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Photo from UMD Aztec, 2002

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. Talk Outline I. Fundamental Properties Importance Reactions

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Photo from UMD Aztec, 2002

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  1. The Atmospheric Chemistry and Physics of AmmoniaRussell DickersonDept. Meteorology, The University of MarylandPresented at theNational Atmospheric Deposition ProgramAmmonia WorkshopOctober 23, 2003 Photo from UMD Aztec, 2002

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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

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

  8. Formation of Aerosols Nucleation – the transformation from the gaseous to condensed phase; the generation of new particles. H2SO4/H2O systemdoes not nucleate easily. NH3/H2SO4/H2Osystem does (e.g., Coffman & Hegg, 1995).

  9. 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.

  10. Cloud ⇗

  11. 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 NH4NO3in the winter in East.

  12. Ammonium NitrateEquilibrium 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.

  13. 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).

  14. 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.

  15. II. Local Observations

  16. Fort Meade, MD Annual mean visibility across the United states (Data acquired from the IMPROVE network)

  17. Fort Meade, MD

  18. Inorganic compounds ~50% (by mass) Carbonaceous material ~40% (by mass) Summer: Sulfate dominates. Winter: Nitrate/carbonaceous particles play bigger roles.

  19. Seasonal variation of 24-hr average concentration of NOy, NO3-, and NH4+ at FME.

  20. 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.

  21. 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. Visibility (Data acquired in July 1999)

  23. (Water amount estimated by ISORROPIA)

  24. 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).

  25. Linearity over five orders of magnitude.

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

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

  28. 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.

  29. Acknowledgements Contributing Colleagues: Antony Chen (DRI) Bruce Doddridge Rob Levy (NASA) Jeff Stehr Charles Piety Bill Ryan (PSU) Lackson Marufu Melody Avery (NASA) Funding From: Maryland Department of the Environment NC Division of Air Quality VA Department of Environmental Quality NASA-GSFC EPRI

  30. The End.

  31. MODIS: August 9, 2001 Highest Ozone of the Summer “Visible” Composite Aerosol Optical Depth at 550 nm Phila Phila Balt Balt GSFC GSFC AOT 0.8 0.0 Robert Levy, NASA

  32. Donora, PA Oct. 29, 1948

  33. Madonna Harten Castle Germany: Ruhr area Portal figure Sandstone Sculptured 1702 Photographed 1908

  34. Madonna Harten Castle Germany: Ruhr area Portal figure Sandstone Sculptured 1702 Photographed 1969

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