Cloud formation by condensation

Clouds develop from condensation of water vapor to water droplets and ice particles. The probability depends critically on the saturated vapor pressure SVP and therefore on the temperature conditions in the saturated atmosphere layer. But it also depends on the size (radius), shape, and surface tension of the condensing water droplet, which is determined by its chemical constituents. The relative humidity U correlates with the temperature and the size of the water droplets that condense at cooler surfaces or other condensation points as expressed in the Kelvin formula.

r: radius of droplet : surface tension of liquid (H2O) - 810-2N/m : density of liquid (H2O) PH2O(r): vapor pressure over convex surface

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Condensation nuclei Random condensation to achieve critical radius is unlikely. The presence of condensation nuclei is necessary for condensation process. This lowers the surface tension (H2O=8·10-2 N/m) and therefore the required saturation level.

Super-saturation level in natural clouds U’≈0.1%! That is mostly insufficient for cloud condensation!

Optimum and most effective condensation nuclei are hygroscopic aerosols (e.g. sea salt, sodium chlorate, ammonium sulfate etc), which lower the relative humidity necessary for providing the critical droplet size. Sea salt provides condensation conditions at relative humidity of <100%, sulphuric and nitric acid particles provide already condensation at a low relative humidity of 75%.

Cloud condensation probability as a function of super-saturation for different condensation conditions.

continental air

arctic air

maritime air

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According to Raoult’s law for an ideal solution, the SVP of a water drop with radius r depends on the concentration of the condensation nuclei no compared to the one of the water molecules n.

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Example: Sea Salt condensation nuclei

For a relative humidity of 100.2% a solution droplet of 0.1m would form

For a relative humidity of 98% a solution droplet of 0.05m would form

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292

An aerosol is a suspension of fine m sized solid particles or liquid droplets in in the atmosphere. This includes natural sources such as sand dust from desert wind, haze and water condensates from the ocean, or volcanic ashes, as well as anthropogenic sources like sulfates from fuel combustion and smog from other kinds of industrial air pollution or also forest fires!

Aerosols

Aerosols and Climate

Aerosols decrease visibility by scattering and absorbing sunlight. This affects the optical depth ·d for electromagnetic radiation. Smaller particles reflect blue to UV light much stronger than larger particles which scatter and reflect all UV, visible, and IR light equally. Aerosol particles affect Earth's climate. They reflect sunlight, increasing the albedo and thus cool Earth's surface. They compensate for the effects of greenhouse gases such as CO2, which absorb the heat escaping from Earth's surface and thus heat Earth's surface and lower atmosphere. This triggers plans of climate engineering by aerosol ignition into the stratosphere. Aerosols serve as in situ condensation nuclei and thus modify cloud amount, cloud distribution, and cloud properties. Aerosols may also influence precipitation amount, distribution, and frequency since they enhance cloud condensation (rain maker technique).

Optical depth Many of the aerosol characteristics are described in terms of the optical depth, which reflects the opacity or the capability to look through a layer of aerosols. This basically correlates with the absorption of light in aerosol filled layers.

I

I

I

I

eIeII d

0

0

00

lnln

: optical depth depends on thickness of layer and on the absorption coefficient (scattering cross section) of aerosols

Beijing on a semi-clear day and a smoggy day

Traffic & Industry

Optical depth over China

Plumes of smoke and regional pollution show large concentrations of small particles, less than 1 m (green areas) of biomass burning sites and urban areas.

Estimates of annual emission of different natural and anthropogenic sources

1Tg=1012g=106tons=1Mton

Ship tracks through aerosol emission

With aerosols from ship exhaust more but smaller droplets are generated for cloud formation. That modifies the reflective power of the cloud since with smaller droplets, you have more condensation points and more backscattering opportunities.

Emission of aerosols in Tg in 2000

Daily variations of aerosol emission in Los Angeles

Subsequent (in situ) chemistry between aerosol molecules and air molecules such as O2 and N2 can lead to the formation of more complex molecules ranging from O3 (ozone) to hydrocarbons and polycylic hydrocarbons (photochemical smog).

Size of aerosols Atmospheric aerosol particles range in size from 10-4 m to 50 m diameter. For most of the aerosol particles, however, the typical diameter is between D=0.01 and 0.1m with concentrations falling rapidly off towards larger sized particles. The distribution follows roughly a logarithmic behavior and can be approximated by:

DCDd

dN

DconstDd

dN

log

loglog

log

C corresponds to the concentration of particles and correlates with the fall-off of the distribution; =2-4, depending on the source and nature of aerosol particles, typically ≈3.

The distribution of aerosols of continental, marine, and urban origin!

Global distribution of fires detected by ESO satellite in September 2000

Carbon monoxide distribution in 4.5 km altitude

Aerosol distribution and optical depth ·d

Sand storm in the Western Sahara in February 2000 extending over the Canary Islands and the Atlantic Ocean. Up to 50% of incoming UV radiation is absorbed. Dust becomes fertilizer for the Amazon jungle!

Dust storm in China. April 1998 with dust transported across the Pacific Ocean to West Coast of North American Continent.

Sources and sinks for nitrogen and sulfur containing molecules in the atmosphere

Numbers give the averaged annual fluxes for emission and absorption in Tg

The balance between emission (sources) and absorption is provided by the chemical cycles: carbon cycle, nitrogen cycle, and sulfur cycle. Balance may shift with enhanced production!

Cooling effects of aerosols

The efficiency of the cooling effect of aerosols is still a matter of debate, yet proponents of geo-engineering methods for controlling climate by aerosol emission view it as a possible tool. The main concern are unforeseen secondary chemistry effects.

Volcano based aerosol emission demonstrates convincingly a cooling effect of aerosols

Climate forcing