ATMOSPHERIC CHEMISTRY OF ORGANIC COMPOUNDS Lecture for NC A&T (part 2) March 9, 2011 John Orlando...

ATMOSPHERIC CHEMISTRYOF ORGANIC COMPOUNDS

Lecture for NC A&T (part 2)March 9, 2011

John [email protected]

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.

IsopreneCH2=CH-C(CH3)=CH2

But also anthropogenic emissions, mostly the types of things we just saw on the previous page (fossil fuel combustion, industrial…)

AlkanesAlkenesAlcohols Etc. Etc. etc.Ethers

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,

DMS, NH3 nitrates, etc. (2ry POLLUTANTS)

NO NO2

THE TROPOSPHERIC “ENGINE”: Now the “Odd Hydrogen” Family: Consider first OH and HO2:

Production: O3 + h O(1D) + O2 O(1D) + H2O OH + OH

Conversion of OH to HO2:

OH + CO (+O2) HO2 + CO2 dominant (when all ‘fuel’ considered)OH + O3 HO2 + O2, usually minor

Conversion of HO2 back to OH:

HO2 + O3 OH + 2 O2 HO2 + NO OH + NO2, (followed by NO2 + h 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

CH3CH2CH2CH2CH3

CH3CH2CH2CH()CH3 + H2O

CH3CH2CH2CH(OO)CH3

CH3CH2CH2CH(O)CH3+ NO2

CH3CH2CH2C(=O)CH3 + HO2

+ NO

+ O2

+ O2

+ OH1

2

3

4

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3



+ NO

+ O2

+ O2

+ OH1

2

3

4

IN GENERAL, REFER TO THE PARENT COMPOUND AS R-H

REFER TO THE ALKYL RADICAL AS R•

REFER TO THE PEROXY RADICAL AS RO2•

NOTE ALSO: THESE BASIC REACTIONSPROPOGATE RADICALS !!

We will refer to this again from time to time, noting that other pathways DO NOT PROPOGATE

REFER TO THE ALKOXY RADICAL AS RO•

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3

CH3CH2CH2CH(O)CH3

+ NO2


CH3CH2CH2 + CH3CHO+ NO

+ O2

+ O2

Ea = 13 kcal

CH2CH2CH2CH(OH)CH3

Ea = 8 kcal

CH3CH2CH2CH(OOH)CH3 CH3CH2CH2CH(ONO2)CH3

+ HO2 + NO

+ OH1

2

3

4

3b

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)

From Wikipedia

REACTION DOES NOT OCCUR ON EVERY COLLISION!!!

Ea

k = A * exp(-Ea/RT)

A is the pre-exponential factor, and accounts for the geometry limitations.Ea is activation energy.

http://en.wikipedia.org/wiki/File:Ac_com.svg

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 + CH4Hr = - 13900 calories

HOH + CH3

FOR OH + CH4:FOR OH + C2H6: (CH3-CH3)

[ HO…H-CH3 ]

Ea = 3525 calories Ea = 2100 calories

OH + CH4 Hr = - 13900 calories OH + CH3-

CH3Hr = - 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

n-PENTANE: CH3CH2CH2CH2CH3 DIETHYL ETHER : CH3CH2-O-CH2CH3

2-PROPANOL:CH3CH(OH)CH3 2-PENTANONE: CH3CH2C(=O)CH2CH3

COMPOUND A-Factor(cm3 molecule-1 s-1)

Activation Energy (calories)

Rate Constant at 298 K

(cm3 molecule-1 s-1)

Approx. Lifetime (OH = 106

molecule cm-3)

METHANE 1.85 10-12 3360 6.4 10-15 8.4 yearsETHANE 8.61 10-12 2080 2.6 10-13 45 days

n-PENTANE 1.81 10-11 900 3.9 10-12 3 days

2-PROPANOL 2.7 10-12 -190 5.1 10-12 2 daysDIETHYL ETHER 4.6 10-12 -290 1.2 10-11 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

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)

400 ppt

200 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)

50 ppt

< 1 ppt

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

log10 (OH Rate Coefficient)

-14.5 -14.0 -13.5 -13.0 -12.5 -12.0 -11.5 -11.0 -10.5

log

10

(Ra

te C

oe

ffic

ien

t)

-18

-17

-16

-15

-14

-13

-12

-11

-10

-9

k(Cl) vs. k(OH)k(NO3) vs. k(OH)

k(O(3P)) vs. k(OH) Cl-atom data, not fitFits to the data

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



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”:

O - OO3 + CH2=CH2 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 –CH2- group, 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) = 4.0F(-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



Addition reaction wins, k 6 10-11 cm3 molecule-1 s-1

Abstraction reactions, k 3 10-12 cm3 molecule-1 s-1

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3



+ NO

+ O2

+ O2

+ OH1

2

3

4

OK, READY FOR STEP #2

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


CH3CH2CH2CH(OO)CH3



+ NO

+ O2

+ O2

+ OH1

2

3

4

OK, ON TO STEP #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


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 CH3ONO2

CH3CH2CH2CH(OO)CH3 + NO 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!

3

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 Approx. RO2 LIFETIME

5 Very remote regions 800 sec. 1000 Rural conditions 4 sec.100000 Mexico City (e.g.) 0.04 sec.

NUMBER OF CARBON ATOMS

0 1 2 3 4 5 6

RA

TE

CO

EF

FIC

IEN

T (

10-1

2 cm

3 m

ole

cu

le-1

s-1

)

4

6

8

10

12

14

16

18

20

22

24

ALKANES ALKENES OXYGENATESACYLPEROXY ALKANES (avg) ALKENES (avg) ACYLPEROXY (avg)


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 ROOH + O2

Radical termination.

3

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3

CH3CH2CH2CH(O)CH3

+ NO2



+ O2

+ O2

Ea = 13 kcal

CH2CH2CH2CH(OH)CH3

Ea = 8 kcal


+ HO2 + NO

+ OH1

2

3

4

3b

RATE CONSTANTS FOR REACTION OF PEROXY RADICALS WITH HO2 (Boyd et al., JPCA, 107, 818, 2003)

Similar values to RO2 + NO reactions.

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3



+ NO

+ O2

+ O2

+ OH1

2

3

4

OK, ON TO STEP #4, WE CAN DO IT !!!

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 CH2O + HO2

CH3CH2CH2CH(O)CH3 + O2 CH3CH2CH2C(=O)CH3 + HO2

Rate coefficient typically about 10-14 cm3 molecule-1 s-1

So lifetime is about 20 s

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

4

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3

CH3CH2CH2CH(O)CH3

+ NO2


CH3 + CH3CH2CH2CHO

H + CH3CH2CH2C(=O)CH3


+ O2

+ O2

Ea = 13 kcal

Ea > 20 kcal

Ea = 17 kcal

+ OH

H CH3CH2CH2C(=O)CH3 + H

CH3CH2CH2 C O CH3 CH3CH2CH2CHO + CH3

CH3CHO + CH3CH2CH2

•CH2CH2CH2CH(OH)CH3

(Isomerization via 6-Member Transition State)

H3C

H2C

CH2

CH

CH3

O.

.H2C

H2C

CH2C

H2

CH

CH3

O.H2C

H2C

H

CH

OH

CH3

ISOMERIZATION

CH3CH2CH2CH2CH3


CH3CH2CH2CH(OO)CH3

CH3CH2CH2CH(O)CH3

+ NO2



+ O2

+ O2

Ea = 13 kcal

CH2CH2CH2CH(OH)CH3

Ea = 8 kcal


+ HO2 + NO

+ OH1

2

3

4

2-Pentoxy Chemistry vs. Altitude

0

2

4

6

8

10

12

14

16

18

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06

Rate (per second)

Alt

itu

de

(km

)

Reaction with O2 Isomerization

Methyl Elimination Propyl elimination

CH3CH2-O-CH2CH3

CH3CH2-O-CH()CH3 + H2O

CH3CH2-O- CH(OO)CH3

CH3CH2-O-CH(O)CH3 + NO2

CH3CH2-O-C(=O)CH3 + HO2

CH3 + CH3CH2-O-CHO

H + CH3CH2-O-C(=O)CH3

CH3CH2O + CH3CHO

+ NO

+ O2

+ O2

Ea = 7 kcal?

Ea ≤ 11 kcal?

Ea = 15 kcal?

Orlando, 2007; Cheema et al., 1999; Wallington and Japar, 1991; Eberhard et al., 1993

DIETHYL ETHER

+ OH

4

CH3CH2-O-CH2CH3

CH3CH2-O-CH()CH3 + H2O

CH3CH2-O- CH(OO)CH3

CH3CH2-O-CH(O)CH3 + NO2

CH3CH2-O-C(=O)CH3 + HO2

CH3 + CH3CH2-O-CHO

H + CH3CH2-O-C(=O)CH3

CH3CH2O + CH3CHO

+ NO

+ O2

+ O2

Ea = 7 kcal?

Ea ≤ 11 kcal?

Ea = 15 kcal?

Orlando, 2007; Cheema et al., 1999; Wallington and Japar, 1991; Eberhard et al., 1993

DIETHYL ETHER

+ OH

[CH3CH2OCH(O)CH3 ]‡

4

[ 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 !!!)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

5 7.5 10 12.5 15 17.5 20

ENERGY BARRIER

FR

AC

TIO

N O

F A

CT

IVA

TE

D R

AD

ICA

LS

SOME GENERALITIES ON ALKOXY RADICALS

1. There is almost always a reaction with O2 to produce HO2 and a carbonyl, time constant about 20 s.

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.

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

O O

OO

CH3CO

+

OO OOH

O

O2

OOH

O2

OH

NO2O

OOH

O

OH

CO2

+

CH3CHO + HCHO + HO2

OH

O2, NO

dissociation

OH O2, NO

O2, NO

isomerization

HCHO+

dissociation

O

O2

NO2NO

O2, NO

dissociation

dissociationO2

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 = F() d

(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,

DMS, NH3 nitrates, etc. (2ry POLLUTANTS)

NO NO2

OH HO2

RO

RO2

R

Parent NMHC In + Oxidized Species

Out

Nitrates, Peroxides Out

NO, HO2

NO, HO2O2

NO

O2

Unimolecular Reaction

OZONE PRODUCTION

NO2

HONO2

OZONE PRODUCTION

ATMOSPHERIC CHEMISTRY OF ORGANIC COMPOUNDS Lecture for NC A&T (part 2) March 9, 2011 John Orlando...

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