrations had ranges of 3.2 – 4.3 μg/m3 and 0.3 – 0.9 μg/m3 respectively, indicating a 

prominence of sulfate.

Blanchard et al. [2013b] indicated that particulate sulfate concentrations decreased in tandem 

with decreases in SO2 emissions reported by electrical generating units. A one-to-one 

relationship was not observed, likely due to the exclusion of other sources emitting SO2. 

Decreases in SO2 emissions and SO4 concentrations were also observed within the larger 

Clean Air Status and Trends Network (CASTNet) indicating a robust geographic range in the 

regional downward trend of sulfate. However, a reduction in the conversion of SO2 to sulfate 

potentially increases the availability of ammonia for the conversion of NOx to nitrate. 

The formation of particulate nitrate is a function of NOx, NH3, and SO4 interactions as well as 

atmospheric conditions such as temperature and humidity, thus causing additional complexity in 

understanding the relationships associated with particulate formation. The use of thermal 

equilibrium models [Blanchard 2007] indicate that, within the SEARCH network, the formation of 

nitrate is limited by the abundance of NH3, a compound which showed no significant trend. 

However, nitrate concentrations did not increase with decreasing sulfate concentrations, 
Background PM2.5 Analysis, EGOM 10 ENVIRON

suggesting that, despite reductions in SO2 emissions, the reaction of SO2 with NH3 dominates 

the region. This notion is further validated by the correlation of decreasing sulfate particulates 

and ammonium particulate concentrations within the SEARCH network. Therefore, the addition 

of a new source of NOx would likely have little impact on the formation of secondary nitrate 

within the region. 

Figure 5. SEARCH Monitor Locations [Blanchard 2013b]

5.1 Summary

A significant increase in PM2.5 concentrations near the eastern Gulf of Mexico from a source of 

nitrogen oxides is unlikely. The region is dominated by the conversion of sulfur oxides to sulfate 

by reactions with ammonia. Analysis of the SEARCH network has shown that as sulfate 

concentrations and sulfur oxide emissions have declined, there has been no reciprocal increase 

in nitrate, suggesting that the sulfur oxide and ammonia reaction remains dominant. Further, 

assessment of the EPA FRM monitors shows a declining trend in PM2.5. The most recent design 

values allow for an increase of combined primary and secondary PM2.5 of 14.41 μg/m3 before 

the 24-hour NAAQS level is met.
Background PM2.5 Analysis, EGOM 11 ENVIRON

References

Blanchard, C. L., Tanenbaum, S., & Hidy, G. M. (2007). Effects of Sulfur Dioxide and Oxides of 

Nitrogen Emission Reductions on Fine Particulate Matter Mass Concentrations: Regional 

Comparisons. Journal of the Air & Waste Management Association, 57(11), 1337–1350. 

doi:10.3155/1047-3289.57.11.1337

Blanchard, C. L., Hidy, G. M., Tanenbaum, S., Edgerton, E. S., & Hartsell, B. E. (2013a). The 

Southeastern Aerosol Research and Characterization (SEARCH) study: Spatial variations 

and chemical climatology, 1999–2010. Journal of the Air & Waste Management 

Association, 63(3), 260–275. doi:10.1080/10962247.2012.749816

Blanchard, C. L., Hidy, G. M., Tanenbaum, S., Edgerton, E. S., & Hartsell, B. E. (2013b). The 

Southeastern Aerosol Research and Characterization (SEARCH) study: Temporal trends in 

gas and PM concentrations and composition, 1999–2010. Journal of the Air & Waste 

Management Association, 63(3), 247–259. doi:10.1080/10962247.2012.748523