ATMOSPHERIC CHEMISTRY
OF ORGANIC COMPOUNDS
Lecture for NC A&T (part 2)
March 9, 2011
John Orlando
orlando@ucar.edu
REVIEW:
Geoff showed something about the types of compounds:
CH4
CH3-CH(CH3)2
CH3-CH=CH-CH3
CH3CH2CH2C(=O)CH3
CH3CH2CH2OH
CH3CH2-O-CH2CH3
REVIEW:
Where they come from:
Biogenic sources the largest – isoprene, terpenes,etc.
Isoprene
CH2=CH-C(CH3)=CH2
But also anthropogenic emissions, mostly the types of things we just saw
on the previous page (fossil fuel combustion, industrial…)
Alkanes
Alkenes
Alcohols
Ethers
Etc. Etc. etc.
REVIEW:
How they are distributed (and how we know - measurements):
T. Karl et al. (ACD), J. Geophys. Res., 112, D18302, 2007.
REVIEW:
What are the impacts?
Ozone
“Chemical Weather” – From Louisa Emmons (ACD), Mozart-4 Global CTM
REVIEW:
What are the impacts?
Secondary Organic Aerosol
From Alma Hodzic (ACD) et al., Atmos. Chem. Phys., 9, 6949, 2009.
SO NOW LET’S TALK ABOUT THE CHEMISTRY:
RECALL:
The atmosphere (particularly the troposphere) acts as a low-temperature,
slow-burning combustion engine.
Takes all the emissions (reduced compounds) and ‘burns’ (oxidizes) them:
OH
HO2
CH4
CO2 + H2O
Isoprene
Other by-products, such
as O3, particles, acids,
nitrates, etc.
(2ry POLLUTANTS)
DMS, NH3
NO
NO2
THE TROPOSPHERIC “ENGINE”:
Now the “Odd Hydrogen” Family:
Production:
Consider first OH and HO2:
O3 + hn  O(1D) + O2
O(1D) + H2O  OH + OH
Conversion of OH to HO2:
OH + CO (+O2)  HO2 + CO2
OH + O3  HO2 + O2,
dominant (when all ‘fuel’ considered)
usually minor
Conversion of HO2 back to OH:
HO2 + O3  OH + 2 O2
HO2 + NO  OH + NO2,
(followed by NO2 + hn  NO + O, O + O2 + M  O3 + M,
which generates O3 !!)
Losses of HOx via two processes:
HO2 + HO2 + M  HOOH + O2 + M
OH + NO2 + M = HNO3 + M
OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail:
Talked about this in my seminar at NC A&T: What are the basic steps
(there are four)? (Let’s start with methane, CH4).
CH4
OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail:
Talked about this in my seminar at NC A&T: What are the basic steps
(there are four)? (Let’s start with methane, CH4).
CH4
1.
Starts with reaction with OH:
OH
CH3 + H2O
OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail:
Talked about this in my seminar at NC A&T: What are the basic steps
(there are four)? (Let’s start with methane, CH4).
CH4
1.
Starts with reaction with OH:
OH
CH3 + H2O
2.
The alkyl radical adds O2, to make a peroxy radical.
O2
CH3O2
OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail:
Talked about this in my seminar at NC A&T: What are the basic steps
(there are four)? (Let’s start with methane, CH4).
CH4
1.
Starts with reaction with OH:
OH
CH3 + H2O
2.
The alkyl radical adds O2, to make a peroxy radical.
O2
CH3O2
3. Peroxy radical often reacts with NO, making an alkoxy
radical. (There are other pathways, see later).
NO
CH3O + NO2
OK, Let’s go beyond CO and look at ORGANIC SPECIES in more detail:
Talked about this in my seminar at NC A&T: What are the basic steps
(there are four)? (Let’s start with methane, CH4).
CH4
1.
Starts with reaction with OH:
OH
CH3 + H2O
2.
The alkyl radical adds O2, to make a peroxy radical.
O2
CH3O2
3. Peroxy radical often reacts with NO, making an alkoxy
radical. (There are other pathways, see later).
NO
CH3O + NO2
4. Alkoxy radical reacts with O2, to make a carbonyl
compound. (There are other pathways, see later).
O2
CH2O + HO2
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
2
+ O2
3
CH3CH2CH2CH(OO)CH3
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
4
+ O2
CH3CH2CH2C(=O)CH3 + HO2
CH3CH2CH2CH2CH3
1
+ OH
IN GENERAL, REFER TO THE PARENT
COMPOUND AS R-H
CH3CH2CH2CH()CH3
+ H2O
REFER TO THE ALKYL RADICAL
AS R•
2
+ O2
3
CH3CH2CH2CH(OO)CH3
REFER TO THE PEROXY
RADICAL AS RO2•
REFER TO THE ALKOXY
RADICAL AS RO•
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
4
+ O2
NOTE ALSO: THESE BASIC REACTIONS
PROPOGATE RADICALS !!
We will refer to this again from time to time,
noting that other pathways DO NOT PROPOGATE
CH3CH2CH2C(=O)CH3 + HO2
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
2
+ O2
3
Ea = 13 kcal
CH3CH2CH2CH(OO)CH3
Ea = 8 kcal
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
CH3CH2CH2 + CH3CHO
CH2CH2CH2CH(OH)CH3
4
+ HO2
+ NO
+ O2
3b
CH3CH2CH2C(=O)CH3 + HO2
CH3CH2CH2CH(OOH)CH3
CH3CH2CH2CH(ONO2)CH3
OK, LET’S START WITH STEP #1
– REACTION OF OH WITH HYDROCARBONS
(Also applies to NO3, and Cl-atoms)
CAN HAVE TWO KINDS OF REACTIONS –
1) ABSTRACTION:
OH + CH4  •CH3 + H2O
- Occurs when the hydrocarbon is “saturated” (no double bonds)
2) ADDITION:
OH + CH2=CH2  HOCH2-CH2•
OK, LET’S START WITH STEP #1
– REACTION OF OH WITH HYDROCARBONS
(Also applies to NO3, and Cl-atoms)
Go back to our old friend, OH + Methane (CH4)
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.
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.
FOR OH + CH4:
[ HO…H-CH3 ]
Ea = 3525 calories
OH + CH4
DHr = - 13900 calories
HOH + CH3
FOR OH + CH4:
FOR OH + C2H6: (CH3-CH3)
[ HO…H-CH3 ]
Ea = 3525 calories
Ea = 2100 calories
OH + CH4
OH + CH3-CH3
DHr = - 13900 calories
DHr = - 17800 calories
HOH + CH3
HOH + CH3-CH2
SO, IN GENERAL: The more substituted (complicated) the molecule, the
weaker the C-H bond, and the faster the rate coefficient
COMPOUND
A-Factor
(cm3 molecule-1 s-1)
Activation Energy
(calories)
METHANE
ETHANE
n-PENTANE
1.85  10-12
8.61  10-12
1.81  10-11
3360
2080
900
6.4  10-15
2.6  10-13
3.9  10-12
8.4 years
45 days
3 days
2-PROPANOL
DIETHYL ETHER
2.7  10-12
4.6  10-12
-190
-290
5.1  10-12
1.2  10-11
2 days
1 days
2-PENTANONE
3.2  10-13
-1430
3.6  10-12
3 days
CH3CF3
1.06  10-12
3975
1.3  10-15
> 25 years
n-PENTANE: CH3CH2CH2CH2CH3
2-PROPANOL:CH3CH(OH)CH3
Rate Constant at Approx. Lifetime
298 K
(OH = 106
(cm3 molecule-1 s-1) molecule cm-3)
DIETHYL ETHER : CH3CH2-O-CH2CH3
2-PENTANONE: CH3CH2C(=O)CH2CH3
400 ppt
200 ppt
Figure I-F-1g. The annual mean surface distribution of a synthetic alkane with a man-made source strength of 1 Tg yr-1
and an OH reaction rate coefficient of 1.0 ×10-14 cm3 molecule-1 s-1.
(From Calvert et al., Mechanisms of the Atmospheric Oxidation of the Alkanes, OUP, 2008)
50 ppt
< 1 ppt
Figure I-F-1a. The annual mean surface distribution of a synthetic alkane with a man-made source strength of 1 Tg yr-1
and an OH reaction rate coefficient of 1.0 ×10-11 cm3 molecule-1 s-1.
(From Calvert et al., Mechanisms of the Atmospheric Oxidation of the Alkanes, OUP, 2008)
THERE ARE OTHER OXIDANTS BESIDES OH:
- One of the them is the “NITRATE RADICAL”, NO3
- Photolyzes rapidly, so only active at nighttime.
- Can abstract, though energetics not as favorable.
As an example,
OH + Isobutane (C4H10)  •C(CH3)3 + H2O
k = 7.0  10-12 exp(-350/T) cm3 molecule-1 s-1
NO3 + Isobutane (C4H10)  •C(CH3)3 + H2O
k = 3.9  10-12 exp(-3150/T) cm3 molecule-1 s-1
-9
-10
log10 (Rate Coefficient)
-11
-12
-13
-14
k(Cl) vs. k(OH)
k(NO3) vs. k(OH)
-15
k(O( P)) vs. k(OH)
Cl-atom data, not fit
Fits to the data
3
-16
-17
-18
-14.5
-14.0
-13.5
-13.0
-12.5
-12.0
-11.5
-11.0
-10.5
log10 (OH Rate Coefficient)
Figure III-F-1. Plots of logarithm of the rate coefficients (cm3 molecule-1 s-1) for reaction of Cl, O(3P) and
NO3 with the alkanes versus those for reaction of OH with the corresponding alkane. Solid lines are
unweighted least-squares fits to the data. (From Calvert et al., Mechanisms of Atmospheric Oxidation of
the Alkanes, OUP, 2008)
SO FAR, We have only dealt with abstraction.
Can also have ‘addition’ reactions, when the hydrocarbon is ‘unsaturated’:
(i.e., contains a C=C double bond, alkenes)
Occurs for OH, NO3, Cl-atoms too:
Generally very fast reactions:
OH + CH2=CH2 (ethene) 
HOCH2-CH2•
For OH + ethene, k = 8.1  10-12 cm3 molecule-1 s-1
Ethene lifetime  1.5 days
====
Again, more substituted species react even faster.
k(OH + isoprene) = 1.0  10-10 cm3 molecule-1 s-1
Isoprene lifetime  (1-2) hours
Generally, when multiple choices, addition will win over abstraction.
CH3CH2-CH=CH-CH(CH3)2
Generally, when multiple choices, addition will win over abstraction.
CH3CH2-CH=CH-CH(CH3)2
Addition reaction wins, k  6  10-11 cm3 molecule-1 s-1
Abstraction reactions, k  3  10-12 cm3 molecule-1 s-1
OZONE CAN ALSO ACT AS AN OXIDANT – Adds to double bonds:
Chemistry is a bit weird, producing something called “Criegee Biradicals”:
O3 + CH2=CH2

O-O
CH2
CH2

CH2=O + •CH2-OO•
O
Chemistry of Criegee radicals is complex (and not totally understood):
•CH2-OO• undergoes numerous types of reactions that form CO, CO2,
HCOOH
THERE ARE METHODS FOR ESTIMATING RATE COEFFICIENTS FOR
REACTION OF VARIOUS OXIDANTS WITH HYDROCARBONS
“STRUCTURE-REACTIVITY” RELATIONSHIPS
(e.g., Kwok & Atkinson, Atm. Env., 1995)
Consider only OH abstraction today,
but they exist for addition reactions and also for other reactants (NO3, Cl, O3)
How does it work?
First:
Assign ‘starting values’ for reaction of OH with a –CH3 group, and –CH2group, and a –CH< group (298 K):
k(-CH3) = 1.36  10-13 cm3 molecule-1 s-1
k(-CH2-) = 9.34  10-13 cm3 molecule-1 s-1
k(-CH<) = 19.4  10-13 cm3 molecule-1 s-1
MODIFY THE INITIAL VALUE ACCORDING TO WHAT IS BONDED TO IT
(“Substituent factors”)
CH3 – X
k = k(-CH3) * F(X)
Y – CH2 – X
k = k(-CH2-) * F(X) * F(Y)
Y – CH – X
k = k(-CH<) * F(X) * F(Y) * F(Z)
Z
CONSIDER PROPANOL:
HO – CH2 – CH2CH3
k = k(CH2) * F(X) * F(Y)
k(-CH2-) = 9.34  10-13 cm3 molecule-1 s-1
F(-OH) = 4.0
F(-CH2CH3) = 1.23
So, estimated k for reaction at the one particular -CH2- group is:
k = k(-CH2-) * F(X) * F(Y)
= 9.34  10-13 cm3 molecule-1 s-1 * (4.0) * (1.23)
= 4.2  10-12 cm3 molecule-1 s-1
Generally, when multiple choices, addition will win over abstraction.
CH3CH2-CH=CH-CH(CH3)2
Addition reaction wins, k  6  10-11 cm3 molecule-1 s-1
Abstraction reactions, k  3  10-12 cm3 molecule-1 s-1
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
OK, READY FOR STEP #2
2
+ O2
3
CH3CH2CH2CH(OO)CH3
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
4
+ O2
CH3CH2CH2C(=O)CH3 + HO2
No worries, this one is EASY PEASY LEMON SQUEEZY
Take alkyl radical, e.g., CH3-CH2•
And add O2,
CH3-CH2 + O2 + M 
CH3-CH2O2 + M
Voila, instant peroxy radical !!
Typical k = 7 x 10-12 cm3 molecule-1 s-1
[O2] = 5 x 1018 molecule cm-3
So, time scale for the reaction is about 30 ns at Earth’s surface !!!
Nothing else has much of a chance, except in extremely rare circumstances
that we will not pursue today.
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
OK, ON TO STEP #3 !!!
2
+ O2
3
CH3CH2CH2CH(OO)CH3
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
4
+ O2
CH3CH2CH2C(=O)CH3 + HO2
3
PEROXY RADICAL CHEMISTRY
MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL
NO Reaction (MAIN PATHWAY):
RO2 + NO

RO + NO2
CH3O2 + NO

CH3O + NO2
This reaction propogates radicals.
3
PEROXY RADICAL CHEMISTRY
MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL
NO Reaction (MAIN PATHWAY):
RO2 + NO

RO + NO2
CH3O2 + NO

CH3O + NO2
This reaction propogates radicals.
BUT, ALSO ANOTHER MINOR CHANNEL THAT COMPETES:
RO2 + NO

RONO2
CH3O2 + NO
CH3CH2CH2CH(OO)CH3 + NO


CH3ONO2
CH3CH2CH2CH(ONO2)CH3
The larger and more complex the peroxy radical, typically the higher the nitrate yield
(up to about 40% in some cases). NB: This channel is a radical TERMINATION!
22
20
ALKANES
ALKENES
OXYGENATES
ACYLPEROXY
ALKANES (avg)
ALKENES (avg)
ACYLPEROXY (avg)
18
3
-1
-1
cm molecule s )
24
RATE COEFFICIENT (10
-12
16
14
12
10
8
6
4
0
1
2
3
4
5
6
NUMBER OF CARBON ATOMS
Rate coefficient independent of structure, all k  10-11 cm3 molecule-1 s-1
So what are typical lifetimes for an RO2 (peroxy) radical in the real world (Earth’s
surface)?
[NO] (pptv)
LOCATION
5
1000
100000
Very remote regions
Rural conditions
Mexico City (e.g.)
Approx. RO2 LIFETIME
800 sec.
4 sec.
0.04 sec.
3
PEROXY RADICAL CHEMISTRY
MAIN REACTION IS WITH NO, CONVERTS PEROXY TO ALKOXY RADICAL.
ALSO HAVE THE NITRATE FORMING CHANNEL, WHICH TERMINATES.
ALSO, a reaction with HO2, main channel
RO2 + HO2
Radical termination.

ROOH + O2
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
2
+ O2
3
Ea = 13 kcal
CH3CH2CH2CH(OO)CH3
Ea = 8 kcal
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
CH3CH2CH2 + CH3CHO
CH2CH2CH2CH(OH)CH3
4
+ HO2
+ NO
+ O2
3b
CH3CH2CH2C(=O)CH3 + HO2
CH3CH2CH2CH(OOH)CH3
CH3CH2CH2CH(ONO2)CH3
RATE CONSTANTS FOR REACTION OF PEROXY RADICALS WITH HO2
(Boyd et al., JPCA, 107, 818, 2003)
Similar values to RO2 + NO reactions.
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
OK, ON TO STEP #4,
WE CAN DO IT !!!
2
+ O2
3
CH3CH2CH2CH(OO)CH3
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
4
+ O2
CH3CH2CH2C(=O)CH3 + HO2
4
ALKOXY RADICAL CHEMISTRY
MAIN REACTION IS WITH O2, CONVERTS ALKOXY RADICAL TO A CARBONYL
COMPOUND, ALSO GET HO2 (a peroxy radical) formed. PROPOGATION!!
CH3O + O2

CH3CH2CH2CH(O)CH3 + O2
CH2O + HO2

CH3CH2CH2C(=O)CH3 + HO2
Rate coefficient typically about 10-14 cm3 molecule-1 s-1
So lifetime is about 20 ms
For larger alkoxy radicals, like 2-pentoxy, can have competing reactions:
Decomposition
4
H
CH3CH2CH2
C O
CH3CH2CH2C(=O)CH3 + H
CH3
CH3CH2CH2CHO + CH3
CH3CHO + CH3CH2CH2
(Baldwin et al., 1977; Choo and Benson, 1981;
Atkinson, 1999)
Energy
k = 5e13 * exp (-Ea/RT) sec-1
CH3CH2CH2CH2CH3
+ OH
CH3CH2CH2CH()CH3
+ H2O
+ O2
Ea > 20 kcal
Ea = 17 kcal
Ea = 13 kcal
CH3CH2CH2CH(OO)CH3
+ NO
H + CH3CH2CH2C(=O)CH3
CH3 + CH3CH2CH2CHO
CH3CH2CH2 + CH3CHO
CH3CH2CH2CH(O)CH3
+ NO2
+ O2
CH3CH2CH2C(=O)CH3 + HO2
H
CH3CH2CH2
CH3CH2CH2C(=O)CH3 + H
C O
CH3
CH3CH2CH2CHO + CH3
CH3CHO + CH3CH2CH2
•CH2CH2CH2CH(OH)CH3
(Isomerization via 6-Member Transition State)
ISOMERIZATION
O
H2
C
H3C
.
H
H2C
O
H2C
CH
OH
.
H2
C
CH
C
H2
CH3
C
H2
CH3
.H2C
CH
C
H2
CH3
CH3CH2CH2CH2CH3
1
+ OH
CH3CH2CH2CH()CH3
+ H2O
2
+ O2
3
Ea = 13 kcal
CH3CH2CH2CH(OO)CH3
Ea = 8 kcal
+ NO
CH3CH2CH2CH(O)CH3
+ NO2
CH3CH2CH2 + CH3CHO
CH2CH2CH2CH(OH)CH3
4
+ HO2
+ NO
+ O2
CH3CH2CH2C(=O)CH3 + HO2
CH3CH2CH2CH(OOH)CH3
CH3CH2CH2CH(ONO2)CH3
2-Pentoxy Chemistry vs. Altitude
18
16
14
Altitude (km)
12
10
8
6
4
2
0
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
Rate (per second)
Reaction with O2
Methyl Elimination
Isomerization
Propyl elimination
DIETHYL ETHER
CH3CH2-O-CH2CH3
+ OH
CH3CH2-O-CH()CH3
+ H2O
+ O2
Ea = 15 kcal?
Ea ≤ 11 kcal?
CH3CH2-O- CH(OO)CH3
Ea = 7 kcal?
+ NO
CH3CH2-O-CH(O)CH3
+ NO2
4
CH3CH2O + CH3CHO
H + CH3CH2-O-C(=O)CH3
CH3 + CH3CH2-O-CHO
+ O2
Orlando, 2007; Cheema et al., 1999; Wallington and Japar, 1991;
Eberhard et al., 1993
CH3CH2-O-C(=O)CH3 + HO2
DIETHYL ETHER
CH3CH2-O-CH2CH3
+ OH
CH3CH2-O-CH()CH3
+ H2O
+ O2
[CH3CH2OCH(O)CH3 ]‡
Ea = 15 kcal?
Ea ≤ 11 kcal?
CH3CH2-O- CH(OO)CH3
Ea = 7 kcal?
+ NO
CH3CH2-O-CH(O)CH3
+ NO2
4
CH3CH2O + CH3CHO
H + CH3CH2-O-C(=O)CH3
CH3 + CH3CH2-O-CHO
+ O2
Orlando, 2007; Cheema et al., 1999; Wallington and Japar, 1991;
Eberhard et al., 1993
CH3CH2-O-C(=O)CH3 + HO2
[ CH3CH2OCH(O)CH3 ] ‡
10-15 %
35-40 %
CH3CH2OC(=O)CH3 + H
CH3CH2OCH=O + CH3
deactivation
(50%)
CH3CH2OCH(O)CH3
dissoc., minor
EA ~ 6 kcal, major
+ O2
CH3CH2OC(=O)CH3 + H
CH3CH2OCH=O + CH3
CH3CH2OC(=O)CH3 + HO2
[Orlando, 2007]
CHEMICAL ACTIVATION:
About 20 occurrences documented !
(alkenes, halogenates, ketones, ethers, esters, even alkanes !!!)
FRACTION OF ACTIVATED RADICALS
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
7.5
10
12.5
ENERGY BARRIER
15
17.5
20
SOME GENERALITIES ON ALKOXY RADICALS
1. There is almost always a reaction with O2 to produce
HO2 and a carbonyl, time constant about 20 ms.
2. There can be competing unimolecular reactions –
decompositions and isomerizations.
3. Chemical activation might also be important (if barrier
is low enough).
OK, Let’s step back a minute and review: We have a set of four
reactions that occur for essentially every organic species.
E.g., we saw methane (CH4) get converted to CH2O.
Also, pentane to 2-pentanone.
CH4
1.
Starts with reaction with OH:
OH
CH3 + H2O
2.
The alkyl radical adds O2, to make a peroxy radical.
O2
CH3O2
3. Peroxy radical often reacts with NO, making an alkoxy
radical. (There are other pathways, see later).
NO
CH3O + NO2
4. Alkoxy radical reacts with O2, to make a carbonyl
compound. (There are other pathways, see later).
O2
CH2O + HO2
OK, Let’s step back a minute and review: We have a set of four
reactions that occur for essentially every organic species.
E.g., we saw methane (CH4) get converted to CH2O.
Also, pentane to 2-pentanone.
So, what happens to the CH2O, and to the 2-pentanone.
Well, they go through the same processes:
e.g., OH + CH2O  HCO + H2O
HCO + O2  HO2 + CO
Figure V-B-10. Main routes in the OH-initiated oxidation mechanism of 2-pentanone under high NOx conditions.
(From “The Mechanisms of Atmospheric Oxidation of the Oxygenates, J. Calvert et al., Oxford Univ. Press, 2011)
BUT, ONE OTHER THING CAN HAPPEN IN THE GAS-PHASE:
Photolysis !!
Because in general carbonyl compounds (species
containing C=O double bonds) absorb near-UV photons !!
From Sasha’s Lecture:
Photolysis frequency (s-1)
J=

l
F(l) s(l) f(l) dl
(From “The Mechanisms of Atmospheric Oxidation of the Oxygenates, J. Calvert et al., Oxford Univ. Press, 2011)
So, photolysis of CH3CHO to CH3 and HCO occurs at a rate of about 10-5 sec-1
for overhead sun.
(From “The Mechanisms of Atmospheric Oxidation of the Oxygenates, J. Calvert et al., Oxford Univ. Press, 2011)
AND, ONE OTHER THING CAN HAPPEN : Deposition !!
RECALL: We are converting an emitted hydrocarbon (say pentane,
CH3CH2CH2CH2CH3) to oxidized products, CH3CH2CH2C(=O)CH3.
As the process continues, the partially-oxidized products become
increasingly SOLUBLE, and also LESS VOLATILE.
So, they are more prone to uptake into clouds, into aqueous aerosols, to
deposition to the ground, etc…
Big issue these days:
Formation of secondary organic aerosol !!
Species like CH3(CH2)15C(=O)CH3 actually form aerosol !
OH
HO2
CH4
CO2 + H2O
Isoprene
Other by-products, such
as O3, particles, acids,
nitrates, etc.
(2ry POLLUTANTS)
DMS, NH3
NO
NO2
OZONE
PRODUCTION
HONO2
NO2
Parent
NMHC In
NO
OH
HO2
+ Oxidized Species
Out
O2
Unimolecular Reaction
R
RO
O2
RO2
NO, HO2
NO, HO2
Nitrates, Peroxides
Out
OZONE
PRODUCTION