Nucleation, cloud

Shulman, M., M. Jacobson, R. Charlson, R. Synovec, and T. Young, Dissolution Behavior and Surface Tension Effects of Organic Compounds in Nucleating Cloud Droplets, Geophys. Res. Lett., 23, 277-280 (1996). 

Dimethyl sulfoxide reductases (DMSOR) of bacteria and fungi that catalyze the reduction of DMSO to dimethyl sulfide (DMS). These enzymes play a significant role in the global sulfur cycle, not least because DMS is volatile and is the precursor of the methylsulfonate aerosols that nucleate cloud formation (29). Furthermore, the distinctive smell of DMS acts as a guide to certain seabirds who use it to locate productive regions of the ocean (30). 

In determining their ability to nucleate clouds, the chemical composition of aerosol particles is much less important than their size, a result that will clarify aerosol effects on climate (Rosenfeld 2006). This is what is expected from theory because the radius-to-volume ratio determines the molecular transfer from the gas phase, whereas the hygroscopicity (surface characteristics of CCN) determines the uptake coefficient (Chapter 4.3.7.4). The other very important parameter for cloud formation is the CCN number, determining the cloud droplet number (cf Fig. 2.20). As shown originally by Twomey (1991), and recently reviewed by Lohmann and Feich-ter (2005), the sensitivity of climate to CCN number density is nonlinear, with the effect being much stronger at low particle numbers. 

Shulman ML, Jacobson MC, Carlson RJ, Synovec RE, Young TE (1996) Dissolution behavior and surface tension effects of organic compounds in nucleating cloud droplets. Geophys Res Lett 23 277-280... 

Nucleation in a cloud chamber is an important experimental tool to understand nucleation processes. Such nucleation by ions can arise in atmospheric physics theoretical analysis has been made [62, 63] and there are interesting differences in the nucleating ability of positive and negative ions [64]. In water vapor, it appears that the full heat of solvation of an ion is approached after only 5-10 water molecules have associated with... 

Fig. 9.1. Rain falls when the water droplets in clouds turn to ice. This con only happen if the clouds are below 0°C to begin with. If the droplets are clean, ice can form only in the unlikely event that the clouds cool down to the homogeneous nucleation temperature of -40°C. When dust particles are present they can catalyse nucleation at temperatures quite close to 0°C. This is why there is often heavy rainfall downwind of factory chimneys.

Fig. 9.2. The excellent crystallographic matching between silver iodide and ice makes silver iodide a very potent nucleating agent for ice crystals. When clouds at sub-zero temperatures are seeded with Agl dust, spectacular rainfall occurs.

Thus we proceed to examine the physical-chemical nature of the cloud nucleation process. 

Cloud nucleation also has chemical consequences. The soluble material of the CCN introduces solute into cloud droplets which, in many instances, is a major and even dominant ingredient of cloud and rainwater. A simple but useful expression for the amount of solute from CCN is... 

The condensed phases also are important to the physical processes of the atmosphere however, their role in climate poses an almost entirely open set of scientific questions. The highest sensitivity of physical processes to atmospheric composition lies within the process of cloud nucleation. In turn, the albedo (or reflectivity for solar light) of clouds is sensitive to the number population and properties of CCN (Twomey, 1977). At this time, it appears impossible to predict how much the temperature of the Earth might be expected to increase (or decrease in some places) due to known changes in the concentrations of gases because aerosol and cloud effects cannot yet be predicted. In addition, since secular trends in the appropriate aerosol properties are not monitored very extensively there is no way to know... 

SO4 in cloudwater Washout, rainout Dry deposition Cloud nucleation COS + OH multistep - HzS MSA - sol by some mechanism... 

Greenhouse - effect Albedo, cloud nucleation Stratospheric Ozone Reduction Direct Indirect... 

The interplay of these two basic rates determines the size of the resulting particles. For instance, the reason that snow flakes reach sizes of several cm at lower latitudes but arrive as extremely small crystals, called diamond dust in Antarctica, is that the nuclei that are formed in a cloud, will grow during their voyage to earth by adsorbing water molecules. Obviously, this growth will be more important in the moist atmosphere at low latitudes than in the extremely dry atmosphere above Antarctica. The same interplay of nucleation and growth determine the size of metal particles that are formed on a support by chemical reduction of adsorbed precursors, such as metal ions. Here... 

A flow stream produced from boiling water appears white in color. Similar to cloud in the sky, condensed water vapor shows a white color in the atmosphere. Humid air leads to condensation when nucleating materials are present in the atmosphere, producing a white-colored fog. However, condensed water vapor and fog appear as black smoke when the background is brighter than the foreground. 

However, one finds that, in cooling a liquid below its freezing point, the liquid may not always turn into solid phase at the freezing point. In fact, in some cases, such as water, even at around -40°C, liquid water does not turn into a solid phase. It stays in what is called a supercooled state. A major phenomena is the freezing of supercooled clouds. However, if certain so-called nucleating agents are used, then the clouds would turn into liquid droplets (and form rain). The nucleation process is a surface phenomena and is observed in transitions from... 

The explanation for this behavior is similar to that given in the preceding section for nonionic surfactant mixtures. Adding a hydrophihc anionic surfactant raises the temperature at the cloud point and other phase transitions above those for pure Ci2(EO)4. If the amount of anionic added exceeds only slightly that needed for complete solubility, the final stages of the dissolution process are slow because preferential dissolution of the anionic causes the remaining drop to rise above its cloud point and nucleate small droplets of surfactant-rich liquid. But if the amount added is sufficiently large, drop composition remains below the cloud point in spite of preferential dissolution, with the result that dissolution is fast as with pure nonionic surfactants below their cloud points. 

It is possible to dilute diesel fuel such as 2-D low sulfur with kerosene, 1 fuel oil, or jet fuel to reduce the fuel cloud point. Also, additives are also marketed which have the ability to inhibit nucleation of wax crystals in some fuels, thereby lowering the cloud point of the fuel. These products are called cloud point improvers. 

Certain cloud point improvers function by effectively inhibiting the nucleation of wax crystals. This can be accomplished by dispersion of the wax, thus interfering with nucleation. By functioning as an effective dispersant, certain cloud point improvers can help to solubilize water into fuel to give the fuel a cloudy, hazy appearance. As little as 200 ppm of a cloud point improver can create an opaque, relatively stable haze in treated distillate fuel. 

For a review of nucleation in the atmosphere, the reader is referred to Nucleation and Atmospheric Aerosols (Fukuta and Wagner, 1992 Kulmala and Wagner, 1996) and Microphysics of Clouds and Precipitation (Pruppacher and Klett, 1997). 

This has important implications for nucleation in the atmosphere. Condensation of a vapor such as water to form a liquid starts when a small number of water molecules form a cluster upon which other gaseous molecules can condense. However, the size of this initial cluster is very small, and from the Kelvin equation, the vapor pressure over the cluster would be so large that it would essentially immediately evaporate at the relatively small supersaturations found in the atmosphere, up to 2% (Prup-pacher and Klett, 1997). As a result, clouds and fogs would not form unless there was a preexisting particle upon which the water could initially condense. Such particles are known as cloud condensation nuclei, or CCN. 

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