Contribution of ChemicalConstituents to Visibility Reduction

During the California RegionalPM10/PM2.5 Air Quality Study

Steven Heisler, ENSR InternationalCamarillo, CA

Presented at

AAAR Annual Meeting, October 22, 2003Anaheim, CA

Approach• Applied light extinction efficiencies to 24-hour

average particulate matter chemical composition datato estimate constituent contributions to the lightextinction coefficient

• Constituents included:Fine Soil = 1.89[Al] + 2.14[Si] +1.4[Ca] + 1.43[Fe]NH4NO3 = 1.29[NO3

-](NH4)2SO4 = 1.375[SO4

=]Organic Compounds (OCM) = 1.4[OC]Elemental Carbon (EC) = measured EC

Light Extinction Efficiencies

• Used 10 m2/g for light absorption by EC• Evaluated light scattering efficiencies developed for

IMS95 and for IMPROVE (Interagency Monitoring ofProtected Visual Environments) with 24-hour averagechemical composition, light scattering coefficient (bs),and relative humidity (RH) data from Fresno FirstStreet (FSF) site

• Chose FSF data because only site with open-air,unheated nephelometer (NGN-2)

IMS95 and IMPROVE LightScattering Efficiencies

Constituent IMS95(m2/g)

IMPROVE(m2/g)

Fine Soil 2 1NH4NO3 2.1/(1-RH)0.7 3f(RH)

(NH4)2SO4 2.1/(1-RH)0.7 3f(RH)OC 2.8/(1-RH)0.2 4

IMPROVE f(RH)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

RH (%)

f(RH)

IMS95 and IMPROVE EfficienciesProduced Essentially Identical Results

IMPROVE vs. IMS95

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600 700 800 900 1000

IMPROVE Calculated bs (Mm-1)

IMS9

5 Ca

lcul

ated

bs (

Mm

-1) Calc. = 1.00 Meas. - 1.33 Mm-1

R2 = 0.9999n = 54

IMPROVE Light Scattering Efficiencies

0

500

1000

1500

2000

2500

3000

3500

4000

0 500 1000 1500 2000 2500 3000 3500 4000

Measured bs (Mm-1)

Calc

ulat

ed b

s (M

m-1

)

Measured bs is Under-predicted,Especially at High Values

IMPROVE Light Scattering Efficiencies

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600 700 800 900 1000

Measured bs (Mm-1)

Calc

ulat

ed b

s (M

m-1

)

Calc. = 0.670 Meas. + 27.2 Mm-1

R2 = 0.977n = 48

Correlation is Good at Lower Values, butMeasured bs is Under-Predicted

Calculated Seasonal Adjustments toIMPROVE Efficiencies

• Only adjusted efficiencies for NH4NO3 and (NH4)2SO4

• Seasonal adjustment calculated as average of:(measured bs - soil bs - OCM bs) /(NH4NO3 bs + (NH4)2SO4 bs)

• Also calculated adjustment factor for high lightscattering (measured bs > 1000 Mm-1)

NH4NO3 and (NH4)2SO4 LightScattering Efficiency Adjustment

Factors

Season Adjustment FactorWinter (Dec. – Feb.) 2.17Spring (Mar. – May) 0.74

Summer (Jun. – Aug.) 0.81Fall (Sep. – Nov.) 1.51High Measured bs 5.09

Adjustment Improves AgreementAdjusted IMPROVE Light Scattering Efficiencies

0

500

1000

1500

2000

2500

3000

3500

4000

0 500 1000 1500 2000 2500 3000 3500 4000

Measured bs (Mm-1)

Calc

ulat

ed b

s (M

m-1

)

Calc. = 0.933 Meas. + 23.9 Mm-1

R2 = 0.919n = 54

Agreement Is Best at Lower ValuesAdjusted IMPROVE Light Scattering Efficiencies

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600 700 800 900 1000

Measured bs (Mm-1)

Calc

ulat

ed b

s (M

m-1

)

Calc. = 0.929 Meas. + 9.5 Mm-1

R2 = 0.983n = 48

Application of Light ExtinctionEfficiencies

• Applied to 11 sites with annual relative humidity andPM2.5 chemical composition data

• Data available from 12/99 - 1/01• Calculated f(RH) for every hour and averaged over

24-hour filter sampling periods• “Capped” f(RH) at value for 95% relative humidity to

avoid “instabilities”• Did not apply adjustment for “high” values (i.e., only

used seasonal adjustment)

Merced

Sacramento

Stockton

Modesto

Fresno (3)Selma

Visalia

BakersfieldOildale

Seasonal Average Total Calculated Light Extinction

0

100

200

300

400

500

600

700

800

900

Sacram

ento

Stockton

Modesto

Merced

Fresno-A

Fresno-B

Fresno-C

Selma

Visalia

Oildale

Bakersf

ield

Seas

onal

Ave

rage

Cal

cula

ted

Ligh

t Ext

inct

ion

(Mm

-1)

Winter Spring Summer Fall

Seasonal Average Total CalculatedLight Extinction Coefficient

• Winter average values are higher than other seasonsat all sites

• Fall average values are higher than spring or summer• Values are highest at Visalia, Merced and Modesto• Values are lowest at Sacramento and Stockton

Winter Constituent Contributions

0

10

20

30

40

50

60

70

80

90

Sacram

ento

Stockton

Modesto

Merced

Fresno-A

Fresno-B

Fresno-C

Selma

Visalia

Oildale

Bakersf

ield

Perc

ent o

f Sea

sona

l Ave

rage

Lig

ht E

xtin

ctio

n

Ammonium Nitrate Organic Compounds Elemental Carbon Ammonium Sulfate Other

Fall Constituent Contributions

0

10

20

30

40

50

60

70

80

Sacram

ento

Stockton

Modesto

Merced

Fresno-A

Fresno-B

Fresno-C

Selma

Visalia

Oildale

Bakersf

ield

Perc

ent o

f Sea

sona

l Ave

rage

Lig

ht E

xtin

ctio

n

Ammonium Nitrate Organic Compounds Elemental Carbon Ammonium Sulfate Other

Spring Constituent Contributions

0

5

10

15

20

25

30

35

40

45

Sacram

ento

Stockton

Modesto

Merced

Fresno-A

Fresno-B

Fresno-C

Selma

Visalia

Oildale

Bakersf

ield

Perc

ent o

f Sea

sona

l Ave

rage

Lig

ht E

xtin

ctio

n

Ammonium Nitrate Organic Compounds Elemental Carbon Ammonium Sulfate Other

Summer Constituent Contributions

05

101520253035404550

Sacram

ento

Stockton

Modesto

Merced

Fresno-A

Fresno-B

Fresno-C

Selma

Visalia

Oildale

Bakersf

ield

Perc

ent o

f Sea

sona

l Ave

rage

Lig

ht E

xtin

ctio

n

Ammonium Nitrate Organic Compounds Elemental Carbon Ammonium Sulfate Other

Seasonal Average ConstituentContributions

• NH4NO3 is largest contributor at all sites during winterand fall, accounting for about 60% to about 80%during winter and about 40% to about 70% during fall

• Organic compounds are largest contributor at mostsites during spring, accounting for about 30% toabout 40%, and at all sites during summer,accounting for about 30% to about 45%

• Elemental carbon is a significant contributor at mostsites during spring and summer, accounting for asmuch as 25%

Why is Winter Calculated LightExtinction Higher at Visalia, Mercedand Modesto than at Other Sites?

Winter Average NH4NO3 Concentrationsare Higher at Other Sites

0

5

10

15

20

25

Sacram

ento

Stockton

Modesto

Merced

Fresno - A

Fresno - B

Fresno - C

Selma

Visalia

Oildale

Bakersf

ield

Win

ter A

vera

ge N

H4N

O3 (

ug/m

3 )

But Average f(RH) is Lower at Other Sites

Winter Average f(RH)

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Sacramento Stockton Modesto Merced Fresno &Selma

Visalia Bakersfield

Aver

age

f(RH)

Summary• Previously developed light scattering efficiencies

under-predicted measured light scattering, butseasonal adjustment factors improved agreement

• Calculated total light extinction is highest duringwinter and lowest during summer

• NH4NO3 is the largest contributor at all sites duringwinter and fall

• Organic compounds are the highest contributor atmost sites during spring and summer

• Winter average light extinction was higher at Visalia,Merced and Modesto than at other sites because ofhigher reported relative humidity