Bridging the Gap October 30 - 31 , 1999. Generation of Sea-Salt Aerosols. Magdalena Anguelova. Polluted (urban) air. Clean (rural) air. What are Aerosols?. Aerosols are minute stable particles, solid or liquid, suspended in the atmosphere. Tiny Particles. - PowerPoint PPT Presentation
Generation of Sea-Salt Aerosols
Generation of Sea-Salt AerosolsMagdalena Anguelova
Bridging the GapOctober 30 - 31 , 19991What are Aerosols?Aerosols are minute stable particles, solid or liquid, suspended in the atmosphere.
Clean (rural) air
Polluted (urban) air2Tiny Particles
Seen as...Why red?3 Rayleigh scattering: At sunset the sunlight travels longer path through the atmosphere. The coming light loses most of the blue component due to scattering and retains most of the longer wavelengths, i.e., red light. Mie scattering: At sunset the light travels dominantly through the lower atmospheric layers where larger particles reside. Though the Mie scattering does not depend strongly on the wavelength, it tends to be more efficient for longer wavelengths.
Aerosol TypesThe aerosols are:Natural:Man-made: Soil dust;Sea salt;Volcanic dust;Organic particles.Industrial sulfates;Burning biomass;Soot;Organic particles.The most numerous are:
4Climate Effects of the Aerosols
Aerosols can influence climate:Directly:Indirectly:- by reflecting incoming sunlight back to space.- by modifying the properties and lifetime of the clouds.Cloud Properties
More aerosols = more cloud droplets.Clouds reflect more sunlight.Net result: cooling.6Cloud Lifetime
More aerosols inhibit the growth of cloud droplets.Droplet condensation and rain are delayed.Net result: prolonged cloud lifetime.Aerosols in Climate ModelsInclusion of aerosol effects in climate models improves the predictions.
Assess the effect of man-made aerosols.
The effect of background aerosols.Cooling effect.Aerosol CoolingMan-made sulfates. Natural sulfates.
Sulfate aerosols dominate in polluted air.Sea-salt aerosols dominate in clean air.Sea-Salt AerosolsMajor constituent of a remote marine atmosphere.
Sea-Salt Aerosols and ClimateThe major contributor to sunlight reflection.
Facilitate the formation of cloud drops. Medium for chemical reactions in the atmosphere. Oceanic Whitecaps
The major source for sea-salt drops.1.3 GT (=1.3109 kg) sea salt annually.13Wave BreakingAs waves break,air mixes into the water,and forms clouds of bubbles.
14Sea Spray Formation
The large bubbles rise to the surface,and while floating on the water, burst.
Film DropsUpon bursting, bubble caps shatter into thousands of small droplets.
Film drops form.Jet DropsThe bubble cavity collapses.
Jet drops form.Spume Drops
Under very high winds drops are torn from the wave crests and blown directly into the air. Spume drops form.Sea SprayIn the air, this sea spray:sea-salt aerosols. evaporates, shrinks, and forms So, whitecaps fill the air with film, jet, and spume drops.
Sea Spray SizesFilm drops: 0.5 to 5 mJet drops: 3 to 50 mSpume drops: 20 to > 500 mr, m5001001010.1Depend on the generation mechanism:Sizes Relevant to Aerosol CoolingLong residence time.Gravitational settling.Heat exchangeAerosol coolingr, m5001001010.1Modeling Sea SpraydF/dr = dF0 /dr + dF1 /drr, m5001001010.1 Indirect; Direct.(Monahan et al., 1986)Explicit Expressionsd F0 /d r = 1.373 U103.41 r -3(1 + 0.057r1.05) 101.19e-B2B = (0.38 - log r) / 0.65d F1 /d r = 0 r 10 md F1 /d r = 8.6010-6 e 2.08U r -2 10 m r 75 md F1 /d r = 4.83 10-2 e 2.08U r -475 m r 100 m1010d F /d r = f (U10 , r)
Wind DependenceFitzgerald, 1991Size Dependence
Radius, mNumber of aerosols , cm-3ODowd et al., 1997
Surface area , m2 cm-3In Climate Modelsd F/d r = d F0 /d r + d F1 /d rImprovements Best availableMy Researchd F0 /d r = f (U10 , r)W (U10 , T, Tw , S, f , d , C) W (U10)