Georgia Institute of Technology

Estimation of NH4+/SO4

2- Molar Ratios Using URM

Modeling Outputs

Jim Boylan, Talat Odman, Ted Russell

April 10, 2001

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Outline

• SEAVS Data– IMPROVE– Harvard

• Methodology • Model Results

– July 11 - 19, 1995– February 8 - 13, 1994

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Fine PM Ions from IMPROVE

19.2 g/m3

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Fine PM Ions from HEADS

19.2 g/m3

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NH4+/SO4

2- Molar Ratios

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NH4+ vs. SO4

2- from IMPROVE

19.2 g/m3

28.8 g/m3

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NH4+ vs. SO4

2- from HEADS

17.3 g/m3

24.0 g/m3

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Aerosol Species• Aerosol species that are possible in the ISORROPIA aerosol

module:(NH4)2SO4 NaHSO4 NH4NO3

(NH4)HSO4 Na2SO4NH4Cl

(NH4)3H(SO4)2 H2SO4 NaCl

NaNO3

• URM Model Output (individual concentration): Sulfate (SO4

2-) - SUL1, SUL2, SUL3, SUL4

Nitrate (NO3-) - NIT1, NIT2, NIT3, NIT4

Ammonium (NH4+) - AMN1, AMN2, AMN3, AMN4

Chloride (Cl-) - CHL1, CHL2, CHL3, CHL4Sodium (Na+) - SOD1, SOD2, SOD3, SOD4

• Therefore, we can not directly determine the form of the sulfate aerosols by looking at the URM output.

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Derivation of Equation

• The aerosol concentrations are in g/m3. • To get the moles of each species, we must

sum the concentrations in all four size bins and divide by the Molecular Weights.

• We need to subtract out the NH4+ and Na+

associated with NO3- and Cl-, leaving only the

NH4+ and Na+ associated with SO4

2-.

• We cannot subtract out the Na+ associated with the SO4

2- because we do not now what form it is in (Na2SO4 or NaHSO4).

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Molar Ratio Equation

• If we assume or show that the moles of Na+ are small compared to the moles of NH4

+, we can estimate the molar ratio of NH4

+ to SO42-.

• If the moles of Na+ and NH4+ are similar, we can

assume that both are in the same sulfate form.

96

5.3562231824

34

24

4

SO

ClNONaNH

SOmoles

NaNHmoles

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Molar Ratios

• We can either look at the NH4+/SO4

2- molar ratio on an hour by hour basis or a 24-hour averaged ratio.

Ratio Species2.0 100% (NH4)2SO4

1.5 50% (NH4)2SO4 + 50% (NH4)HSO4

1.0 100% (NH4)HSO4

0.5 50% (NH4)HSO4 + 50% H2SO4

0.0 100% H2SO4

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July 11, 1995

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July 11, 1995

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July 12, 1995

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July 13, 1995

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July 14, 1995

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July 15, 1995

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July 16, 1995

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July 17, 1995

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July 18, 1995

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July 19, 1995

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February 8, 1994

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February 8, 1994

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February 9, 1994

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February 10, 1994

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February 10, 1994

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February 11, 1994

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February 12, 1994

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February 13, 1994

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Summary

• Generally, the SEAVS Data show:– no H2SO4 when [SO4

2-] < 20 g/m3

– 25% H2SO4 when [SO42-] between 20 g/m3 and 25 g/m3

• Generally, the URM Model Results show:– no H2SO4 when [SO4

2-] < 20 g/m3

– 25% H2SO4 when [SO42-] between 20 g/m3 and 24 g/m3

• Presence of some H2SO4 in West Virginia during the July 1995 Episode.

• No H2SO4 in the February 1994 Episode.