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3) FUNDAMENTALS OF GAS-PHASE REACTIONS
AND PHOTOCHEMISTRY
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3) FUNDAMENTALS OF GAS-PHASE REACTIONS
AND PHOTOCHEMISTRY
What do we know so far?
LOTS OF REACTIVE TRACE GASES !!
Natural sources – NO from soil and lightning, many hydrocarbons
(isoprene) from plants, sulfur species from oceans, …
Anthropogenic sources – hydrocarbons, NO, …
YIPPEE!! Chemistry!
The atmosphere needs a way to remove
these species.
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3) FUNDAMENTALS OF GAS-PHASE REACTIONS
AND PHOTOCHEMISTRY
Some basic concepts:
Stable Molecules versus “Free Radicals”
Stable species – usually all electrons paired up. Atmospheric lifetimes
hours, days, years.
e.g.,
Oxygen wants two things bonded to it.
H-O-H, O=C=O
Nitrogen wants three...
NH3
Carbon wants four…
CH4, CH3-CH2-CH3, CH3-CH=CH-CH3
Often represented as
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3) FUNDAMENTALS OF GAS-PHASE REACTIONS
AND PHOTOCHEMISTRY
Stable Molecules versus “Free Radicals”
Free radicals – have unpaired electron(s).
Atmospheric lifetimes seconds, minutes.
e.g., •O-H radical, missing one bond (H), wants to steal one from
somewhere. Similar story for •CH3 radical, missing one bond. Or the
HO2 radical, H-O-O•
These free radicals are usually generated by sunlight (photochemistry).
Initiates chemistry, chain reactions, etc !
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
First, a couple of definitions.
ELEMENTARY REACTION
From Wikipedia - An elementary reaction is a chemical reaction in which one or
more of chemical species react directly to form products in a single reaction step.
Usually involves 1-3 molecules, with bimolecular most common:
e.g., •OH + CH4  •CH3 + H2O
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
COMPLEX REACTION SCHEME OR MECHANISM
- Made up of a bunch of elementary reactions
- e.g., the oxidation of CH4 to CH2O in the polluted troposphere leads to the
following net effect:
CH4 + 4 O2  CH2O + H2O + 2 O3
•OH + CH4  •CH3 + H2O
•CH3 + O2  CH3O2•
CH3O2• + NO  CH3O• + NO2
CH3O• + O2  CH2O + HO2
HO2• + NO  •OH + NO2
NO2 + hn  NO + O
NO2 + hn  NO + O
O• + O2  O3
O• + O2  O3
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
BACK TO ELEMENTARY REACTIONS (BIMOLECULAR)
Bimolecular reactions are the most common type of elementary reaction in the
atmosphere
Typically are of the form
A-B + C  A + B-C
CH4 + OH  CH3 + HOH
Rate of the chemical reaction (disappearance of reactants or appearance of
products):
Rate =
k is the rate constant, units of (time)-1 (concentration)-1
[AB] and [C] are concentrations
Then rate in units of (concentration) (time)-1
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MACROSCOPIC : (1044 molecules in the atmosphere)
MICROSCOPIC : (about 25 molecules in a 10 nm cube)
KINETIC THEORY OF GASES
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KINETIC THEORY OF GASES:
(most any Physical Chemistry text, including Adamson, 1979; Atkins, 1986).
1) Molecules Move !! (they have kinetic energy):
Average Velocity:
For N2, can show that c is about 4 x 104 cm/sec at 298 K (225 miles / hr).
They traverse our 10 nm cube in 25 picosec!
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KINETIC THEORY OF GASES:
(most any Physical Chemistry text, including Adamson, 1979; Atkins, 1986).
2) Molecules collide with each other!!
Frequency of collisions is about 5 x 109 sec-1 / atm at 298 K.
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KINETIC THEORY OF GASES:
(most any Physical Chemistry text, including Adamson, 1979; Atkins, 1986).
2) Molecules collide with each other!!
Frequency of collisions is about 5 x 109 sec-1 / atm at 298 K.
3) Molecules can react with each other when they collide !
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KINETIC THEORY OF GASES:
(most any Physical Chemistry text, including Adamson, 1979; Atkins, 1986).
2) Molecules collide with each other!!
Frequency of collisions is about 5 x 109 sec-1 / atm at 298 K.
3) Molecules can react with each other when they collide !
4) The rate at which collisions occur defines the maximum rate at which two
molecules can react with each other. The maximum (“Gas Kinetic”) rate
coefficient for a bimolecular reaction is about 5 x 109 sec-1 / atm.
We usually use molecular units, 1 atm = 2.5  1019 molec cm-3, so collision
rate (max. rate coefficient) is 2 x 10-10 s-1 / (molec cm-3), or
2 x 10-10 cm3 molec-1 s-1
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
What determines the value of the rate constant???
Recall:
Frequency of collisions is about
2 x 10-10 cm3 molec-1 s-1 at 298 K.
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
BUT IN REALITY, REACTION DOES NOT OCCUR ON EVERY COLLISION!!!
CH4 + OH  CH3 + HOH
Why NOT? – any ideas?
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
BUT IN REALITY, REACTION DOES NOT OCCUR ON EVERY COLLISION!!!
Two main reasons:
1) There are energetic limitations. Colliding molecules must possess
sufficient energy to overcome an ‘activation energy’ that typically exists.
2) Also, there may be ‘geometrical limitations’. Molecules must approach
each other in such a way that the appropriate bonds can break / form.
k = A * exp(-Ea/RT)
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
BUT IN REALITY, REACTION DOES NOT OCCUR ON EVERY COLLISION!!!
Ea
From Wikipedia
k = A * exp(-Ea/RT)
A is the pre-exponential factor, and accounts for the geometry limitations.
Ea is activation energy.
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
k = A * exp(-Ea/RT)
So, Let’s go back to the •OH / CH4 reaction.
IF REACTION OCCURRED ON EVERY COLLISION,
k = 2 x 10-10 cm3 molecule-1 s-1
Turns out that k = 2.45 x 10-12 * exp(- 3525 cal / RT)
k = 6.3 x 10-15 cm3 molecule-1 s-1 at 298 K
k = 5.2 x 10-16 cm3 molecule-1 s-1 at 210 K
Only about 1 in 30000 OH/CH4 collisions results in reaction at 298 K.
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
Another example
HO2• + NO  •OH + NO2
– two radical species; no barrier to reaction (attractive forces).
HO2• + NO
HOO-NO
•OH + NO2
Reaction turns out to have a “negative activation energy”.
k = 3.5 x 10-12 exp(500 cal / RT) cm3 molecule-1 s-1
(Colder molecules more likely to react – less able to overcome attraction). 18
REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
ELEMENTARY REACTIONS (BIMOLECULAR)
REACTION
A-Factor
(cm3 molecule-1 s-1)
Activation
Energy (cal.)
Rate constant k at 298 K
(cm3 molecule-1 s-1)
•OH + CH4  •CH3 + H2O
•OH + H2  H• + H2O
•OH + CH3OH  •CH2OH + H2O
2.45e-12
5.5e-12
6.7e-12
3525
3975
1200
6.3e-15
6.7e-15
8.9e-13
NO3• + CH3CHO  HNO3 + CH3CO•
1.4e-12
3800
2.4e-15
HO2• + CH3O2•  O2 + CH3OOH
NO + CH3O2•  NO2 + CH3O•
CH3C(O)OO• + CH3C(O)OO• 
CH3C(O)O• + CH3C(O)O• + O2
3.8e-13
4.2e-12
2.9e-12
-1600
-360
-1000
5.6e-12
7.7e-12
1.5e-11
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
EQUILIBRIUM – All elementary reactions are reversible. At equilibrium, rate
of forward and reverse reactions must be equal.
[ HO…H-CH3 ]
Ea = 3525 calories
Ea = 17450 calories
•OH + CH4
kf [OH] [CH4] = kr [CH3] [H2O]
HOH + •CH3
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
EQUILIBRIUM – All elementary reactions are reversible. At equilibrium, rate
of forward and reverse reactions must be equal.
[ HO…H-CH3 ]
Ea = 3525 calories
kf = 6.3e-15 cm3 molec-1 s-1
Ea = 17450 calories
kr = 1.2e-25 cm3 molec-1 s-1
•OH + CH4
kf [OH] [CH4] = kr [CH3] [H2O]
HOH + •CH3
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
“Equilibrium” and “Steady-State” are different:
Equilibrium is a very precise, physical concept - established when forward
and reverse rates of all reactions in a system are equal.
Steady-State is more conceptual and approximate
- A (short-lived) species, often an intermediate in a chemical
scheme, is being produced and destroyed at roughly the same rate.
Production Rate = Loss Rate
O(1D) + H2O
•OH
Reaction with CH4
Reaction with CO
Reaction with Isoprene
HO2 + NO
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
Let’s look at this in more detail.
Consider a reaction scheme like this:
OH + CH4  CH3 + H2O
CH3 + O2  CH3O2
CH3O2 + NO  CH3O + NO2
CH3O + O2  CH2O + HO2
HO2 + NO  OH + NO2
NO2 + hn  NO + O
NO2 + hn  NO + O
O + O2  O3
O + O2  O3
Can assume CH3 to be in ‘steady-state’. (CH3O, CH3O2, OH, HO2, NO, NO2 too).
What is the steady-state [CH3]? (What do we need to know?)
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
OH + CH4  CH3 + H2O
CH3 + O2  CH3O2
CH3O2 + NO  CH3O + NO2
CH3O + O2  CH2O + HO2
HO2 + NO  OH + NO2
NO2 + hn  NO + O
NO2 + hn  NO + O
O + O2  O3
O + O2  O3
Can assume CH3 to be in ‘steady-state’. (CH3O, CH3O2, OH, HO2, NO, NO2 too).
Assume [OH] = 106, [CH4] = 4.6 x 1013, [O2] = 5 x 1018, all in molecule cm-3
Appropriate rate constants:
k1 = 6.3 x 10-15 cm3 molecule-1 s-1
k2 = 1 x 10-12 cm3 molecule-1 s-1
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
OH + CH4  CH3 + H2O
CH3 + O2  CH3O2
CH3O2 + NO  CH3O + NO2
CH3O + O2  CH2O + HO2
HO2 + NO  OH + NO2
NO2 + hn  NO + O
NO2 + hn  NO + O
O + O2  O3
O + O2  O3
Can assume CH3 to be in ‘steady-state’. (CH3O, CH3O2, OH, HO2, NO, NO2 too).
Assume [OH] = 106, [CH4] = 4.6 x 1013, [O2] = 5 x 1018, all in molecule cm-3
Appropriate rate constants:
k1 = 6.3 x 10-15 cm3 molecule-1 s-1
k2 = 1 x 10-12 cm3 molecule-1 s-1
k1[OH][CH4] = k2[O2][CH3]ss
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
OH + CH4  CH3 + H2O
CH3 + O2  CH3O2
CH3O2 + NO  CH3O + NO2
CH3O + O2  CH2O + HO2
HO2 + NO  OH + NO2
NO2 + hn  NO + O
NO2 + hn  NO + O
O + O2  O3
O + O2  O3
Can assume CH3 to be in ‘steady-state’. (CH3O, CH3O2, OH, HO2, NO, NO2 too).
Assume [OH] = 106, [CH4] = 4.6 x 1013, [O2] = 5 x 1018, all in molecule cm-3
Appropriate rate constants:
k1 = 6.3 x 10-15 cm3 molecule-1 s-1
k1[OH][CH4] = k2[O2][CH3]ss
k2 = 1 x 10-12 cm3 molecule-1 s-1
[CH3]ss = 0.06 molecule cm-3 (Very small !!)
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
Termolecular reactions (three-body reactions).
e.g.,
NO3 + NO2 + M  N2O5 + M
Here, M is the ‘bath gas’ (air – usually N2, O2).
Not as simple as it looks.
N2O5 initially formed with ‘excess energy’. Collisions help stabilize it.
NO3 + NO2  N2O5*
N2O5* + M  N2O5 + M
Reverse reaction – N2O5 decomposition. Collisions with the bath gas
‘activates’ some of the N2O5 molecules, leading to decomposition.
N2O5 + M  N2O5* + M
N2O5*  NO3 + NO2
Can involve an equilibrium (in this case between NO3, NO2 and N2O5).
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
One More Concept – Atmospheric lifetime.
If a compound is emitted into the atmosphere, how long (on average) will it
take to remove it (assuming no other production or emission).
Consider CH4 and its reaction with OH:
•OH + CH4  •CH3 + H2O
Let’s assume that, on average, [OH] = 106 molecule cm-3. Then k x [OH] is a constant,
let’s call it k.
In this particular case, with k = 3.7 x 10-15 cm3 molecule-1 s-1 at 273 K (roughly the average
tropospheric temperature), k = 3.7 x 10-9 s-1.
The atmospheric lifetime is then defined as the inverse of this k value.
1/ k = t (CH4) = 2.7 x 108 s, or about 8.5 years.
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
One More Concept – Atmospheric lifetime.
If a compound is emitted into the atmosphere, how long (on average) will it
take to remove it (assuming no other production or emission).
Consider CH4 and its reaction with OH:
•OH + CH4  •CH3 + H2O
Could think about this in reverse. What is the lifetime of OH?
Let’s assume that, on average, [CH4] = 4.75 x 1013 molecule cm-3.
Then k x [CH4] is a constant. With k = 6.3 x 10-15 cm3 molecule-1 s-1 at 298 K,
k = 0.3 s-1.
The atmospheric lifetime is then defined as the inverse of this k value.
1/ k = t(OH) = 3.3 s.
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
One More Concept – Atmospheric lifetime.
One more example – Isoprene, C5H8.
Reacts with OH (daytime only), O3 (day and night) and NO3 (nighttime only).
Calculate isoprene lifetime during daylight hours and during nighttime hours,
assuming the following rate constants and concentrations:
[OH] = 106 molecule cm-3, 12-hr daylight; k(OH+Isoprene) = 1 x 10-10 cm3 molecule-1 s-1
[NO3] = 108 molecule cm-3, 12-hr nighttime; k(NO3+Isoprene) = 7 x 10-13 cm3 molecule-1 s-1
[O3] = 1012 molecule cm-3, 24-hr average; k(O3+Isoprene) = 1.3 x 10-17 cm3 molecule-1 s-1
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
One More Concept – Atmospheric lifetime.
One more example – Isoprene, C5H8.
Reacts with OH (daytime only), O3 (day and night) and NO3 (nighttime only).
Daytime loss via OH and O3 reactions.
[OH] = 2 x 106 molecule cm-3, 12-hr daylight; k(OH+Isoprene) = 1 x 10-10 cm3 molecule-1 s-1
[O3] = 1012 molecule cm-3, 24-hr average; k(O3+Isoprene) = 1.3 x 10-17 cm3 molecule-1 s-1
Then Isoprene Loss Rate = kOH [OH] + kO3[O3] = 2 x 10-4 s-1 + 1.3 x 10-5 s-1 = 2.13 x 10-4 s-1.
Then, lifetime t = 1/ (Loss Rate) = 4700 sec. = 1.3 hrs.
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REACTION KINETICS: (follows Brasseur, Orlando and Tyndall, pp. 95-114.)
One More Concept – Atmospheric lifetime.
One more example – Isoprene, C5H8.
Reacts with OH (daytime only), O3 (day and night) and NO3 (nighttime only).
Nighttime loss via NO3 and O3 reactions.
[NO3] = 108 molecule cm-3, 12-hr nighttime; k(NO3+Isoprene) = 7 x 10-13 cm3 molecule-1 s-1
[O3] = 1012 molecule cm-3, 24-hr average; k(O3+Isoprene) = 1.3 x 10-17 cm3 molecule-1 s-1
Then Isoprene Loss Rate = kNO3 [NO3] + kO3[O3] = 7 x 10-5 s-1 + 1.3 x 10-5 s-1 = 8.3 x 10-5 s-1.
Then, lifetime t = 1/ (Loss Rate) = 12000 sec. = 3.3 hrs.
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Photolysis Reactions (Gas-Phase)
Some generalities:
- Sunlight provides energy across the electromagnetic spectrum,
which can be absorbed by molecules.
- Energies of chemical bonds typically correspond to UV photons.
- Absorption of UV (sun)light can lead to photolytic destruction of
certain molecules.
e.g.,
NO2 + hn  NO + O(3P)
Threshold l: 392 nm (near UV)
i.e., only photons at wavelengths < 392 nm or so can dissociate NO2
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Photolysis Reactions (Gas-Phase)
Some generalities:
- Sunlight provides energy across the electromagnetic spectrum,
which can be absorbed by molecules.
- Energies of chemical bonds typically correspond to UV photons.
- These photolytic processes can be seen as the initiators of
atmospheric chemistry. Energy from the sun breaks chemical bonds, creates
reactive free radicals:
e.g.,
O3 + hn  O2 + •O(1D)
O(1D) + H2O  •OH + •OH
Essentially, we have split apart a water molecule and created two reactive OH
radicals using one UV solar photon.
Net:
O3 + H2O + hn  •OH + •OH + O2
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Photolysis Reactions (Gas-Phase)
Some generalities:
- Sunlight provides energy across the electromagnetic spectrum,
which can be absorbed by molecules.
NO2 + hn  NO + O(3P)
Can consider these as unimolecular reactions, for given intensity and
distribution of sunlight.
“j-value” – unimolecular ‘rate coefficient’, units of s-1. Vary with spectral
properties of the molecule of interest, but also with solar intensity (as a fn of
wavelength)
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Quantifying Photolysis Processes
AB + hn  A + B
Photolysis reaction:
Photolysis rates:
Photolysis frequency (s-1)
J=

l
F(l) s(l) f(l) dl
(other names: photo-dissociation rate coefficient, J-value)
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CALCULATION OF PHOTOLYSIS COEFFICIENTS
J (s-1) = l F(l) s(l) f(l) dl
F(l) = spectral actinic flux, photons cm-2 s-1 nm-1
 probability of photon near molecule.
s(l) = absorption cross section, cm2 molec-1
 probability that photon is absorbed.
f(l) = photodissociation quantum yield, molec photons-1
 probability that absorbed photon causes dissociation.
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CALCULATION OF PHOTOLYSIS COEFFICIENTS
J (s-1) = l F(l) s(l) f(l) dl
F(l) = spectral actinic flux, photons cm-2 s-1 nm-1
 probability of photon near molecule.
s(l) = absorption cross section, cm2 molec-1
 probability that photon is absorbed.
f(l) = photodissociation quantum yield, molec photons-1
 probability that absorbed photon causes dissociation.
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Measurement of Absorption Cross Section, s(l)
Pressure
gage
Gas
supply
Pump for
clean out
Gas
in
Light
detector
Prism
Absorption cell
Lamp
L
Transmittance = I / Io = exp(-s n L)
s = -1/(nL) ln( I/Io )
Easy: measure pressure (n = P/RT), and relative change in light: I/Io
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O2 Absorption Cross Section
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O3 Absorption Cross Section
http://www.atmosphere.mpg.de
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NO2 Absorption Cross Section
http://www.atmosphere.mpg.de
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CH2O Absorption Cross Section
H2C=O
http://www.atmosphere.mpg.de
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CALCULATION OF PHOTOLYSIS COEFFICIENTS
J (s-1) = l F(l) s(l) f(l) dl
F(l) = spectral actinic flux, photons cm-2 s-1 nm-1
 probability of photon near molecule.
s(l) = absorption cross section, cm2 molec-1
 probability that photon is absorbed.
f(l) = photodissociation quantum yield, molec photons-1
 probability that absorbed photon causes dissociation.
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Measurement of Quantum Yields f(l)
Pressure
gage
Gas
supply
Pump for
clean out
Gas
in
Light
detector
Prism
Absorption cell
Lamp
L
Transmittance = I / Io = exp(-s n L)
Quantum Yield = number of breaks per photon absorbed
f = Dn / DI
Difficult: must measure absolute change in n (products) and I (photons absorbed)
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Measured Quantum Yields
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Spectral Range For Tropospheric Photochemistry
J (s-1) = l F(l) s(l) f(l) dl
- What do these integrals look like ?!?
1
0.9
0.8
dJ/d l (rel)
0.7
0.6
O3->O2+O1D
NO2->NO+O
0.5
H2O2->2OH
HONO->HO+NO
0.4
CH2O->H+HCO
0.3
0.2
0.1
0
280
300
320
Earth’s surface, overhead sun
340
360
380
400
420
Wavelength, nm
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Aerosol Fundamentals
Aerosol - A gaseous suspension of fine solid or liquid particles.
http://www.thefreedictionary.com/aerosol
“We have in this fine dust [aerosols] a most
beautiful illustration of how the little things in
the world work great effects by virtue of their
numbers.”
John Aitken (1839-1919)
-John Aitken, 1880
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Why should we care about aerosol?
Air quality and Human health
Global Climate
Radiation, Chemistry, Rainfall
Reactive surfaces/volume where chemistry can
happen
Aerosols are important from the molecular to the
global scale
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Aerosols are the principle component of what
we perceive as “smog”
Submicron aerosols are primarily
responsible for visibility reduction.
Pasadena, CA, on a clear day (hills are 7 km away)
Environmental Protection Agency (EPA)
PM2.5 15 g /m3 (annual average)
PM10 150 g /m3 (24 hour)
regulations.
See national map of compliance at:
http://en.wikipedia.org/wiki/File:Pm25-24asuper.gif
Pasadena, CA, on a bad smog day
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Aerosols and human health
Submicron aerosols can penetrate to the deepest parts of the lung
whereupon they can affect the pulmonary part of the respiratory system.
Deposition Fraction
Aerosol Deposition in Human Respiratory Tract
1
0.8
Total
Pulmonary
Nasal
Tracheo-bronchial
0.6
0.4
0.2
0
0.01
0.1
1
10
Particle Diameter (m)
100
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Aerosols and climate
 Drives
Global Warming
Direct effect –
Light is scattered
and absorbed
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IPCC, AR4
Aerosol “indirect effect” on climate
clean cloud (few particles):
large cloud droplets
• low albedo
• efficient precipitation
polluted cloud (many particles):
small cloud droplets
• high albedo
• suppressed precipitation
(very controversial)
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Polar Stratospheric Cloud (PSC)
g, gas
s, particle
ClONO2 (g) + HCl (s) (PSC)  Cl2(g) + HNO3(s)
Cl2 + hv  2Cl
Leads to the “Ozone Hole” every Spring
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Aerosols : solid and liquid particles
suspended in the air
Size: nm to 100 microns (range of 105)
Lifetime: Troposphere (hours to days) Stratosphere (year)
Primary aerosol: emitted directly into the air
Secondary aerosol: gas to particle conversion
• Composition: sulfate, nitrate, ammonium, sodium, chloride,
trace metals, crustal, water, carbonaceous:
elemental: emitted directly into the air (e.g. diesel soot)
organic: a) directly by sources (e.g combustion, plant leaf)
b) condensation of low volatile organic gases
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“fine”
diameters D < 2.5 microns
sulfate, ammonium, organic carbon, elemental carbon
Nucleation (Aitken) mode - 0.005 to 0.01 microns
condensation of vapors
Accumulation mode
0.1 to 2.5 microns coagulation
“coarse”
diameters D > 2.5 microns
natural dust (e.g. desert)
mechanical processes
crustal materials
biogenic (pollen, plant fragments)
57
Aerosols come in a variety of sizes,
have variable lifetimes and compositions
Seinfeld and Pandis
58
Size Distribution - “remote continental air”
<100 nm: ultrafine
Number Density
(n)
100 nm<dp<1 m:
accumulation
Surface Area
(π r2 n)
sub-2.5 m: fine
coarse
Volume Density
(4/3 π r3 n)
Seinfeld and Pandis
59
Seinfeld and Pandis
60
The chemical properties of atmospheric aerosols: North America
http://eosweb.larc.nasa.gov/PRODOCS/narsto/table_narsto.html
Annual mean
PM2.5
concentrations
(NARSTO, 2004)
61
Secondary Organic Aerosol (SOA)
SOA accounts for large fraction of submicron particulate mass
Arises from nucleation / condensation of low volatility products of
oxidation…
We’ll talk more about organic chemistry later, but basic idea is that
more oxygen on a molecule usually means lower vapor pressure
E.g., Biogenic terpenes, C10H16, can be oxidized to make things like
C10H16O2, then C10H16O4.
Some of these products nucleate new particles, or add on to
existing ones.
Jimenez, Science, 326, Dec 2009
Odum, Env Sci Tech, 30, 2580, 1996
Ervens, JGR, 109, 2004
62
Secondary Organic Aerosol (SOA)
M. Kulmala et al., Nature Protocols, 2012
63
Modeling Organic Aerosol:
What are the challenges?
Partitioning
Semi-Volatile
Organic Vapors
SOA
Nucleation
Cloud Processing
Oxidation
by OH, O3, NO3
Deposition
Evaporation
upon dilution
VOCs
Isoprene
Monoterpenes
Sesquiterpenes
Forest
POA
Aromatics
Alkanes
For MILARGO
application see:
Hodzic, ACP,2009
SOA is
underestimated
Direct
Emission
Traffic Industries
Surface / multiphase
reactions
Biomass
Burning
Biological
Debris
Alma Hodzic
Cloud Chemistry
(more generally, Aqueous Phase Chemistry)
Chemistry occurring in or on liquid particles (cloud drops, rain drops, fog
droplets, aerosols)
Cloud
droplets
Will focus on two aspects of this in terms of atmospheric composition:
1) Scavenging of oxidized gases (nitric acid, sulfuric acid)
2) Oxidative Chemistry (sulfur species, carbon species)
65
Aqueous Phase Chemistry
1) Scavenging of oxidized gases
- Governed by Henry’s Law equilibrium
HNO3
HNO3 (aq)
(gas)
[HNO3]aq = KH,HNO3 * pHNO3
KH ≡ Henry’s Law Constant
pHNO3 ≡Partial Pressure (atm)
[HNO3]aq ≡ Aq. Phase Conc (mol/l)
66
Aqueous Phase Chemistry
1) Scavenging of oxidized gases
- Governed by Henry’s Law equilibrium
Some Henry’s Law Constants
of Atmospheric Relevance
HNO3
HNO3 (aq)
Chemical
Species
Henry’s Law Constant @
25°C (mol/L/atm)
HNO3
2.1x105
H2O2
7.5x104
HCHO
3.5x103
NH3
57.5
SO2
1.2
pHNO3 ≡Partial Pressure (atm)
O3
0.0113
[HNO3]aq ≡ Aq. Phase Conc (mol/l)
CO
9.6x10-4
(gas)
[HNO3]aq = KH,HNO3 * pHNO3
KH ≡ Henry’s Law Constant
67
Phase Ratio between Gas and Liquid
Px = L KH RT
268 K
293 K
HNO3
1.6x109
6.4x106
H2O2
6.4
0.8
SO2
0.072
0.014
CO2
6.3x10-7
2.9x10-7
O3
1.9x10-7
9.1x10-8
68
Aqueous Phase Chemistry
SO2 • H2O
H+ + HSO3H+ + SO3=
Dissociation in water increases the
effective solubility of the gas
NB: Solubility of SO2 strong function
of droplet acidity (i.e., pH)
69
S(IV) Solubility and Composition Depends Strongly on pH
Strongly acidic cloud water Neutral / slightly basic cloud water
70
[Seinfeld & Pandis]
Aqueous Phase Chemistry
Chemical reaction in drop
HSO3- + oxidant
SO4=
SO3= + oxidant
SO2, HSO3-, SO32- collectively called S(IV).
The atmosphere is oxidizing
– wants to push S(IV) to S(VI) {H2SO4 / HSO4- / SO42-}
In aqueous phase, H2O2 and O3 can do the trick. Relative contribution of
these two oxidants is a complex function of the solubilities, reaction rates,
etc.
Bottom Line:
71
Aqueous Phase Chemistry
Reaction rates for S(IV) oxidation by H2O2 and O3
H2O2
268 K
278 K
288 K
298 K
O3 + SO3=
O3 + HSO3SO2 = 2 ppbv
H2O2 = 1 ppbv
O3 = 50 ppbv
72
Aqueous Phase Chemistry
GLOBAL CLIMATE MODEL
SIMULATIONS:
50-55% of sulfate in
troposphere is from
aqueous-phase chemistry
The remainder is from
gas-phase chemistry,
reaction of OH with SO2
Barth et al., 2000
73
Aqueous Phase Chemistry
CH4
CH2O
S(IV) chemistry is not only aqueous
chemistry going on!
CH2(OH)2+ OH
CO2
HCOO- + OH
HCOOH + OH
74
Aqueous Phase Chemistry
Chemical reaction in drop
Organic aqueous chemistry is a
source of secondary organic aerosol
Chen et al. (2007) ACP
75