Cloud Physical Properties Pertinent To Cloud Chemistry

Clouds consist of a suspension of liquid or solid (ice) particles in air. Thus, formally, a cloud is an aerosol, a suspension of particles in air. However, it is useful to distinguish clouds from clear-air (noncloud) aerosols. The cloud environment is slightly supersaturated with respect to liquid water or ice, respectively. The typical amount of condensed-phase water is 0.1 to 1 g/m3 (roughly equivalent to 0.1 to 1 /kg

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of air). The amount of condensed-phase water is substantially lower in cirrus clouds and in polar stratospheric clouds. For condensed-phase amounts substantially exceeding 1 g/m3, coagulation occurs and precipitation rapidly develops, removing condensed-phase water from the cloud.

A liquid water content of 1 g/m3 corresponds (within the approximation that the density of water is 1 kg/m3) to a liquid water volume fraction L = 1 x 10~6, or one part per million by volume. On dimensional grounds the separation between cloud droplets is ~L3 times the diameter of the droplets; fori = 1 x 10~6, the average interdrop separation is ~100 times the drop diameter. Thus clouds must be considered a sparse suspension of condensed-phase water. Clouds are mostly air. Thus any consideration of cloud chemistry must deal with both the gas phase and the condensed phase.

Despite this sparseness, clouds still contain much more condensed-phase material than cloud-free air. Consider a clear-air aerosol of mass loading of 100 |ig/rrT3; within the approximation of density equal to 1 kg/m3, the corresponding condensedphase volume fraction is 1 x 10~10. The much greater mass loading of a cloud leads among other things to its greater light scattering, the most distinguishing feature of clouds.

Clouds form when air, containing water vapor, is cooled to a temperature below its dew point. Typically this occurs when air is lifted, for example, buoyant rise of a convective parcel, or larger scale gentle upward motion of warm air over denser cooler air. Cooling by conduction can also be important, for example, in ground fogs, as can radiative cooling. The condensation process defines the number concentration of cloud droplets by activating a certain fraction of preexisting aerosol particles into cloud droplets (see Chapter 19). The number concentration is typically 100 to 1000/cm3 or 108 to 109/m3. Thus within the cloud the condensed-phase water is finely suspended. For droplet concentration of 1 x 109/m3 and liquid water volume fraction of 1 x 10-6 m3/m3, the corresponding volume of an individual droplet is 1 x 10"15 m3 and the corresponding diameter x 109 m or 10 |im.

Invariably there is a dispersion in the diameter of drops; that is, there is a spectrum of cloud droplet sizes. This influences mass transport processes, which are faster for smaller droplets, affecting uptake and reaction of gases in clouds. Typically cloud droplet distributions are rather sharply peaked. This is a consequence of the fact that mass transport of condensing water is faster for smaller droplets thereby allowing the smaller droplets, to "catch up" with the larger ones early in the cloud formation process.

Clouds persist in the atmosphere for a few tens of minutes (short-lived cumulus) to a few tens of hours (persistent stratus). Most clouds evaporate, rather than precipitate, thereby returning dissolved nonvolatile material to the clear air as aerosol particles.

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