Shell Chukchi Sea Exploration Plan

Revision 2

Environmental Impact Assessment

Attachment C

Air Quality Technical Report

Offshore Subsistence Area

Prepared for:

Bureau of Ocean Energy Management

Anchorage, Alaska

On behalf of:

Shell Gulf of Mexico Inc.

Anchorage, Alaska

Prepared by:

ENVIRON International Corporation

Lynnwood, WA

Date:

March 2015

Project Number:

2922291A
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Contents

Page

List of Tables................................................................................................................................................................2

List of Figures..............................................................................................................................................................2

Acronyms, Abbreviations, and Definitions................................................................................................................3

Preface .........................................................................................................................................................................6

1 Summary ...............................................................................................................................................................7

2 Project Description...............................................................................................................................................8

2.1 Exploration Program Activity Area.................................................................................................................8

2.2 Offshore Drilling Program Activities and Emissions Units ..........................................................................10

2.3 Spatial and Temporal Relationships of Offshore Drilling Program Emission Units.....................................11

3 Affected Environment........................................................................................................................................13

3.1 Evaluation Criteria ........................................................................................................................................13

3.2 Existing Air Quality Conditions....................................................................................................................13

3.3 Meteorological Conditions and Climate........................................................................................................17

4 Offshore Exploration Program Analytical Methods.......................................................................................19

4.1 Model Configuration of Emission Units .......................................................................................................19

4.2 Dispersion Modeling .....................................................................................................................................20

4.2.1 Dispersion Model Selection..............................................................................................................20

4.2.2 Methods............................................................................................................................................21

4.2.3 Domain .............................................................................................................................................21

4.2.4 Mesoscale Model Interface Format and Weather Research Forecast ...............................................22

4.2.5 Secondary Aerosols ..........................................................................................................................23

4.2.6 Downwash ........................................................................................................................................24

4.2.7 CALPUFF.........................................................................................................................................25

5 Drilling Program Modeling Results..................................................................................................................26

6 References ...........................................................................................................................................................33
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List of Tables

Page

Table 1. Candidate Drilling Sites1..........................................................................................................................8

Table 2. Evaluation Criteria for Offshore Subsistence Area Users......................................................................13

Table 3. Maximum Existing Ambient Air Concentrations...................................................................................16

Table 4. Maximum Predicted Concentrations at Offshore Subsistence Area Receptors (μg/m3) ........................26

List of Figures

Figure 1. Project Vicinity Map ................................................................................................................................9

Figure 2. Regional Air Quality Monitoring Site Locations...................................................................................14

Figure 3. Wind Rose for Shell Burger Lease Area 2009-2010..............................................................................18

Figure 4. Orientation of Model Emissions Sources...............................................................................................20

Figure 5. Subsistence Area Receptors Used in the CALPUFF Modeling .............................................................22

Figure 6. Locations of Receptors for Peak Model Predictions in Subsistence Area..............................................27

Figure 7. Isopleths of Peak 1-hour NO2 Concentration in Subsistence Area in micrograms per cubic meter 

(μg/m3

)...................................................................................................................................................28

Figure 8. Isopleths of Peak 1-hour PM10 Concentration in Subsistence Area in micrograms per cubic meter 

(μg/m3

)...................................................................................................................................................29

Figure 9. Isopleths of Peak 1-hour PM2.5 Concentration in Subsistence Area in micrograms per cubic meter 

(μg/m3

)...................................................................................................................................................30

Figure 10. Isopleths of Peak 1-hour CO Concentration in Subsistence Area in micrograms per cubic meter 

(μg/m3

)...................................................................................................................................................31

Figure 11. Isopleths of Peak 1-hour SO2 Concentration in Subsistence Area in micrograms per cubic meter 

(μg/m3

)...................................................................................................................................................32
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Acronyms, Abbreviations, and Definitions

Note that in this section and throughout the rest of this report there are active hyperlinks that will jump to 

the referenced material or section. General hyperlinks are formatted like this. Hyperlinks for tables and 

figures are highlighted like this.

Air quality standard................Health-based standard representing a pollutant concentration in the 

ambient air usually over some averaging period (e.g. 1-hour), intended 

to protect the health and welfare of people with a margin of safety.

Ambient air.............................the air in outdoor locations to which the public has ready access

Area source............................. an emission source type defined in CALPUFF. Area source emissions 

are released from two-dimensional four-sided areas 

Attainment/Nonattainment ..... a determination and classification made by EPA indicating whether 

ambient air quality in an area complies with (i.e., attains) or fails to 

meet (i.e., nonattainment) the requirements of one or more NAAQS

Averaging time ....................... a specific period of time (e.g., 1 hour, 24-hours, 1 year) over which 

concentrations of an air pollutant are measured or model-calculated. 

Note that some NAAQSs are also based on multi-year averages of 

certain percentiles of measured or calculated concentrations.

BACT .....................................Best Available Control Technology

BOEM ....................................Bureau of Ocean Energy Management, the regulatory agency with air 

quality authority in portions of the Outer Continental Shelf, part of the 

Department of Interior.

CALPUFF ..............................Air quality dispersion modeling system used in this analysis. The 

CALPUFF modeling system consists of pre-processors, a dispersion 

model and post-processors. The meteorological preprocessor 

(CALMET) was not used to provide meteorological information. 

Instead meteorological data was provided by the Weather Research and 

Forecasting Model (WRF) and prepared for CALPUFF input using a 

specially designed program called the Mesoscale Model InterFace 

(MMIF).

CO .......................................... carbon monoxide, a criteria air pollutant

CO2 ......................................... carbon dioxide

CO2e .......................................Carbon dioxide equivalents (emissions of all GHGs expressed in terms 

of their "global warning potential")

Criteria air pollutant ............... an air pollutant specifically governed by the Federal Clean Air Act for 

which ambient air quality standards have been set. Criteria air 

pollutants include carbon monoxide, particulate matter, sulfur dioxide, 

nitrogen dioxide, ozone, and lead.

Dispersion model...................A computerized calculation tool used to estimate pollutant concentrations in the ambient air based on numeric simulations that consider the 

locations and rates of pollutant emissions and the effects of meteorological conditions, usually over specific averaging times (e.g., 8-hours)
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EPA ........................................US Environmental Protection Agency

hp............................................ horsepower

Meteorological data set........... a compilation of meteorological data representing conditions over 

some period of time and including such things as wind speed and wind 

direction, and formatted as required by the dispersion model being 

used. This analysis used a meteorological data set covering 5 years.

Micrometer/Micron ................ one millionth of a meter; typically used to distinguish particle size; 

typical human hair is 100 about microns in diameter

MLC .......................................Mud Line Cellar, an excavation below the level of the sea floor to 

protect drilling equipment from moving ice floes. 

MCL ROV System.................A remotely operated submersible excavation unit proposed for 

establishing MLCs.

MLC ROV System Vessel......A support vessel used to operate and support the MLC ROV System.

Modeling domain ...................the area included in the dispersion-modeling analysis

Modeling receptor .................. a theoretical (i.e., often non-specific) location used in computer 

modeling at which air pollutant concentrations are calculated. 

Modeling may also use site-specific receptors representing individual 

locations.

NAAQS ..................................National Ambient Air Quality Standard

NSPS ......................................New Source Performance Standard; rules that pertain to air pollution 

emission sources subject to air quality permits and newly manufactured 

equipment

NO2 ......................................... nitrogen dioxide, a criteria air pollutant

NOX ........................................ oxides of nitrogen, a general class of air pollutant without a specific air 

quality standard but used in monitoring air quality

Particulate matter (PM) .......... air pollutant comprised of solid or liquid particles; PM is usually 

characterized based on the particle size. See also PM10 and PM2.5.

PM10........................................ "Coarse" inhalable particulate matter with an aerodynamic size less 

than or equal to 10 micrometers (microns)

PM2.5 ....................................... "Fine" inhalable particulate matter with an aerodynamic size less than 

or equal to 2.5 micrometers (microns)

Point source ............................ an emission source type defined in AERMOD. Point source emissions 

are released from a single location.

ppm......................................... parts per million (a metric used in quantifying concentrations of air 

pollutants)

Receptor..................................See modeling receptor.

Release height......................... a CALPUFF term defining the height above ground at which source 

emissions are released

SO2..........................................Sulfur dioxide, a criteria air pollutant

tpy...........................................tons per year, an estimate of annual emissions
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μg/m3 .....................................micrograms per cubic meter (a metric used in quantifying 

concentrations of air pollutants)
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Preface

Shell Gulf of Mexico Inc. (Shell) has requested authorization from the United States Department of the 

Interior, Bureau of Ocean Energy Management (BOEM), to drill exploration wells in the Chukchi Sea 

beyond the 3-mile seaward boundary of Alaska. Exploration drilling will consist of the operation of two 

drilling units with support vessels on the Outer Continental Shelf (OCS) of the Chukchi Sea. Shell 

already has an approved Exploration Plan (EP) for drilling in the Chukchi Sea at the Burger Prospect (EP 

for Revision 1), but is making changes to that approved EP. 

This report was developed for Shell’s EP Revision 2 and its supporting Environmental Impact 

Assessment (EIA), for exploration drilling operations for Shell’s future seasons of operations. This Air 

Quality Technical Report presents the analytical methods and results of an analysis to estimate offshore 

air pollutant concentrations in subsistence use areas that may result from the drilling units and their 

support vessels.

BOEM implements its authority to protect air quality under 30 CFR Part 550 Subpart C. This program is 

referred to as the BOEM Air Quality Regulatory Program (AQRP). BOEM also has the responsibility to 

evaluate potential impacts of the exploration drilling program pursuant to the National Environmental 

Policy Act (NEPA). BOEM Alaska indicates that air quality modeling is required to evaluate potential 

impacts under NEPA.1

 

This report is in response to a specific request by BOEM to evaluate potential air quality impacts of the 

exploration program in certain offshore areas that are used by native communities for subsistence 

activities. The basic assumptions, methods and analyses used in this report are identical to those in 

Attachment B (the onshore analysis) of the EIA for EP Revision 2, with the following exceptions:

• The locations of predicted concentrations are offshore in a specific area defined as the 

Subsistence Use Area.

• The criteria used for evaluation of these impacts are based on occupational criteria rather than the 

ambient air quality standards that are designed to protect sensitive populations such as the elderly, 

sick or very young.

Much of the detail in this report is identical to the onshore assessment but has been repeated here for the 

convenience of the reader.

1 Meeting between BOEM and Shell, May 15, 2013, held in BOEM office, Anchorage, Alaska
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1 Summary

This air quality analysis of the Shell OCS Exploration Program described in Shell’s EP Revision 2 

identifies air pollutant emissions and onshore concentrations that may result from the air emissions from 

the exploration program. 

The air quality assessment of the exploration drilling program, as described in EP Revision 2, includes 

development of detailed emission inventories based on spatially and temporally distributed emissions 

from the following emissions units:

Offshore Drilling Program

• Two drilling units, including

o Main generators

o Propulsion engines

o Various internal-combustion engines

o Heaters and boilers

o On-board incinerators

• Ice-management vessels (includes anchor handlers), including

o Propulsion and generator engines

o Boilers

o Incinerators

• Oil spill response vessels

• Offshore supply vessels

• Support Tugs

• Oil storage tanker

• Science vessels

• MLC ROV System vessel

Emissions from these units and activities were evaluated with air quality dispersion modeling. The air 

quality analysis considered emissions and concentrations of “criteria” air pollutants, including oxides of 

nitrogen, oxides of sulfur, particulate matter and carbon monoxide. The air quality analysis indicates 

emissions from the exploration drilling program, including all offshore activities, would not result in any 

subsistence area air pollutant concentrations above the proposed significance criteria. 
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2 Project Description

2.1 Exploration Program Activity Area

Shell proposes to continue the exploration drilling program in the Chukchi Sea, Alaska, that began in the 

2012 drilling season and is detailed in EP Revision 1. Shell’s proposed exploration drilling operations 

will take place on federal OCS leases in the vast Chukchi Sea, an area of approximately 230,000 square 

miles (mi2

). Shell’s EP Revision 2 proposes to conduct exploration drilling activities on six lease blocks 

all located within what is known as the Burger Prospect, acquired in federal OCS Lease Sale 193. The 

drill sites are remote from any infrastructure or human habitation and are located more than 60 miles 

offshore in Arctic waters that are inaccessible for eight months or more of the year due to pack ice. 

Shell’s seasonal exploration drilling operations would begin on or about July 1st and extend no later than 

October 31st. The sites are identified “Burger” A, F, J, R, S and V in Table 1. The general location is 

depicted in Figure 1. 

Table 1. Candidate Drilling Sites1

Prospect Well Area Lease 

Number

Lease 

Block

Latitude Longitude UTM Coordinates3

X(m) Y(m)

Burger A2 Posey OCS-Y-2280 6764 N71° 18' 30.92" W163° 12' 43.17" 563945 7912759

Burger F Posey OCS-Y-2267 6714 N71° 20' 13.96" W163° 12' 21.75" 564063 7915957

Burger J Posey OCS-Y-2321 6912 N71° 10' 24.03" W163° 28' 18.52" 555036 7897424

Burger R Posey OCS-Y-2294 6812 N71° 16' 06.57" W163° 30' 39.44" 553366 7907999

Burger S Posey OCS-Y-2278 6762 N71° 19' 25.79" W163° 28' 40.84" 554391 7914199

Burger V Posey OCS-Y-2324 6915 N71° 10' 33.39" W163° 04' 21.23" 569401 7898125

1 Identified under Table 1.a-1 of Chukchi Sea EP Revision 2.

2 Burger A drill site where a partial well was begun in 2012.

3

 The Universal Transverse Mercator (UTM) coordinate system quoted here is from BOEM’s OCS Official Protraction Diagram and are based 

on the North American Datum 1983 (NAD-83), zone 3. The actual coordinates shown are the expected drill locations within each block.
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Figure 1. Project Vicinity Map
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2.2 Offshore Drilling Program Activities and Emissions Units

The offshore drilling program will be conducted by two drilling units, the M/V Noble Discoverer

(Discoverer) and the Transocean Polar Pioneer (Polar Pioneer), with support from ice management 

vessels, anchor handlers, oil-spill response (OSR) vessels, offshore supply vessels (OSVs), tugs, tankers, 

science vessels, MLC ROV System vessel, and aerial transport. For the drilling units, the Discoverer and 

Polar Pioneer, the actual vessel to be used and the types of emission units on board are defined. Support 

vessels are contracted on a yearly basis and multiple vessels are able to meet the duty requirements for the 

needed tasks. As such, it is not certain that the support vessels currently considered for the exploration 

program will be available. Therefore, for the support vessels, a candidate vessel and the anticipated 

emission units are identified.

The Discoverer is a turret-moored drilling unit that underwent significant upgrades in 2007 and 2013 so 

that it could operate in the Arctic. The Discoverer has its own propulsion engine for self-transport. The 

Discoverer is equipped with electrically powered thrusters, engine-driven electrical generators for the 

drilling motors, and other self-powered equipment such as hydraulic pumps, cranes, boilers, an 

incinerator, and other (mostly emergency-related) small sources. There will be no flares and no 

hydrocarbon-venting sources except for minor amounts associated with the drill cuttings. 

The Polar Pioneer is a semisubmersible vessel designed to operate in the arctic environment. The Polar 

Pioneer is transported by a towing vessel. The Polar Pioneer operates main generator engines, heaters 

and an incinerator. There will be no flares and no hydrocarbon-venting sources except for minor amounts 

associated with the drill cuttings.

The support vessels are equipped with diesel-fueled primary and emergency power generation engines, 

and in some cases incinerators and/or diesel-fueled boilers. 

Ice management vessels and anchor handlers will assist with management of the drilling unit anchors, 

bow washing of any ice buildup on the drilling units, and some ice floe fragmenting in support of the ice 

management vessel. One anchor handler and one ice management vessel provide primary close support 

for each drilling unit with regard to these tasks. The ice management vessels are needed when there are 

ice features that require disruption in their path or fragmentation in order to provide protection for the 

drilling units, or other assets critical to the safety of the exploration drilling program (i.e., mooring buoys, 

etc.). Up to two ice management vessels may be tasked to fragment any manageable ice flows so that the 

ice will flow around the drilling units. These ice management vessels may work at distances of 25 miles

or more upwind of the drilling unit to monitor the leading edge of any ice floe of possible concern. These 

activities are necessary for managing ice at distances that provide adequate response time for drilling 

units to get off a well and anchor in case of encroaching ice that cannot be managed. These response 

times may vary depending on the drilling stage of the well hole. Furthermore, the anchor handlers and ice 

management vessels may have other tasks to conduct beyond 25 miles of the drilling units. This 

operation is accounted for in the fuel consumption limits used to calculate emissions, but ENVIRON has 

assumed they will be within 25 miles for the entire season. As discussed below, this assumption is 

conservative and tends to over-predict concentrations. 

An oil-spill response vessel or vessels will be anchored nearby, typically between the two drilling units. 

During season, these vessels will primarily be used during refueling operations to protect against possible 

spills and will be located near the refueling Arctic oil fuel storage tanker. The OSR vessels are expected 

to be used in the unplanned and unlikely event of an oil discharge to the water. These vessels will be 

available to both the Discoverer and the Polar Pioneer.

Other support vessels include those for resupply and material transfer to shore. The OSVs would travel to 

the drilling units, then “park” in dynamic positioning (DP) mode beside the drilling unit for material or 
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personnel transfer. The OSVs may operate in DP mode beside the drilling unit and would remain there 

for approximately one day. 

It is anticipated that up to two vessels similar to the OSVs will be primarily used to monitor discharges 

from each drilling unit as required by the National Pollutant Discharge Elimination System (NPDES) 

General Permit. These vessels may remain within a few miles of each drilling unit for these sampling 

periods or conduct secondary tasks, as needed. 

Two tugs will be operated in standby mode on-location in case the Polar Pioneer must leave location 

quickly because of encroaching ice. Another tug will escort the Discoverer to the drill site, assist during 

mooring, and depart the season to conduct other activities outside the Chukchi Sea.

A fuel and oil tanker is expected to be located in an area between the two drilling units to resupply the 

drilling units and support vessels. 

During the 2012 drilling season, mudline cellars (MLC) were excavated with large drills aboard the 

Discoverer. As described in Shell's EP Revision 2, an MLC may also be excavated by a separate vessel

(MLC ROV System vessel) supporting a specially designed subsurface excavator. This MLC operation 

may proceed ahead of the Discoverer and Polar Pioneer to future well sites within the same lease blocks 

identified in Table 1.

If the ice management vessels or OSVs travel beyond 25 miles from the drilling units, air emissions 

would be dispersed to a greater extent than when the vessels are closer to the drilling units. Because the 

dispersion modeling conducted in this analysis concentrates all ice management and OSV emissions 

within 25 miles of the drilling units, it results in predictions that are higher than those expected if some 

vessels venture outside the 25 mile radius. Consequently, additional modeling of that scenario is not 

warranted.

2.3 Spatial and Temporal Relationships of Offshore Drilling Program Emission Units

For air impact analysis purposes, there are three emission unit groups: the drilling units, the on-location 

support vessel support, and the onshore activities. Emissions units that are physically close together can 

have additive impacts, whereas, if the same emissions units are located over a large area, the impacts will 

be smaller at any one location (and distributed over a larger area). 

The emission units on the Discoverer and Polar Pioneer are concentrated on each drilling unit. The 

support vessels will be spread over a 25 mile radius of the Discoverer and Polar Pioneer; therefore, 

emissions will be dispersed over this large area and will not be concentrated. At large distances from the 

drilling units, of up to 50 miles, the collective emissions will be well dispersed. 

The Discoverer and the Polar Pioneer will operate in separate lease blocks. Based on Table 1, the 

distance between drill sites varies from 3 kilometers to 21 kilometers, depending on the two drill sites that 

are active at any given time. In order to evaluate the greatest potential onshore impact from the 

exploration activities, it is assumed that the drilling units are operating at the two lease blocks closest to 

the subsistence use area (see Figure 3.11.6-11 of EIA for EP Revision 2), which are blocks J and V. This 

assumption will result in an analysis of the greatest potential impacts to subsistence activities because at 

distances larger than a few miles where there is no elevated terrain, air quality concentrations are 

inversely proportional to the distance between the emission unit and the receptor. In this instance, the two 

drilling units are approximately 14 kilometers apart.

As discussed, the support vessels operate in different locations and operate in different ways over large 

areas. The anchor handler and ice management vessels will operate in areas determined by the drilling 

unit locations. When they are managing or scouting ice, they may typically be 25 miles or more upwind 
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of the drilling unit. When these vessels are not working they could be anchored in warm-stack mode, 

transitioning or working in support of activities inside or outside of the OCS program area. 

The OSR vessels will normally be in an area between the Discoverer and the Polar Pioneer. These 

vessels will be in a stand-by mode or supporting refueling operations associated with exploration 

program.

Some emissions units may not operate concurrently. Only emissions units that operate concurrently can 

have additive short-term (1-hour and 24-hour) concentrations. Those that do not operate concurrently 

will not have additive short-term concentrations, although all will contribute to concentrations averaged 

over the season. Drilling and use of the drilling units’ smaller internal combustion (IC) engines will take 

place only after the drilling unit is fully anchored and connected to its anchors. The Discoverer’s 

propulsion engine will only be used intermittently once it is anchored. The cementing and logging 

equipment will only be used when setting casing or logging a well when the drilling unit is anchored at a 

drill site. None of the smaller diesels are operated during ship transit to and from the drill site. None of 

the smaller IC engines are used more than occasionally. However, since the precise times when 

individual emission units are expected to operate are not known at present, all units have been 

conservatively assumed to be operating concurrently in the air quality analysis.
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3 Affected Environment

3.1 Evaluation Criteria

Air quality is generally assessed in terms of whether concentrations of air pollutants are higher or lower 

than prescribed criteria. As discussed in Attachment A of the EIA for EP Revision 2, national ambient air 

quality standards (NAAQS), which are established at levels determined to protect human health with a 

margin of safety, including the health of sensitive individuals like asthmatics, the elderly, the chronically 

ill, and the very young, are not appropriate criteria for offshore locations because the area is not readily 

accessible to the public and those who are able to reach the area are more apt to be healthy individuals 

capable of hunting, fishing, or working on commercial vessels. These individuals are present, if at all, 

only for limited periods of time and are comparatively healthier than the more susceptible population that 

NAAQS are designed to protect. See Attachment A of the EIA for EP Revision 2. In addition, under the 

clean air act, NAAQS are only implemented in air quality control regions, which do not extend beyond 

the State’s 3-mile seaward boundary. The criteria applied in the current evaluation have been selected to 

protect subsistence area users in offshore locations and are more protective than Occupational Safety and 

Health Administration’s (OSHA) exposure standards, and thus have a built-in margin of safety . These 

criteria are presented in Table 2. 

Table 2. Evaluation Criteria for Offshore Subsistence Area Users

Pollutant Averaging 

Time

Offshore Subsistence Area 

Criteria (μg/m3)

Nitrogen Dioxide (NO2) 1-hour 3,760

Particulate Matter (PM10 and PM2.5) 1-hour 500

Carbon Monoxide (CO) 1-hour 55,000

Sulfur Dioxide (SO2) 1-hour 5,200

3.2 Existing Air Quality Conditions

There are no existing sources of air pollution near the Chukchi Sea lease area because it is more than 60 

miles from land and there are no other oil exploration or development sources in the Chukchi Sea at this 

time. In the absence of sources of emissions, the air quality in the project area is expected to be good. 

The points of land nearest the proposed drill sites are in the remote parts of the Arctic coast of Alaska, 

and are mostly uninhabited except for occasional subsistence hunting and fishing. The nearest native 

villages are at Wainwright and Point Lay, approximately 66 and 86 nautical miles away, respectively. 

Because the drill site location will be far from the Alaska shoreline and away from significant sources of 

pollution, existing air quality concentrations can be represented with a regional value. According to the

EPA’s Guideline on Air Quality Models (40 CFR 51, Appendix W, Section 8.2.2c), a “regional site” may 

be used to determine background concentrations if there are no monitors located in the vicinity of the 

source. A “regional site” is one that is located away from the area of interest, but is impacted by similar 

natural and distant man-made sources. The majority of the air quality data on the North Slope have been 

collected by various industrial developments associated with the oil and gas resources of the area. 

Shell and ConocoPhillips began monitoring NO2, PM2.5, PM10, SO2, CO, and O3 concentrations at 

Wainwright, Alaska in November 2008 and Point Lay, Alaska in June 2010. Both monitoring stations are
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remotely located (minimal influence of industry and other human activities) and are the most 

representative “regional sites” on the North Slope for estimating offshore pollutant concentrations in the 

Chukchi Sea. However, both monitoring sites are located onshore and are located adjacent to villages and 

therefore may be exposed to high concentrations of pollutants from nearby combustion or unpaved road 

dust from vehicles. Consequently, it is expected that the measured concentrations of pollutants at these 

stations are higher than the concentrations that actually occur offshore because the combustion and dust 

sources are not present on the open sea. A map of the ambient monitoring stations on the North Slope is 

provided in Figure 2. 

Figure 2. Regional Air Quality Monitoring Site Locations
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Table 3 summarizes the background pollutant concentrations measured at the Wainwright and 

Point Lay monitoring stations during the drilling seasons (July 1 – Nov. 3).Note that annual 

concentrations best represent the average concentrations onshore, and likely overstate 

concentrations in the subsistence area because the onshore areas adjacent to developed areas with 

local sources of air pollution as opposed to the undeveloped environment of the Arctic Ocean. 

Table 3. Maximum Existing Ambient Air Concentrations

Pollutant Averaging 

Period

How standard is applied Wainwright 

Maximum 

Measured

Concentration b

(μg/m3)

Pt. Lay 

Maximum 

Measured 

Concentration c

(μg/m3)

NAAQS 

(μg/m3)

NO2

1-hour 3-year average of 98th percentile 

daily maximum 1-hour averages 40.1 43.8 188

Annual Maximum arithmetic average 1.4 1.4 100

PM2.5

24-hour 3-year average of 98th percentile 

daily averages 10.7 5.5 35

Annual 3-year average of annual arithmetic 

averages

2.9 2.0 12

PM10

24-hour

Not to be exceeded more than once 

per year (2nd highs), averaged over 3 

years

73.0 23.6

150

Annuald -- 16.6 6.2 --

SO2

1-hour 3-year average of 99th percentile 

daily maximum 1-hour averages 8.1 11.6 190

3-hour Not to be exceeded more than once

per year (2nd high) 12.8 14.1 1,300

24-hour Not to be exceeded more than once 

per year (2nd high) 2.2 13.4 365

Annual Maximum arithmetic average 0.4 4.8 80

CO

1-hour Not to be exceeded more than once 

per year (2nd high) 953 1490 40,000

8-hour Not to be exceeded more than once 

per year (2nd high) 946 1280 10,000

a Maximum background ambient air concentrations are calculated for the July 1st

- Nov. 30th “drill season” period, using the most recent available 

data from Wainwright and Point Lay monitoring stations. b Maximum concentrations measured at Wainwright from 2009 through 2012, as calculated according to the method described in “how standard 

is applied” column

c Maximum concentrations measured at Point Lay monitoring site from 2011 through 2013, as calculated according to the method described in 

“how standard is applied” column. 

d

There is currently no NAAQS for annual PM10

The criteria that have been identified in Table 2 all refer to a one-hour average concentration. 

Comparison of the 1-hour concentrations in Table 3 (NO2, SO2, and CO) all show measured 

concentrations well below the criteria. There are no 1-hour ambient standards for either designation of 

particulate matter (PM10 or PM2.5), therefore, the 24-hour values are adjusted by the persistence ratio of 

24-hour to 1-hour concentration of 0.4 from the SCREEN3 model. In this comparison, Point Lay PM

backgrounds were deemed more representative to use. Wainwright particulate matter backgrounds were

not considered representative because of contamination from road dust from the adjacent unpaved road 

discussed in SLR (2011) and SLR (2012). Thus for PM10 the peak measured 24-hour concentration from 

Table 3 of 24 μg/m3 is multiplied by 2.5 to get 60 μg/m3 while for PM2.5 the peak measured 24-hour 

concentration of 5.5 μg/m3 is multiplied by 2.5 to get 14 μg/m3. Both of these values are well below the 

1-hour criteria in Table 2 of 500 μg/m3

. 
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3.3 Meteorological Conditions and Climate

Climate in the project study area is unique to the polar region. The climate is dominated by severe cold 

temperatures during winter and a brief period of warming in late summer and early fall. 

From an air pollution perspective, the most important meteorological parameters are wind speed and 

direction because they determine the transport and dispersion of airborne contaminants. Wind conditions 

are commonly represented by a figure known as a wind rose. Figure 3 depicts a wind rose figure 

constructed from the meteorological data used in the current analysis. The figure has a series of bars 

emanating from the center of the drawing. The bars represent the relative frequency of wind directions 

with the length of each bar representing the relative frequency of the wind direction. In this case it shows 

the most frequent wind directions at the Burger Lease are coming from the east-northeast.

The colors in the figure illustrate the relative frequencies of wind speeds at the project site. The color 

code in the figure can be used to interpret the wind speeds.

Wind roses are indicative of dominant wind directions and thus also, the most frequent transport 

directions of air pollutants. Due to the higher transport frequencies, in many cases peak long-term 

average concentrations can be found downwind in these directions, because they occur more often and 

when averaged over a longer period of time can typically have higher average concentrations. This is not 

always the case, as other factors can contribute to the determination of peak downwind concentrations, 

such as the presence or absence of receptors in each downwind direction, terrain elevations in the 

downwind directions, wind speeds associated with each wind direction and air turbulence associated with 

each wind direction. Thus wind roses are useful when interpreting long-term average concentrations, 

with the above qualifiers. However, the wind rose gives little information on the peak short-term (e.g., 

hourly or daily) concentrations. Short-term concentrations can occur in any wind direction, regardless of 

how common or frequent the wind direction is. As a result, it is common for peak short-term impacts to 

be in different locations than the peak long-term concentrations, and the peak short term concentrations 

are not necessarily aligned with the most common wind directions. 
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Figure 3. Wind Rose for Shell Burger Lease Area 2009-2010
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4 Offshore Exploration Program Analytical Methods

An air quality impact analysis includes two basic steps: (1) development of an emission inventory that 

identifies short-term and annual emissions related to the exploration drilling program, and (2) dispersion 

modeling to estimate resulting air contaminant concentrations in the ambient air. Appendix K of EP 

Revision 2 provides detailed documentation of projected emissions, including the basis for all 

calculations, attributable to the exploration activities described in EP Revision 2. Please refer to Appendix 

K of EP Revision 2 for further details. The following sections discuss the methods employed and the 

critical assumptions involved in the dispersion modeling analysis, which are based on the information 

provided in Appendix K of EP Revision 2.

4.1 Model Configuration of Emission Units

All of the emission units associated with the exploration drilling program are to some extent mobile. The 

most stationary of the units are those on the drilling units. During the drilling of any individual well, the 

drilling unit remains fixed over the well. However, the Discoverer rotates about the drilling stem, placing 

the bow of the ship in the direction of the oncoming wind, which is usually also the direction any moving 

ice would come from. The Discoverer does not rotate as a result of the wind acting on it, but rather is 

moved by a cranking system aboard the Discoverer. As the vessel is rotated, the locations of many or all 

the emission units on the drilling unit are moved. The Polar Pioneer does not rotate with the wind.

Although the Discoverer emission units are mobile, for purposes of the modeling study, both drilling 

units are assumed to be point sources at a fixed location. Given that the nearest onshore receptors are 

more than 100 kilometers from the Discoverer, the effect of the actual rotation of the ship is insignificant 

in the modeled concentrations. Hence the drilling unit is assumed to be pointing in the direction of the 

prevailing wind for the entire drill season. The prevailing wind direction was assumed to be coming from 

60 degrees measured clockwise from north based on buoy measurements taken at the Burger site during 

July to November 2009.

In contrast to the drilling units, the support vessel emissions units are much more mobile. Section 2.4 

describes the various types of movement and operation from each type of support vessel.

Given the highly mobile nature of these support vessels it is inappropriate to model them as fixed point 

sources, but rather as area sources where emissions are distributed over an area. For the ice management 

vessels, the area source is modeled as a large triangular area approximately 25 miles long and located 

upwind of the drilling unit. For the other vessels, a square area source, 2 kilometers on each side, is 

assumed to represent the remaining emission units. It is recognized that ice management vessels may be 

located further from the drilling units when managing ice, but the dimensions and locations used for these 

vessels have been selected to conservatively represent the spatial extent of their emissions.

Figure 4 is a schematic drawing that shows the location of these point and area sources. Note that the 

large triangular areas are intended to represent the ice management vessel emissions. The four smaller 

square areas shown in the figure are used to represent the support vessels. One of these square areas is 

used to represent the MLC ROV System and its associated MLC ROV System vessel. The two areas 

shown directly downwind of the Discoverer and Polar Pioneer represent the close support vessels 

including the science vessel and any tugs dedicated to the drilling unit. The central area source, located 

between the two drilling units is the common fleet, including the Oil Spill Recovery vessels and the oil 

supply tanker. 
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Figure 4. Orientation of Model Emissions Sources

4.2 Dispersion Modeling

ENVIRON International Corporation (ENVIRON) used air quality dispersion modeling simulations to 

estimate ambient concentrations attributable to emission units associated with the exploration program. 

This section discusses the methods used to develop these simulations.

Air quality models are computer programs designed to mathematically represent atmospheric transport 

and dispersion of airborne contaminants. The purpose of the proposed air quality modeling in this impact 

analysis is to provide estimates of ambient concentrations of air pollutants emitted by the various engines 

boilers, and other emission units that are part of the exploratory drilling program described in EP 

Revision 2. 

4.2.1 Dispersion Model Selection

The two air quality models most commonly recommended in the Guideline for industrial sources of 

emissions are the AERMOD model and the CALPUFF model. The AERMOD model is recommended by 

EPA for computation of concentrations within 50 kilometers of a source, while the CALPUFF model is 

recommended for locations farther than 50 kilometers from a source. Discussions with BOEM2 indicated 

2 Meeting between Shell and BOEM held on May 15, 2013 at BOEM’s offices in Anchorage Alaska.
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the agency’s intention to follow EPA’s Guideline on Air Quality Models. While the Guideline provides 

no specific guidance on modeling offshore vessels, the guidance does suggest that when source-receptor 

distances are more than 50 kilometers, CALPUFF should be used.

BOEM requires a demonstration that emissions from the exploration program will not exceed certain 

levels onshore. For that analysis, provided in Attachment B to the EIA, BOEM has indicated to Shell that 

the CALPUFF modeling system is appropriate. For this special analysis of offshore impacts in the 

subsistence area, the same logic is used. Because the nearest subsistence area is located more than 70 

kilometers from the closest candidate drilling location, the CALPUFF model is the appropriate model to 

use.

ENVIRON applied CALPUFF to predict pollutant concentrations from emissions associated with Shell’s 

offshore drilling program in the Chukchi Sea. The CALPUFF predictions were used to display potential 

regional pollutant concentrations, and to assess compliance with the subsistence area criteria. The 

remainder of this section describes the long-range transport dispersion modeling techniques.

4.2.2 Methods

ENVIRON applied the regulatory version of the CALPUFF modeling system to simulate emissions from 

the offshore drilling program described in EP Revision 2. CALPUFF (Version 5.8) is the EPA 

recommended dispersion model for long-range transport analyses and source-to-receptor distances 

beyond 50 km.3

 For the application of CALPUFF, Shell followed the techniques recommended by the 

Federal Land Managers for Class I area assessments with a few modifications for Arctic conditions and 

available datasets. The simulations were performed based on a representative set of meteorological 

conditions from July to November 2007, 2008, and 2009, so as to best reflect drilling season conditions. 

EPA and ADEC practice is to accept data sets less than 10 years old for use with modeling. Shell 

considers this data set to be representative because meteorology does not change dramatically on short 

time scales. The methods used to prepare the meteorological fields and perform the dispersion model 

analysis are described below.

4.2.3 Domain 

The area included in the dispersion modeling analysis, CALPUFF modeling domain, is shown in

Figure 5. This area corresponds with the known subsistence use areas documented under Figure 3.11.6-

11 of the EIA of EP Revision 2. The Burger site, several villages, and a 4-km receptor grid mesh size are 

posted on the domain plot. The 4 kilometer grid size for receptors was selected in order to give coverage 

to the entire domain while still giving adequate resolution to concentrations. The analysis assumes the 

Discoverer and Polar Pioneer are located at the anticipated drill sites in Lease Blocks V and J, 

respectively. The CALPUFF domain is a rectangular 167-by-118 grid with a horizontal mesh size of 

4 km and 10 vertical layers ranging geometrically from the surface to 4,000 m. A Polar Stereographic 

(PS) projection was used for the coordinate system with an origin at (70 N, 155 W) and standard latitude 

of 70 N. 

3 40 CFR Part 51 Appendix W: Guideline on Air Quality Models
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Note: Receptor locations indicated in blue.

Figure 5. Subsistence Area Receptors Used in the CALPUFF Modeling

4.2.4 Mesoscale Model Interface Format and Weather Research Forecast

The CALPUFF model requires meteorological data inputs to predict how potential emissions will behave 

in the specified subsistence area. This is achieved through the use of a meteorological data set and a 

preprocessor that prepares the information in a format that CALPUFF can accept. ENVIRON used the 

Weather Research Forecast (WRF) model to construct the meteorological fields and the Mesoscale Model 

Interface Format tool (MMIF)4 to process and reformat the WRF output for input to CALPUFF. EPA 

provided ENVIRON with WRF model simulations for the Chukchi Sea, which were then processed with 

MMIF (Version 2.3). These WRF simulations for July to November of 2007 to 2009 supported previous 

ConocoPhillips permitting activities in the Chukchi Sea.5

 The WRF simulations have the three domains 

with grid mesh sizes of 36/12/4-km and 37 vertical levels. The boundary layer, nudging and other options 

4 Brashers, B., and C. Emery, 2013. Draft User’s Manual: The Mesoscale Model Interface Program (MMIF), Version 2.3, 2013-

4-30. Prepared by Environ International Corp. for U.S. EPA, OAQPS, Air Quality Assessment Division, Air Quality Modeling 

Group, Mail Code C439-01, Research Triangle Park, NC, 27771, Accessed at 

http://www.epa.gov/ttn/scram/dispersion_related.htm#mmif. 5 McNally, D. and Wilkinson, J.G., 2011. Model Application and Evaluation – ConocoPhillips Chukchi Sea WRF Modeling 

Application, Prepared by Alpine Geophysics, 7341 Poppy Way, Arvada, CO, 8007, November 21, 2011.
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selected for the WRF simulations are based on comparisons to Arctic meteorological data and the results 

of ongoing studies sponsored by BOEM.6

The WRF meteorological dataset for the Chukchi Sea has been evaluated against surface and upper air 

observations within the model 4-km domain including analyses focused on the coastal stations as a group, 

and at Wainwright and Barrow individually. The WRF simulations replicated observed temperatures, 

mixing ratios, wind speeds, wind directions, and precipitation totals within well-established model 

performance benchmarks.5 WRF also predicted the observed atmospheric structure aloft when compared 

to the observations at Barrow. The developers of dataset found the overall WRF model performance was 

better than previous datasets they evaluated for the intermountain west, Gulf Coast or eastern seaboard of 

the United States.5

ENVIRON used the following MMIF options, which follow modeling guidance, to process and reformat 

the WRF meteorological fields for CALPUFF:

• Use only the 4-km WRF inner domain;

• Select the GOLDER option for calculation of the Pasquill-Gifford stability class;

• Use layer mapping of the 37 vertical WRF levels to 10 layers with tops of 20, 40, 160, 320, 640, 

1200, 2000, 3000, and 4000m;

• No recalculation of the mixing height, the WRF diagnostic output will be used directly; and

• Trim five cells along from the outer edge of the WRF 4-km mesh size domain to account for

potential edge effects in the WRF simulations

ENVIRON used MMIF to prepare daily input files for CALPUFF to account for changing sea-ice 

coverage in the Arctic Ocean. The corresponding changes to the hourly energy fluxes and other 

important variables predicted by WRF governing dispersion and transport are already incorporated 

directly into the MMIF data provided to CALPUFF. However, several algorithms in CALPUFF (e.g. 

deposition velocity calculations) need to distinguish between over water and over land characteristics 

based on land use that is only provided at the start of each meteorological input file. Surface roughness 

changes from hour to hour over the water and seasonally over the land. Daily input files allow CALPUFF 

to consider more refined changes to land use for these algorithms. It should be noted that the surface 

roughness is calculated independently by WRF based on a two dimensional representation of the surface 

and is different for every grid cell in the surface domain. 

4.2.5 Secondary Aerosols 

CALPUFF incorporates algorithms to consider secondary aerosols formed from interaction and chemical 

transformations of emitted NOX and SO2. Total PM10 and PM2.5 were calculated from the sum of the 

emitted primary species, ammonium nitrate, and ammonium sulfate. The primary PM10 emissions for 

each source were divided into six species, including: soot or elemental carbon (EC), fine soil particles 

(PMF), coarse particles (PMC), organic carbon (OC), sulfate (SO4), and nitrate (NO3). PMC fractions 

were calculated from the difference between PM10 and PM2.5 emission rates. PM2.5 emissions were 

categorized into the remaining five species using the source profiles for diesel engines and incinerators 

6 Zhang, J., Liu, F., Krieger, J., Tao, W, and X. Zhang, 1981. Project Report for the 5-year Experimental Mesoscale Meteorology 

Reanalysis for the Beaufort/Chukchi Seas for Beaufort and Chukchi Mesoscale Meteorology Model Study. Prepared for US 

DOI, Bureau of Ocean Energy managements, Alaska Outer Continental Shelf Region, Anchorage Alaska, Contract 

0106CT39787, November 2011.
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based on profiles recommended by the EPA for the Community Multi-Scale Air Quality (CMAQ) 

model.7

 

Reaction rates and aerosol formation in the CALPUFF chemistry algorithms are influenced by 

background ozone and ammonia concentrations. ENVIRON used the maximum hourly ozone 

observations from the NOAA Barrow Observatory and BP’s Pad A monitoring site to represent 

background ozone concentrations in the simulations. The background ammonia concentration was 

assumed to be 0.5 ppb for all hours based on the Alaska Regional Haze Best Available Retrofit 

Technology (BART) modeling simulations.8

ENVIRON used the above quantitative methods to assess the contribution of emitted SO2 and NOX

emissions to form secondary aerosols and assess PM2.5 concentrations. Although OCS activities in the 

arctic are not under EPA’s jurisdiction, EPA Guidance for PM2.5 Permit Modeling recommends an 

assessment of secondary aerosols when PM2.5 precursor emissions (NOX and/or SO2) emissions are 

greater than 40 tons per year.9 EPA- recommended procedures include both quantitative and qualitative 

analyses. EPA’s Guidance provides a qualitative example based on a Region 10 Office assessment of 

Shell’s Discoverer drill ship and support fleet in the Chukchi Sea. EPA’s qualitative assessment:

“examined the regional background PM2.5 monitoring data and aspects of secondary PM2.5

formation from existing sources; the relative ratio of the combined modeled primary PM2.5

impacts and background PM2.5 concentrations to the level of the NAAQS; the spatial and 

temporal correlation of the primary and secondary PM2.5 impacts; meteorological characteristics 

of the region during periods of precursor pollutant emissions; the level of conservatism associated 

with the modeling of the primary PM2.5 component and other elements of conservatism built into 

the overall NAAQS compliance demonstration; aspects of the precursor pollutant emissions in the 

context of limitations of other chemical species necessary for the photochemical reactions to form 

secondary PM2.5; and an additional level of NAAQS protection through a post-construction 

monitoring requirement.”

Region 10 found the formation of secondary aerosols would be limited in the Arctic and the NAAQS 

would be protected accounting for both the primary PM2.5 impacts and potential contributions due the 

PM2.5 precursors from the Discoverer and Associated Fleets.9 These same qualitative arguments apply to 

the exploration activities examined in this study.

4.2.6 Downwash

The tendency for exhaust plume rise to be reduced by wind flowing across nearby structures is referred to 

as downwash. Because the nearest receptors are located more than 100 km from the source at the 

shoreline, building downwash effects will not significantly affect the modeled results. However, previous 

modeling analyses for the Shell exploration program have developed building downwash parameters. 

The modeling did use the previous downwash values developed for the Shell exploration program in the 

7 CMAQ is the preferred regulatory model for PM2.5 and regional haze simulations. The EPA website containing PM speciation 

by source categories is: http://www.epa.gov/ttn/chief/emch/speciation/.

8 The Alaska BART and Regional Haze programs are described at http://www.dec.state.ak.us/air/anpms/rh/rhhome.htm. In the 

original BART simulations a background of 0.1 ppb was assumed. In the more refined simulations performed by applicants 

seeking exemption from BART, a more conservative 0.5 ppb ammonia concentrations was assumed.

9 EPA, 2014. Guidance for PM2.5 Permit Modeling. U.S. EPA, Office of Air Quality Planning and Standards, Air Quality 

Assessment Division, Research Triangle Park, NC, EPA-454/B-14-001, May 2014.
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CALPUFF modeling analysis for the Discoverer sources. New downwash parameters were created for 

the Polar Pioneer sources using EPA’s Building Profile Input Program (BPIP).

4.2.7 CALPUFF 

ENVIRON performed three CALPUFF simulations for the Chukchi Sea using short-term emissions for 

each July to November period of 2007 to 2009. Short-term emissions were used in the analysis to address 

the one-hour maximum concentrations for each pollutant in accordance with the criteria presented in 

Table 2. The respective short-term emission rates for emission units included in the simulations are 

discussed in Chapter 2.4 of Appendix K of EP Revision 2. It should be noted that drilling may occur 

between July 1 and November 30 (153 days), but each drilling season is limited to 120 days. As the 

critical impacts are short term ambient standards and increments, not annual averages, Shell wanted to 

address concerns that drilling could occur at any time during the five month window. Therefore, in 

order to make certain the peak meteorological scenarios were evaluated, short-term emissions were 

conservatively modeled to occur during all 3,672 hours in the July 1 to November 30 drilling season for 

each of the three years in the simulations. 

It should be noted that CALPUFF addresses all pollutants in a combined single run. The interactions 

between pollutants, such as the secondary aerosol formation of particulate matter from gaseous oxides of 

nitrogen or sulfur are treated as chemical transformations in the model. These non-linear processes are 

modeled dynamically by CALPUFF for each hour and no aggregate or averaging methods are used.

The secondary aerosol is most commonly sulfates and nitrates, formed from sulfur dioxide and nitrogen 

oxides. To address secondary aerosol formation, ENVIRON applied POSTUTIL a program developed by 

the EPA, that converts particulate species such as sulfate and nitrate into the most common solid forms of 

these species, ammonium sulfate and ammonium nitrate, by converting mass to account for the difference 

in molecular weight between sulfate and ammonium sulfate, and nitrate and ammonium nitrate. It then 

sums all the particulate species to give total concentrations of PM10 and PM2.5. 

CALPOST (Version 6.221) was used to calculate the maximum 1-hour concentrations of each pollutant. 

The results used in this analysis are the maximum 1-hour values for each pollutant, without any statistical 

analysis to determine the 98th or 99th percentile that is commonly used in analyses for onshore criteria.

ENVIRON conservatively assumed all NOX predicted at downwind receptor is NO2 for comparisons to 

the criteria. A second tier approach assuming a conversion factor of 0.8 is also appropriate and could be 

applied in any future analyses. It should be noted, although not performed here, a Tier 3 approach can be 

used to limit the potential formation of NO2 by the amount of ozone available. The Ozone Limiting 

Method (OLM) could be applied by post-processing the CALPUFF output files and assuming a constant 

NO2/NOX in-stack ratio, an equilibrium ratio of 0.8. The amount of NO2 formed will be limited using the 

same hourly ozone input file used in the CALPUFF simulations.
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5 Drilling Program Modeling Results

A total of 1,800 receptors were selected to represent the subsistence area depicted in Figure 5. Maximum 

predicted concentrations from all 1,800 receptors are presented in Table 4 for each pollutant. All modelpredicted concentrations are well below the impact criteria for offshore locations.

Table 4. Maximum Predicted Concentrations at Offshore Subsistence Area Receptors

(μg/m3)

Pollutant Averaging 

Time

Maximum

Modeled

Concentration 

Background 

Concentration1

Total 

Concentration 

Offshore 

Subsistence 

Area Criteria 

Nitrogen 

Dioxide (NO2) 1-hour 52.1 40.1 92.2 3,760

Particulate 

Matter (PM10) 1-hour 24.4 60.0 84.4 500

Particulate 

Matter (PM2.5) 1-hour 24.4 14.0 38.4 500

Carbon 

Monoxide (CO) 1-hour 16.2 953 969 55,000

Sulfur Dioxide 

(SO2) 1-hour 0.1 8.1 8.2 5,200

1 Background concentrations are documented in Table 3.1.3-1 of the EIA of EP Revision 2. It should be noted that Table 3.1.3-1 provides 1-hour 

background concentrations only for gaseous pollutants but for particulate matter, the 24-hour values from Table 3.1.3-1 are adjusted by the ratio 

of 24-hour to 1-hour concentration of 0.4 from the SCREEN3 model. Thus for PM10 the peak measured 24-hour concentration from Table 3 of 

24 μg/m3 is multiplied by 2.5 to get 60 μg/m3 while for PM2.5 the peak measured 24-hour concentration of 5.5 μg/m3 is multiplied by 2.5 to get 14

μg/m3

The most representative maximum background air quality values available were selected for this 

evaluation. These values are highly conservative, given that the monitors are located onshore and adjacent 

to villages containing sources of air pollutants. Maximum offshore concentrations determined by 

CALPUFF occur at the seaward edge of the subsistence zone west of Wainwright. The background 

concentrations from the nearest monitoring station to the modeled maximum concentration for each 

pollutant and averaging period were selected to estimate total concentrations reported in Table 4. 

Wainwright background concentrations were selected for all pollutants and categories except for PM, 

instead, Point Lay PM backgrounds were deemed more representative.

Wainwright PM backgrounds were not considered representative because of contamination from road 

dust from the adjacent unpaved road discussed in SLR (2011) and SLR (2012). A persistence factor was 

used to estimate 1-hour average maximum background values for particulate matter (24-hour values 

multiplied by 2.5 to estimate 1-hour values, based on persistence factors used in the EPA SCREEN3 

model).
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To provide further detail on the model predictions, maps of peak 1-hour model-predicted concentrations 

have been prepared for each of the 5 pollutants listed in Table 2. These figures are presented in Figure 

7, Figure 8, Figure 9, Figure 10 and Figure 11 for NOX, PM10, PM2.5, CO, and SO2, respectively. These 

figures, called “isopleth” plots, are created by drawing contours of the peak model prediction at each 

location on the surface in the offshore subsistence area. As such, these do not represent conditions that 

occur at the same time, but rather reflect the peak potential concentration at each location, regardless of 

the time and date of the predicted concentrations. 

These isopleth maps will not reflect the frequency of winds as seen in a wind rose, such as that in 

Figure 3, because the isopleths reflect the peak 1-hour concentration, not the annual average 

concentration. A wind rose shows how often a particular meteorological condition occurs, while these 

plots show the worst case, typically a very infrequent condition, at each location. As seen in Figure 3, 

winds most frequently come from the east, carrying emissions to the west of the drill site locations. 

Under these most frequent meteorological conditions, concentrations in the offshore subsistence areas are 

typically low. But as the wind rose also shows, on rare occasions, the wind will come from the north or 

northwest, carrying emissions toward the closest parts of the subsistence area. Under these infrequent 

conditions, the peak 1-hour concentrations are predicted and the isopleths reflect the magnitude of impact 

that occurs at each receptor under the most adverse conditions for that receptor location.

Figure 6. Locations of Receptors for Peak Model Predictions in Subsistence Area
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Figure 7. Isopleths of Peak 1-hour NO2 Concentration in Subsistence Area in micrograms 

per cubic meter (μg/m3)
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Figure 8. Isopleths of Peak 1-hour PM10 Concentration in Subsistence Area in micrograms 

per cubic meter (μg/m3

)
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Figure 9. Isopleths of Peak 1-hour PM2.5 Concentration in Subsistence Area in 

micrograms per cubic meter (μg/m3

)
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Figure 10.Isopleths of Peak 1-hour CO Concentration in Subsistence Area in micrograms 

per cubic meter (μg/m3

)
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Figure 11.Isopleths of Peak 1-hour SO2 Concentration in Subsistence Area in micrograms 

per cubic meter (μg/m3)
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6 References

US Environmental Protection Agency (EPA)

2004. AERMOD User's Guide. EPA-454/B-03-001. September 2004

CALPUFF User’s Guide