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

1

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

2

3

photo-oxidation

formation of intermediates

transportEmissions: VOCs, NOx, SO2

solar UV radiation

Products:CO2,H2O, COO3, H2O2, CH2OH2SO4, HNO3

SOA

Increasing solubility

dry and wetdeposition

Atmospheric “Life Cycle”

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

4

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

ADD SUNLIGHT !!

5

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

ADD SUNLIGHT !!

O3 + h 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.

6

Let’s Add a Couple of More Things - N2, O2, H2O, O3, NO and NO2

ADD SUNLIGHT !!

O3 + NO NO2 + O2 (k)

NO2 + h 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

7

8

Tropospheric O3 FormationRecall 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 + OO + O2 + M O3 + M

But: There are hydrocarbons out there!(Role of NOx in ozone production)

OH + CO CO2 + HH + O2 +M HO2 + MHO2 + NO NO2 + OH

NO2 + h NO + OO + O2 + M O3 + M

______________________CO + 2 O2 + h CO2 + O3

9

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)

10

Chemistry is analogous (conceptually) to CO !

11

OH + RH R + H2OR + O2 + M RO2 + MRO2 + NO RO + NO2

RO + O2 R'C=O + HO2

HO2 + NO OH + NO2

2 (NO2 + h NO + O)2 (O + O2 + M O3 + M)

______________________RH + 4 O2 + 2 h 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) conservedPropagation !!

Do these radicals ‘propagate’ forever?

12

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

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

Emissions

Deposition

NO2 + O3 NO3 + O2

NO3 + NO2 + M N2O5 + MNO2 + NO2 + H2Oliq HONOg + HNO3liq

NOy: nitrogen “reservoir” species

NO2 + OH + M HNO3 + MNO2 + RO2 + M ROONO2 + M

NO2 + RC(O)O2 + M RC(O)OONO2 + MNO + RO2 + M RONO2 + M (0-30%)

RONO2 + hv RO + NO2 NO + OH + M HONO + M

15

Losses of HOx, ROx radicals into “reservoirs”

NO2 + OH + M HNO3 + MNO2 + RO2 + M ROONO2 + M

NO2 + RC(O)O2 + M RC(O)OONO2 + MNO + RO2 + M RONO2 + M (0-30%)

HO2 + HO2 H2O2 + O2

RO2 + HO2 ROOH

16

17

Initiation by photo-dissociationO3 + h + H2O 2 OH + O2

Oxidation of hydrocarbonsOH + RH + O2 + M ROO + H2O + M

NO NO2 conversionsROO + NO RO + NO2

O3 + NO NO2 + O2

Actual O3 formationNO2 + h + O2 O3 + NO

PropagationRO + O2 HOO + R’COHOO + NO OH + NO2

TerminationOH + NO2 + M HNO3 + MHOO + HOO + M H2O2 + O2 + MHOO + O3 OH + 2 O2

Summary of Key Steps In Tropospheric O3 Formation

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

Near-Zero NOx troposphere

OH + CO CO2 + HH + O2 +M HO2 + MHO2 + O3 2 O2 + OHHO + O3 O2 + HO2

___________________CO + O3 CO2 + O2

HO2 + HO2 H2O2

HO2 + HO H2O + O2

H2O2+hv 2 OHH2O2 + H2O(liq) H2O2(liq) 19

What if there is “too much” NOx?

Not quite this simple, but chemistry ‘suppressed’ by

OH + NO2 HNO3

20

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

22

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

23

What kinds of compounds?

• Characterized by Functional Groups– e.g. double bonds, hydroxyl, nitrate, etc.

CH=CH2-CH3 CH3OH CH3ONO2

• The presence of functional groups affects their chemistry (and hence lifetime).

24

Atmospheric VOC’s: Hydrocarbons

AlkanesCH4

CH3CH3

CH3CH2CH3

C4H10 (2 isomers)C5H12 (3 isomers)C6H14 (5 isomers)C7H16 (9 isomers)C8H18 (18 isomers)….

methaneethanepropanebutanepentanehexaneheptaneoctane….

25

Atmospheric VOC’s: Hydrocarbons

AlkenesCH2=CH2

CH2=CHCH3

…CH2=C(CH3)CH=CH3

AromaticsC6H6

C6H5CH3

C6H5(CH3)2 (3 isomers)…

TerpenesC10H16

ethene (ethylene)propene (propylene)…2-methyl 1,3 butadiene

(isoprene)

BenzeneTolueneXylenes…

-pinene, -pinene…

26

Examples of Monoterpenes

Atkinson &Arey, 2003

Natural ProductsFrom PlantsAnd Trees

C10H16

27

Examples of Sesquiterpenes

Atkinson &Arey, 2003

Natural ProductsFrom PlantsAnd Trees

C15H24

28

Atmospheric VOC’s:Substituted Hydrocarbons

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)

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)

29

OH + Hydrocarbon Reactions

Abstraction of H

•OH + CH3CH3 CH3CH2• + H2O

Addition to double bonds (actually more facile!)

•OH + CH2=CH2 HOCH2CH2•

30

NO3 + VOC Reactions

H atom abstraction:

CH3CHO + NO3 CH3CO + HNO3

Addition to double bond:

CH2=CH2 + NO3 + M •CH2CH2ONO2 + M

31

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) Products32

O O O

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

(cm3 molecule-1 s-1)

Approx. Lifetime (OH = 106

molecule cm-3)

METHANE 1.85 10-12 3360 6.4 10-15 8.4 years

33

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

(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

CH3CF3 1.06 10-12 3975 1.3 10-15 > 25 years

PROPENE 1.2 10-11 -264 2.8 10-11 3 days

2-PROPANOL 2.7 10-12 -190 5.1 10-12 2 days

34

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

35

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

36

Chemistry is analogous (conceptually) to CO !

37

OH + RH R + H2OR + O2 + M RO2 + MRO2 + NO RO + NO2

RO + O2 R'C=O + HO2

HO2 + NO OH + NO2

2 (NO2 + h NO + O)2 (O + O2 + M O3 + M)

______________________RH + 4 O2 + 2 h 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) conservedPropagation !!

38

39

40

41

42

CH3CH2CH2CH2CH3

CH3CH2CH2CH()CH3 + H2O

CH3CH2CH2CH(OO)CH3

CH3CH2CH2CH(O)CH3+ NO2

CH3CH2CH2C(=O)CH3 + HO2

+ 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 !!

REFER TO THE ALKOXY RADICAL AS RO•

43

Oxidation Schemes – IsopreneD. Taraborrelli et al. – replace with GECKO SLIDE

44

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.

45

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 !!!

Troposphere - lots of stuff

Stratosphere

46

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;

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.

The “CHAPMAN” MECHANISM (Pure oxygen chemistry)

The Only Production: O2 + h ( < 242 nm) O + OChapman 1930 O + O2 + M O3 + M

Destruction Reactions:Chapman 1930 O3 + h ( < 800 nm) O + O2

O + O3 2 O2

    

48

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:

There are loss mechanisms:

Catalytic cycles – one reactive species ‘cycles around’ to destroy more than one ozone.

From D. Jacob,

Stratospheric Ozone Chemistry• The Only Production: O2 + h ( < 242 nm) O + OChapman 1930O + O2 + M O3 + M

• Several Destruction Reactions:Pure oxygen chemistry: O3 + h ( < 800 nm) O + O2

Chapman 1930O + O3 2 O2

Catalytic Cycles:

Odd hydrogen (HOx = OH + HO2) O3 + OH O2 + HO2

Bates and Nicolet 1950 O + HO2 O2 + OHO3 + HO2 2 O2 + OH

Odd nitrogen (NOx = NO + NO2) O3 + NO O2 + NO2

Crutzen 1970 O + NO2 O2 + NO

Halogens (Cl, Br) O3 + Cl O2 + ClORowland and Molina 1974 O + ClO O2 + Cl

50

 

POLAR REGIONS

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

 51

 

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.

52

Ozone “hole” chemistryLower 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 + h Cl + Cl

HOCl + h Cl + OH

ClNO2 + h Cl + NO2 2 (Cl + O3 ClO + O2)

ClO + ClO + M ClOOCl + M ClOOCl + h Cl + Cl + O2

2 O3 3 O2

53

Ozone “hole” chemistryFrom 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)

54

Ozone “hole” chemistry

55

R. Garcia et al., JGR, 2012.