1. Spectroscopy and Photochemistry AOSC 620 R. Dickerson Copyright © 2013 R. R. Dickerson1

2. Outline Additional details on spectroscopy and photochemistry as they relate to atmospheric chemistry. Direct measurements of photolysis rate coefficients (frequencies). Tropospheric ozone. Copyright © 2013 R. R. Dickerson2

3. Copyright © 2010 R. R. Dickerson & Z.Q. Li 3 Spectroscopy - The study of the interaction of substances with electromagnetic radiation. The energy can be very great such as that of gamma rays or relatively weak such as that of microwaves. Different substances have such differing spectra that spectroscopy is usually used for positive identification. For example when new elements were being discovered the visible emission spectra were used for confirmation. Finlayson - Pitts, Chapters 2 & 3 McEwan & Phillips, Chapter 1 Wayne, Chapter 2.6, 3.1 - 3.3 Seinfeld, Chapt. 4.1 Recap from AOSC 620

4. Copyright © 2010 R. R. Dickerson & Z.Q. Li 4 Photochemistry - The study of chemical reactions caused by the absorption of light. Laws of Photochemistry 1. Only light absorbed by a molecule or atom can effect a chemical change. 2. Absorption of light is a one quantum process therefore the sum of the efficiencies of the primary processes must be unity. This law holds for atmospheric processes, but not for some laboratory processes in which the photon flux is so great that a second photon can be absorbed before the energy from first photon is expelled.

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12. Copyright © R. R. Dickerson12 So what are those funny symbols behind the O atoms and O 2 molecules? Term Symbols. Spectroscopy: A Quick Qualitative Description Term symbols show the energy state of atoms and molecules, as described by the quantum numbers. Atomic Quantum Numbers: n – principal quantum number. Value: 1, 2, 3,.... Tells which shell of an atom the e - resides. The farther from the nucleus the higher the n. l the azimuthal quantum number. Value: 0 to n-1. Describes the orbital angular momentum of the shape of the orbital. s – the spin quantum number. Value: ±½. j – the total (spin plus azimuthal) quantum number. Important for heavier atoms.

13. Copyright © R. R. Dickerson13 Spectroscopy: A Quick Qualitative Description, cont. Energy states of Molecules: Molecular Quantum Numbers  – the azimuthal quantum number. Value: 0 to n-1. Orbital angular momentum s – the spin quantum number. Value: ±½. Same as in atoms. J – rotational quantum number. Value: 1, 2, 3,.... Tells which shell of an atom the e - resides. The farther from the nucleus the higher the n. – vibrational quantum number. Value: 1, 2, 3,.... K – vertical component of the total angular momentum. This QN only exists for polyatomic molecules. g/u – gerade/ungerade; symmetry terms. Reflection through the center of symmetry of molecule. +/- – plus/minus; symmetry terms. Reflection through the plane of symmetry of molecule. Only for diatomics.

14. Copyright © R. R. Dickerson14 Start 10/28/14 Internal Energy of Molecules E total = E rot + E vib + E elect The equipartition principle says that the total energy of a molecule will be the sum of the internal energy terms (rotational, vibrational, and electronic) and the external (translational) energy. Rotational energy can be expressed as: E rot = B J(J + 1) Where B = h / (8  2 Ic), often in units of cm -1, with I as the moment of inertia. Vibrational energy of an anharmonic oscillator can be expressed as: E vib = h vib ( + ½) – h vib ( + ½) 2 + h vib ( + ½) 3 + … Where vib is a constant dependent on the bond strength and length.

15. Copyright © R. R. Dickerson15 Term Symbols for Atoms and Molecules S L j S  ± g/u Where S = 2s + 1. When the value of S is 1, 2, 3, the spectra appear as singlets, doublets, triplets etc. L or  = 0 1 2 3 4 5 … Atoms = S P D F G H … Molecules =  Atoms and molecules tend toward the lowest energy levels. Finding the lowest levels for molecules is complicated, but for atoms: 1. Lowest n 2. Highest l 3. Highest s (no two electrons in the same shell until they are all occupied by at least one electron). 4. Lowest j

16. Copyright © R. R. Dickerson16 Dr. Salawitch asked: Why don’t O( 1 D) atoms relax to O( 3 P) in the troposphere and stratosphere? (wait for answer) Where do O( 1 D) atoms relax to O( 3 P)? and O ( 1 S 0 ) relax to O( 1 D 2 ) ?

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19. Copyright © 2011 R. R. Dickerson19 Transitions in oxygen atoms.

20. Copyright © R. R. Dickerson20 Selection Rules for Atomic Transitions  n = 0, 1, 2, … (no restrictions)  l = ± 1  j = 0, ± 1  S = 0 This is the strongest rule: no multiplicity change.  S ≠ 0 is a “forbidden” transition. Let us examine O atoms as an example.

21. Copyright © 2013 R. R. Dickerson21 Electrons in ground state Oxygen atoms: O( 3 P).

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23. 23 Fraunhofer Lines in the solar spectrum

24. Copyright © 2013 R. R. Dickerson & Z.Q. Li 24 The Fraunhofer lines in the solar spectrum are a good example of absorption spectroscopy. Elements in the solar and terrestrial atmospheres absorb radiation. They have funny historical names not to be confused with spectroscopic designations. D 1 & D 2 are Na doublets, a is O 2 ; C, F, G', and h are H-atoms.

25. Copyright © R. R. Dickerson25 Selection Rules for Molecular Transitions   = 0, ±1  J = ±1 in monatomic molecules 0, ±1 in polyatomic molecules  ±1 for fundamental vibrations and rotations ±2, ±3… for overtones (Frank-Condon principle for vibronic transitions)  K = 0 for polyatomic molecules only. g and u, no change. +/- must change.  S = 0 This is the strongest rule: no multiplicity change.  S ≠ 0 is a “forbidden” transition. Ross already examined O 2 molecules as an example.

26. Copyright © R. R. Dickerson26 For a purely rotational transition, the molecule must have a permanent dipole. N 2 and O 2 have no long-wave IR purely rotational spectra while CO, NO, HCl, and H 2 O do and are thus greenhouse gases. For a combination vibration/rotation, the molecule must have at least an induced dipole. CO 2 and CH 4 have easily induced dipoles. The stronger the dipole the greater the absorption coefficient.

27. Copyright © 2013 R. R. Dickerson & Z.Q. Li 27 Line shapes and Energy TransitionWavelength (  m) Energy (kcal/mole) Natural Line shape Pure rotation  301Very sharp Vibrations (with rotations) 1-301-10Thin Electronic0.1 – 110-250Broad What causes these line shapes?

28. Copyright © 2013 R. R. Dickerson28 Line Shapes Natural broadening, an inherent property of all atoms and molecules, is the result of the Heisenberg uncertainty Principle.  E  t = h/2   N = 2 /(2  ct)  N = (2  t) -1 The slowest transitions (rotations) must be accompanied by the least uncertainty in energy and are thus sharpest. For similar types of transitions the line width depends on the stability of the upper level. In the emission of light from an excited molecule, if the higher energy state is stable it will have a long lifetime and a small energy spread leading to sharp lines. Conversely if the excited state is unstable and the emission happens in a short time the line will be relatively broad.

29. Copyright © 2011 R. R. Dickerson29 Line Shapes Doppler broadening, caused by thermal motions toward or away from the observer, is the same as thermal broadening. an inherent property of all atoms and molecules, is the result of the Heisenberg uncertainty Principle.  D   RT  ½ Pressure broadening, caused by collisions between molecules, is also called Lorentz broadening. Collisions perturb the energy level of excited molecules and generally reduce their energy thus broadening and red shifting the lines. Ozone in the troposphere is subject to more pressure broadening than in the stratosphere and thus has broader (and asymmetric) absorptions lines, allowing ozone near the tropopause to absorb radiation that passes through the stratospheric ozone maximum. The 9.6  m band of O 3 adds to the greenhouse effect.

30. Copyright © 20110 R. R. Dickerson 30 Spectroscopy of Simple Molecules Example 1. HCl HCl has a strong dipole and strong transitions near 3.5 mm. There is only one degree of vibration freedom, and the observed transition corresponds to = 0  = 1. Rotations have such a low energy that they are already excited at room temperature with the maximum J = 3 and J = 12 common. In diatomics,  J = 0 is forbidden and there is no Q branch. R branch P branch

31. Copyright © 2011 R. R. Dickerson31 Energy levels associated with the IR Spectrum of HCl Centered at 3.5  m ↑ Big Gap  Selection rules:  J = ± 1, not 0 for diatomics  v = ± 1

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33. Copyright © 2013 R. R. Dickerson & Z.Q. Li 33 Transmission spectrum of CO 2 This is the bend near 15  m; there is a a Q-Branch because  J = 0 is allowed. Strong absorption means CO 2 is a greenhouse gas and NDIR spectroscopy is a great technique for detection. How are the wings related to temperature?

34. Copyright © 2013 R. R. Dickerson & Z.Q. Li 34 Energy levels in molecular oxygen, O 2 Ground state is 3  g

35. Copyright © 2013 R. R. Dickerson & Z.Q. Li 35 O 2 + hv   2O( 3 P) Schumann-Runge bands  E ≥ 40,000 cm -1 or ≤ 250 nm. Potential Energy Curves for O 2 ≥ O 2 + hv   O( 3 P) + O( 1 D) Herzberg band  E ≥ 57,000 cm -1 or ≤ 175 nm.

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38. 38 According to the Spin Conservation Rule (Wayne 1991, p. 86-94) the products of ozone ( 1 A) must both be singlets or both triplets. This is also critical for O( 1 D) and OH production. Spin angular momentum sums vectorially: Products |S O2 + S O |, |S O2 + S O – 1|,…|S O2 – S O | = 2, 1, 0 for 3 P + 3 S, but can only be zero for 1 D + 1 , so the latter is favored.

39. Copyright © 2013 R. R. Dickerson & Z.Q. Li 39 Absorption Spectrum of Ozone.

40. Copyright © 2013 R. R. Dickerson & Z.Q. Li 40 Atmospheric radiation absorption as a fnx of wavelength. Chappuis

41. Copyright © 2013 R. R. Dickerson41 The Dobson Spectrometer. Courtesy of: Ulf Köhler, DWD Hohenpeissenberg

42. Copyright © 2013 R. R. Dickerson & Z.Q. Li 42 Spectroscopy and Photochemistry Take Home Messages 1.The spectra of atoms and molecules are related to their ability to interact with electromagnetic radiation, and to their shape and structure. 2.We use the observed spectra to determine the energy levels and geometry of atoms and molecules. 3.Extraterrestrial radiation is absorbed by the atmosphere except in window regions such as the visible and IR near 10  m. 4.Transitions and reactions are influenced by selection rules, esp. spin conservation. 5.The energy and lifetime set the natural line shape: a.Rotations are slow, low energy, and very sharp. b.Vibrations are intermediate. c.Electronic transitions are very fast, high energy, and broad.

43. Copyright © 2013 R. R. Dickerson & Z.Q. Li 43 Spectroscopy and Photochemistry Take Home Messages, cont. 1.Oxygen:Schumann Runge Continuum <175 nm strong allowed. Schumann Runge Bands < 200 nm Herzberg Continuum < 242 nm forbidden weak. 2.Ozone:Hartley ~250 nm, allowed, strong. Huggins < forbidden, weaker ~330 nm Chappuis ~ 600 nm Forbidden, weak. 3. The production of OH and thus all of atmospheric chemistry depends strongly on the wavelength dependent absorption of UV radiation.

44. International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) NCAR Marshall Field Site, 39°N 105°W, elevation: 1.8 km; June 15–19, 1998 Objectives: j [NO 2  NO + O], j [O 3  O 2 + O( 1 D)], spectral actinic flux. Measurements by 21 researchers from around the world. Photolysis Frequency of NO 2 : Measurement and Modeling During the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI), R. E. Shetter, W. Swartz, et al., J. Geophys. Res., 108(D16), 10.1029/2002JD002932, 2003.

45. UMD j NO2 Actinometer Schematic NO 2 + h  NO + O

46. Copyright © R. R. Dickerson 201346 Problem for the student: Show that for 1.00 ppm NO 2, 1.00 atm pressure, exposure times of 1.00 s, and j(NO 2 ) values of 10 -2 s -1 the errors to: from complicating reactions are less than 1%. 1. O + O 2 + M → O 3 + M k 1 = 6.0 x10 -34 cm 6 s -1 2. O 3 + NO → NO 2 + O 2 k 2 = 1.9×10 –14 cm 3 s -1 3. O + NO 2 → NO + O 2 k 3 = 1.04×10 – 11 cm 3 s -1

47. Trailer UMD Actinometer

48. inside on top quartz photolysis tube

49. UMD j NO2 Actinometer Data

50. Copyright © 2009 R. R. Dickerson50 DIRTY AIR (3') OH + CO  H + CO 2 (4') H + O 2 + M  HO 2 + M (5') HO 2 + NO  NO 2 + OH (6') NO 2 + h  NO + O (7') O + O 2 + M  O 3 + M ------------------------------------------------- (3'-7') CO + 2 O 2  CO 2 + O 3 NET

51. Smog Machine Smog O 3 PAN etc NO 2 NO NO 2 CO 2, H 2 O, HNO 3 NOx, VOCs

52. Copyright © 2009 R. R. Dickerson52 EKMA. Empirical Kinetic Modeling Approach, or EKMA. See Finlayson & Pitts page 892.

53. Copyright © 2012 R.R. Dickerson based on Chameides et al., 1992 53 Urban ~1990 Rural~1990 How fast do precursor pollutants make ozone (ppb/hr)? Where is the Balt/Wash area? (boundary layer ) Smog chamber and modeling results on O 3 formation rates. VOC’s (reactivity) Where is Western MD?

54. Copyright © 2013 R. R. Dickerson54 Reactions in Solution (Also called multiphase or heterogeneous reactions) Dr. Salawitch also showed that the reaction N 2 O 5 + H 2 O → 2HNO 3 Has favorable enthalpy and Gibbs Free Energy, but proceeds only in the condensed (aqueous) phase. Why does it not go in the gas phase? This reaction involves breaking two bonds and forming two new bonds. That is too many. There is no gas- phase configuration that puts the two molecules into a favorable configuration.

55. Copyright © 2013 R. R. Dickerson55 Reactions Rate Guidelines Breaking bonds requires energy. Forming bonds releases energy. Both processes require proximity. Reactions where only one bond is broken (such as thermal dissociation or photolysis) proceed quickly if there is enough energy. HO 2 NO 2 → HO 2 + NO 2 Reactions where only one bond if formed (such as ozone formation) have negative activation energy and proceed more quickly at high pressures and low temperatures. O + O 2 + M → O 3 + M Reactions where only one bond is broken and one other is formed (such as thermal dissociation or photolysis) proceed quickly if there is enough energy released. OH + VOC → R  + H 2 O

56. Copyright © 2013 R. R. Dickerson56 Reactions Rate Guidelines, continued Reactions where there are two bonds broken or two bonds formed proceed more slowly. NO + NO + O 2 → 2N O 2 Reactions where the total of bonds broken and formed exceeds three proceed slowly or not at all in the gas phase. Here  G is a ‘false friend’. O 3 + H 2 S → H 2 O + SO 2 NH 3 + O 3 → H 2 O + HNO 2 SO 2 + H 2 O 2 → H 2 SO 4 (Count the bonds broken and formed.) None of these reactions proceed quickly in the gas phase. The final reaction is important in cloud water however.

57. Copyright © R. R. Dickerson 201357 Lecture Summary Changes in enthalpy and  entropy,  H and  S, are powerful indicators of reaction probabilities and rates and are nearly independent of temperature. Gibbs free energy provides the criterion of feasibility, but does not dictate rates. The residence time (lifetime), t, is the inverse of the first order rate constant, k. If second or third order reactions can be approximated as first order then lifetimes can be estimated. For reversible reactions, k f /k r = K eq Photolysis rates can be both calculated and measured directly. Tropospheric ozone production depends on the rate of formation of NO 2 from NO + RO 2 and on UV flux, j(NO 2 ). The concentration of NO an UV flux are usually limiting.