The Atmosphere. Giovanna Jerse

Transcription

1 The Atmsphere Givanna Jerse

2 Thickness f the Atmsphere Apprximately 80% f the atmsphere ccurs in the lwest 20km abve the Earth. Radius f the Earth is ver 6,000 km Atmsphere is a thin shell cvering the Earth.

3 But what is the atmsphere? Cmprised f a mixture f invisible permanent and variable gases as well as suspended micrscpic particles (bth liquid and slid) Permanent Gases Frm a cnstant prprtin f the ttal atmspheric mass Variable Gases Distributin and cncentratin varies in space and time Aersls Suspended particles and liquid drplets (excluding clud drplets)

4 Cmpsitin f Earth s Atmsphere Imprtant gases in the Earth s Atmsphere (Nte: Influence nt necessarily prprtinal t % by vlume!)

5 Permanent Gases 78% Nitrgen (N 2 ) 21% Oxygen (O 2 ) <1% Argn (Ar) Relative percentages f the permanent gases remain cnstant up t km high (~ 60 miles!) This layer is referred t as the Hmsphere (implies gases are relatively hmgeneus)

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7 2. Atmspheric divisins in terms f temperature structure The atmsphere f a planet can be divided int different regins. In terms f temperature structure atmsphere is divided int Trpsphere Lwermst part f the atmsphere where primary heat surce is the planetary surface. The heat is cnvected by turbulent mtin leading t a cnvective r adiabatic distributin. Adiabatic lapse rate is given by g/c p where g is the acceleratin due t gravity and c p the specific heat at cnstant pressure. (depends n planet s acceleratin due t gravity and cmpsitin). Fr earth the theretical estimate is 10 K/km: bserved is 6.5 /km due t presence f water vapur & large-scale circulatin. Trppause: where the decrease in temperature ceases. Fr earth trppause level varies frm 18 km at equatr t 8 km at ples.

8 Temperature Layers f the Atmsphere: Trpsphere Lwer part f the atmsphere Energy surce is heating f the earth s surface by the sun. Temperature generally decreases with height. Air circulatins (weather) take place mainly here. Trpsphere ges frm surface t abut 30,000 ft. (10 km).

9 Temperature Layers f the Atmsphere: Stratsphere Sun s ultravilet light is absrbed by zne, heating the air. Heating causes increase f temperature with height. Bundary between trpsphere and stratsphere is the trppause. Stratsphere ges frm abut 10 t 50 km abve the surface.

10 Temperature Layers f the Atmsphere: Messphere Abve 50 km, very little zne, s n slar heating Air cntinues t cl with height in messphere Messphere extends frm abut 50 km t 90 km abve the surface

11 Temperature Layers f the Atmsphere: Thermsphere Abve 90 km, residual atmspheric mlecules absrb slar wind f nuclear particles, x-rays and gamma rays. Absrbed energy causes increase f temperature with height. Air mlecules are mving fast, but the pressure is very lw at these heights.

12 2. Atmspheric divisins in terms f temperature structure 3 Thermsphere Abve mespause slar EUV radiatin is absrbed and part used in heating s in thermsphere temperature increases with altitude. In the lwer thermsphere cnvectin is principal prcess f heat transprt while in upper thermsphere heat is transprted by cnductin leading t an isthermal regin where temperature is cnstant abve abut 500 km (thermpause). Heating due t the absrptin f slar EUV by atmspheric cnstituents, heat transprt due t cnductin and cnvectin, lss by IR radiatins by cnstituents gvern the energy budget. Cllisins between charged particles and neutrals, Jule heating, atmspheric waves (tides, internal gravity waves), hydrmagnetic waves and slar wind are ther surces f heating. Temperatures shw diurnal (minimum at 06h, maximum at 17h LT) and slar cycle variatins.

13 2. Atmspheric divisins in terms f temperature structure -4 Exsphere In the regin abve 500 km (fr earth) mean free path becmes large and cllisins becme negligible s that the light atmspheric cnstituents whse velcity exceeds the gravitatinal escape velcity can escape the atmsphere. This regin is als called exsphere. The exbase is als called barpause since the atmsphere belw this is als referred t as barsphere, the regin where barmetric law hlds. In the exsphere velcity distributin becmes nn-maxwellian due t the escape f high velcity particles. Fr Mars and Venus the expause is arund 200 km (due t lwer scale height).

14 The Main Pints Planetary atmspheres as a balancing act: Gravity vs. thermal mtins f air mlecules Heating by Sun vs. heat radiated back int space Weather as a way t equalize pressures at different places n a planet s surface Atmspheres f terrestrial planets are very different nw frm the way they were brn Frmatin: vlcanes, cmets Destructin: escape, incrpratin int rcks, ceans Huge changes ver a billin years r less Prspect f human-induced glbal warming n Earth is a serius issue. Can be apprached scientifically.

15 Unique Features f Earth s Atmsphere Atmspheric cmpsitin high Oxygen cntent, lw Carbn Dixide cntent. Greenhuse gases cntribute t livable surface temperatures Mst imprtant greenhuse gas is water vapr! Withut an atmsphere, Earth s surface temp wuld nly be apprximately 0 F! Water in all three phases: slid, liquid, gas. Patchy clud fields extensive up and dwn cnvective mtins in atmsphere. Circular mtins with strms.

16 Origin f the atmsphere a) Earth frmed 4.6 billin years ag, withut atmsphere b) Outgassing frm vlcanes f N 2, CO 2, H 2 O, H 2, Cl 2, SO 2, CH 4, but nt xygen c) Water cndensed in ceans, H 2 escaped, acid rain weathered rcks. CO 2 at 100 times present level larger greenhuse effect kept the atmsphere warmer and cmpensated fr a 30% weaker Sun d) Emergence f life in ceans prduced O 2 and sequestered CO 2 in rcks e) Once O 2 was present O 3 culd be frmed, giving UV prtectin and allwing life t expand its range

17 Where d planetary atmspheres Three primary surces Primrdial (slar nebula) Outgassing (trapped gases) Later delivery (mstly cmets) cme frm? Hw can we distinguish these? Slar nebula cmpsitin well knwn Nble gases are useful because they dn t react Istpic ratis are useful because they may indicate gas lss r surce regins (e.g. D/H) 40 Ar ( 40 K decay prduct) is a tracer f utgassing

18 Atmspheric Cmpsitins Earth Venus Mars Titan Pressure 1 bar 92 bar bar 1.5 bar N 2 77% 3.5% 2.7% 98.4% O 2 21% H 2 O 1% 0.01% 0.006% - Ar 0.93% 0.007% 1.6% 0.004% CO % 96% 95% ~1ppb CH 4 1.7ppm -? 1.6% 40 Ar 6.6x10 16 kg 1.4x10 16 kg 4.5x10 14 kg 3.5x10 14 kg H/D N/ 15 N Istpes are useful fr inferring utgassing and atms. lss

19 Nt primrdial! Terrestrial planet atmspheres are nt primrdial (Hw d we knw?) Why nt? Gas lss (due t impacts, rck reactins r Jeans escape) Chemical prcessing (e.g. phtlysis, rck reactins) Later additins (e.g. cmets, asterids) Giant planet atmspheres are clse t primrdial: Slar Jupiter Saturn Uranus Neptune H He CH Values are by number f mlecules Why is the H/He rati nt cnstant?

20 Atmspheric Lss Atmspheres can lse atms frm stratsphere, especially lw-mass nes, because they exceed the escape velcity (Jeans escape) Escape velcity v e = (2 g R) 1/2 (where s this frm?) Mean mlecular velcity v m = (2kT/m) 1/2 Bltzmann distributin negligible numbers f atms with velcities > 3 x v m Mlecular hydrgen, 900 K, 3 x v m = 11.8 km/s Jupiter v e =60 km/s, Earth v e =11 km/s H cannt escape gas giants like Jupiter, but is easily lst frm lwer-mass bdies like Earth r Mars A cnsequence f Jeans escape is istpic fractinatin heavier istpes will be preferentially enriched

21 Atmspheric Evlutin Earth atmsphere riginally CO 2 -rich, xygen-free Hw d we knw? CO 2 was prgressively transferred int rcks by the Urey reactin (takes place in presence f water): MgSiO CO MgCO SiO Rise f xygen began ~2 Gyr ag (phtsynthesis & phtdissciatin) Venus never underwent similar evlutin because n free water present (greenhuse effect, t ht) Venus and Earth have ~ same ttal CO 2 abundance Urey reactin may have ccurred n Mars (water present early n), but very little carbnate detected

22 In an atmsphere in equilibrium, pressure gradient balances gravity Pressure = Net Frce / Area Frce [P(h) P(h dh)] Area P A mass Gravitatinal frce Mg vlume Ah g g (Ah) P A g Ah vlume P h g r, in calculus language, dp dh g P(h) P(h+h) Area A

23 Prfile f density with altitude (a calculus-based derivatin) P nkt m kt P dp dh d dh kt m g If temperature cnst, d dh kt m kt m Divide bth sides by kt m : 1 d dh mg kt cnst Slutin: = 0 e (h/h 0 ) where h0 kt mg d dh g P 0 /e h 0 h Pressure, density fall ff expnentially with altitude Higher temperature T larger scale height h 0 Strnger gravity g shrter scale height h 0

24 Hw big is pressure scale height? h 0 = kt / mg height at which pressure has fallen by 1/e = Earth h 0 = 8 km the thin blue line Venus h 0 = 15 km (g a bit lwer, T higher) Mars h 0 = 16 km (bth g and T lwer)

25 Hw big is pressure scale height? h 0 = kt / mg height at which pressure has fallen by 1/e = Earth h 0 = 8 km the thin blue line Venus h 0 = 15 km (g a bit lwer, T higher) Mars h 0 = 16 km (bth g and T lwer)

26 Tabella: Parametri Pianeta Densità Temp.Sup Gravità Vel.fuga Cmpsizine atmsferica g/cm3 K Terra =1 m/s Mercuri Trace? Venere CO 2 (96%) +N 2 (3.5%) + SO 2 (130 ppm) Terra N 2 (78%) + O 2 (21%) +Ar (.9%) Marte CO 2 (95%) + N 2 (2.7%) Give H 2 (86%), H e (14%), CH 4 (0.2%) Saturn H 2 (97%), H e (3%), CH 4 (0.2%) Uran H 2 (83%), H e (15%), CH 4 (2%) Nettun H 2 (79%), H e (18%), CH 4 (3%)

27 E in 2 ( 1 A) R FE r Surface Temperature (1) What determines a planet s surface temperature? Incident energy Reflected energy Absrbed energy warms surface T eq Energy re-radiated frm warm surface r r E r 2 E 2 F E E 4 R T rad (1 A) 4 1/ A is albed, F E is slar flux at Earth s surface, r E is distance f Earth t Sun, r is distance f planet t Sun, is emissivity, is Stefan s cnstant (5.67x10-8 Wm -2 K -4 ) Balancing energy in and energy ut gives: R a Sun

28 Surface Temperature (2) Slar cnstant F E =1370 Wm -2 Earth (Bnd) albed A=0.28, =1 Equilibrium temperature = 255 K Hw reasnable is this value? T eq r r E 2 F E (1 A) 4 is Stefan s cnstant 5.67x10-8 in SI units 1/ 4 Bdy Mercury Venus Earth Mars A T eq Actual T Hw t explain the discrepancies? Has the Sun s energy stayed cnstant with time?

29 Surface temperature (3) At distance d frm Sun, a planet f radius R receives: P abs L sun W 4d 2 R2 where L sun is the slar luminsity in W. At 1 AU, Flux (F) = L sun / 4 d 2 = 3.85 x / 4 (1.49 x ) 2 = 1370 W/m 2. But, a fractin (A) f pwer is reflected - A called planetary albed (0 A 1). A = 1: Ttal reflectin. A = 0: Ttal absrptin. Fractin (1 - A) is absrbed by surface f planet. Rcks are pr reflectrs and have lw albeds, ice is a mderate reflectr.

30 Cmparisn with terrestrial planets

31 Cmparisn with Venus Cmpsitin f Venus Atmsphere: 96% CO 2, 3% N 2 (cmpare t Earth.04% CO 2, 78% N 2 ) Pressure at surface: 90,000 mbar (by cmparisn, Earth s mean sea-level pressure is apprximately 1,013 mbar Venus surface pressure is 90x greater!) Temperature at surface: ~ 900 F (by cmparisn, Earth s mean sfc temperature is abut 59 F) Extreme atmspheric pressures n Venus due large amunt f gaseus CO 2. N mechanisms t remve CO 2 frm atmsphere (e.g., phtsynthesis, disslutin in water).

32 Earth and Venus nearly same size velcity required t escape gravitatinal pull similar fr bth.

33 Why the drastic difference? Venus is clser t Sun Warmer temperatures prevented liquid water frm frming. With n liquid water, n means t disslve the carbn dixide. Result is a rich carbn dixide atmsphere.

34 Earth and Venus CO 2 and N 2 Earth actually has mre CO 2 than Venus (as fractin f ttal planet m Earth and Venus have similar amunts f N 2. CO 2 is 96% f Venus atmsphere and nly.04% f Earth s. Venus has CO 2 in atmsphere, while Earth has CO 2 in limestne.

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38 Atmspheric retentin (1) Energy f a mlecule in atmsphere can be written: E ttal E k E p 1/2mv 2 GMm 0 r A particle will escape frm planet if has enugh KE. Escape speed v = v esc, needed t escape frm r = R is therefre: v esc 2GM R 2 Frm kinetic thery, 1/2mv therm 3/2kT therefre, v therm 3kT m Lightest particles (H and He) have highest speeds and escape preferentially if T is large enugh fr particles t have v therm > v esc.

39 Atmspheric retentin (2) A planet will retain its atmsphere if The escape cnditin ccurs when The regin where this cnditin is met is called the exsphere. 3kT m 2GM R T esc 2GMm 3kR v therm v esc Exsphere Grund Escape Atmsphere Randm cllisins If surface temperature is large, planet will lse atmsphere. Als, small planets find it difficult t hld nt atmspheres. Fr a given planet r satellite f mass M and radius R the atmspheric retentin cnditin is T atm < T esc

40 Atmspheric retentin (3) Fr a given mlecule t be retained: 2GM R 3kT m Definitin: m = m H where m is mlecular weight and m H is mass f H-atm (m H = 1.67 x kg). s, fr hydrgen m = 1, and fr helium m = 4 hence at a given temperature the He atms will be mving slwer than H atms Fr Earth T atm = 288 K and v esc = 11.2 km s -1 Hence, escape fr all mlecules with m 4 S, dn t expect t find much H r He. m 3kTR 2GM Fr Jupiter T atm = 134 K and v esc = 59.5 km s -1 Hence, escape fr all mlecules with m < 0.06 S, nthing escapes, since hydrgen with m = 1 is the lightest gas element. Observatins shw that Jupiter is a H and He gas giant.

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42 Tabella: Parametri Pianeta Densità Temp.Sup Gravità Vel.fuga Cmpsizine atmsferica g/cm3 K Terra =1 m/s Mercuri Trace? Venere CO 2 (96%) +N 2 (3.5%) + SO 2 (130 ppm) Terra N 2 (78%) + O 2 (21%) +Ar (.9%) Marte CO 2 (95%) + N 2 (2.7%) Give H 2 (86%), H e (14%), CH 4 (0.2%) Saturn H 2 (97%), H e (3%), CH 4 (0.2%) Uran H 2 (83%), H e (15%), CH 4 (2%) Nettun H 2 (79%), H e (18%), CH 4 (3%)

43 Ozne frms a layer in the stratsphere, thinnest in the trpics (arund the equatr) and denser twards the ples measured in Dbsn units (DU) ~260 DU near the trpics What is zne?

44 What is a Dbsn unit? 1 Dbsn Unit (DU) is defined t be 0.01 mm thickness at STP - (0C and 1 atms press). A slab 3mm thick crrespnds t 300 DU

45 Hw is zne frmed? UV radiatin strikes the O 2 mlecule and splits it, atmic xygen assciates itself with anther O 2 mlecule simplistic versin

46 Grund level clean air : main cnstituents Mlecule Mle fractin ppmv (parts per millin by vlume) N r 78 % O r 21 % H 2 O 0.03 (25 C,100 % humidity) 0.01 (25 C, 50 % humidity) Ar 0.01 r 1 % 9300 CO r % 380 Ne CH 4 O x 10-6 Trace gases 2.0 x

47 The Ozne stry : distributin in the stratsphere befre ca Altitude / km thermsphere stratsphere trpsphere Temperature / K messphere Ozne [O 3 ] max = cm -3 at 30 km Temperature / K

48 Vertical variatin f zne

49 Chapman Reactins Ozne is frmed by: O 2 + hv -> O + O (1) Ozne can refrm resulting in n net lss f zne: O 3 + hv -> O 2 + O O + O 2 -> O 3 Ozne is als destryed by the fllwing reactin: (3) (2) O + O 3 -> O 2 + O 2 (4)

50 What is the zne hle? News media cnfuses it with the prblem f glbal warming zne cntributes t the greenhuse effect ver Antarctica (and the Arctic), stratspheric zne depleted ver past 15 years at certain times f the year hle presently size Antarctica, 10km altitude - lwer stratsphere

51 Develpment f Antarctic Ozne Hle,

52 The Antarctic Ozne Hle Antarctic Ozne Levels in Fall 2003 (Surce: NOAA TOVS satellite ) The zne hle is represented by the purple, red, burgundy, and gray areas that appeared ver Antarctica in the fall f The zne hle is defined as the area having < 220 Dbsn units (DU) f zne in the verhead clumn

53 The Antarctic Ozne Hle The zne hle is a well-defined, large-scale destructin f the zne layer ver Antarctica that ccurs each Antarctic spring. The wrd "hle" is a misnmer; the hle is really a significant thinning, r reductin in zne cncentratins, which results in the destructin f up t 70% f the zne nrmally fund ver Antarctica.

54 The Antarctic Ozne Hle

55 The Antarctic Ozne Hle The illustratin abve shws a clumn f air, 10 deg x 5 deg, ver Labradr, Canada. If all the zne in this clumn were t be cmpressed t standard temperature and pressure (STP) (0 deg C and 1 atmsphere pressure) and spread ut evenly ver the area, it wuld frm a slab apprximately 3mm thick. 1 Dbsn Unit (DU) is defined t be 0.01 mm thickness at STP; the zne layer ver Labradr then is 300 DU. The unit is named after G.M.B. Dbsn, ne f the first scientists t investigate atmspheric zne (~ ). He designed the 'Dbsn Spectrmeter' - the standard instrument used t measure zne frm the grund. The Dbsn spectrmeter measures the intensity f slar UV radiatin at fur wavelengths, tw f which are absrbed by zne and tw f which are nt.

56 Basic Chemistry f Ozne Depletin Frmatin and destructin f zne (Up t 98% f the sun's high-energy ultravilet light (UV-B and UV-C) are absrbed) The glbal exchange between zne and xygen is n the rder f 300 millin tns per day.

57 Ozne-Depleting Substances (ODS) Ozne-Depleting Substance(s) (ODS): a cmpund that cntributes t stratspheric zne depletin include CFCs, HCFCs, halns, methyl brmide, carbn tetrachlride, and methyl chlrfrm. very stable in the trpsphere and nly degrade under intense ultravilet light in the stratsphere. When they break dwn, they release chlrine r brmine atms, which then deplete zne. Chlrflurcarbn (CFC, Fren): a cmpund cnsisting f chlrine, flurine, and carbn CFCs are cmmnly used as refrigerants, slvents, and fam blwing agents.

58 Ozne lss recipe - summary Plar winter plar vrtex islates air within Cld temperatures Plar Stratspheric Cluds vrtex air islated cld temperatures & PSC s persist Hetergeneus reactins allw reservir species f chlrine & brmine - rapidly cnverted t mre active frms. N zne lss until sunlight returns prductin active chlrine initiates catalytic zne destructin zne lss rapid

59 The Antarctic plar vrtex

60 Ozne lss ver Antarctica mst dramatic in the lwer stratsphere nearly all the zne depleted area the size f Antarctica many km thick mst prnunced in spring/octber persists tw mnths December mves Falklands, S Gergia, S Am

61 Why is the lss mre dramatic at the ples? Plar meterlgy Plar vrtex winter plar night Plar stratspheric cluds (-80C) nitric acid trihydrate Chemical reactins ccur n surface PSC s Occur very fast