CFC Numbering System Add 90 to the number; the three digits represent the numbers of C, H, F atoms; make up the rest of the "unused bonds" with Cl.

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Aubrey Newman
5 years ago
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1 The CFC Story CFC - chlorofluorocarbon Saturated carbon compounds with carbon bonded to chlorine and fluorine Were/are used as: Refrigerants Propellants for foams Propellants for aerosols CFC Numbering System Add 90 to the number; the three digits represent the numbers of C, H, F atoms; make up the rest of the "unused bonds" with Cl. Example: CFC = 202, 2 C, 0 H, 2 F Saturated, so 2(#C)+2=6 bonds to C total Cl = 6-(0+2) = 4 C 2 F 2 Cl 4 Week3-1

2 CFCs do not absorb radiation < ~250 nm (not present in troposphere) Chemically unreactive: no tropospheric sinks Slow migration to stratosphere; globally well mixed In stratosphere, slow reaction with t ½ many decades: e.g. CF 2 Cl 2 + h< 6 CF 2 Cl + Cl weakest bond breaks Hence CFCs are a source of stratospheric chlorine atoms which are catalysts for destroying stratospheric ozone Photolysis initiates the chain reactions; up to 104 cycles per chlorine atom CFCs increase an existing "sink strength", thereby lowering the steady state concentration of ozone. [originally, it was thought that this was a new sink] Catalytic reactions are affected by: 1) temporary reservoirs 2) between cycle interactions 3) null cycles 4) initiation/ termination reactions Week3-2

3 The Road to the Montreal Protocol CFCs first discovered in the atmosphere 1974 Many were increasing at 6%/year in the late 1970s Travel to the stratosphere takes 5-10 years N. American and European ban on aerosol use 1978 Montreal Protocol 1987 with a ban on "hard" CFCs + CH 3 CCl 3 + CCl 4 January 1, year extension for developing countries (originally by 2010) Halons (e.g., CF 3 Br) phased out January 1, 1994 The Montreal Protocol is historic because it is the first instance of international action on the basis of a future anticipated environmental threat. Mario Molina and Sherwood Rowland received the 1995 Nobel Prize in Chemistry for their seminal work on stratospheric ozone depletion: Nature, 1974, 249, 810 Week3-3

4 Why have CFCs been banned? CFC lifetimes 100+ years: photolysis is inefficient Loss of stratospheric ozone allows more UV-B radiation to reach the earth's surface Relationship between UV radiation and skin cancer (melanoma deaths among white US males) High UV at noon, in summer, high altitude, in the tropics Week3-4

5 CFC Replacement Compounds HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons): desired properties are chemical stability, low toxicity, appropriate boiling point CFC replacements must undergo tropospheric oxidation Since CFCs deplete stratospheric ozone replacements should avoid or minimize Cl atoms Since hydrocarbons oxidize in the atmosphere replacement compounds should include H atoms. As will be discussed in Chapter 3, HCFCs and HFCs will react with tropospheric OH radicals. Example: CH 3 CHF 2 + OH 6 CH 3 CF 2 + H 2 O followed by further reactions of CH 3 CF 2 CFCs and their replacements are also greenhouse gases. Fully fluorinated compounds must be avoided (no hydrogen atoms to react with OH) Week3-5

6 Ozone Depleting Potential (ODP) CFCs, HFCs, HCFCs: Week3-6

7 ODP vs Global Warming Potential: Compound ODP (CFCl 3 = 1) GWP (CO 2 = 1) HCFC-22 CHF 2 Cl HCFC-123 CF 3 CHCl HCFC-124 CF 3 CHFCl HFC-125 CF 3 CHF HFC-134a CF 3 CH 2 F HCFC-141b CH 3 CFCl HCFC-142b CH 3 CF 2 Cl HFC-152a CH 3 CHF Note: HFCs have ODP = zero; GWPs taken from the IPCC Report and /60fr52357.pdf+global+warming+potential+HCFC&hl=en (20 year horizon). They are much larger than the values in the text Week3-7

8 Where are we now? Halogen loading has peaked, ozone levels should begin to recover. Chlorine burden near the stratopause is decreasing, driven by tropospheric decreases of methyl chloroform. Bromine is increasing. CFCs no longer considered a major player in recent total ozone variations. Certain greenhouse gases (i.e. N 2 O and NO x )will become key factors. Present O 3 concentrations still only 50% of pre 1970 values. Response of industry to CFC phase-out aerosol propellants: CO 2 ; CH 3 OCH 3 -H 2 O; C 4 H 10 -CH 2 Cl 2 foam blowing agents: [HCFC-22]; HCFC-141b; cyclopentane refrigerants: HFC-134a; hydrocarbons (in Europe) Note: concern about CF 3 CO 2 H as a highly stable breakdown product of several HFCs, including HFC-134a Week3-8

9 Figure from Week3-9

10 Polar Ozone Holes (the precipitating event for the Montreal Protocol) "Polar Sunrise" experiments have shown that this effect is also seen in the Arctic (but not every year) The curious effect of the break-up of the Ross Antarctic ice shelf (January-March 2002): D.W.J. Thompson and S. Solomon, Science, 2002, 296, 895 attributed to stratospheric cooling from the polar ozone hole having two effects: tropospheric warming; changing polar vortex W Antarctic becomes warmer, E Antarctic cooler. The other cycles for ozone depletion: Week3-10

11 X = Cl, NO, OH, H CH 3 Cl: a natural source of atmospheric chlorine; long tropospheric t ½ NO: not much transport from the troposphere (short t ½ ); mostly made in the upper atmosphere, but injection into stratosphere from e.g., volcanic eruptions, supersonic aircraft Another route is photolysis of N2O N 2 O + h< 6 N 2 + O* (excited oxygen atom) O* + N 2 O 6 2 NO Note: O* also comes from high energy dissociation of ozone O 3 + h< 6 O* + O 2 * OH arises from photolytic cleavage of H 2 O in the stratosphere (not troposphere) and from reaction of excited state oxygen atoms with water vapour O* + H 2 O 6 2 OH H atom chemistry is most important in the upper stratosphere, where p(o 2 ) is less, because: H + O 2 6HO 2 Week3-11

12 Chemistry in the Troposphere: Photochemical Smog The atmosphere as an oxidizing system Tendency of organic components (natural and anthropogenic) to be oxidized to CO 2 + H 2 O. These are called VOCs (volatile organic compounds) Formation of toxic intermediates en route to CO 2 + H 2 O Formation of ozone as a byproduct of VOC oxidation Central role of the hydroxyl radical, OH C, in initiating oxidation Interaction between VOC oxidation and NO x chemistry Conditions for photochemical smog formation strong sunlight temperature > ~20 C polluted air, i.e., greater than background levels of both VOCs and NO x (Note Table 3.3, text, p. 81) Week3-12

13 Tropospheric formation and reactions of OH radical Formation by photolysis of ozone in the UV-B region ( nm) The reaction: O 3 + h< (8 < 325 nm) 6 O 2 * + O* O* + H 2 O 6 2 OH O 3 + h< 6 O 2 + O also occurs but does not lead anywhere because ground state O cannot react with H 2 O Tropospheric concentration of OH global average = molec cm -3 (day/night; winter/summer; poles/tropics) experimental measurements very difficult, but day-time maxima in southern Canada ~ 10 5 molec cm -3 in winter and ~ 10 7 molec cm -3 in summer with high ozone in darkness, concentration of OH falls to near zero (no photolysis of ozone) because OH is so reactive that it disappears in seconds Week3-13

14 Characteristic reactions of OH radicals 1. Abstracts a hydrogen atom Prototype reaction: CH 4 + OH 6 CH 3 + H 2 O 2. Adds to a double bond Prototype reactions: SO 2 + OH 6 HO-SO 2 C H 2 C=CH 2 + OH 6 HOCH 2 -CH 2 C 3. Terminates with another odd-electron species Prototype reaction: NO 2 + OH 6 HNO 3 Before considering oxidation of VOCs further we must consider NO x chemistry Be careful to distinguish the OH C radical from the hydroxide ion OH -. Their chemistries are quite different! Week3-14

15 Question from old mid-term: The reaction: O 3 (g) + NO (g) 6 O 2 (g) + NO 2 (g) has k = 2.3x10-12 e -1450/T cm 3 molecule -1 s -1. Calculate the RATE of the reaction above at 295K, under the conditions that the concentration of ozone is 100 ppbv and that of NO is 1.0 ppbv [just divide c(o 3 ) by 100 to get c(no)]. Answer: rate = k[no][o 3 ] Calculate k: k = 2.3x10-12 e -1450/295 cm 3 molecule -1 s -1 = 1.7x10-14 cm 3 molecule -1 s -1 Calculate c(o 3 ) = P/RT = 100x10-9 atm / [1.36x10-22 cm 3 atm molecule -1 K -1 x 295K] = 2.5x10 12 molecule cm -3 Hence c(no) = 2.5x10 10 molecule cm -3 Reaction rate: = k[no][o 3 ] =(1.7x10-14 cm 3 molecule -1 s -1 )(2.5x10 12 molecule cm -3 ) x (2.5x10 10 molecule cm -3 ) =1.1x10 9 molecule cm -3 s -1 Week3-15

16 The formation of ozone: the NO/NO 2 cycle Clean troposphere: no significant parallel VOC oxidation Formation: 1. NO 2 + h< (8 < 400 nm) 6 NO + O 2. O + O 2 6 O 3 fast reaction Destruction: 3. NO + O 3 6 NO 2 + O 2 Net reaction = "null cycle" - sunlight is degraded to heat Steady state: NO 2 formed and destroyed at equal rates k 3 [NO][O 3 ] = J(NO 2 )[NO 2 ] (note reaction (2) is fast) or [O 3 ] = J(NO 2 )[NO 2 ] [A] k 3 [NO] Steady state - not equilibrium! - achieved in minutes in strong sunlight. [O 3 ]ss depends on solar intensity of solar radiation, on latitude, season, and time of day. Because they are rapidly interconverted, the sum {[NO] + [NO 2 ]} is known as NO x. Question: What happens at night? More NO 2, less NO 2? Week3-16

17 Photochemical rate constants: J(X) Important tropospheric examples: O 3 + h< (8 < 325 nm) 6 O 2 + O NO 2 + h< (8 < 420 nm) 6 NO + O To estimate the rate of such a process we have to know: the strength (intensity) of the photon flux, I 0 : photons per second per cm 2 of the Earth's surface. I 0 is a function of 8 and zenith angle, Z the efficiency with which the photons are absorbed to give excited molecules, e.g., F [NO 2 ]. F is the absorption cross section (cm 2 per molecule), and is wavelength dependent (compare Beer's law) the efficiency with which the excited NO 2 molecules dissociate, N, the quantum yield of dissociation, also wavelength dependent. N is dimensionless N = number of molecules dissociated number of photons absorbed rate of photolysis (of NO 2 ) = (I 0 F N)[NO 2 ] (I 0 F N) 8 = J, units photons per molecule per second (effectively s -1 ). Depends on 8 and Z, and must be summed over all 8 relevant to the particular zenith angle Week3-17

18 Let us reconsider Equation [A] [O 3 ] = J(NO 2 )[NO 2 ] [A] k 3 [NO] Actual concentrations of O 3 in urban air are larger than Eq. [A] predicts, there must be other sources Photodissociation of NO 2 is the only important reaction by which O 3 is formed in the troposphere. In the polluted troposphere, ozone is formed as a byproduct of VOC oxidation. Specifically NO is converted to NO 2 by reactions that do not consume O 3 Null Cycle 1. NO 2 + h< (8 < 420 nm) 6 NO + O 2. O + O 2 6 O 3 3. NO + O 3 6 NO 2 + O 2 ˆ Need another oxidant for NO besides O 3 : peroxy radicals R-O-O C Every NO molecule that is oxidized to NO 2 while bypassing Reaction [3] yields one additional ozone molecule, because the "extra" NO 2 is photolyzed to restore the steady state. Week3-18

19 Peroxy radical R-O-O C chemistry RO 2 (or HO 2 ) + NO 6 RO + NO 2 Modified scheme 1. NO 2 + h< (8 < 400 nm) 6 NO + O 2. O + O 2 6 O 3 3. NO + RO 2 6 NO 2 + RO Net reaction: RO 2 + O 2 6 RO + O 3 Conclusion: If NO is oxidized by O 3, then the steady state predicted by Equation [A] is valid. If NO is oxidized by peroxy radicals, there will be an excess of O 3 compared with Equation [A]. Source of peroxy radicals addition of O 2 to a carbon-based radical Example: CH 4 + OH 6 CH 3 + H 2 O CH 3 + O 2 6 CH 3 -O-O Week3-19

20 Summary comments so far: NO x is indispensable to the formation of O 3 in the troposphere (the only other tropospheric source of ozone is downwards transport from the stratosphere) Peroxy radicals (ROO) are formed as intermediates in the oxidation of VOCs In polluted air (NO x present), ROO radicals oxidize NO; O 3 is formed when the NO:NO 2 steady state is restored O 3 is a byproduct of VOC oxidation in polluted air Week3-20

21 Oxidation of Methane to Formaldehyde as a prototype [only] VOC (oversimplified) 1. CH 4 + OH C 6 H 2 O + CH 3 C OH much more reactive than RO 2, HO 2, NO 3 2. CH 3C + O 2 6 CH 3 -O-O C 3A. CH 3 -O-O C + NO 6 CH 3 -O C + NO 2 3B. CH 3 -O-O C + RO 2 C 6 CH 3 -O C + RO C + O 2 R = CH 3 or H 4. CH 3 -O C + O 2 6 CH 2 =O + HO 2 CH 3 O reacts much faster with O 2 than it abstracts H from CH 4 Reactions 3A and 3B are in competition - both always occur Reaction 3B predominates (low [NO x ]; unpolluted air): 1. CH 4 + OH C 6 H 2 O + CH 3 C 2. CH 3 C + O 2 6 CH 3 -O-O C 3B. CH 3 -O-O C + HO 2 C 6 CH 3 -O C + OH C + O 2 peroxy radicals tend to self-annihilate rather than abstract hydrogen 4. CH 3 -O C + O 2 6 CH 2 =O + HO 2 Net reaction I: CH 4 + O 2 6 CH 2 =O + H 2 O Under these conditions, methane production produces no ozone as a byproduct Week3-21

22 Reaction 3A predominates (high [NO x ], polluted air) 1. CH 4 + OH C 6 H 2 O + CH 3 C 2. CH 3C + O 2 6 CH 3 -O-O C 3A. CH 3 -O-O C + NO 6 CH 3 -O C + NO 2 4. CH 3 -O C + O 2 6 CH 2 =O + HO 2 Net reaction II: CH O 2 + OH + NO 6 CH 2 =O + NO 2 + H 2 O + HO 2 This is inconvenient, because of the reactive intermediates that are present in the equation use of cycles in which the concentrations of reactive intermediates are eliminated. Note: NO 6 NO 2 ; OH 6 HO 2 Recycle NO 2 6 NO NO 2 + O 2 + h< 6 NO + O 3 Recycle HO 2 6 OH HO 2 + NO 6 OH + NO 2, then NO 2 + O 2 + h< 6 NO + O 3 New net reaction II: CH O 2 6 CH 2 =O + 2 O 3 + H 2 O Methane oxidation produces ozone as a byproduct Week3-22

23 Oxidation of VOCs 70% of OH reacts with CO in the unpolluted troposphere CO naturally present at approx. 0.1 ppmv tropospheric gas-phase sink = oxidation by OH OH + CO 6 H + CO 2 H + O 2 6 HO 2 M Are all VOCs created equal? No!! The main issue is their reactivity with OH radicals. Fast reaction more tendency to form ozone locally, especially if t ½ is only a few hours. Generally, alkenes and aromatics (OH reacts by addition) are more reactive than alkanes (OH reacts by H abstraction). Biogenic hydrocarbons (alkenes) are highly reactive. Week3-23

24 Photochemical Ozone Creation Potential (POCP) Was developed by the Organization for Economic Cooperation and Development (OECD) as an approximate approach to rank VOCs in terms of ozone forming ability based on the emission of equal masses of the VOC and ethylene POCP = k(voc) M(C 2 H 4 ) 100 k(c 2 H 4 ) M(VOC) at 298 K, k(ethene) = cm 3 molecule -1 s -1 Compound koh POCP ethylene propylene hexene toluene p-xylene styrene cyclohexane butanone isoprene methylpentane Week3-24

25 Sources of nitrogen oxides in the troposphere The primary pollutant is always NO: N 2 + O 2 W 2NO )H/ = +180 kj Natural background of NO x is < 1 ppbv (lightning). All combustion processes increase [NO x ] above background US data: transportation 40-45%; power generation, 30-35%; industrial, 20% Low [O 3 ] in major cities at night 1. NO 2 + h< (8<400 nm) 6 NO + O daytime only 2. O + O 2 6 O 3 3. NO + O 3 6 NO 2 + O 2 day and night NO formation continues, but ozone formation shuts down, the concentration of ozone declines Low [O 3 ] in major cities during pollution episodes High NO emissions "eat up" O 3 NO + O 3 6 NO 2 + O 2 NO 2 "eats up" OH NO 2 + OH 6 6 HNO 3 (g) Low OH means a low rate of VOC oxidation, hence low production of O 3 as a byproduct Highest O 3 occurs down-wind of city centre - in the suburbs Week3-25

26 The problem of advection Vancouver or Los Angeles: due to westerly air flow, "up-wind" air is "clean". Excess ozone above the nocturnal boundary layer dissipates by morning, when mixing brings in clean air. Southern Ontario: upwind air brings high [O 3 ] from Detroit, Windsor, Ohio Valley. Air above the nocturnal boundary is ozone-rich. [O 3 ] at ground level falls at night due to NO + O 3 reaction, but morning mixing raises [O 3 ] because air aloft is still ozone-rich. Seen in data from CN Tower. The effect of temperature Ozone episodes are promoted by high temperature The reason is that PAN (peroxyacetyl nitrate) and related congeners act as storage reservoirs for NO x Formation of PAN (text, p. 82) CH 3 CH=O + OH 6 CH 3 C=O + O 2 6 CH 3 C(=O)O-O + NO 2 6 CH 3 C(=O)O-O-NO 2 PAN has a large E act for decomposition (113 kj/mol) CH 3 C(=O)O-O-NO 2 6 CH 3 C(=O)O-O + NO 2 Tying up NO 2 inhibits O 3 formation (NO 2 =precursor) Tropospheric chemistry at night NO 3 C (nitrate radical) initiates chain reactions, but is much less reactive than OH Week3-26

27 NO 3C + R-H 6 HNO 3 + R C 6 etc NO C 3 is indirectly made from NO 2 NO 2 + O 3 6 NO 3 + NO 2 W N 2 O 5 NO 3 is readily decomposed by sunlight, but N 2 O 5 is not Emission controls Concepts: lower emissions of NO x and VOCs by adjusting airfuel ratios and using catalytic converters (text, pp ) The failure to control ground level ozone (US National Research Council Report, 1991) due to underestimation of VOC inventories Concept: either NO x or VOCs can be the limiting reactant Week3-27

Measurements of Ozone. Why is Ozone Important?

Measurements of Ozone. Why is Ozone Important? Anthropogenic Climate Changes CO 2 CFC CH 4 Human production of freons (CFCs) Ozone Hole Depletion Human production of CO2 and CH4 Global Warming Human change of land use Deforestation (from Earth s Climate: