Condensed Water Molecules Change in number of liquid molecules Critical Radius and Supersaturation Integrate then find maximum Cloud Droplet Nucleation particle activation - process by which droplets (several micron in size) are formed (or activated) from primarily submicron particles; also called heterogeneous nucleation or just nucleation by cloud physicists process illustrates the conditions required for growth to droplets the approach used assumes that this formation is an equilibrium process Bubbles Liquid (H 2 O/EtOH) supersaturated with vapor (CO 2 ) nucleates on salt to form bubbles Clouds Vapor (air) supersaturated with liquid (H 2 O) nucleates on particles to form droplets 1
7.1 Activation on Particles Soluble salts dissolve Kelvin effect S v,w = exp 2M wσ s / a RTρ w a υφ sm s M w / M s 4πa 3 3 ρ " s m s Activation of CCN to Droplets Variation of the equilibrium vapor pressure of an aqueous solution drop containing ammonium sulfate and insoluble material Initial dry particle diameter 0.1 µm Soluble mass fractions 0.2, 0.4, 0.6, 1.0 at 293K Seinfeld and Pandis, Fig. 15.8 Köhler Curves Köhler curves for sodium chloride and ammonium sulfate Dry diameters 0.05, 0.1, 0.5 µm Supersaturation of 1% is equivalent to 101% relative humidity Seinfeld and Pandis, Fig. 15.5 2
Lecture Ch. 5b Chapter 5, Problem 3 homework Aerosol particles CCN Indirect effect Chemical and Surface Effects Curry and Webster, Ch. 5 (skip 5.6, 5.7) For Thursday: Read Ch. 5 For Exam: Homework Problem 7, p. 158 (7d misprint) Simplified Köhler Equation Maximum ds/dr=0 Critical Radius and Supersaturation S(r) =1+ a r b r 3 ds dr = a r + 3b 2 r 4 @S*,r*: ds dr = 0 = a r * + 3b 2 r * 4 ar * 2 = 3b r* = 3b a S* =1+ a r * b r * 3 =1+ a3 3b a 3 27b =1+ 4a3 27b 3
Aerosol-Cloud Interactions Activation: Köhler 1921 (Aitken) CCN/Droplets: Twomey 1959 (Conover) Particle Growth: Hoppel 1986 (Frick) Precipitation Suppression: Albrecht 1989 (Rosenfeld, Ackerman, Ramanathan) Cloud Condensation Nuclei (CCN) cloud condensation nuclei - those particles which have large enough radii and enough solute content to activate to particles at a prescribed supersaturation in marine air, only 50% of all aerosol particles may be CCN for typical clouds, with supersaturations ~ 0.05%-0.5% 7.1 1.1 1.1 What is an Aerosol? Particle Sources Suspension of liquid or solid particles in a vapor phase Colloidal suspension that may be stable for <1 s or > 1 yr combustion incineration road dust power plants automobiles sea salt biogenic volcanoes Particle + Vapor = AEROSOL 1.1 Size Range for Particle Sources Particle Type automotive emissions Size Range 0.01 µm to 1 µm bacteria 0.2 µm to 10+ µm tobacco smoke 0.01 µm to 1 µm viruses 0.002 µm to 0.05 µm 4
Classification of Pollutants Fine Particles less than 2.5 µm in diameter Coarse Particles greater than 2.5 µm in diameter Particle Types and Sizes Bacteria Black Carbon (Soot) Tobacco Smoke Viruses Gas Molecules 0.0001 0.001 0.01 0.1 1 10 Particle size (microns) Number (dn/dlogd p ), cm -3 x10 3 Volume (dv/dlogd p ), µm 3 /cm 3 Fine and Coarse s 40 30 20 10 0 40 30 20 10 Ultrafine Particles Nucleation Fine Particles Condensation Submode Aitken Accumulation Coarse Particles Droplet Submode Coarse 0 0.01 0.1 1 10 Diameter (micrometers) 7.1 Cloud Condensation Nuclei Cloud Processing Hoppel Minimum dn dlogd Activation Evaporation Particle evolution in remote marine conditions cloud processing growth of particles due to coalescence and solute condensation in cloud 0.1 10 Diameter (µm) 0.1 10 Diameter (µm) 0.1 10 Diameter (µm) Seinfeld and Pandis, Fig. 15.23 (Hoppel et al., 1990) How do Aerosols cool? Aerosol direct effects cause cooling by reflecting more light (e.g. smog). clear more reflection smoggy Ship Tracks Coakley et al., Science, 1987 Aerosol indirect effects cause cooling by clouds that reflect more light (e.g. tracks). normal more reflection whiter 5
Turkey City Tracks Manitoba, Canada South Australia Yellow indicates polluted clouds >>Polluted clouds cause cooling Rosenfeld, Science, 2000 6