1. Atmospheric chemistry
Lecture 3:
Tropospheric Oxidation Chemistry
Dr. David Glowacki
University of Bristol,UK

2. Yesterday… Today… We discussed photochemistry and kinetics
The earth’s atmosphere is a huge low temperature chemical reactor with variable temperature, pressure, and actinic flux
All of these variables affect the rates of individual chemical reactions
Today…
Atmospheric chemistry is largely driven by free radical chain reactions
We will discuss some of the important individual chemical reactions that are important in the troposphere

3. Why is atmospheric chemistry important?
Human activity is changing the composition of the atmosphere
Regulatory policy requires an understanding of pollutant impact
Atmospheric pollutants impact living organisms
Health
Vegetation (e.g., farming) & animals
Climate change
Atmospheric pollutants & their subsequent chemistry are responsible for:
Acid rain
Photochemical smog (e.g., arctic haze)
Vegetation & animals
Ozone hole

4. Atmospheric chemistry and Climate Change
Atmospheric chemistry plays an important role in radiative forcing processes
Source:
IPCC 4th assessment

5. Tropospheric Oxidation Starts with OH
O3 + h  O1D + O2
O1D + M  O1D + M
O1D + H2O  2OH
Degradation of atmospheric pollutants starts with the OH radical
OH is often called ‘the detergent of the atmosphere’
OH is very reactive because it has an unpaired electron:
O-H
Measuring OH is hard! There’s not much of it, and it doesn’t live for long
Tropospheric oxidation results in ground level O3, which is a greenhouse gas harmful to health
FAGE OH detection instrument in Halley Base, Antarctica
See:

6. O3 Photolysis makes OH
O3 + hn g O2 + O(1D)

7. OH sinks OH Sinks: oxidation of reduced species
CO + OH g CO2 + H
CH4 + OH g CH3 + H2O
HCFC + OH g H2O + …
Major
OH sinks
GLOBAL MEAN [OH] ~ 1.0x106 molecules cm-3

8. Initiation
High NOx O3
sunlight
NO2 NO OH
HO2
VOC
RO2 RO NO
NO2

9. Initiation
High NOx
NO2 NO OH
HO2
VOC
RO2 RO NO
NO2

10. Initiation
High NOx
NO2 NO OH
HO2
VOC
RO2 RO O2 NO
NO2

11. Propagation
High NOx
NO2 NO OH
HO2
VOC
RO2 RO NO
NO2

12. O3 High NOx VOC Ozone Formation NO2 NO OH HO2 RO2 RO NO NO2 O2
sunlight

13. O3 High NOx VOC Propagation NO2 NO OH HO2 oxidation product RO2 RO O2

14. O3 High NOx VOC Propagation NO2 NO OH HO2 oxidation product RO2 RO NO

15. O3 O3 High NOx VOC Ozone Formation sunlight O2 NO2 NO OH HO2
oxidation product
VOC
RO2 RO NO
NO2 O3

16. High NOx O3
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2 O3

17. O3 High NOx VOC Run Cycle NO2 NO OH HO2 oxidation product RO2 RO NO

18. High NOx O3
sunlight
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2

19. High NOx
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2

20. High NOx
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO O2 NO
NO2

21. High NOx
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2

22. O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO NO NO2 O2
sunlight

23. High NOx
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO O2 NO
NO2 O3

24. High NOx
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2 O3

25. O3 O3 High NOx VOC sunlight O2 NO2 NO OH HO2 oxidation product RO2 RO

26. High NOx O3
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO O2 NO
NO2 O3

27. High NOx O3
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2 O3

28. O3 O3 O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO NO NO2 O2
sunlight O3

29. High NOx O3
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO O2 NO
NO2 O3 O3

30. High NOx O3
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2 O3 O3

31. O3 O3 O3 O3 High NOx VOC sunlight O2 NO2 NO OH HO2 oxidation product

32. O3 O3 O3 O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO O2 NO

33. High NOx O3 O3
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO NO
NO2 O3 O3

34. O3 O3 O3 O3 O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO NO
sunlight O3 O3

35. O3 O3 O3 O3 O3 O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO

36. O3 O3 O3 O3 O3 O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO

37. O3 O3 O3 O3 O3 O3 O3 High NOx VOC sunlight O2 NO2 NO OH HO2
oxidation product
VOC
RO2 RO NO
NO2 O3 O3 O3 O3

38. O3 O3 O3 O3 O3 O3 High NOx VOC NO2 NO OH HO2 oxidation product RO2 RO

39. Ozone Production O3 O3 O3 O3 O3 O3 High NOx VOC NO2 NO OH HO2
oxidation product
VOC
RO2 RO NO
NO2 O3 O3 O3

40. Chemistry of ozone formation
sunlight O2
sunlight
NO2 NO OH
HO2
oxidation product
VOC
RO2 RO O2 O2 NO
NO2 O2 O3
sunlight

41. Initiation
Low NOx O3 O3 O3
sunlight OH

42. Initiation
Low NOx O3 O3 OH
VOC
RO2

43. Termination
Low NOx O3 O3 OH
VOC
RO2
HO2
ROOH

44. General VOC oxidation scheme
O3 + h  O1D + O2
O1D + H2O  2OH
OH + RH (+O2)  RO2 + H2O
RO2 + NO  NO2 + RO
RO + O2  HO2 +R’CHO
HO2 + NO  OH + NO2
NO2 + h  NO + O; O + O2  O3
OVERALL
NOx + VOC + sunlight  ozone
The same reactions can also lead to formation of secondary organic aerosol (SOA)

45. PO3  [NO] & independent of [RH]
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed
NOx limited
PO3  [NO] & independent of [RH]
VOC limited
PO3  [NO2]-1; PO3  [RH]

46. Polluters: Mobile Transportation: Generates NOx and VOC.
Reductions focus on catalytic converters and fuel additives as well as congestion abatement strategies
Stationary industrial sources of VOC and NOx:
Reductions involve scrubbing of pollutants from chimney stacks.
Biogenic Emissions:
Generate VOCs, no feasible reduction strategy,
Can propose urban landscapes that reduce emissions

47. NOx sources

48. Spatial distribution of NOx emissions

49. NOx sinks & transport NOx lifetime ~1 day NOx sinks – primarily HNO3
HNO3 is water soluble
PAN allows locally produced NOx to be transported on global scales

50. Other oxidizing species
NO3
NO2 + O3  NO3 + O2
NO3 is rapidly lost in the day by photolysis and reaction with NO ( NO2), so that its daytime concentration is low. It is an important night time oxidant. It adds to alkenes to form nitroalkyl radicals which form peroxy radicals in the usual way. O3
Ozone reacts with alkenes to form a carbonyl + an energised Criegee biradical. The latter can be stabilised or decompose. One important reaction product is OH: O3 reactions with alkenes can act as a source of OH, even at night.

51. VOC removal by reaction with OH
VOC Lifetime with respect to OH:
Atmospheric distribution depends on lifetime. The Northern Hemisphere (NH) is a major source of anthropogenic pollutants. CH4 is distributed globally with a slight NH/SH difference. Isoprene is found only close to its sources.
The oxidising capacity of the atmosphere refers to its capacity to remove VOCs and depends on [OH] (and the concentrations of other oxidants like O3 and NO3
OH + CH × 10-3
OH + CO × 10-1
OH + isoprene 1.1 × 102
OH + ethane 2.4 × 10-1
k(298K) in units of
10-12 cm3 molecule-1 s-1

52. HCHO + OH (+O2)  HO2 + CO + H2O
CH4 Oxidation Scheme
CH OH (+O2)  CH3O H2O
CH3O NO  CH3O NO2
CH3O O2  HO HCHO
HO NO  OH NO2
HCHO OH (+O2)  HO CO H2O
HCHO hn  H CO
HCHO hn (+2O2)  2HO CO
Note:
2 × (NO  NO2) conversions
HCHO formation provides a route to HO2 radical formation.

53. Global budget for methane (Tg CH4 yr-1)
Sources:
Natural 160
Anthropogenic 375
Total 535
Natural Sources:
wetlands, termites, oceans…
Anthropogenic Sources:
natural gas, coal mines, enteric fermentation, rice paddies
Sinks:
Trop. oxidation 445
by OH
Transfer to 40
stratosphere
Uptake by soils 30
Total 515
Notes:
The rate of oxidation is k5[CH4][OH], where the concentrations
are averaged over the trop.
2. Concentrations of CH4 have increased from 800 to 1700 ppb since pre-industrial times
3. Methane is a greenhouse gas.

54. HISTORICAL TRENDS IN METHANE
Historical methane trend
Recent methane trend
Recent measurements at Mace
Head in W Ireland.
1mg m-3 = 0.65 ppb
NB – seasonal variation –
higher in winter

55. GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements
Seasonal dependence – higher in winter than summer (maximum in NH correlates with minimum in SH).
NH concentrations > SH – main sources are in SH; slow transport across the intertropical conversion zone

56. General description of a chemical mechanism

57. Can we model oxidation results of other VOCs? …The MCM
Constructed by University of Leeds, in collaboration with Imperial College and UK Met Office
Explicit mechanism, based on a protocol which describes the chemistry. Includes reactions of OH, NO3 and O3 and photolysis. For development protocol see: M.E.Jenkin et al. Atmos. Env., 1997, 31, 81.
Describes the oxidation of 123 VOCs, based on the UK emissions inventory.
It can be accessed via the web:
The MCM is used by the UK Department of the Energy and Climate Change (DECC) to help develop its air quality strategy.