1. The Atmospheric Oxidation System:
Ox, HOx, NOx, etc.

2. 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
CH CO2 + H2O
Isoprene Other by-products, such as O3, particles, acids, DMS, NH3 nitrates, etc (2ry POLLUTANTS)
NO NO2

3. Atmospheric “Life Cycle”
solar UV radiation
Products:
CO2,H2O, CO
O3, H2O2, CH2O
H2SO4, HNO3
SOA
photo-oxidation
formation of intermediates
transport
Emissions:
VOCs, NOx, SO2
dry and wet
deposition
Increasing solubility

4. Start with a simple atmosphere – N2, O2, H2O, O3

5. Start with a simple atmosphere – N2, O2, H2O, O3
ADD SUNLIGHT !!

6. Start with a simple atmosphere – N2, O2, H2O, O3
ADD SUNLIGHT !! O3 + hn  O(1D) + O2 O(1D) + N2  O(3P) + N2 O(3P) + O2  O3 O(1D) + O2  O(3P) + O2 O(1D) + H2O  OH + OH !!! OH is central to the whole tropospheric chemistry picture !! OH + CO  OH + CH4  It acts to initiate oxidation! OH + Almost Everything  “The atmospheric cleanser” Hold this thought for now ! We’ll follow through in a min.

7. Let’s Add a Couple of More Things - N2, O2, H2O, O3, NO and NO2
ADD SUNLIGHT !!
O3 + NO  NO2 + O (k)
NO2 + hn  NO + O (jNO2)
O + O2 + M  O3 + M
______________________
Null Cycle (t ≈ 100 seconds)
(No net O3 production /loss) k[NO] [O3] = JNO2 [NO2]
[NO2]/[NO] = k[O3] / JNO2

8. Tropospheric O3 Formation Recall we have made OH
Tropospheric O3 Formation Recall we have made OH ! And we have NO and NO2 in SS Let’s add something else – a HYDROCARBON Use a surrogate for simplicity – Carbon Monoxide (CO)
Initiation by UV radiation (Levy, 1970):
O3 + h ( < 330 nm)  O*(1D) + O2
O*(1D) + H2O  OH + OH
Hydrocarbon consumption (oxygen entry point):
OH + CO  H + CO2
H + O2 + M  HOO + M Introducing HO2 (HOO), the other HOx member
Single-bonded oxygen transferred to NOx:
HOO + NO  OH + NO2
NOx gives up oxygen atoms (as before):
NO2 + h ( < 420 nm)  NO + O
O + O2 + M  O3 + M

9. But: There are hydrocarbons out there
But: There are hydrocarbons out there! (Role of NOx in ozone production)
OH + CO  CO2 + H H + O2 +M  HO2 + M HO2 + NO  NO2 + OH NO2 + hn  NO + O O + O2 + M  O3 + M ______________________ CO + 2 O2 + hn  CO2 + O3

10. So far we have only considered CO There are 1000’s of them out there More in a little bit, but CH4, C5H8, C12H26, etc etc. Let’s lump them all together and call them R-H, where R is some hydrocarbon fragment (e.g., CH3)

11. Chemistry is analogous (conceptually) to CO !
OH + RH  R + H2O
R + O2 + M  RO2 + M
RO2 + NO  RO + NO2
RO + O2  R'C=O + HO2
HO2 + NO  OH + NO2
2 (NO2 + hn  NO + O)
2 (O + O2 + M  O3 + M)
______________________
RH + 4 O2 + 2 hn  R'C=O + H2O + 2 O3
Introducing RO2 (e.g., CH3OO), does same job as HO2
Again, O3 produced, HOx (OH, HO2), ROx (R, RO, RO2), NOx (NO, NO2) conserved
Propagation !!

12. Do these radicals ‘propagate’ forever?
NO !! We have seen “INITIATION” (Production of OH from O3, H2O and sunlight)
“PROPAGATION” (All the nasty chemistry stuff on the previous slide)
“TERMINATION” - removal of our reactive species (NO, NO2, OH, HO2, RO2)

13. NOx chemistry in the troposphere
Our NO and NO2 are converted into
(somewhat temporary) ‘reservoirs’
- typical lifetimes of hrs., days
Things like HNO3 (nitric acid) can be removed by scavenging (clouds), deposition

14. NOx chemistry in the troposphere
Emissions
Our NO and NO2 are converted into
(somewhat temporary) ‘reservoirs’
- typical lifetimes of hrs., days
Things like HNO3 (nitric acid) can be removed by scavenging (clouds), deposition
Deposition

15. NOy: nitrogen “reservoir” species
NO2 + OH + M  HNO3 + M NO2 + RO2 + M ROONO2 + M NO2 + RC(O)O2 + M RC(O)OONO2 + M NO + RO2 + M  RONO2 + M (0-30%) RONO2 + hv  RO + NO2 NO + OH + M  HONO + M
NO2 + O3  NO3 + O2
NO3 + NO2 + M N2O5 + M
NO2 + NO2 + H2Oliq  HONOg + HNO3liq

16. Losses of HOx, ROx radicals into “reservoirs”
NO2 + OH + M  HNO3 + M NO2 + RO2 + M ROONO2 + M NO2 + RC(O)O2 + M RC(O)OONO2 + M NO + RO2 + M  RONO2 + M (0-30%) HO2 + HO2  H2O2 + O2 RO2 + HO2  ROOH

17. Summary of Key Steps In Tropospheric O3 Formation
Initiation by photo-dissociation
O3 + h + H2O  2 OH + O2
Oxidation of hydrocarbons
OH + RH + O2 + M  ROO + H2O + M
NO  NO2 conversions
ROO + NO  RO + NO2
O3 + NO  NO2 + O2
Actual O3 formation
NO2 + h + O2  O3 + NO
Propagation
RO + O2  HOO + R’CO
HOO + NO  OH + NO2
Termination
OH + NO2 + M  HNO3 + M
HOO + HOO + M  H2O2 + O2 + M
HOO + O3  OH + 2 O2

18. We have seen that a mix of hydrocarbons (CO, others) and nitrogen oxides (NO, NO2) in sunlight
 CHEMISTRY!
Making Ozone (other stuff, like nitric acid and particles)
Relative availability of Hydrocarbons and NOx is critical to efficiency.
Let’s consider an extreme case – no (or at least very little) NOx

19. Near-Zero NOx troposphere
OH + CO  CO2 + H H + O2 +M  HO2 + M HO2 + O3  2 O2 + OH HO + O3  O2 + HO2 ___________________ CO + O3  CO2 + O2 HO2 + HO2  H2O2 HO2 + HO  H2O + O2 H2O2+hv  2 OH H2O2 + H2O(liq)  H2O2(liq)

20. What if there is “too much” NOx?
Not quite this simple, but chemistry ‘suppressed’ by OH + NO2  HNO3

21. Figure from Grewe et al., Atmos. Env., 2012

23. Organics in the Atmosphere
Some definitions:
VOC - Volatile Organic Compounds
Hydrocarbons – just HYDROgen and CARBON
(e.g., CH4, C2H6, …)
Oxygenates – alcohols, aldehydes, ketones… CH3OH CH3CH=O CH3C(=O)CH3

24. What kinds of compounds?
Characterized by Functional Groups
e.g. double bonds, hydroxyl, nitrate, etc.
CH=CH2-CH CH3OH CH3ONO2
The presence of functional groups affects their chemistry (and hence lifetime).

25. Atmospheric VOC’s: Hydrocarbons
Alkanes
CH4
CH3CH3
CH3CH2CH3
C4H10 (2 isomers)
C5H12 (3 isomers)
C6H14 (5 isomers)
C7H16 (9 isomers)
C8H18 (18 isomers) ….
methane
ethane
propane
butane
pentane
hexane
heptane
octane ….

26. Atmospheric VOC’s: Hydrocarbons
Alkenes
CH2=CH2
CH2=CHCH3 …
CH2=C(CH3)CH=CH3
Aromatics
C6H6
C6H5CH3
C6H5(CH3)2 (3 isomers)
Terpenes
C10H16
ethene (ethylene)
propene (propylene) …
2-methyl 1,3 butadiene (isoprene)
Benzene
Toluene
Xylenes
a-pinene, b-pinene

27. Examples of Monoterpenes
Natural
Products
From Plants
And Trees
C10H16
Atkinson &
Arey, 2003

28. Examples of Sesquiterpenes
Natural
Products
From Plants
And Trees
C15H24
Atkinson &
Arey, 2003

29. Atmospheric VOC’s: Substituted Hydrocarbons
Organic hydroperoxides, -OOH
methyl hydroperoxide, CH3(OOH)
Organic peroxy acids, -CO(OOH)
peracetic, CH3CO(OOH)
Organic nitrates, -ONO2
methyl nitrate, CH3(ONO2)
Ethyl nitrate, CH3CH2(ONO2)
Peroxy nitrates, -OONO2
methyl peroxy nitrate, CH3(OONO2)
Acyl peroxy nitrates, -CO(OONO2)
PAN, CH3CO(OONO2)
Alcohols, -OH
methanol, CH3OH
ethanol, CH3CH2OH
Aldehydes, -CHO
formaldehyde,CH2O
acetaldehyde, CH3CHO
Ketones, -CO-
acetone, CH3COCH3
MEK, CH3COCH2CH3
Carboxylic acids, -CO(OH)
formic, HCO(OH)
acetic, CH3CO(OOH)

30. OH + Hydrocarbon Reactions
Abstraction of H
•OH + CH3CH3  CH3CH2• + H2O
Addition to double bonds (actually more facile!)
•OH + CH2=CH2  HOCH2CH2•

31. NO3 + VOC Reactions H atom abstraction: Addition to double bond:
CH3CHO + NO3  CH3CO + HNO3
Addition to double bond:
CH2=CH2 + NO3 + M  •CH2CH2ONO2 + M

32. O3 + Hydrocarbon Reactions
Ozone addition across double bond
O3 + CH2=CH2  CH2 – CH2  CH2O + (CH2OO)*
Fate of excited Criegee diradical:
(CH2OO)*  CO + H2O
 CO2 + H2
 CO2 + 2 H
 …
+ M  CH2OO (stabilized Criegee diradical)
CH2OO + (H2O, NO, NO2, SO2)  Products O

33. Activation Energy (calories)
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)
Rate Constant at 298 K
Approx. Lifetime
(OH = 106 molecule cm-3)
METHANE
1.85  10-12
3360
6.4  10-15
8.4 years

34. Activation Energy (calories)
SO, IN GENERAL: The more substituted (complicated) the molecule, the weaker the C-H bond, and the faster the rate coefficient
n-PENTANE: CH3CH2CH2CH2CH3 PROPENE: CH3CH=CH2
2-PROPANOL:CH3CH(OH)CH3
COMPOUND
A-Factor
(cm3 molecule-1 s-1)
Activation Energy (calories)
Rate Constant at 298 K
Approx. Lifetime
(OH = 106 molecule cm-3)
METHANE
1.85  10-12
3360
6.4  10-15
8.4 years
ETHANE
8.61  10-12
2080
2.6  10-13
45 days
n-PENTANE
1.81  10-11
900
3.9  10-12
3 days
CH3CF3
1.06  10-12
3975
1.3  10-15
> 25 years
PROPENE
1.2  10-11
-264
2.8  10-11
2-PROPANOL
2.7  10-12
-190
5.1  10-12
2 days

35. 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)

36. 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)

37. Chemistry is analogous (conceptually) to CO !
OH + RH  R + H2O
R + O2 + M  RO2 + M
RO2 + NO  RO + NO2
RO + O2  R'C=O + HO2
HO2 + NO  OH + NO2
2 (NO2 + hn  NO + O)
2 (O + O2 + M  O3 + M)
______________________
RH + 4 O2 + 2 hn  R'C=O + H2O + 2 O3
Introducing RO2 (e.g., CH3OO), does same job as HO2
Again, O3 produced, HOx (OH, HO2), ROx (R, RO, RO2), NOx (NO, NO2) conserved
Propagation !!

38. OK, Let’s look at ORGANIC SPECIES in more detail:
Thankfully, oxidation of all hydrocarbons follow similar pattern. There are FOUR BASIC STEPS. Let’s use methane as our example.
CH4

39. OK, Let’s look at ORGANIC SPECIES in more detail:
Thankfully, oxidation of all hydrocarbons follow similar pattern. There are FOUR BASIC STEPS. Let’s use methane as our example.
CH4
Starts with reaction with OH: OH
CH3 + H2O

40. OK, Let’s look at ORGANIC SPECIES in more detail:
Thankfully, oxidation of all hydrocarbons follow similar pattern. There are FOUR BASIC STEPS. Let’s use methane as our example.
CH4
Starts with reaction with OH: OH
CH3 + H2O
The alkyl radical adds O2, to make a peroxy radical O2
CH3O2

41. OK, Let’s look at ORGANIC SPECIES in more detail:
Thankfully, oxidation of all hydrocarbons follow similar pattern. There are FOUR BASIC STEPS. Let’s use methane as our example.
CH4
Starts with reaction with OH: OH
CH3 + H2O
The alkyl radical adds O2, to make a peroxy radical O2
CH3O2
Peroxy radical often reacts with NO, making an alkoxy NO
radical. (There are other pathways, see later).
CH3O + NO2

42. OK, Let’s look at ORGANIC SPECIES in more detail:
Thankfully, oxidation of all hydrocarbons follow similar pattern. There are FOUR BASIC STEPS. Let’s use methane as our example.
CH4
Starts with reaction with OH: OH
CH3 + H2O
The alkyl radical adds O2, to make a peroxy radical O2
CH3O2
Peroxy radical often reacts with NO, making an alkoxy NO
radical. (There are other pathways, see later).
CH3O + NO2
4. Alkoxy radical reacts with O2, to make a carbonyl O2
compound. (There are other pathways, see later).
CH2O + HO2

43. IN GENERAL, REFER TO THE PARENT COMPOUND AS R-H1
CH3CH2CH2CH2CH3
IN GENERAL, REFER TO THE PARENT
COMPOUND AS R-H
+ OH
CH3CH2CH2CH()CH H2O 2
REFER TO THE ALKYL RADICAL
AS R•
+ O2 3
REFER TO THE PEROXY
RADICAL AS RO2•
CH3CH2CH2CH(OO)CH3
REFER TO THE ALKOXY
RADICAL AS RO•
+ NO
CH3CH2CH2CH(O)CH3+ NO2 4
+ O2
NOTE ALSO: THESE BASIC REACTIONS
PROPOGATE RADICALS !!
CH3CH2CH2C(=O)CH3 + HO2

44. Oxidation Schemes – Isoprene D. Taraborrelli et al
Oxidation Schemes – Isoprene D. Taraborrelli et al. – replace with GECKO SLIDE

45. 6. Stratospheric Chemistry (we’re almost done)
Central Feature of the Stratosphere – The “Ozone Layer”
“Good Ozone” – shields us from harmful UV rays
More about where it all comes from in a slide or two.

46. What is in our atmosphere?
Troposphere also acts as a ‘filter’ for what can reach the stratosphere:
Many reactive species (almost entirely) removed in the troposphere
Only non-reactive, long-lived species (generally months-to-years)
survive the “trip” to the stratosphere
e.g.,
Methane (CH4) Nitrous Oxide (N2O) Water (H2O) Halocarbons (e.g., CF2Cl2)
OZONE !!!
Stratosphere
Troposphere
- lots of stuff

47. 6. Stratospheric Chemistry (we’re almost done)
Central Feature of the Stratosphere – The “Ozone Layer”
“Good Ozone” – shields us from harmful UV rays
More about where it all comes from in a slide or two.
Raw Material to work with – what makes it from the troposphere – stable species not (completely) destroyed in the trop. :
H2O – ‘freeze-drying’ – start with about 2 ppm? Source of HOx
CH4 – 10 year lifetime – some transport to the stratosphere, source of ROx
N2O – no gas-phase destruction in troposphere – source of stratospheric NOx
Halocarbons – CFCs (CF2Cl2) – no tropospheric destruction;
Natural compounds (CH3Cl, CH3Br) – partial destruction in troposphere;

48. 6. Stratospheric Chemistry (we’re almost done)
Central Feature of the Stratosphere – The “Ozone Layer”
“Good Ozone” – shields us from harmful UV rays
More about where it all comes from.
The “CHAPMAN” MECHANISM (Pure oxygen chemistry)
The Only Production: O2 + hn (l < 242 nm)  O + O
Chapman O + O2 + M  O3 + M
Destruction Reactions:
Chapman O3 + hn (l < 800 nm)  O + O2
O + O3  2 O2

49. THE (QUANTITATIVE) PROBLEM: The Chapman Mechanism predicts twice as much ozone as is observed, with an incorrect altitude distribution.
OK, so what is the problem:
From D. Jacob,
There are loss mechanisms:
Catalytic cycles – one reactive species ‘cycles around’ to destroy more than one ozone.

50. Stratospheric Ozone Chemistry
The Only Production: O2 + hn (l < 242 nm)  O + O
Chapman O + O2 + M  O3 + M
Several Destruction Reactions:
Pure oxygen chemistry: O3 + hn (l < 800 nm)  O + O2
Chapman O + O3  2 O2
Catalytic Cycles:
Odd hydrogen (HOx = OH + HO2) O3 + OH  O2 + HO2
Bates and Nicolet O + HO2  O2 + OH
O3 + HO2  2 O2 + OH
Odd nitrogen (NOx = NO + NO2) O3 + NO  O2 + NO2
Crutzen O + NO2  O2 + NO
Halogens (Cl, Br) O3 + Cl  O2 + ClO
Rowland and Molina O + ClO  O2 + Cl

51.   POLAR REGIONS
Big surprise – mid 1980’s – Oops, no ozone in the lower stratosphere in Antarctic Spring !

52. PSCs – surfaces for ‘activation’ of reactive ClOx
POLAR REGIONS
Big surprise – mid 1980’s – Oops, no ozone in the lower stratosphere in Antarctic Spring !
J. Anderson et al., JGR, vol. 94 , 1989
PSCs – surfaces for ‘activation’ of reactive ClOx
from J. Anderson et al., JGR JG.

53. Ozone “hole” chemistry
Lower Stratosphere “denitrified” and chlorine activated
ClONO2 + HCl(s)  Cl2 + HNO3(s)
ClONO2 + H2O(s)  HOCl + HNO3(s)
N2O5 + HCl(s)  ClNO2 + HNO3(s)
N2O5 + H2O(s)  2 HNO3(s)
Cl2 + hn  Cl + Cl
HOCl + hn  Cl + OH
ClNO2 + hn  Cl + NO (Cl + O3  ClO + O2)
ClO + ClO + M  ClOOCl + M
ClOOCl + hn  Cl + Cl + O2
2 O3  3 O2

54. Ozone “hole” chemistry
From Wikipedia –
The Montreal Protocol on Substances that Deplete the Ozone Layer … is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances believed to be responsible for ozone depletion. The treaty was opened for signature on 16 Sept. 1987, and entered into force on 1 January 1989.
Types of compounds phased out:
CF2Cl2 (Freon-12; F-12) CFCl3 (Freon-11; F-11)
No tropospheric loss (no H-atoms for OH to abstract).
Replacement compounds:
CF3CFH2 (HFC-134a) C4F9OCH2CH3 (HFE-7200)
No chlorine!! - so no “ozone-depletion potential”;
Contain H-atoms, so (some) tropospheric removal. (NB: also greenhouse gases)

55. Ozone “hole” chemistry
R. Garcia et al., JGR, 2012.