Condensation cloud nuclei

TABLE 15.3 Updraft Velocities and Maximum Supersaturations for Clouds and Fogs 

Cloud Type Updraft Velocity (m s ) Maximum Supersaturation (%) Reference 

Supersaturations of several hundred percent are necessary for the formation of water droplets in particle free air (see Chapter 10). The need for such high supersaturations indicates the necessity of particles for cloud formation in the ambient atmosphere. The ability of a given particle to serve as a nucleus for water droplet formation, as we have seen in the previous sections, will depend on its size, chemical composition, and the local supersaturation. 

Particles that can activate at a given supersaturation are defined as cloud condensation nuclei (CCN) for this supersaturation. In the cloud physics literature one often defines as condensation nuclei (CN) those particles that form droplets at supersaturations of 400% and therefore CN include all the available particles. One can therefore assume that the CN concentration is equal to the total aerosol number concentration. This CN definition should be contrasted with the CCN definition where supersaturations often well less than 2% are used. Therefore CCN represent the particles that can form cloud droplets under reasonable atmospheric supersaturations. We caution the reader that CCN concentrations always refer to a specific supersaturation, for example, CCN(1%) or CCN(0.5%) and one should be careful when comparing CCN concentrations measured or estimated at different supersaturations. 

The CCN concentration of a given supersaturation corresponds under ideal cloud formation conditions (e.g., spatial uniformity) to the number concentrations of droplets if the cloud had the same supersaturation. We will use the symbol CCN(5) for CCN at s% supersaturation. 

For a given aerosol population CCN(s) depends on both the size and composition of the particles. In the simple case of an aerosol population that has uniform size-independent composition, by definition, 

FIGURE 17.16 Schematic of an aerosol size distribution with the shaded area indicating the CCN for (a) uniform chemical composition and (b) a typical multicomponent aerosol population that is neither externally nor internally mixed and has a size-dependent composition. 

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Only two possibilities exist for explaining the existence of cloud formation in the atmosphere. If there were no particles to act as cloud condensation nuclei (CCN), water would condense into clouds at relative humidities (RH) of around 300%. That is, air can remain supersaturated below 300% with water vapor for long periods of fime. If this were to occur, condensation would occur on surface objects and the hydrologic cycle would be very different from what is observed. Thus, a second possibility must be the case particles are present in the air and act as CCN at much lower RH. These particles must be small enough to have small settling velocity, stay in the air for long periods of time and be lofted to the top of the troposphere by ordinary updrafts of cm/s velocity. Two further possibilities exist - the particles can either be water soluble or insoluble. In order to understand why it is likely that CCN are soluble, we examine the consequences of the effect of curvature on the saturation water pressure of water. 

If the reaction is between two gas-phase species, then this reaction could be a source of cloud condensation nuclei, or simply a means to neutralize an acidic aerosol. Although there are some questions concerning the measurement of atmospheric HNO3, (Lawson, 1988) most measurements indicate that gaseous HNO3 concentrations predominate over particle NOi". 

These particles can be a major fraction of the cloud condensation nuclei. 

Fig. 8.3 Dimethylsulfoniopropionate and dimethyl sulfide cycling in the open ocean. DMSO dimethylsulfoxide, CCN cloud condensation nuclei, MMPA 3-methylpropionate, ji-HP (S-hydrox-ypropionate, 3-MPA 3-mercaptopropionate, MeSH methanethiol, X-CH unidentified molecule with a terminal methyl group. (Reprinted from Yoch 2002, with permission from D. Yoch and the American Society for Microbiology)...

Halocarbon, Alkyinitrate, and DMS emissions to atmosphere Radiative forcing and production of cloud condensation nuclei (CCN)... 

Rogers, C. F J. G. Hudson, B. Zielinska, R. L. Tanner, J. Hallett, and J. G. Watson, Cloud Condensation Nuclei from Biomass Burning, in Global Biomass Burning Atmospheric, Climatic, and Biospheric Implications (J. S. Levine, Ed.), Chap. 53, pp. 431-438, MIT Press, Cambridge, MA, 1991. 

Quinn, P. K., D. S. Covert, T. S. Bates, V. N. Kapustin, D. C. Ranisey-Bell, and L. M. Mclnnes, Dimethylsulfide Cloud Condensation Nuclei Climate System Relevant Size-Resolved Measurements of the Chemical and Physical Properties of Atmospheric Aerosol Particles, J. Geophys. Res., 98, 10411-10427 (1993). 

Rivera-Carpio, C. A., C. E. Corrigan, T. Novakov, J. E. Penner, C. F. Rogers, and J. C. Chow, Derivation of Contributions of Sulfate and Carbonaceous Aerosols to Cloud Condensation Nuclei from Mass Size Distributions, . /. Geophys. Res., 101, 19483-19493 (1996). 

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. 

In short, while anthropogenically produced particles can act as cloud condensation nuclei, only a fraction of them actually do so. This fraction can be close to one... 

Andreae, M. O., W. Elbert, and S. J. de Mora, Biogenic Sulfur Emissions and Aerosols over the Tropical South Atlantic. 3. Atmospheric Dimethylsulfide, Aerosols, and Cloud Condensation Nuclei, J. Geophys. Res., 100, 11335-11356 (1995). 

Andrews, E., S. M. Kreidenwels, J. E. Penner, and S. M. Larson, Potential Origin of Organic Cloud Condensation Nuclei Observed at Marine Site, J. Geophys. Res., 102, 21997-22012 (1997). 

Bigg, E. K., Discrepancy between Observation and Prediction of Concentrations of Cloud Condensation Nuclei, Atmos. Res., 20, 82-86 (1986). 

Cruz, C. N., and S. N. Pandis, The Effect of Organic Coating on the Cloud Condensation Nuclei Activation of Inorganic Atmospheric Aerosol, J. Geophys. Res., 103, 13111-13123 (1998). 

Hegg, D. A., Heterogeneous Production of Cloud Condensation Nuclei in the Marine Atmosphere, Geophys. Res. Lett., 17, 2165-2168 (1990). 

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