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The Atmospheric Chemistry and Physics of Ammonia

Russell DickersonDept. Meteorology, The University of Maryland

Presented at the National Atmospheric Deposition Program

Ammonia WorkshopOctober 23, 2003

Photo from UMD Aztec, 2002

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Talk Outline

I. Fundamental PropertiesImportanceReactionsAerosol formationThermodynamicsRole as ccn

II. Local Observations Observed concentrations Impact on visibility Box Model results New Detection Technique

III. Fun Stuff – if there’s time.

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Atmospheric Ammonia, NH3

I. Fundamental Properties

Importance• Only gaseous base in the atmosphere.

• Major role in biogeochemical cycles of N.

• Produces particles & cloud condensation nuclei.• Haze/Visibility• Radiative balance; direct & indirect cooling• Stability wrt vertical mixing.• Precipitation and hydrological cycle.

• Potential source of NO and N2O.

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Fundamental Properties, continued

Thermodynamically unstable wrt oxidation.

NH3 + 1.25O2 → NO + 1.5H2O

H°rxn = −53.93 kcal mole-1

G°rxn = −57.34 kcal mole-1

But the kinetics are slow:NH3 + OH· → NH2 + H2O

k = 1.6 x 10-13 cm3 s-1 (units: (molec cm-3)-1 s-1)Atmospheric lifetime for [OH] = 106 cm-3

τNH3 = (k[OH])-1 ≈ 6x106 s = 72 d. Compare to τH2O ≈ 10 d.

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Fundamental Properties, continued

Gas-phase reactions:

NH3 + OH· → NH2· + H2O

NH2· + O3 → NH, NHO, NO

NH2· + NO2 → N2 or N2O (+ H2O)

Potential source of atmospheric NO and N2O in low-SO2 environments.

Last reaction involved in combustion “deNOx” operations.

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Fundamental Properties, continued

Aqueous phase chemistry:

NH3(g) + H2O ↔ NH3·H2O(aq) ↔ NH4 + + OH−

Henry’s Law Coef. = 62 M atm-1

Would not be rained out without atmospheric acids.

Weak base: Kb = 1.8x10-5

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Aqueous ammonium concentration as a function of pH for 1 ppb gas-phase NH3. From Seinfeld and Pandis (1998).

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Formation of Aerosols

Nucleation – the transformation from the gaseous to condensed phase; the generation of new particles.

H2SO4/H2O system does not nucleate easily.

NH3/H2SO4/H2O system does (e.g., Coffman & Hegg, 1995).

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Formation of aerosols, continued:

NH3(g) + H2SO4(l) → NH4HSO4(s, l) (ammonium bisulfate) NH3(g) + NH4HSO4(l) → (NH4)2SO4(s, l) (ammonium sulfate)

Ammonium sulfates are stable solids, or, at most atmospheric RH, liquids.

Deliquescence – to become liquid through the uptake of water at a specific RH ( 40% RH for ∽ NH4HSO4).

Efflorescence – the become crystalline through loss of water; literally to flower.

We can calculate the partitioning in the NH4/SO4/NO3/H2O system with a thermodynamic model; see below.

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Cloud⇗

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Formation of aerosols, continued

NH3(g) + HNO3(g) ↔ NH4NO3(s)

G°rxn = −22.17 kcal mole-1

[NH4NO3] Keq = ------------------ = exp (−G/RT) [NH3][HNO3]

Keq = 1.4x1016 at 25°C; = 1.2x1019 at 0°C

Solid ammonium nitrate (NH4NO3) is unstable except at high [NH3] and [HNO3] or at low temperatures. We see more NH4NO3 in the winter in East.

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Ammonium Nitrate Equilibrium in Air = f(T)

NH3(g) + HNO3(g) ↔ NH4NO3(s)

– ln(K) = 118.87 – 24084 – 6.025ln(T) (ppb)2

1/Keq 298K = [NH3][HNO3] (ppb)2 = 41.7 ppb2

(√41.7 ≈ 6.5 ppb each)

1/Keq 273K = 4.3x10-2 ppb2

Water in the system shifts equilibrium to the right.

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Radiative impact on stability: Aerosols reduce heating of the Earth’s surface, and can increase heating aloft. The atmosphere becomes more stable wrt vertical motions and mixing – inversions are intensified, convection (and rain) inhibited (e.g., Park et al., JGR., 2001).

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Additional Fundamental Properties

• Radiative effects of aerosols can accelerate photochemical smog formation.

• Condensed–phase chemistry tends to inhibit smog production.

• Too many ccn may decrease the average cloud droplet size and inhibit precipitation.

• Dry deposition of NH3 and HNO3 are fast; deposition of particles is slow.

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II. Local Observations

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Annual mean visibility across the United states

(Data acquired from the IMPROVE network)

Fort Meade, MD

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Fort Meade, MD

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Summer: Sulfate dominates.

Winter: Nitrate/carbonaceous particles play bigger roles.

Inorganic compounds ~50% (by mass)

Carbonaceous material ~40% (by mass)

19• Seasonal variation of 24-hr average concentration of NOy, NO3-, and NH4

+ at FME.

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ISORROPIA Thermodynamic Model (Nenes, 1998; Chen 2002)

Inputs: Temperature, RH, T-SO42-, T-NO3

-, and T-NH4+

Output: HNO3, NO3-, NH3, NH4

+, HSO4-, H2O, etc.

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ISORROPIA Thermodynamic Model (Nenes, 1998; Chen, 2002)

Inputs: Temperature, RH, T-SO42-, T-NO3

-, and T-NH4+

Output: HNO3, NO3-, NH3, NH4

+, HSO4-, H2O, etc.

22(Data acquired in July 1999)

23(Water amount estimated by ISORROPIA)

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Interferometer for NH3 Detection

Schematic diagram detector based on heating of NH3 with a CO2 laser tuned to 9.22 μm and a HeNe laser interferometer (Owens et al., 1999).

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Linearity over five orders of magnitude.

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Response time (base e) of laser interferometer ∽ 1 s.

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28*Emissions from vehicles can be important in urban areas.

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Summary:• Ammonia plays a major role in the chemistry of the atmosphere.

• Major sources – agricultural.

• Major sinks – wet and dry deposition.

• Positive feedback with pollution – thermal inversions & radiative scattering.

• Multiphase chemistry

• Inhibits photochemial smog formation.

• Major role in new particle formation.

• Major component of aerosol mass.

• Thermodynamic models can work.

• Rapid, reliable measurements will put us over the top.

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AcknowledgementsAcknowledgementsContributing Colleagues:

Antony Chen (DRI) Bruce DoddridgeRob Levy (NASA) Jeff StehrCharles Piety Bill Ryan (PSU)Lackson Marufu Melody Avery (NASA)

Funding From:Maryland Department of the EnvironmentNC Division of Air QualityVA Department of Environmental QualityNASA-GSFCEPRI

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The End.

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MODIS: August 9, 2001MODIS: August 9, 2001

“Visible” Composite Aerosol Optical Depth at 550 nm

AOT

0.8

0.0

Phila

BaltGSFC GSFC

Balt

Phila

Highest Ozone of the Summer

Robert Levy, NASA

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Donora, PA Oct. 29, 1948

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Madonna

Harten Castle

Germany: Ruhr area

Portal figure

Sandstone

Sculptured 1702

Photographed 1908

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Madonna

Harten Castle

Germany: Ruhr area

Portal figure

Sandstone

Sculptured 1702

Photographed 1969

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