Transcript The Atmospheric Chemistry and Physics of Ammonia

The Atmospheric Chemistry and Physics of
Ammonia
Russell Dickerson
Dept. Meteorology, The University of Maryland
Presented at the
National Atmospheric Deposition Program
Ammonia Workshop
October 23, 2003
Photo from UMD Aztec, 2002
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Talk Outline
I. Fundamental Properties
Importance
Reactions
Aerosol formation
Thermodynamics
Role 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,
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convection (and rain) inhibited (e.g., Park et al., JGR., 2001).
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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Fort Meade, MD
Annual mean visibility across the United states
(Data acquired from the IMPROVE network)
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Fort Meade, MD
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Inorganic compounds ~50% (by mass)
Carbonaceous material ~40% (by mass)
Summer: Sulfate dominates.
Winter: Nitrate/carbonaceous
particles play bigger roles.
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• Seasonal variation of 24-hr average concentration of NOy, NO3-, and NH4+ at FME.
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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(Data acquired in July 1999)
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(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
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tuned to 9.22 μm and a HeNe laser interferometer (Owens et al., 1999).
Linearity over five orders of magnitude.
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Response time (base e) of laser interferometer ∽ 1 s.
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*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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Acknowledgements
Contributing Colleagues:
Antony Chen (DRI)
Rob Levy (NASA)
Charles Piety
Lackson Marufu
Bruce Doddridge
Jeff Stehr
Bill Ryan (PSU)
Melody Avery (NASA)
Funding From:
Maryland Department of the Environment
NC Division of Air Quality
VA Department of Environmental Quality
NASA-GSFC
EPRI
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The End.
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MODIS: August 9, 2001
Highest Ozone of the Summer
“Visible” Composite
Aerosol Optical Depth at 550 nm
Phila
Balt
GSFC
Phila
Balt
GSFC
AOT
0.8
0.0
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Robert Levy, NASA
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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