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

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Presentation on theme: "Atmospheric chemistry Lecture 3: Tropospheric Oxidation Chemistry Dr. David Glowacki University of Bristol,UK"— Presentation transcript:

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

2 Yesterday… We discussed photochemistry and kinetics The earths 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 4 th assessment

5 Tropospheric Oxidation Starts with OH 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! Theres not much of it, and it doesnt live for long Tropospheric oxidation results in ground level O 3, which is a greenhouse gas harmful to health O 3 + h O 1 D + O 2 O 1 D + M O 1 D + H 2 O 2OH FAGE OH detection instrument in Halley Base, Antarctica See:

6 O 3 Photolysis makes OH O 3 + h O 2 + O( 1 D)

7 OH sinks OH Sinks: oxidation of reduced species CO + OH CO 2 + H CH 4 + OH CH 3 + H 2 O HCFC + OH H 2 O + … Major OH sinks GLOBAL MEAN [OH] ~ 1.0x10 6 molecules cm -3

8 Initiation VOC OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x sunlight O3O3

9 Initiation OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x VOC

10 Initiation VOC OHHO 2 RO 2 RO NONO 2 NONO 2 O2O2 High NO x

11 Propagation OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x VOC

12 Ozone Formation OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x O3O3 O2O2 sunlight VOC

13 Propagation OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x O3O3 O2O2 oxidation product VOC

14 Propagation OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 VOC

15 Ozone Formation OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 sunlight O2O2 VOC

16 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x O3O3 oxidation product O3O3 VOC

17 Run Cycle OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 VOC

18 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 sunlight VOC

19 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product VOC

20 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O2O2

21 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product VOC

22 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O2O2 sunlight VOC

23 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O2O2 VOC

24 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 VOC

25 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 sunlight O2O2 VOC

26 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O2O2 O3O3 O3O3

27 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 VOC

28 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O2O2 sunlight O3O3 O3O3 VOC

29 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O2O2 O3O3 O3O3 VOC

30 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 O3O3 VOC

31 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 sunlight O2O2 O3O3 O3O3 VOC

32 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O2O2 O3O3 O3O3 O3O3 O3O3

33 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 O3O3 O3O3 VOC

34 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O2O2 sunlight O3O3 O3O3 O3O3 O3O3 O3O3 VOC

35 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O2O2 O3O3 O3O3 O3O3 O3O3 O3O3 VOC

36 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 O3O3 O3O3 O3O3 O3O3 VOC

37 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 sunlight O2O2 O3O3 O3O3 O3O3 O3O3 O3O3 VOC

38 OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 O3O3 O3O3 O3O3 O3O3 O2O2 VOC

39 Ozone Production VOC OHHO 2 RO 2 RO NONO 2 NONO 2 High NO x oxidation product O3O3 O3O3 O3O3 O3O3 O3O3 O3O3

40 Chemistry of ozone formation VOC oxidation product OHHO 2 RO 2 RO NONO 2 NONO 2 O2O2 O2O2 sunlight O3O3 O2O2 O2O2 O3O3

41 Initiation OH sunlight Low NO x O3O3 O3O3 O3O3

42 Initiation VOC OH RO 2 Low NO x O3O3 O3O3

43 Termination VOC OH RO 2 Low NO x HO 2 ROOH O3O3 O3O3

44 General VOC oxidation scheme O 3 + h O 1 D + O 2 O 1 D + H 2 O 2OH OH + RH (+O 2 ) RO 2 + H 2 O RO 2 + NO NO 2 + RO RO + O 2 HO 2 +RCHO HO 2 + NO OH + NO 2 NO 2 + h NO + O; O + O 2 O 3 OVERALL NO x + VOC + sunlight ozone The same reactions can also lead to formation of secondary organic aerosol (SOA)

45 OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed NOx limited P O3 [NO] & independent of [RH] VOC limited P O3 [NO 2 ] -1 ; P O3 [RH]

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

47 NO x sources

48 Spatial distribution of NOx emissions

49 NO x sinks & transport NO x lifetime ~1 day NO x sinks – primarily HNO 3 HNO 3 is water soluble PAN allows locally produced NO x to be transported on global scales

50 NO 3 NO 2 + O 3 NO 3 + O 2 NO 3 is rapidly lost in the day by photolysis and reaction with NO ( NO 2 ), 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. O 3 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: O 3 reactions with alkenes can act as a source of OH, even at night. Other oxidizing species

51 VOC Lifetime with respect to OH: Atmospheric distribution depends on lifetime. The Northern Hemisphere (NH) is a major source of anthropogenic pollutants. CH 4 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 O 3 and NO 3 VOC removal by reaction with OH k(298K) in units of cm 3 molecule -1 s -1 OH + CH × OH + CO 2.4 × OH + isoprene1.1 × 10 2 OH + ethane2.4 × 10 -1

52 CH 4 + OH (+O 2 ) CH 3 O 2 + H 2 O CH 3 O 2 + NO CH 3 O + NO 2 CH 3 O + O 2 HO 2 + HCHO HO 2 + NO OH + NO 2 HCHO + OH (+O2) HO 2 + CO + H 2 O HCHO + h H 2 + CO HCHO + h (+2O 2 ) 2HO 2 + CO Note: 2 × (NO NO 2 ) conversions HCHO formation provides a route to HO 2 radical formation. CH 4 Oxidation Scheme

53 Global budget for methane (Tg CH 4 yr -1 ) Sources: Natural160 Anthropogenic375 Total535 Natural Sources: wetlands, termites, oceans… Anthropogenic Sources: natural gas, coal mines, enteric fermentation, rice paddies Sinks: –Trop. oxidation445 by OH –Transfer to 40 stratosphere –Uptake by soils 30 Total515 Notes: 1.The rate of oxidation is k 5 [CH 4 ][OH], where the concentrations are averaged over the trop. 2. Concentrations of CH 4 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. 1 g 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 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, NO 3 and O 3 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. Can we model oxidation results of other VOCs? …The MCM


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