IMPROVE Algorithm for Estimating Light Extinction

Draft Recommendations to the IMPROVE Steering Committee

Current IMPROVE Algorithm

10

6.0

1

10

4

)(3

)(3

MassCoarse

SoilFine

CarbonElemental

CarbonOrganic

NitrateRHf

SulfateRHfbext

Assumptions include that • Six particle component terms plus a constant Rayleigh scattering term are sufficient for a good estimate of light extinction; • Constant dry extinction efficiency terms for each of the six particle components works adequately for all locations and times; and • Light extinction by the individual particle components can be adequately estimated as separate terms (like externally mixed components).

Reasons for Revision

• Regional Haze Rule utilizes the IMPROVE algorithm for calculating the RHR index– Haze index is in deciview (a logarithmic

transformation of light extinction)– Calls for improvement in 20% worst haze to natural

levels by 2064– Rate of progress for each 10-year SIP period is linear

between current and natural haze levels– So haze estimates need to be good at the upper and

lower extremes, should have no fixable biases and should include all important contributions even if exclusively from natural sources

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Measured Bsp

IMP

RO

VE

Bsp

(ppb)NO0.33

Specific)iteS(ScatteringRayleigh

SaltSea(RH)f1.7

CarbonOrganicargeL6.1CarbonOrganicSmall2.8

NitrateargeL(RH)f5.1NitrateSmall(RH)f2.4

SulfateargeL(RH)f4.8SulfateSmall(RH)f2.2

2

SS

LS

LS

MassCoarse

SoilFine

CarbonElemental

bext

6.0

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10

Recommended Revised IMPROVE Algorithm(Changed terms are in Bold Font)

20,20

arg SulfateTotalforSulfateTotalSulfateTotal

SulfateeL

20,arg SulfateTotalforSulfateTotalSultateeL

SulfateeLSulfateTotalSulfateSmall arg

where

and nitrate and organic are split using the same process

Split of Small & Large Sulfate Concentrations versus

Total Sulfate Concentration

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Sulfate concentration (ug/m3)

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mp

on

en

t S

ulf

ate

Co

ncen

trati

on

Overall Extinction Efficiency vs. Total Mass

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Sulfate concentration (ug/m3)

Dry

Exti

ncti

on

Eff

icie

ncy

Split Component Extinction Efficiency Model for Sulfate, Nitrate, and Organic components

• Composite dry extinction efficiency varies linearly from 2.2 to 4.8m2/g

• Small: Dg = 0.2μm & g = 2.2

• Large: Dg = 0.5μm & g = 1.5

• Different size distributions dictate different water grow function, f(RH)

• Small size dominates at low concentrations, while large size dominates at high concentrations

• Small size is likely young aerosol produce by homogenous gas to particle processes.

• Large size is likely aged aerosol involving cloud processing.

Water Growth Curves

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Relative Humidity (%)

f(R

H)

fS(RH)

fL(RH)

Original

fSS(RH)

Other New or Revised Terms

• Organic Compound Mass to Organic Carbon Mass Ratio changed from 1.4 to 1.8

• Sea Salt = 1.8 x [Chlorine] and has its own water growth term fSS(RH). Important at coastal locations

• Rayleigh Scattering is calculated for the monitoring site elevation and annual mean temperature and integer rounded. Ranges from 8Mm-1 at 10,000’ to 12Mm-1at sea level

• NO2 light absorption in the visible is included for sites that have such data (not routinely available at IMPROVE sites)

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Measured Bsp

Sp

lit C

om

po

nen

t M

eth

od

Bsp

Normalized Bias for all data

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Overall Top 20% Mid 60% Bottom 20%

Current

New

Bias = (estimated – measured)/measured

Normalized Bias for CENRAP Sites

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-0.1

0

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Overall Top 20% Mid 60% Bottom 20%

Current

New

Normalized Bias for MANE-VU Sites

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Overall Top 20% Mid 60% Bottom 20%

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Normalized Bias for VISTAS Sites

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-0.1

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Overall Top 20% Mid 60% Bottom 20%

Current

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Normalized Bias for WRAP Sites

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Overall Top 20% Mid 60% Bottom 20%

Current

New

Implementation Steps for Regional Haze Rule Application

• IMPROVE Steering Committee approval – 11/05• Calculation of water growth functions for monthly

& annual averaged conditions for each monitoring site – SAIC task-1 to 2 months

• Recalculation of current (5-year baseline) and natural haze levels – VIEWS-1 month

• EPA modifies the regional haze guidance, so states can choose – 6 to 12 months

• VIEWS will support both versions of the algorithm for the foreseeable future