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Copyright © 2010 R. R. Dickerson 1 Lecture 15 AOSC/CHEM 637 Atmospheric Chemistry R. Dickerson Ammonia, NH 3, and Nitrous Oxide, N 2 O And The Nitrogen.

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Presentation on theme: "Copyright © 2010 R. R. Dickerson 1 Lecture 15 AOSC/CHEM 637 Atmospheric Chemistry R. Dickerson Ammonia, NH 3, and Nitrous Oxide, N 2 O And The Nitrogen."— Presentation transcript:

1 Copyright © 2010 R. R. Dickerson 1 Lecture 15 AOSC/CHEM 637 Atmospheric Chemistry R. Dickerson Ammonia, NH 3, and Nitrous Oxide, N 2 O 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]

2 Copyright © 2010 R. R. Dickerson 2 What color was dinosaur poop?

3 Copyright © 2010 R. R. Dickerson 3 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 CO 2, H 2 O, and NH 3. Ammonia is toxic. Ammonia moderately soluble. Urea, costs 4 ATP molecules, but is highly soluble.

4 Copyright © 2010 R. R. Dickerson 4 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.

5 Copyright © 2010 R. R. Dickerson 5 Mammals excrete urea: (NH 2 ) 2 CO

6 Copyright © 2010 R. R. Dickerson 6 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. (NH 2 ) 2 CO + H 2 O → 2NH 3 + CO 2

7 Copyright © 2010 R. R. Dickerson 7 The Nitrogen Cycle NO

8 The Nitrogen Cascade Atmospheric Terrestrial Agricultural animals crops soils Vegetated grasslands forests soils Groundwater Aquatic Surface Water & Wetlands Coastal Bays & Estuaries Oceans NO y, NH x, N org NO x NH 3, N org NO y, NH x, N org N2ON2O Populated landscape people soils biogeochemical cycling greenhouse gases eutrophication ecosystem productivity denitrification potential N2ON2O troposphere stratosphere NO y, NH x N org ozone Food Production & creation of synthetic fertilizers Energy Production & combustion of fossil fuels “new” nitrogen NH x NO x ozone depletion particulate matter acidification

9 Copyright © 2010 R. R. Dickerson 9

10 10

11 Copyright © 2010 R. R. Dickerson 11 Atmospheric Ammonia, NH 3 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 N 2 O.

12 Copyright © 2010 R. R. Dickerson 12

13 Copyright © 2010 R. R. Dickerson 13 Fundamental Properties, continued Thermodynamically unstable wrt oxidation. NH O 2 → NO + 1.5H 2 O  H° rxn = −53.93 kcal mole -1  G° rxn = −57.34 kcal mole -1 But the kinetics are slow: NH 3 + OH · → NH 2 + H 2 O k = 1.6 x cm 3 s -1 (units: (molec cm -3 ) -1 s -1 ) Atmospheric lifetime for [OH] = 10 6 cm -3 τ NH3 = (k[OH]) -1 ≈ 6x10 6 s = 72 d. Compare to τ H2O ≈ 10 d.

14 Copyright © 2010 R. R. Dickerson 14 Fundamental Properties, continued Gas-phase reactions: NH 3 + OH· → NH 2 · + H 2 O NH 2 · + O 3 → NH, NHO, NO NH 2 · + NO 2 → N 2 or N 2 O (+ H 2 O) Potential source of atmospheric NO and N 2 O in low-SO 2 environments. Last reaction involved in combustion “deNOx” operations.

15 Copyright © 2010 R. R. Dickerson 15 Fundamental Properties, continued Aqueous phase chemistry: NH 3(g) + H 2 O ↔ NH 3 ·H 2 O (aq) ↔ NH OH − Henry’s Law Coef. = 62 M atm -1 Would not be rained out without atmospheric acids. Weak base: K b = 1.8x10 -5

16 Copyright © 2010 R. R. Dickerson 16 Formation of Aerosols Nucleation – the transformation from the gaseous to condensed phase; the generation of new particles. H 2 SO 4 /H 2 O system does not nucleate easily. NH 3 /H 2 SO 4 /H 2 O system does (e.g., Coffman & Hegg, 1995).

17 Copyright © 2010 R. R. Dickerson 17 Formation of aerosols, continued: NH 3(g) + H 2 SO 4(l) → NH 4 HSO 4(s, l) (ammonium bisulfate) NH 3(g) + NH 4 HSO 4(l) → (NH 4 ) 2 SO 4(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 NH 4 HSO 4 ). Efflorescence – the become crystalline through loss of water; literally to flower. We can calculate the partitioning in the NH 4 /SO 4 /NO 3 /H 2 O system with a thermodynamic model; see below.

18 Copyright © 2010 R. R. Dickerson 18 Formation of aerosols, continued NH 3(g) + HNO 3(g) ↔ NH 4 NO 3(s)  G° rxn = −22.17 kcal mole -1 [NH 4 NO 3 ] K eq = = exp (−  G/RT) [NH 3 ][HNO 3 ] K eq = 1.4x10 16 at 25°C; = 1.2x10 19 at 0°C Solid ammonium nitrate (NH 4 NO 3 ) is unstable except at high [NH 3 ] and [HNO 3 ] or at low temperatures. We see more NH 4 NO 3 in the winter in East.

19 Copyright © 2010 R. R. Dickerson 19 Ammonium Nitrate Equilibrium in Air = f(T) NH 3(g) + HNO 3(g) ↔ NH 4 NO 3(s) – ln(K) = – – 6.025ln(T) (ppb) 2 1/K eq 298K = [NH 3 ][HNO 3 ] (ppb) 2 = 41.7 ppb 2 (√41.7 ≈ 6.5 ppb each) 1/K eq 273K = 4.3x10 -2 ppb 2 Water in the system shifts equilibrium to the right.

20 Copyright © 2010 R. R. Dickerson 20 Aqueous ammonium concentration as a function of pH for 1 ppb gas-phase NH 3. From Seinfeld and Pandis (1998).

21 Copyright © 2010 R. R. Dickerson 21 Cloud ⇗

22 Copyright © 2010 R. R. Dickerson 22 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).

23 Copyright © 2010 R. R. Dickerson 23 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 NH 3 and HNO 3 are fast; deposition of particles is slow.

24 Copyright © 2010 R. R. Dickerson 24 Nitrogen Deposition Past and Present mg N/m 2 /yr Galloway et al., 2003

25 Copyright © 2010 R. R. Dickerson 25 II. Local Observations

26 Copyright © 2010 R. R. Dickerson 26 Annual mean visibility across the United states (Data acquired from the IMPROVE network) Fort Meade, MD

27 Copyright © 2010 R. R. Dickerson 27 Fort Meade, MD

28 Copyright © 2010 R. R. Dickerson 28 Summer: Sulfate dominates. Winter: Nitrate/carbonaceous particles play bigger roles. Inorganic compounds ~50% (by mass) Carbonaceous material ~40% (by mass)

29 Copyright © 2010 R. R. Dickerson 29 Seasonal variation of 24-hr average concentration of NO y, NO 3 -, and NH 4 + at FME.

30 Copyright © 2010 R. R. Dickerson 30 ISORROPIA Thermodynamic Model (Nenes, 1998; Chen 2002) Inputs: Temperature, RH, T-SO 4 2-, T-NO 3 -, and T-NH 4 + Output: HNO 3, NO 3 -, NH 3, NH 4 +, HSO 4 -, H 2 O, etc.

31 Copyright © 2010 R. R. Dickerson 31 ISORROPIA Thermodynamic Model (Nenes, 1998; Chen, 2002) Inputs: Temperature, RH, T-SO 4 2-, T-NO 3 -, and T-NH 4 + Output: HNO 3, NO 3 -, NH 3, NH 4 +, HSO 4 -, H 2 O, etc.

32 Copyright © 2010 R. R. Dickerson 32 (Data acquired in July 1999)

33 Copyright © 2010 R. R. Dickerson 33 (Water amount estimated by ISORROPIA)

34 Copyright © 2010 R. R. Dickerson 34 Interferometer for NH 3 Detection Schematic diagram detector based on heating of NH 3 with a CO 2 laser tuned to 9.22 μm and a HeNe laser interferometer (Owens et al., 1999).

35 Copyright © 2010 R. R. Dickerson 35 Linearity over five orders of magnitude.

36 Copyright © 2010 R. R. Dickerson 36 Response time (base e) of laser interferometer ∽ 1 s.

37 Copyright © 2010 R. R. Dickerson 37

38 Copyright © 2010 R. R. Dickerson 38 *Emissions from vehicles can be important in urban areas.

39 Copyright © 2010 R. R. Dickerson 39 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.

40 Copyright © 2010 R. R. Dickerson 40 Nitrous Oxide, N 2 O 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 SO 2 and NO in the grab sampling cans can produce artifact N 2 O. Biomass burning, atmospheric ammonia oxidation, and industrial processes are minor sources.

41 Copyright © 2010 R. R. Dickerson41 Global averages of the concentrations of the major, well-mixed, long-lived greenhouse gases.

42 Copyright © 2010 R. R. Dickerson42

43 Copyright © 2010 R. R. Dickerson 43 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. N 2 O + hυ → N 2 + O(1) N 2 O + O( 1 D) → 2 NO (2a) → N 2 + O 2 (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.

44 Copyright © 2010 R. R. Dickerson 44 Troposphere N 2 O 1600 TgN HNO 3 Earth’s surface Stratosphere N 2 O + O( 1 D) → 2 NO Oceans 3 Soils 6.6 Mankind (ag) 8.1 Atmospheric Nitrous Oxide Budget

45 Copyright © 2010 R. R. Dickerson 45 BUDGET: In pretty good shape because N 2 O 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 [N 2 O] 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/[N 2 O] = 0.5%) over the entire globe. 320x10 -9 * 1.8x10 20 * 28 = 1.6x10 15 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 N 2 O is a greenhouse gas with a global warming potential 300 times that of CO 2.

46 Copyright © 2010 R. R. Dickerson 46 An Unbalanced BUDGET: When fertilizer is applied to soils, about 0.5% of the N is quickly released as N 2 O 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 N 2 O destruction rate is 11.9 Tg N yr -1. The rate of increase in the global atmospheric N 2 O 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.

47 Copyright © 2010 R. R. Dickerson 47 Mammals excrete urea: (NH 2 ) 2 CO

48 Copyright © 2010 R. R. Dickerson 48 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.

49 Copyright © 2010 R. R. Dickerson 49 H Uric Acid C 5 N 4 H 4 O 3 An insoluble semi-solid that requires no water as a carrier.

50 Copyright © 2010 R. R. Dickerson 50 Nest made of guano.


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