637L15

Lecture 15AOSC/CHEM 637Atmospheric ChemistryR. Dickerson: 

Lecture 15 AOSC/CHEM 637 Atmospheric Chemistry R. Dickerson Ammonia, NH3, and Nitrous Oxide, N2O And The Nitrogen Cycle -or- Reading: Finlayson-Pitts Ch 14; Seinfeld and Pandis Chapters 2, 7 & 10. [Cicerone, 1989; Mosier and Kroeze, 1998; Dentener and Crutzen, 1994; Galloway, et al., 2004; Mosier, et al., 1998; NRC, 2003]

What color was dinosaur poop?: 

What color was dinosaur poop?

Life requires Nitrogen: 

Life requires Nitrogen Proteins, chains of amino acids, are central to life. Only lightning and a few organisms can fix N. Plants use nitrates to make amino acids. Amino acids decompose to CO2, H2O, and NH3. Ammonia is toxic. Ammonia moderately soluble. Urea, costs 4 ATP molecules, but is highly soluble.

Slide4: 

Ammonia is toxic to most animals; 100 ppm begins to cause adverse effects and 5000 ppm is rapidly fatal. Fish can easily expel ammonia because it is moderately soluble and lost to the water passing through their gills. But ammonia with a Henry’s Law coefficient of 60 M atm-1 is not soluble enough for us. You would have to drink at least 1000 L of water per day to get rid of 100 g of ammonia. To solve this problem, your body expends 4 ATP molecules (~15% of the total available energy of an amino acid) to make each molecule of urea. The solubility of urea exceeds 1000 g/L, so you can get rid of your excess ammonia that way. Because urea lies uphill thermodynamically it is easily converted back to ammonia and carbon dioxide. In soils ammonia/ammonium can be nitrified and used by plants.

Slide5: 

Mammals excrete urea: (NH2)2CO

Slide6: 

SOURCES: Direct emissions from industrial processes and cars with catalytic converters are minor. The main sources are fertilized soils and hydrolysis of urea in animal waste. Urease enzymes in manure quickly hydrolyze urea to ammonia and carbon dioxide. (NH2)2CO + H2O → 2NH3 + CO2

The Nitrogen Cycle: 

The Nitrogen Cycle NO

Slide11: 

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.

Slide13: 

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.

Slide14: 

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.

Slide15: 

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

Slide16: 

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).

Slide17: 

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.

Slide18: 

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.

Slide19: 

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.

Slide20: 

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

Slide21: 

Cloud ⇗

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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).

Slide23: 

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.

Nitrogen DepositionPast and Presentmg N/m2/yr: 

Nitrogen Deposition Past and Present mg N/m2/yr 1860 1993 5000 2000 1000 750 500 250 100 50 25 5 Galloway et al., 2003

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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)

Slide29: 

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

Slide30: 

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.

Slide31: 

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.

Slide32: 

(Data acquired in July 1999)

Slide33: 

(Water amount estimated by ISORROPIA)

Interferometer for NH3 Detection: 

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).

Slide35: 

Linearity over five orders of magnitude.

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

Slide38: 

*Emissions from vehicles can be important in urban areas.

Slide39: 

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 photochemical 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.

Slide40: 

Nitrous Oxide, N2O SOURCES: Bacterial nitrification in soils and waters. Emissions from fertilized soils and animal feeding operations now dominate the global budget. Combustion was thought to be a major source (e.g., Hao et al. J. G. R. 1987), but work by Muzio and Kramlich (G. R. L., 1988) showed that SO2 and NO in the grab sampling cans can produce artifact N2O. Biomass burning, atmospheric ammonia oxidation, and industrial processes are minor sources.

Global averages of the concentrations of the major, well-mixed, long-lived greenhouse gases. http://www.esrl.noaa.gov/gmd/aggi/: 

Global averages of the concentrations of the major, well-mixed, long-lived greenhouse gases. http://www.esrl.noaa.gov/gmd/aggi/

Slide43: 

CHEMISTRY: In the troposphere there is none! In the stratosphere nitrous oxide is broken down to molecular nitrogen or odd nitrogen, 90% through photolysis and about 10% through attack by electronically excited oxygen atoms. N2O + hυ → N2 + O (1) N2O + O(1D) → 2 NO (2a) → N2 + O2 (2b) Rxn 2a is the principal source of odd nitrogen and thus ozone destruction in the stratosphere. SINKS: Nitrous oxide in the stratosphere is converted to nitric oxide that eventually oxidizes to nitric acid. This nitric acid diffuses down to the troposphere where it can be rained out.

Slide45: 

BUDGET: In pretty good shape because N2O is long lived, and can be accurately measured. Note in general the longer the lifetime of a species, the better the global budget. Atmospheric burden is given by [N2O] times the number of moles of air in troposphere times the molecular weight of N. The mean mixing is about 320 ppb, and relatively constant (σ/[N2O] = 0.5%) over the entire globe. 320x10-9 * 1.8x1020 * 28 = 1.6x1015 g = 1600 TgN Estimated source strength = 9-17 Tg(N) / yr Lifetime = 1600/17 to 1600/9 = 100 to 180 yr The mixing ratio (concentration) is growing at a rate of about 0.2% (1.4 ppb) per year, and N2O is a greenhouse gas with a global warming potential 300 times that of CO2.

Slide46: 

An Unbalanced BUDGET: When fertilizer is applied to soils, about 0.5% of the N is quickly released as N2O and then the emission rate drops to a low level found in most soils. This number has been used to estimate that agriculture (crops plus animals) accounts for about 3 Tg N yr-1 The current N2O destruction rate is 11.9 Tg N yr-1. The rate of increase in the global atmospheric N2O burden is3.9 Tg N yr-1, thus the total emission rate has to be equal to the sum of these two or about 15.8 Tg N yr-1. Natural sources add up to about 10.2 Tg N yr-1 thus anthropogenic sources have to total 15.8 minus 10.2, or 5.6 Tg N yr-1. This is about 4% of the total N fixed by man each year of 127 Tg N yr-1. Crutzen et al. (2007) have used these facts to conclude that long-term N recycling in soils and waters leads to a total leakage of 4% of the originally applied N. If correct, this implies that N-rich biofuels have a greater warming impact than fossil fuels.

Slide47: 

Mammals excrete urea: (NH2)2CO

What color was dinosaur poop?: 

What color was dinosaur poop? Many birds, snakes, and lizards, under great pressure to minimize their water use, burn a few additional ATP molecules to excrete uric acid rather than urea.

Uric Acid C5N4H4O3 : 

Uric Acid C5N4H4O3 An insoluble semi-solid that requires no water as a carrier.

Nest made of guano.: 

Nest made of guano.