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ATM 507 – Lecture 15

Text reading – Chapter 6

Homework #5 – Due Nov. 8

Paper – Due Dec. 11

Today’s topics –Tropospheric Oxidation

Chemistry

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Outline of Study I. Radical Sources

II. Photooxidation of CO in the presence of NOX (PROTOTYPE MECHANISM)

III. Formaldehyde Photooxidation

IV. Methane Photooxidation

V. Odd Hydrogen Chemistry

VI. Aldehyde Oxidation

VII. Alkane Oxidation

VIII. Alkene Oxidation

IX. Aromatic Oxidation

X. Isoprene Oxidation

XI. Alcohols and Organic Acids

XII. Summary of Organic/NOX Chemistry

Why spend time on this?

• Future predictions (projections) are based on model calculations, which are based on rate constants and mechanisms from lab and theoretical studies.

• There is a whole field within Atm. Chem. that seeks to understand and explain the details of how species interact.

• What we know and what we learn does make a difference in how well the models describe the real world!

• That said, this is not the primary focus of this course!

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Oxidation Mechanisms for Aldehydes/Ketones

• Aldehydes and ketones are subgroups of carbonyls – hydrocarbons that contain a C=O group.

• Aldehydes (alkanals) have an HC=O (or CHO) group at the end.

• Ketones (alkanones) have functional groups (denoted R or R’) on each side of the C=O group.

• Formaldehyde (HCHO or H2CO) is the simplest aldehyde.

• This is the general pattern – carbonyls are photolyzed and/or react with OH.

HCHO hv

hv

CO + H2

CO + 2HO2 OH CO + HO2 + H2O

Photolysis I – no effect on radicals

Photolysis II – increase in radicals, potential O3 formation OH Rxn – Potential O3 formation

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Acetaldehyde Photooxidation • As noted – initial step is either photolysis or

OH reaction (see figure)

• Note especially the production of peroxyacetyl

nitrate or PAN.

• Properties of PAN:

1. Eye irritant and plant phytotoxicant

2. Significant component of urban photochemical

smog.

3. Storage reservoir for NOX and PA radicals – one of

very few non-photolytic free radical sources.

4. Slow loss due to surface reaction.

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Acetaldehyde Oxidation Mechanism

minor

minor

Methyl peroxy

methoxy

Methyl nitrate

acetyl

Peroxy acetyl

PAN

Radicals like CH3 and HCO keep reacting

Note NO to NO2 conversions, how CH3 results from both channels.

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General aldehyde-OH reaction mechanism

• RCHO + OH → RCO + H2O

• RCO + O2 (+M) → RC(O)O2 (+M)

• RC(O)O2 + NO2 RC(O)O2NO2

The competition for the RC(O)O2 comes from

• RC(O)O2 + NO → RC(O)O + NO2

(peroxy → oxy radical along with NO → NO2)

Followed by RC(O)O → R + CO2

Note that for the overall mechanism, some reaction pathways involve one or more NO → NO2 conversions (i.e. potential ozone production), and some pathways don’t.

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Acyl radical formation

Acyl peroxy radical

peroxyacyl nitrate

Acyl oxy radical

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Acetone Photooxidation

• Acetone is the simplest ketone.

• Again there is photolysis and OH attack.

• Photolysis channel larger, but both important.

• Both pathways produce peroxy radicals (photolysis produces two rapidly).

• PAN production occurs via the photolysis channel

• Through the photolysis channel, acetone is an important source of HOX radicals in the upper troposphere.

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Acetone Photooxidation

Methyl glyoxal

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Alkane photooxidation

• Alkanes are “fully saturated” hydrocarbons – all

carbon-carbon bonds are single bonds.

• Alkanes are not strong absorbers of sunlight

(very energetic photons are require to break the

strong C-C and C-H bonds).

• Oxidation of these compounds begins with “OH

attack” – abstraction reaction with OH.

• Reaction rates are relatively slow (~ 10 times

slower than reactions with alkenes, or those

hydrocarbons with carbon-carbon double bonds.)

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Propane Oxidation - Example CH3CH2CH3 + OH → CH3CHCH3+ H2O (R = CH3CHCH3)

CH3CHCH3 + O2 → CH3CH(O2)CH3 (RO2 formation)

CH3CH(O2)CH3 + NO → CH3CH(O)CH3+ NO2 (RO formation)

CH3CH(O)CH3+ O2 → CH3C(O)CH3+ HO2 (carbonyl)

Abbreviate the mechanism by folding in the fast reactions

CH3CH2CH3 + OH → CH3CH(O2)CH3+ H2O

CH3CH(O2)CH3 + NO → CH3C(O)CH3+ NO2 + HO2

NET: CH3CH2CH3 + OH + NO → CH3C(O)CH3+ NO2 + HO2

If we further assume that all the HO2 reacts according to HO2 + NO → OH + NO2, then

NET: CH3CH2CH3 + OH + 2NO → CH3C(O)CH3+ OH + 2NO2

M fast

O2

O2

fast

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General Alkane Mechanism

1. RH + OH → R + H2O

2. R + O2 (+M) → RO2 (+M) – fast

[RH + NO3 → R + HNO3 – minor, nighttime, later]

3. a. RO2 + NO → RO + NO2

b. → RONO2 – alkyl nitrate

4. RO2 + NO2 (+M) ↔ ROONO2 – peroxy nitrate

5. RO2 + HO2 → ROOH + O2 - peroxide

6. a. R1R2CHO2 + R1R2CHO2 → 2R1R2CHO + O2

b. → R1R2CHOH + R1R2CO + O2

c. → R1R2CHOOCHR1R2 + O2

Reactions 3-6 involve peroxy radicals

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Oxy radical reactions (from 3a) 7. RO + O2 → R’CHO + HO2

8. a. RCH2O → R + HCHO

b. RR1CHO → R + R1CHO

c. RR1R2CO → R + R1C(O)R2

9. Isomerization – Section 6.10.1, p 244 – the unpaired electron initially associated with the oxygen (oxy radical) migrates to a carbon atom and the oxygen atom picks up the hydrogen.

Reactions 7-9 involve oxy radicals.

Unimolecular decompositions

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General Features of the Alkane Mechanism

1. OH abstracts an H atom preferentially at the weakest C-H bond, usually an internal secondary or tertiary H.

2. Alkyl nitrate formation (Reaction 3b) increases with the size of the hydrocarbon.

3. Peroxynitrate formation does not increase with the size of the hydrocarbon, and larger peroxynitrates decompose back to reactants readily.

4. Peroxy-peroxy radical reactions play an important role – and even more so in NOX limited conditions.

5. Oxy radical reaction pathways include A. Reaction with O2

B. Unimolecular decomposition

C. Isomerization

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Butane Photooxidation

Peroxy radicals,

Oxy radicals,

Nitrates,

Aldehydes

Ketones

2nd Edition: Figure 6.13.

Methyl Ethyl Ketone

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Alkene (or Olefin) Oxidation • Alkenes make up about 10% of the hydrocarbon

mix in urban areas.

• They are more reactive than alkanes – due to the carbon-carbon double bond.

• Like alkanes, photolysis is unimportant.

• Reaction with OH is very important (addition to the double bond instead of abstraction), but not the only mechanism

• Reaction with O3 – ozonolysis

• Reaction with NO3

• In each case the oxidizing species adds to the electron-rich double bond.

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Alkene Oxidation Examples • Simplest alkene is ethylene or ethene

C2H4 + OH → HOCH2CH2 (O atom end of OH “adds” at C=C double bond)

HOCH2CH2 + O2 → HOCH2CH2O2

HOCH2CH2O2+ NO → HOCH2CH2O+ NO2

• For this “simple” oxy radical, there are two important product channels

HOCH2CH2O → HCHO + CH2OH

HOCH2CH2O + O2 → HOCH2CHO + HO2

Also the CH2OH radical reacts with O2

CH2OH + O2 → HCHO + HO2

• Net result – oxygenated hydrocarbons (aldehydes in this case) and NO → NO2 conversions (via HO2 reaction).

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fast

0.72

0.28

fast

peroxy radical

oxy radical

Unimolecular decomposition

reaction w/ O2 ; product is glycol aldehyde

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OH-Propene Mechanism • There are more options here – the OH can add

to either end of the C=C double bond – and the products are different.

• CH3CH=CH2 + OH(+M) → CH3CHCH2OH

→ CH3CH(OH)CH2

• With available NOX, the intermediate products in each case are formaldehyde and acetaldehyde.

• Decomposition dominates for the oxy radical.

0.65

0.35

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O3-Alkene Reaction Mechanism • The ozone molecule adds “across” the

double bond in the alkene molecule to form a “primary ozonide” (molozonide).

• The ozonide is relatively unstable, and decomposes to a carbonyl species (which retains one O atom double bonded to a carbon), and a biradical species (with two unpaired electrons).

• The biradical is called a “Criegee biradical” and is formed with excess energy – it is “energy-rich” – and unstable.

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O3 – Alkene Reaction (cont.) • We have seen how the carbonyl species is

further oxidized, but what happens to the energy-rich Criegee biradical? The important pathways are collisional stabilization and unimolecular decomposition.

• OH radicals have been observed with fractional yields (based on the initial alkene concentration) between 0.18 and 0.90. This is a “dark” source of HOX radicals (i.e., no photons involved as part of the reaction sequence).

This channel is negligible

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OH radical yields

Key point: Whenever there is adequate ozone and alkene hydrocarbons, there will be production of OH radicals.

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Biradical stabilization

Stabilized biradicals react with H2O, CO, SO2, NO, NO2, and HCHO to form aldehydes and organic acids. See the text for details. An interesting detail is that alcohols and organic acids start to appear as products.

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NO3 (Nitrate Radical) Chemistry

• Gaseous NO3 is formed via

1. NO2 + O3 → NO3 + O2

• It can react with NO2 (“holding cycle”)

2. NO2 + NO3 ↔ N2O5 (equilibrium)

• It is rapidly destroyed via photolysis

3a. NO3 + hv (λ < 700 nm) → NO + O2

3b. NO3 + hv (λ < 580 nm) → NO2 + O

• and via reaction with NO

4. NO + NO3 → 2NO2

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NO3 Chemistry (cont.) • Because of the rapid photolysis, NO3 only builds

up to significant levels during nighttime.

• N2O5 is a reservoir for NOX (two NO2 molecules are required to produce each N2O5), so heterogeneous loss of N2O5 could be (and is!) a significant pathway for the conversion of NOX to HNO3.

5. N2O5 + H2O(l or s) → 2HNO3

• (Most of the N2O5 formed in reaction 2 listed above simply decomposes back to NO2 and NO3, but the small fraction converted to HNO3 via 5 has an important impact on the chemistry.)

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NO3 – Alkene Reaction • Like OH (and similar to O3), NO3 adds to the

electron-rich double bond

• This forms a β-nitratoalkyl radical which 1. Adds O2 to form a “nitrato” peroxy radical

2. Reacts with NO to make the corresponding oxy radical

3. The oxy radical mainly reacts with O2 or decomposes, with a minor NO2 reaction channel.

• For the propene + NO3 reaction, products are oxygenated nitrates and aldehydes.

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Propene-NO3 Mechanism