Lecture 7: Photochemistry of Important Atmospheric Species

Transcription

1 Lecture 7: Photochemistry of Important Atmospheric Species Required Reading: FP Chapter 4 Atmospheric Chemistry CHEM-5151 / ATC-5151 Spring 2005 Prof. Jose-Luis Jimenez General remarks 2 3 Nitrogen species Aldehydes and ketones CFCs utline of Lecture We won t cover everything in class, read the rest in the book Have to know how to find + interpret quickly 1

2 Reminder from last time Sunlight drives chemistry of trop. & strat. Hot photons break bonds and create free radicals Radicals can react with many molecules bj: calculate rates of photolysis & product generation This lecture Generic reaction: A + hν B + C d[ A] = J A [ A] = A( ) A( ) F( ) d [ A] dt σ λ φ λ λ λ J A first order photolysis rate of A (s -1 ) σ A (λ) wavelength dependent cross section of A (cm 2 /molec) φ A (λ) wavelength dependent quantum yield for photolysis F(λ) spectral actinic flux density (#/cm 2 /s) Last lecture General Remarks Photodissociation is the most important class of photochemical process in the atmosphere: AB + hv A + B In order to photodissociate a molecule it must be excited above its dissociation energy (D 0 ). In the lower troposphere, only molecules with D 0 corresponding to λ > 290 nm are photochemically active. Most common atmospheric molecules, including N 2, C, 2, C 2, CH 4, N, etc. are stable against photodissociation in the troposphere. In addition, the molecule should have bright electronic transitions above D 0. For example, HN 3 has a low dissociation energy (D 0 = 2.15 ev) but it needs UV for its photolysis because it does not have appropriate electronic transitions in the visible. In general, both the absorption cross sections and photodissociation quantum yields are wavelength dependent. Photoionization processes are generally not important in the lower atmosphere (ionization potentials for most regular molecules > 9 ev). From S. Nidkorodov 2

3 2 Electronic Transitions Always start in ground state X 3 Σ g - nly transitions to triplet states are spin-allowed X 3 Σ g- A 3 Σ u+ forbbiden because - + ccurs weakly, Herzberg continuum, nm X 3 Σ g- B 3 Σ u- is allowed Schumann-Runge system, nm, bands due to different vib-rot states B 3 Σ u- crossed by 3 Π u repulsive state, dissociates to ( 3 P)+( 3 P) Later (< 175 nm) spectrum is continuum, dissociation of B 3 Σ u- to ( 3 P)+( 1 D) xygen Spectrum From Brasseur and Solomon Hole in the spectrum coincident with Lyman α line of H-atom 2 photolysis in the nm range is the major source of 3 in the stratosphere 2 can absorb nearly all radiation with λ = nm high up in the atmosphere Solve in class: Estimate the length of air column at P = 0.01 Torr and T= 200 K (characteristic of 80 km altitude) required to reduce the radiation flux at 150 nm by 10 orders of magnitude. Neglect the fact that a substantial portion of oxygen is atomized at this altitude. (A: 230 m) 3

4 UV absorption by 2 and 3 2 Photochemistry Schumann continuum very efficient screening radiation below 200 nm Solar radiation more intense towards longer λ Dissociation of 2 in Herzberg continuum ( nm) is very important for 3 in the stratosphere 2 + hv ( 3 P) + ( 3 P) ( 3 P) + 2 (+ M) 3 Troposphere λ > 290 nm Not enough energy for 2 dissociation 3 from N 2 + hv From Brasseur and Solomon 4

5 Importance of 3 Central role in atmospheric chemistry Highly reactive Highly toxic => health effects in humans Crop degradation => billions of $ in losses Absorbs UV Shield surface from hard UV Its photolysis produces ( 1 D), which yields H H is most important tropospheric oxidant Photolysis to ( 3 P) regenerates 3, not important! Absorbs IR Greenhouse gas zone: Electronic States Transitions into triplet states: extremely weak (forbidden) All (allowed) excited electronic transitions lead to 3 above its lowest dissociation threshold! Multiple dissociation pathways are available for 3 : 3 + hv ( 1 D) + 2 (a 1 g ) 3 + hv ( 1 D) + 2 (X 3 Σ g ) 3 + hv ( 3 P) + 2 (a 1 g ) 3 + hv ( 3 P) + 2 (X 3 Σ g ) lowest Transition into the 1 B 2 state ~255 nm (Hartley band) is strongest. Weaker transitions into other singlet states at ~600 nm (Chappuis bands) and 330 nm (Huggins bands). 3 ( 1 B 2 ) 1 A 2, 1 B 1 3 A 2, 3 B 2, 3 B 1 3 (X 1 A 1 ) 310 nm ( 1 D) + 2 (a 1 g ) 411 nm ( 1 D) + 2 (X 3 Σ g- ) 612 nm ( 3 P) + 2 (a 1 g ) 1180 nm ( 3 P) + 2 (X 3 Σ g- ) From S. Nidkorodov 5

6 3 Absorption Spectrum Hartley bands Huggins bands Hartley bands Chappuis bands From Yung & DeMore 3 Photochemistry Most important aspect is production of ( 1 D) (and thus H) 3 + hv 2 + ( 1 D) φ( 1 D) 90% below 305 nm 3 + hv 2 + ( 3 P) φ( 3 P) 10% below 305 nm ( 1 D) + H 2 2 H H yield 10% (at the surface) ( 1 D) + M ( 3 P) the rest of ( 1 D) atoms are quenched Dissociation threshold for 3 + hv ( 1 D) + 2 (a 1 g ) is at 310 nm. However, ( 1 D) products are observed up to 330 nm because of: Threshold a). dissociation of internally excited 3, which requires less energy to break apart (responsible for nm falloff). Drops as T b). spin-forbidden process 3 + hv ( 1 D) + 2 ( 3 Σ g- ) (sole contributor λ > 325 nm, f(t)) 6

7 Photodissociation Thresholds in N 2 and 3 For N 2 and 3, photodissociation quantum yields do not drop immediately to zero below the dissociation threshold. This effect is explained by channeling the internal energy of molecules into the dissociation process. Threshold Threshold From S. Nidkorodov 3 Photodissociation Channels From Brasseur & Solomon 7

8 Combined UV Shielding by 2 and 3 2 takes care of 90% of deep UV radiation well above 80 km, i.e. in the mesosphere and thermosphere. 3 important below 40 km. Window at 210 nm between 2 and 3 absorption of paramount importance for making 3 in stratosphere via photolysis of 2. From Warneck Fig. 2.9 Fig. 2.9 Elevation at which solar radiation is attenuated by 2 and 3 by one order of magnitude. Summary of 2 & 3 Photochem. From Brasseur & Solomon 8

9 Photolysis Rates for 2, N 2, and 3 From Yung & DeMore Typical values for photodissociation coefficients, J, for 3, 2, & N 2 as a function of altitude. Photolysis rate for 2 is strongly altitude dependent because the lower you go the less UV radiation capable of breaking 2 is available (self-shielding). Photolysis rate for 3 becomes altitude dependent below 40 km for similar self-shielding reasons. n the contrary, visible N 2 photolysis occurs with about the same rate throughout the atmosphere because there is not enough of it for self-shielding. From Warneck Solve in class: Based on the figures shown here, estimate the lifetime of N 2, 2 and 3 against photodissociation at 20 km and 50 km. Structure of Important N-Species From Jacobson (1999) Table B posted on course web page Many other species there, useful when you don t know detailed structure 9

10 Altitude (km) Atm. Profiles of Important N-Species From NASA Total Density Important N- containing molecules in lower atmosphere: extremely photo-stable N 2 and HN 3 easily-degradable N 2, N 3, and N 2 5, H 2 N 2 multitude of poorly quantified organic nitrates, RN 2 (most nitrates are relatively stable towards UV) Generally, concentrations are inversely proportional to photodissociation / reaction rates in the atmosphere From Warneck Absorption Spectra and Photolysis Rates of Selected N-Species Note sensitivity of photolysis rates to shape of absorption spectra Solve in class: Estimate the photodissociation lifetimes of N 2 and HN at 20 km. Compare those with characteristic times for vertical transport in the stratosphere ( 2 years). From S. Nidkorodov 10

11 Nitrogen Dioxide (N 2 ) N 2 is one of a very few atmospheric molecules that absorb & photolyze in the visible range Photolysis of N 2 generates ozone in the troposphere: N 2 + hv N + ( 3 P) ( 3 P) M N 2 Absorption cross sections are structured, and have a non-trivial dependence on T,P. N 2 contributes to the brown color of air in very polluted cities (but most due to aerosols!). 8x10-19 Cross Section (cm 2 molec -1, base e) From S. Nidkorodov Wavelength, nm Photochemistry of N 2 Photolysis occurs with nearly 100% yield below nm. -atom immediately makes 3 : N 2 + hv N + ( 3 P) ( 3 P) + 2 (+ M) 3 Between 398 nm and 415 nm, room temperature N 2 still partially photodissociates because of contributions of internal energy to the process, but the quantum yield declines rapidly with wavelength Above 410 nm, electronic excitation of N 2 can result in the following processes: N * 2 N 2 + hv Fluorescence N * N (a 1 g ) Electronic energy transfer N * 2 + N 2 N 2 + N 2 (v) Electronic-to-vibrational energy transfer N * 2 + N 2 N + N 3 Disproportionation (lab conditions only) 11

12 HN 3 + hv H + N 2 HN 3 + hv + HN HN 3 + hv H + N 3 verall relatively long lifetime against photolysis Nitric Acid (HN 3 ) φ 1 between 200 and 315 nm requires vacuum UV; φ is only 0.03 at 266 nm requires vacuum UV; φ is only at 266 nm vertone Dissociation N H IR N H N H LIF PRBE If one supplies enough vibrational energy into a molecule (more than its dissociation energy D 0 ) the molecule may break. Energy [cm -1 ] HN HN 2 v H = 6 Ex. 1: Near-infrared excitation of 2ν 1 overtone in HN will break it into H and N v H = 5 v H = 4 D 0 Ex. 2: Visible excitation of 6ν 1 in HN 3 will also break it into H and N D 0 ν 1 0 From S. Nidkorodov 2ν 1 3ν 1 v H = 3 v H = 2 v H = 1 v H = 0 ν 1 2ν 1 3ν 1 Such overtone dissociation is a very indirect process involving highly inefficient intermolecular vibrational energy exchange between low and high frequency modes. Therefore it is slow (nanoseconds) and it competes with collisional stabilization. 12

13 Nitrous Acid (HN) HN + hv H + N φ 1 below 400 nm HN, like N 2, has strong absorption in visible and a highly structured spectrum. Its photochemical lifetime in the atmosphere is ~ a few minutes. Very important as source of H radicals in the morning Sources of H x = H + H 2 in Mexico City H x drive smog and secondary aerosol chemistry HN photolyzes at long λ, very important in early morning HCH (formaldehyde) is dominant source 3 source needs to wait for 3 to be produced! (depends on others) From R. Volkamer & W. Brune (MIT & PSU) 13

14 Pernitric Acid (H 2 N 2 ) H 2 N 2 + hv H 2 + N 2 quantum yields uncertain; φ~0.65 recommended H 2 N 2 + hv H + N 3 φ 0.35 at 248 nm; more measurements needed! Unique in that it can be broken by near-infrared radiation. This is important at large SZA, with orders of magnitude larger NIR vs. UV. H 2 N 2 + hv (near-ir) H 2 N 2 (v 1 =2) H 2 + N 2 φ 0.25 (P & T dep.) Nitrate Radical (N 3 ) N 3 is important in nighttime chemistry. It has unusually large cross sections in the red photodissociates in seconds in the morning. N 3 + hv N 2 + ( 3 P) φ is λ dependent; important towards the blue N 3 + hv N + 2 φ is λ dependent; important in the red; competes with fluorescence; this process is nearly thermoneutral but it is inhibited by a tall energy barrier. 14

15 Nitrous xide (N 2 ) N 2 is extremely long-lived because it is unreactive, and it does not absorb much above 200 nm. Below 200 nm: N 2 + hv N 2 + ( 1 D) - Note: this is a popular laboratory method of generating ( 1 D); φ 1 Subsequent reactions of ( 1 D) with N 2 lead to production of other nitrogen oxides in the stratosphere: N 2 + ( 1 D) N + N major source of N in the stratosphere From Calvert & Pitts N competing step Because of its stability, N 2 is used a "tracer". Concentrations of other molecules are often compared to that of N 2 to see is they are well mixed. Solve in class: Find a conversion between absorption coefficient in cm -1 atm -1 an cross sections in cm 2 /# at room temperature. Evaluate N 2 cross sections at 193 nm using the data shown here. Formaldehyde (HCH) Two competing photodissociation channels: HCH + hν H + HC (a) HCH + hν H 2 + C (b) Remember: HCH is dominant H x source in Mexico City Channel (a) leads to H 2 radical production via: H M H 2 (19) HC + 2 H 2 + C (20) Sources of H 2 are effectively sources of H because: H 2 + N H + N 2 (17) All aldehydes have relatively weak transitions at around 300 nm. Photoexcitation of aldehydes normally results in a release of HC, which is quickly converted into H 2 and C in the atmosphere: RCH + hν R + HC (slow) R + 2 R 2 (very fast) HC + 2 H 2 + C (very fast) 15

16 Acetaldehyde and higher aldehydes Four possible photodissociation channels: CH 3 CH + hν CH 3 + HC CH 3 CH + hν CH 4 + C CH 3 CH + hν CH 3 + H CH 3 CH + hν CH 2 C + H 2 (a) (b) (c) (d) Can also be H x source Similarly for higher aldehydes Process Quantum yield H(298 K) Threshold Note a). CH 3 + HC ±1.2 kcal/mol 338 nm - b). CH 4 + C 0 0.5; goes up in UV kcal/mol none Large barrier c). CH 3 + H < 0.01 above 290 nm 95.8 ±1.2 kcal/mol 298 nm - d). CH 2 C + H ±3 kcal/mol 1000 nm Large barrier Acetone (CH 3 -C-CH 3 ) Two main photodissociation channels: CH 3 C()CH 3 + hν CH 3 + CH 3 C (a) Threshold = 338 nm CH 3 C()CH 3 + hν 2 CH 3 + C (b) Threshold = 299 nm (a) more important near the earth surface. Photodissociation competes with collisional relaxation, φ P-dependent. Also a competition between photodissociation and reaction with H: CH 3 C()CH 3 + hν CH 3 C()CH 3 * (+ M) CH 3 C()CH 3 CH 3 C()CH 3 + H CH 3 C()CH 2 + H 2 Quenching Reaction with H 16

17 Chlorofluorocarbons (CFCs) Photolysis Rates Absorption Spectra No other CFC sinks than photolysis Known that Cl would destroy Nobel Prize (M&R) is the idea in this slide: CFCs will provide large source of Cl in stratosphere and lead to 3 destruction From Brasseur and Solomon 17