Outline. 13./ 16. March Introduction. Photochemistry. Ozone chemistry Chapman model Catalytic cycles

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1 Institute f Applied Physics University f Bern 13./ 16. March 2012 Outline

2 The rle f Ozne hle: what causes what? Culd it be that ClO destrys O 3? Atmsphere as a chemical vessel The can be thught f as a cmbustin system at lw temperature where energy frm the sun is used t drive xidatin prcesses We will use basic physical principles t study a few chemical aspects f the Examples: Phtlysis f O 2 in the stratsphere generates zne O 2 + (hν) O + O O + O 2 + M O 3 + M Phtlysis f zne leads t the frmatin f OH O 3 + (hν) O( 1 D) + O 2 O( 1 D) + H 2 O OH + OH OH xidizes many cnstituent e.g. methane OH + CH 4 CH 3 + H 2 O

3 Distributin f mlecules in the Why all these different distributins with altitude? Chemical reactins between stable mlecules are quite slw in planetary s Absrptin f slar UV-radiatin leads t the prductin f radical species: atms, ins, excited mlecules Radicals are extremely reactive Bulk f atmspheric chemistry invlves the reactin between the radicals themselves and between the radicals and stable mlecules Main questin: 1. Is a specific reactin pssible? Therm chemistry 2. Hw fast is a reactin? Chemical kinetics Atmspheric reactins are classified int fur types Unimlecular reactins: A B + C Bimlecular reactins: A + B C + D Termlecular reactins: A + B + M C + M Phtchemical reactins A + (hν) B + C

4 Phtchemical reactins: A + hν B + C Mst cmmn phtchemical event f relevance t atmspheric chemistry is phtdissciatin The reactin rate f a phtchemical reactin is given by d[a] = j[a] dt The inverse f j is the phtchemical lifetime τ In an j is determined by the amunt f phtns, actinic flux, I (λ) = F λ λ/hc, the absrptin crss sectin, σ a and the quantum efficiency Φ j = λ max σ a (λ)φ(λ)i (λ)dλ λ min Imprtant examples in zne chemistry are: O 2 + hν O + O j 2 O 3 + hν O 2 + O j 3 Actinic flux

5 Examples frm zne phtchemsitry Reactins in a pure xygen accrding Chapman: O 2 + hν O + O j 2 (1) O + O 2 + M O 3 + M k 2 (2) O 3 + hν O 2 + O j 3 (3) O + O 3 O 2 + O 2 k 3 (4) There are tw types f reactins: Reactin (1) and (4) create and destry dd xygen Reactin (2) and (3) intercnvert O and O 3 Evaluating reactin rates d[o] d[o dt and 3 ] dt and evaluating steady state i.e. equilibrium, it can be shwn: [O 3 ] = [O 2 ] ( ) 1/2 k2 j2 [M] k 3 j 3

6 Vertical distributin f zne Measurements with balln sndes are perfrmed twice a week in Payerne N. Ka mpfer O3 ver Bern measured by micrwave radimetry N. Ka mpfer

7 O 3 glbal average clumn density Ozne clumn density expressed in Dbsn Units: 100 DU = 1 mm 1 DU = mlecules / cm 2 Seasnal variability in ttal zne

8 O 3 frecast O 3 distributin Measured zne distributin shws: Maximum at apprx. 22 km fr number density Maximum at apprx. 35 km fr vlume mixing rati (remember: VMR=p O3 /p [O 3 ] = VMR p kt ) Clumn density aprrx. 3mm=300 Dbsn units Distributin f zne is variable and changes as functin f time and lcatin is far t simple, particularly it predicts mre zne additinal prcesses must act Chemistry must be mdified Transprt prcesses must be cnsidered

9 In additin t pure xygen chemistry: X + O 3 XO + O 2 XO + O X + O 2 net: O 3 + O O 2 + O 2 X can be a radical as H, OH, NO, Cl, Br,... X stems frm surce gases that are transprted upwards t the stratsphere where they are destryed by UV-radiatin liberating the radicals In additin radicals can be cnverted t s called reservir gases such as HCl r ClONO 2 Als hetergeneus reactins n particles such as n cluds are imprtant zne hle Summary f stratspheric chemistry Nz m D@ =q -q) 0D- (- Cf, m (t) -z ääb Aa' - TlU m a *A+ (, \Js TD 7 ö v) OI ru - zz lu a CD TD I m@m z N) ('t T a 7@ N öz TD z I\) g) m = c g) {rf^".,-s,l l"d.'il r-^l ll YJII,<-\ t ll \st \1 \;._s cl äei '= f -{ =.a de ( 5' I mnm U) -l n T I m7 m cpied frm Brasseur, Atmspheric Chemistry and Glbal Change

10 Stratspheric chemistry STRATOSPHERIC and SYSTEM links 02 Phtlysis Slar UV HOr clx NO* BrO,. Rain-ut HCI HNOs Depsitin Hz Nz cfc cpied frm Brasseur, Atmspheric Chemistry and Glbal Change O 3 n Mars

11 O 3 n Mars Znally averaged zne clumn density in µm-atm O 3 n Ganymed

12 O 3 n an explanet (?) Spectra f O 3 culd serve as a bi-marker The Astrphysical Jurnal, 733:35(12pp),2011May20 Kaltenegger, Segura, & Mhanty Figure Figure 5 frm 5. Detectable Mdelspectral Spectra features f the andfirst bisignatures Ptentially in the emergent Habitable spectra Super-Earth?Gl581d f ver µm, Lisafr Kaltenegger the (B2) mdel et. al Left: ApJ VIS 733t35 NIR di: / x/733/1/35 (0.4 4 µm). Right: MIR (4 40µm). (A clr versin f this figure is available in the nline jurnal.) Detectable spectral features and bisignatures in the emergent spectra f Gl581d ver µm, fr the (B2) Transmissin Spectra mdel. Left: VIS t NIR (0.4 4µm). Right: MIR (4 40µm). alargefractinfmdwarfs;hwever,giventhattherearen signs f such activity in Gl581, as discussed in Sectin 2, we d nt cnsider this pssibility any further here). This, in turn, generates a different phtchemistry n rbiting within the HZ f M stars, cmpared t within the HZ f Sunlike stars (Segura et al. 2005). In particular, the bigenic gases CH 4,N 2 O, and CH 3 Cl have substantially lnger lifetimes and higher mixing ratis than n Earth, making them ptentially bservable by space-based telescpes. In additin, the lw effective temperatures f M dwarfs yield spectra dminated by mlecular absrptin bands that redistribute the radiated energy in a distinctly nn-blackbdy fashin. Bth effects are crucial t determining the bservable spectral features and bisignatures f habitable (see, e.g., Kaltenegger et al. 2010b) arund these cl lw-mass stars. We discuss the resulting emergent spectra in Sectin and the transmissin spectra in Sectin Emergent Spectra Figure 5 shws the spectra and detectable bisignatures in the f Gl581d in emergent spectra. The lwer panels shw the influence f the individual chemicals n the verall spectrum. Each chemical has individual absrptin features that are indicated. The summed effects f the chemicals prduce the verall spectral appearance f the (upper panel). Nte that multiple scattering in the will affect the visible spectrum, what has nt been included in ur mdels. In additin, clud cverage wuld reduce the depth f the spectral features further (see Kaltenegger et al. 2007; Fujiietal.2011). Figure 5 demnstrates that in a dense individual chemicals can dminate and blanket all ther atmspheric features (see als Selsis 2002). The extremely strng CO 2 features dminate the spectra frm 1.2 µm nward.inthemidinfrared (>4 µm), the dense CO 2 als affects the verall flux emitted by the planet by lwering the effective temperature (see Figure 5). The visible is nt sensitive t high 7 CO 2 cncentratins, and is nt influenced by the atmspheric absrptin f CO 2,assumingaclear.Featuresf water and xygen can be seen in the visible. We calculate the transmissin spectra fr Gl581d. Even thugh there is n indicatin yet that Gl581d is a transiting planet, these features culd be detected by a space missin like JWST if either Gl581d r a similar planet transits (see Kaltenegger & Traub 2009 fr details n gemetry). Figure 6 shws the mdel spectrum f Gl581d in transmissin. Even thugh CO 2,H 2 O, O 3,andCH 4 are easily visible in the panel that shws individual absrptin by chemicals, the verall spectrum is dminated by CO 2 dwn t 1 µm and Rayleigh scattering belw that wavelength, nt prviding infrmatin abut the habitability f a rcky planet with a dense CO 2. Clud cverage wuld reduce the depth f the spectral features further (see Sectin 5) Planet Star Cntrast Rati The bservable quantity t remtely derive what atmspheric features exist in a planet s is the cntrast rati versus wavelength, as shwn in Figures 7 and 8 fr emergent and transmissin mdel spectra, respectively. The Sun emits a large fractin f its energy in the visible, a wavelength dmain where the f a habitable planet is highly reflective, because f the Rayleigh backscattering varying like λ 4 and because f the lack f strng H 2 Oabsrptinbands.Theemissinf Gl581, a star with a lw effective temperature, peaks in the near-infrared where the cntributin f Rayleigh scattering t the albed becmes negligible and the strng absrptin bands f H 2 O, CO 2,andCH 4 cause additinal absrptin f stellar radiatin and verall lwer the planet s albed as lng as n additinal reflective clud layer frms (see Sectin 5). Fr the emergent spectra, the larger surface area f a Super-Earth makes the direct detectin and secndary-eclipse

Outline. Chemical lifetime. Photochemistry. Ozone chemistry Chapman model Catalytic cycles Ozone hole. Institute of Applied Physics University of Bern

Outline. Chemical lifetime. Photochemistry. Ozone chemistry Chapman model Catalytic cycles Ozone hole. Institute of Applied Physics University of Bern Institute of Applied Physics University of Bern Outline Introduction Chemical reactions between stable molecules are quite slow in planetary s Absorption of solar UV-radiation leads to the production of