Institute f Applied Physics University f Bern 13./ 16. March 2012 Outline
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
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
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
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
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
O 3 glbal average clumn density Ozne clumn density expressed in Dbsn Units: 100 DU = 1 mm 1 DU = 2.69 10 16 mlecules / cm 2 Seasnal variability in ttal zne
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
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- (- d< @ 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 = (t' @ c g) {rf^".,-s,l l"d.'il r-^l ll YJII,<-\ t ll \st \1 \;._s cl äei '= f -{ v-u @-U =.a de ( 5' I mnm U) -l n = @ T I m7 m cpied frm Brasseur, Atmspheric Chemistry and Glbal Change
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
O 3 n Mars Znally averaged zne clumn density in µm-atm O 3 n Ganymed
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 0.4 20µm, Lisafr Kaltenegger the (B2) mdel et. al. 2011 Left: ApJ VIS 733t35 NIR di:10.1088/0004-637x/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 0.4 20µm, fr the (B2) 4.2.2. 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 4.2.1 and the transmissin spectra in Sectin 4.2.2. 4.2.1. 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). 4.3. 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