Atmospheric chemistry

1. Atmospheric chemistry Lecture 3: Tropospheric Oxidation Chemistry Dr. David Glowacki University of Bristol,UK david.r.glowacki@bristol.ac.uk

2. Yesterday… • 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: http://www.atmos.bham.ac.uk/chablis.htm

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

41. Low NOx Initiation O3 O3 O3 sunlight OH

42. Low NOx Initiation O3 O3 OH VOC RO2

43. Low NOx Termination 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. OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir 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 reactswith alkenes to forma 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.