Cloud formation by condensation

Transcription

1 Cloud formato by codesato U ( r) SV r m RT r: radus of droplet : surface teso of lqud ( ) N/m : desty of lqud ( ) (r): vapor pressure over covex surface e Clouds develop from codesato of water vapor to water droplets ad ce partcles. The probablty depeds crtcally o the saturated vapor pressure SV ad therefore o the temperature codtos the saturated atmosphere layer. But t also depeds o the sze (radus), shape, ad surface teso of the codesg water droplet, whch s determed by ts chemcal costtuets. The relatve humdty U correlates wth the temperature ad the sze of the water droplets that codese at cooler surfaces or other codesato pots as expressed the Kelv formula.

2 Kelv s Law A droplet must grow utl ts SV exceeds the SV of the surroudg gas or t becomes supersaturated U. The crtcal radus for that stuato to occur requres a radus r: r m R T lu( r) U=.0, r 0.00m U=.05, r 0.04m U=.0, r 0.0m U=.00, r.0m Supersaturato U '( %) ( U ) 00 r ( m) 0 3 U ' U SV

3 Codesato ucle Radom codesato to acheve crtcal radus s ulkely. The presece of codesato ucle s ecessary for codesato process. Ths lowers the surface teso ( =8 0 - N/m) ad therefore the requred saturato level. Super-saturato level atural clouds U 0.%! That s mostly suffcet for cloud codesato! ptmum ad most effectve codesato ucle are hygroscopc aerosols (e.g. sea salt, sodum chlorate, ammoum sulfate etc), whch lower the relatve humdty ecessary for provdg the crtcal droplet sze. Sea salt provdes codesato codtos at relatve humdty of <00%, sulphurc ad trc acd partcles provde already codesato at a low relatve humdty of 75%.. cotetal ar martme ar arctc ar Cloud codesato probablty as a fucto of super-saturato for dfferet codesato codtos.

4 Accordg to Raoult s law for a deal soluto, the SV of a water drop wth radus r depeds o the cocetrato of the codesato ucle o compared to the oe of the water molecules r b U r N U U U U U x x x x x x x x x A NaCl NaCl NaCl NaCl NaCl NaCl soluto soluto fracto mole x pressure partal pressure vapor r b r a r b e r b e U r a R T m r Kohler curve

5 relatve humdty Example: Sea Salt codesato ucle r 3b a U 3 4 a 7b.04E+00.0E+00 Kelv Curve Water Raoult Term Soluto Kohler Curve NaCl Soluto a m R T 9 For a relatve humdty of 00.% a soluto droplet of 0.m would form For a relatve humdty of 98% a soluto droplet of 0.05m would form b NaCl N A.00E E E E radus m

6 Aerosols A aerosol s a suspeso of fe m szed sold partcles or lqud droplets the atmosphere. Ths cludes atural sources such as sad dust from desert wd, haze ad water codesates from the ocea, or volcac ashes, as well as athropogec sources lke sulfates from fuel combusto ad smog from other kds of dustral ar polluto or also forest fres!

7 Aerosols ad Clmate Aerosols decrease vsblty by scatterg ad absorbg sulght. Ths affects the optcal depth d for electromagetc radato. Smaller partcles reflect blue to UV lght much stroger tha larger partcles whch scatter ad reflect all UV, vsble, ad IR lght equally. Aerosol partcles affect Earth's clmate. They reflect sulght, creasg the albedo ad thus cool Earth's surface. They compesate for the effects of greehouse gases such as C, whch absorb the heat escapg from Earth's surface ad thus heat Earth's surface ad lower atmosphere. Ths trggers plas of clmate egeerg by aerosol gto to the stratosphere. Aerosols serve as stu codesato ucle ad thus modfy cloud amout, cloud dstrbuto, ad cloud propertes. Aerosols may also fluece precptato amout, dstrbuto, ad frequecy sce they ehace cloud codesato (ra maker techque).

8 ptcal depth May of the aerosol characterstcs are descrbed terms of the optcal depth, whch reflects the opacty or the capablty to look through a layer of aerosols. Ths bascally correlates wth the absorpto of lght aerosol flled layers. I I 0 e d I l l I 0 I 0 I I e 0 : optcal depth depeds o thckess of layer ad o the absorpto coeffcet (scatterg cross secto) of aerosols Bejg o a sem-clear day ad a smoggy day

9 Traffc & Idustry

10 ptcal depth over Cha

11 lumes of smoke ad regoal polluto show large cocetratos of small partcles, less tha m (gree areas) of bomass burg stes ad urba areas.

12 Estmates of aual emsso of dfferet atural ad athropogec sources Tg=0 g=0 6 tos=mto

13 Shp tracks through aerosol emsso Wth aerosols from shp exhaust more but smaller droplets are geerated for cloud formato. That modfes the reflectve power of the cloud sce wth smaller droplets, you have more codesato pots ad more backscatterg opportutes.

14 Emsso of aerosols Tg 000 Daly varatos of aerosol emsso Los Ageles Subsequet ( stu) chemstry betwee aerosol molecules ad ar molecules such as ad N ca lead to the formato of more complex molecules ragg from 3 (ozoe) to hydrocarbos ad polycylc hydrocarbos (photochemcal smog).

15 Sze of aerosols Atmospherc aerosol partcles rage sze from 0-4 m to 50 m dameter. For most of the aerosol partcles, however, the typcal dameter s betwee D=0.0 ad 0.m wth cocetratos fallg rapdly off towards larger szed partcles. The dstrbuto follows roughly a logarthmc behavor ad ca be approxmated by: log d d dn log D dn log D cost C D log D C correspods to the cocetrato of partcles ad correlates wth the fall-off of the dstrbuto; =-4, depedg o the source ad ature of aerosol partcles, typcally 3. The dstrbuto of aerosols of cotetal, mare, ad urba org!

16 Global dstrbuto of fres detected by ES satellte September 000 Carbo mooxde dstrbuto 4.5 km alttude

17 Aerosol dstrbuto ad optcal depth d

18 Sad storm the Wester Sahara February 000 extedg over the Caary Islads ad the Atlatc cea. Up to 50% of comg UV radato s absorbed. Dust becomes fertlzer for the Amazo jugle! Dust storm Cha. Aprl 998 wth dust trasported across the acfc cea to West Coast of North Amerca Cotet.

19 Sources ad sks for troge ad sulfur cotag molecules the atmosphere Numbers gve the averaged aual fluxes for emsso ad absorpto Tg The balace betwee emsso (sources) ad absorpto s provded by the chemcal cycles: carbo cycle, troge cycle, ad sulfur cycle. Balace may shft wth ehaced producto!

20 Coolg effects of aerosols Volcao based aerosol emsso demostrates covcgly a coolg effect of aerosols The effcecy of the coolg effect of aerosols s stll a matter of debate, yet propoets of geo-egeerg methods for cotrollg clmate by aerosol emsso vew t as a possble tool. The ma cocer are uforesee secodary chemstry effects.

21 Clmate forcg