Generation of Sea-Salt Aerosols

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 • Seen as... • Why red?

Aerosol Types • Natural: Soil dust; Sea salt; Volcanic dust; Organic particles. • The aerosols are: • Man-made: Industrial sulfates; Burning biomass; Soot; Organic particles. • The most numerous are:

Climate Effects of the Aerosols • Aerosols can influence climate: • Indirectly: • Directly: - 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.

Cloud Lifetime • More aerosols inhibit the growth of cloud droplets. • Droplet condensation and rain are delayed. • Net result: prolonged cloud lifetime.

The effect of background aerosols. • Assess the effect of man-made aerosols. Aerosols in Climate Models • Cooling effect. • Inclusion of aerosol effects in climate models improves the predictions.

Phytoplankton Pollution Aerosol Cooling • Man-made sulfates. • Natural sulfates.

Relative Abundance • Sea-salt aerosols dominate in clean air. • Sulfate aerosols dominate in polluted air.

Sea-Salt Aerosols • Major constituent of a remote marine atmosphere.

Sea-Salt Aerosols and Climate • The 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 (=1.3109 kg) sea salt annually.

Wave Breaking • As waves break, • and forms clouds of bubbles. • air mixes into the water,

and while floating on the water, burst. Sea Spray Formation • The large bubbles rise to the surface,

Film Drops • Upon bursting, bubble caps shatter into thousands of small droplets. Film drops form.

Jet Drops • The 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 Spray • So, whitecaps fill the air with film, jet, and spume drops. • In the air, this sea spray: • evaporates, • shrinks, • and forms sea-salt aerosols.

r, m 0.1 1 10 100 500 Sea Spray Sizes • Depend on the generation mechanism: • Film drops: 0.5 to 5 m • Jet drops: 3 to 50 m • Spume drops: 20 to > 500 m

r, m 0.1 1 10 100 500 Sizes Relevant to Aerosol Cooling • Long residence time. • Gravitational settling. Aerosol cooling Heat exchange

Indirect; • Direct. r, m 0.1 1 10 100 500 Modeling Sea Spray dF/dr = dF0 /dr + dF1 /dr (Monahan et al., 1986)

d F1 /d r = 0 r 10 m d F1 /d r = 8.6010-6e 2.08U r -2 10 m r 75 m d F1 /d r = 4.83  10-2e 2.08U r -4 75 m r 100 m -B2 d F0 /d r = 1.373 U103.41 r -3(1 + 0.057r1.05) 101.19e 10 B = (0.38 - log r) / 0.65 10 Explicit Expressions d F/d r = f (U10 , r)

Wind Dependence Fitzgerald, 1991

Number of aerosols , cm-3 Surface area , m2 cm-3 Radius, m Size Dependence O’Dowd et al., 1997

Best available Improvements In Climate Models d F/d r = d F0 /d r + d F1 /d r

W (U10) W (U10 , T, Tw , S, f , d , C) My Research d F0 /d r = f (U10 , r)

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Generation of Sea-Salt Aerosols

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