Attainment with the New NAAQS and What You

Need to Know About Air Dispersion Modeling

Dan P. Dix

Pennsylvania Chamber Environmental and Energy Conference & Trade Show

April 18, 2012

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Agenda Summary of NAAQS PSD Modeling Procedures NAAQS Implementation Updates Dispersion Modeling Basics and

Inputs NAAQS Modeling Demonstration

Approach

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About ALL4 Environmental consulting firm Founded 2002 – currently 30+ employees Offices in Kimberton, PA and Columbus, GA Specialize in air quality consulting:

• Complex air permitting and strategy development

• Air dispersion modeling• Ambient air quality monitoring

www.all4inc.com

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About ALL4

ALL4’s customized environmental regulatory update service.

Includes concise tabular summary of changes and events in state, local, and/or federal regulations based on previous months environmental activity.

Also Includes a regular consulting call with an environmental expert to discuss how the activities in report impact your business.

National Ambient Air Quality

Standards (NAAQS)

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NAAQS Background “Backdrop” of the Clean Air Act States design their SIPs and enforce

and implement their regulations to meet the NAAQS

Air quality construction permit programs are designed around NAAQS compliance• PSD: Maintaining NAAQS attainment• NNSR: Getting into NAAQS attainment

NAAQS reevaluated every 5 years

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NAAQS SummaryPollutant

Averaging Period

Primary/Secondary

Historic NAAQS (µg/m3)

Revised NAAQS (µg/m3)

CO1-Hour Primary 40,000 10,0008-Hour Primary 10,000 40,000

Ozone 8-HourPrimary/

Secondary75 ppb Withdrawn

Pb3-Month Rolling

Primary/Secondary

1.5 0.15

PM10 24-HourPrimary/

Secondary150 150

PM2.5

24-HourPrimary/

Secondary65 35

AnnualPrimary/

Secondary15 15

NO2

1-Hour Primary N/A 188

AnnualPrimary/

Secondary100 100

SO2

1-Hour Primary N/A 1963-Hour Secondary 1,300 1,300

24-hour Primary 365 Revoked

AnnualPrimary/

Secondary80 Revoked

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Attainment/Nonattainment Designations

U.S. EPA philosophy on the SO2 NAAQS implementation process:• Proposed NAAQS – designations based

on ambient monitoring data• Final NAAQS – designations based

primarily on air quality modeling data

PSD Air Quality Modeling Analysis

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PSD Modeling Analysis

PollutantPSD Significance

Levels (TPY)

CO 100Pb 0.6PM 25

PM10 15PM2.5 10NO2 40

SO2 40

H2SO4 10TRS 10H2S 10

VOC 40

So what happens when PSD Significance Level are exceeded as part of a PSD permit application?

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SIL Analysis Project related emission increases are

modeled and compared to the Significant Impact Levels (SILs).

A SIL exists for each NAAQS and is used as a screening approach to determine if a full NAAQS and PSD Increment Analysis is required.

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SIL AnalysisPollutant

Averaging Period

SIL(µg/m3)

CO1-Hour 2,0008-Hour 500

Pb 3-Month Rolling 1.5PM10 24-Hour 5

PM2.5

24-Hour 1.2Annual 0.3

NO2 1-Hour 7.5

SO2

1-Hour 7.93-Hour 25

If project related emissions result in predicted concentrations greater than the SIL, a NAAQS and PSD Increment Analysis is required.

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NAAQS Analysis Facility wide PTE emissions must be

modeled for any pollutant resulting in project related emissions greater than the SIL.

Local sources within 50 kilometers plus the Significant Impact Area (SIA) must be included.

Background concentrations from representative monitors must be included.

The cumulative impacts from all three must then be compared to the NAAQS.

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PSD Increment Analysis A facility wide emission inventory must be

prepared for the actual emission increases that occurred since the major source baseline date (the time at which the first PSD project was completed for an area) and compared to the PSD Increment Levels.

Since these emissions are typically difficult to calculate, conservatively use either the NAAQS (PTE) emission rates or actual emissions from the most recent two years.

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PSD Increment Analysis

PollutantAveraging

Period

Class I PSD Increment

(µg/m3)

Class II PSD Increment

(µg/m3)

Class III PSD Increment

(µg/m3)

CO1-Hour None None None

8-Hour None None None

PM10 24-Hour 8 30 None

PM2.5

24-Hour N/A N/A None

Annual N/A N/A None

NO2

1-Hour N/A N/A None

Annual 2.5 25 None

SO2

1-Hour N/A N/A None

3-Hour 25 512 None

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AQRV Analysis An Air Quality Related Value (AQRV)

Analysis must be completed for any Class I area within 300 kilometers of the facility.

The CALPUFF model is used for Class I areas located more than 50 kilometers away.

What is a Class I area?• National parks greater than 6,000 acres that existed

before 1977.• Wilderness areas greater than 5,000 acres that existed

before 1977 and are operated by the U.S. Forest Service or the U.S. Fish and Wildlife Service.

Q/d Screening Approach

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NAAQS Implementation

Updates

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SO2 NAAQS Implementation NAAQS Implementation Schedule:

• June 2011: Initial state nonattainment recommendations to U.S. EPA (most counties were “unclassifiable”)

• June 2012: EPA to finalize attainment status (most states will still be “unclassifiable” or attainment)

• June 2013: Maintenance SIP submittals including individual facility modeling to achieve compliance with the NAAQS (including air quality modeling for individual facilities)

• August 2017: Full NAAQS compliance in all areas

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Implementation Update Draft guidance for states to evaluate designations

using AERMOD was released on September 22, 2011

Numerous comments received on draft guidance. On April 12, 2012 Gina McCarthy (U.S. EPA

Assistant Administrator) issued a letter to all States stating that modeling demonstrations showing attainment of the standard for areas initially designated “unclassifiable” will no longer be required for the June 2013 SIP Submittals.

Instead U.S. EPA will be conducting “Stakeholder Outreach” in order to discuss a workable approach for implementation of the new standard.

http://www.epa.gov/airquality/sulfurdioxide/implement.html

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SO2 Maintenance SIP Submittals

U.S. EPA: Revising PSD/NNSR programs to include new NAAQS is not sufficient. Five components are required:• “Attainment Emission Inventory”• Maintenance Demonstration• Control Strategy• Contingency Plan• Verification of Continued Attainment

Maintenance SIP will list enforceable 1-hour emission limits (August 2017)

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SO2 NAAQS Implementation State SIPs will be based on AERMOD

dispersion modeling for the following individual facilities (by order of priority):• SO2 Actual Emissions > 100 tons per year

• SO2 PTE > 100 tons per year• Smaller facilities “with a potential to

cause or contribute” to a NAAQS violation States are considering other options based

on population

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SO2 NAAQS Implementation Facilities may wish to install on-site

meteorological tower, co-located SODAR, and ambient SO2 measurements• Collection of one-year of on-site meteorological

data• Collection of 3+ years of measurement data for

SO2 NAAQS

• Track concurrent hourly SO2 emissions• Evaluate performance of AERMOD or EPA-

approved alternative model• Propose modeling approach using evaluation

results as guidance

Dispersion Modeling Basics

and Inputs

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AERMOD Process

Hourly Wind Speed

Hourly Wind Direction

Hourly Ambient Temperature

Land Use PatternsTopography

Building DimensionsStack DimensionsExhaust Velocity

Exhaust Temperature

Emission Rates

Predicted Ground Level Ambient Concentrations (µg/m3) for all

averaging times

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AERMOD Modeling System

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Air Quality Modeling Steps

1. Emission Inventory2. Meteorological Data

(AERMET/AERSURFACE)3. Terrain Data (AERMAP)4. Building Downwash (BPIPPRM)

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Emission Inventories Short-term (1-hour) emission rates Potential to be used as permit limits Intermittent emission units (e.g., emergency

generators, intermittent emission scenarios such as startup/shutdown operations or alternative fuels)• Latest guidance indicates following form of

standard as guideline for what to include (i.e., 99th percentile (4th highest) for SO2 or 98th percentile (8th highest) for NO2 & PM2.5)

Stack characteristics (height, temperature, velocity, diameter, location)

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Meteorological Data

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Meteorological Data 5 years of National Weather Service data Minimum of 1 year of onsite data Surface characteristics and topography

surrounding the facility should be similar to (representative of) those surrounding the meteorological station

If no representative meteorological data are available, SO2 implementation guidance suggests possibility of using AERSCREEN (with agency approval)

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Terrain Data “Ambient Air”

Public access must be restricted in some way (e.g., fence, security guard) in order for onsite receptors to be disregarded in the modeling analysis

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Building Downwash

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Building Downwash

NAAQS Modeling Demonstration

Approach

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Full NAAQS Evaluation Includes facility, other local facilities,

and background concentrations Any modeled emission rates should

be acceptable as a 1-hour permit limit (for NO2 and SO2) with the appropriate margin for compliance

Considerations for accounting for emissions during startup and shutdown

Emergency unit considerations

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Local Sources Same emission rate considerations

apply for local sources (although permit limit concerns wouldn’t apply)

State agency typically dictates which local sources to include in evaluation

Typically 50km + Significant Impact Area (SIA)

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NAAQS Modeling Strategy Start with an evaluation of each

individual emission source Each source will have different

factors that drive resulting ambient concentrations

The cumulative ambient concentration from all sources (plus background) will be evaluated against the NAAQS

Evaluate each source against the NAAQS as a first step

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NAAQS Modeling Strategy Big picture factors that will drive

ambient concentrations for individual sources:• Elevated emission rates• Stack velocity (orientation of release

and flowrate)• Stack temperature (plume buoyancy)• Stack height versus surrounding terrain• Surrounding buildings and structures

(i.e., building downwash)

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Hypothetical Modeling Example

Modeling of a hypothetical facility with the following SO2 emission sources:• Process SO2 source• Backup engine source

NAAQS modeling evaluation is based on SO2 potential-to-emit

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Hypothetical Facility Terrain

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“Process” SO2 Source SO2 Emission Rate: 240 lb/hr (CEMS) Stack Height: 290 feet Stack Diameter: 16.5 feet Exhaust Temp: 350 °F Exhaust Flow: 230,000 acfm Elevated emission rate, buoyant

source, tall stack (taller than the tallest buildings at the facility)

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Process SO2 Source Impacts

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Process SO2 Source Impacts Highest impacts in complex terrain far

from facility Wind speed doesn’t match location of

elevated concentrations Impacts occur during periods of

atmospheric stability and low mixing heights (typically early morning, low wind speed conditions)

High concentrations due partially to the limitations of the AERMOD dispersion model

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Combustion SO2 Source SO2 Emission Rate: 20 lb/hr (AP-42) Stack Height: 60 feet Stack Diameter: 2 feet Exhaust Temp: 225 °F Exhaust Flow: 16,000 acfm Buoyant source, short stack (shorter

than the tallest buildings at the facility)

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Engine SO2 Source SO2 Emission Rate: 3 lb/hr (Vendor) Stack Height: 10 feet Stack Diameter: 1.3 feet Exhaust Temp: 935 °F Exhaust Flow: Horizontal Discharge Horizontal discharge, short stack

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Engine SO2 Source Impacts

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Engine SO2 Source Impacts Elevated ambient concentrations at

the facility fenceline for two reasons:• Low stack height (10 feet)• No plume buoyancy due to horizontal

discharge Ambient air considerations become

very important (i.e., public access)

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Modeling Refinements “Process” SO2 Emission Source:

• Stack height increase is technically and economically infeasible

• Raw materials are fixed due to product and consumer demand

• Upgrades to the scrubber could achieve control: ~30% more control (~170 lb/hr)

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Process SO2 Source Impacts (Before)

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Process SO2 Source Impacts (After)

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Modeling Refinements Engine SO2 Emission Source:

• Simplest fix is to change the stack discharge orientation from horizontal to vertical

• No changes to the vendor-guaranteed emission rate of the engine

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Engine SO2 Source Impacts (Before)

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Engine SO2 Source Impacts (After)

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Cumulative Concentrations The facility must cumulatively

comply with the NAAQS Addressing each individual source

helps as a first cut This scenario still exceeds the 1-hour

NAAQS for SO2 when the sources are taken cumulatively

Haven’t even considered ambient background concentrations

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Modeling Strategies Emissions Strategies Actual Distribution of Emissions

• Evaluate adequacy of emission limits• Evaluate emissions control options• Evaluate alternate fuels and fuel

specifications Facility Fence Line Strategies

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Modeling Strategies Stack/Exhaust Strategies:

• Combined source exhausts• Co-located exhaust points to

increase buoyancy• Turn horizontal stacks vertical• Increase stack heights

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Modeling Strategies Plume transport time Surrounding surface

characteristics Wind speed monitor thresholds Mechanical mixing height

considerations Alternative models (e.g.,

CALPUFF)

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Final Thoughts PSD Modeling

• New 1-hour SO2 and NO2 standards extremely stringent Conduct modeling for planning purposes Consider doing through attorney under attorney-client

privilege Your facility could be modeled if another nearby source is

conducting PSD modeling SIP Implementation Modeling

• Be involved in U.S. EPA’s Stakeholder Outreach process• Be Aware that Environmental Group are modeling facilities

with publically available information• Consider collecting site-specific meteorological and SO2

ambient monitoring data

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Questions?

Dan Dixddix@all4inc.com

(610) 933-5246 x182393 Kimberton Road

PO Box 299Kimberton, PA 19442

All4 Inc.www.all4inc.com

www.enviroreview.com