Lecture 15
AOSC/CHEM 637
Atmospheric Chemistry
R. Dickerson
Ammonia, NH3, and Nitrous Oxide, N2O
And
The Nitrogen Cycle
-orReading: 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]
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Dickerson
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What color was dinosaur poop?
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Dickerson
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Life requires Nitrogen
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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.
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Dickerson
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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.
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Dickerson
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Mammals excrete urea: (NH2)2CO
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Dickerson
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• 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
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The Nitrogen Cycle
NO
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The Nitrogen Cascade
Atmospheric
stratosphere
ozone depletion
greenhouse gases
ozone
particulate matter
troposphere
“new” nitrogen
Energy Production
& combustion of fossil fuels
Food Production
NOx
NOy, NHx, Norg
Terrestrial
NHx
& creation of synthetic fertilizers
Vegetated
biogeochemical cycling
ecosystem productivity
soils
NOx
NH3, Norg
Populated
Agricultural
crops
grasslands
forests
landscape
people
animals
soils
soils
N2O
NOy, NHx, Norg
NOy , NHx
Norg
Aquatic
Surface Water
& Wetlands
acidification
eutrophication
N2O
Groundwater
Coastal Bays
& Estuaries
Oceans
denitrification potential
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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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US Ammonia Emissions 2000
Industruial processes
Fuel Combustion
Transportation
Agricurtral crops
Agricurtral livestock
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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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Dickerson
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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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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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Dickerson
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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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Dickerson
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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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Dickerson
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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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Dickerson
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Aqueous ammonium concentration as a function of pH for
1 ppb gas-phase NHCopyright
Seinfeld
© 2010
R. R. and Pandis (1998).
3. From
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⇗
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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,
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© 2010
R. R.
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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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Dickerson
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Nitrogen Deposition
Past and Present
mg N/m2/yr
5000
2000
1000
750
500
250
100
50
25
5
1993
1860
Galloway et al., 2003
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II. Local Observations
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Fort Meade, MD
Annual mean visibility across the United states
(Data acquiredCopyright
from the
IMPROVE
network)
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R. R.
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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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Copyright
© 2010 R. R. of NO , NO -, and NH + at FME.
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• Seasonal variation of 24-hr average
concentration
4
y
3
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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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(DataCopyright
acquired
1999)
© 2010inR.July
R.
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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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2010 R. R.
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tuned to 9.22 μm and a HeNe
laser©interferometer
(Owens et al., 1999).
Dickerson
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
important in urban areas.
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2010 be
R. R.
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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 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.
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Dickerson
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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.
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Global averages of the concentrations of the major, well-mixed,
long-lived greenhouse gases. http://www.esrl.noaa.gov/gmd/aggi/
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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
N2O + O(1D) → 2 NO
→ N2 + O2
(1)
(2a)
(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.
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Dickerson
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Atmospheric Nitrous Oxide Budget
Stratosphere
N2O + O(1D) → 2 NO
HNO3
Troposphere
N2O 1600 TgN
HNO3
Oceans
3
Soils
6.6
Mankind (ag)
8.1
Earth’s surface
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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.
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Dickerson
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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.
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Dickerson
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Mammals excrete urea: (NH2)2CO
Copyright © 2010 R. R.
Dickerson
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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.
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Dickerson
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Uric Acid C5N4H4O3
An insoluble semi-solid
that requires no water as a carrier.
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Dickerson
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Nest made of guano.
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Dickerson
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