the atmospheric chemistry and physics of ammonianadp.slh.wisc.edu/nh4ws/dickerson/dickerson.pdf•...
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The Atmospheric Chemistry and Physics of Ammonia
Russell DickersonDept. Meteorology, The University of Maryland
Presented at the
National Atmospheric Deposition ProgramAmmonia Workshop
October 23, 2003
Photo from UMD Aztec, 2002
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Talk Outline
I. Fundamental PropertiesImportanceReactionsAerosol formationThermodynamicsRole as ccn
II. Local ObservationsObserved concentrationsImpact on visibilityBox Model resultsNew 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-SO2environments.
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)
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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.
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22(Data acquired in July 1999)
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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 & radiativescattering. • 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
AOT0.8
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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MadonnaHarten CastleGermany: Ruhr areaPortal figureSandstoneSculptured 1702Photographed 1908
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MadonnaHarten CastleGermany: Ruhr areaPortal figureSandstoneSculptured 1702Photographed 1969
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