Nucleation in the Atmosphere

The first evidence of in situ particle formation in the atmosphere was provided by John Aitken at the end of the nineteenth century (Aitken 1897). He built the first apparatus to measure the number of dust and fog particles in the atmosphere. However, little progress was made in understanding what causes new particle formation or how widespread it might be for almost a century. In the 1990s the development of instruments capable of measuring the size distribution of particles as small as 3 nm led to the discovery that nucleation and growth of new particles is a rather common event in many areas around the world (Kulmala ct al. 2004). Areas where frequent nucleation bursts have been observed include 

The free troposphere, especially near the outflows of convective clouds and close to the tropopausc 

Eastern United States, Germany, and other European countries 

One surprising finding is that nucleation events take place even in relatively polluted urban areas. For example, Stanier et al. (2004) reported observations of nucleation events in Pittsburgh, PA during 30% of the days of a full year, The events take place on sunny, relatively clean days. A comprehensive review of atmospheric nucleation observations can be found in Kulmala et al. (2004). 

FIGURE 11.15 Evolution of particle size distribution on August 11, 2001 in Pittsburgh, PA, a day with new particle formation and growth. Particle number concentration (z axis) is plotted against time of day (x axis) and particle diameter (y axis). Measurements by Stanier et al. (2004). 

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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. 

Nucleation in the atmosphere is essentially multicomponent process. However, a commonly used classical approach incapable of the quantitative treatment of multicomponent systems due to (a) excessive sensitivity to poorly defined activity coefficients, density and surface tension of multicomponent solutions (b) strong dependence of nucleation rates on thermochemistry of initial growth steps where... 

K.D. Froyd, Ion induced nucleation in the atmosphere Studies of NH3, H2SO4, and H2O cluster ions, Ph.D. thesis, Univ. Colorado, Boulder, (2002). 

This relation suggests that ice nucleation in the atmosphere is a very selective process. For example, at temperatures as low as —20°C the atmosphere typically contains 1INL 1 and 106 particles L1. So, at most, one out of a million particles can serve as an IN. 

Currently, there is much uncertainty about the mechanisms of ice nucleation in the atmosphere, but it is thought that ice nuclei operate by three basic modes. In one mode, water is absorbed from the vapor phase onto the surface of the ice nucleus, and at sufficiently low temperatures, the adsorbed vapor is converted to ice. In another mode, the ice nucleus, which is inside a supercooled droplet either by collection or as a result of its participation in the condensation process, initiates the ice phase from inside the droplet. In the third mode, the ice nucleus initiates the ice phase at the moment of contact with a supercooled droplet (such nuclei are known as contact nuclei). The relative importance of these different modes of operation is not known with certainty, but the latter two modes are thought to be much more common than the first. 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

The secondary source of fine particles in the atmosphere is gas-to-particle conversion processes, considered to be the more important source of particles contributing to atmospheric haze. In gas-to-particle conversion, gaseous molecules become transformed to liquid or solid particles. This phase transformation can occur by three processes absortion, nucleation, and condensation. Absorption is the process by which a gas goes into solution in a liquid phase. Absorption of a specific gas is dependent on the solubility of the gas in a particular liquid, e.g., SO2 in liquid H2O droplets. Nucleation and condensation are terms associated with aerosol dynamics. 

The properties of water near ionic salt surfaces are of interest not only for the understanding of the mechanism of dissolution processes but also for the understanding of the chemistry in the atmosphere next to oceans [205]. Experiments in UHV [205-208] indicate that the water-covered NaCl surface is quite stable at low temperatures. An early simulation study by Anastasiou et al. [209] focused on the arrangements and orientations of water molecules in contact with a rigid NaCl crystal. Ohtaki and coworkers investigated the dissolution of very small cubic crystals of NaF, KF, CsF, LiCl, NaCl, and KCl [210] and the nucleation [211] of NaCl and CsF in a... 

For condensable precursors, particle formation may occur by homogeneous or heterogeneous nucleation. It is generally accepted that heterogeneous processes are most likely in the atmosphere, because of the large number of nuclei present. 

Assuming that the rate of gas-to-particle conversion of any condensable species is greater than its rate of formation in the gas phase (which is the case for heten eneous nucleation predominant in the atmosphere, but may not be valid for homogeneous nucleation in clean-air smog-chamber studies) ... 

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. 

The vapor pressure of H2S04 above solutions with water depends on the solution composition and the temperature. For example, the vapor pressure at 25°C varies from 2.6 X 10-9 Pa for a 54.1 wt% H2S04-H20 solution to 5.9 X 10 6 Pa for a 76.0 wt% solution (Marti et al., 1997). The vapor pressures above solutions partially neutralized with ammonia are also reported by Marti et al. (1997) as discussed in Chapter 9.B.1, the vapor pressures of the partially neutralized solutions are orders of magnitude smaller than those of the acid. As a result, ammonia may play an important role in nucleation of gaseous sulfuric acid in the atmosphere to form new particles. 

The term binary homogeneous nucleation is used to describe the formation of particles from two different gas-phase compounds such as sulfuric acid and water such nucleation can occur when their individual concentrations are significantly smaller than the saturation concentrations needed for nucleation of the pure compounds. It is believed that in the atmosphere, formation of particles from low-volatility gases occurs not by condensation of a single species but rather by the formation and growth of molecular clusters involving at least two, and as described shortly, probably three or more different species. 

The first issue is that of formation of new particles. As discussed in Chapter 9.B, nucleation of gases to form new particles in the atmosphere is not well understood. The observed rates of nucleation of H2S04, for example, greatly exceed the calculated rates. An important contributor to the formation of new particles in the boundary layer (BL) under some conditions appears to be exchange between the BL and the free troposphere (e.g., Davison et al., 1996 Raes et al., 1997 Clarke et al., 1997). For example, some of the DMS from the oceans can be carried to the free... 

Since the production rates of the cosmic ray radionuclides increase rapidly with increasing altitude in the lower atmosphere, the atmospheric concentrations and ratios of short lived cosmic ray radionuclides can be used to study rapid vertical air motions if the equilibrium concentrations of the radionuclides are known. For example, the concentrations of the short lived cosmic ray radionuclides in air which has moved upward recently from a lower altitude will be less than the equilibrium concentrations. The concentrations of the radionuclides will therefore increase with time until equilibrium is reached. However, the concentration of the shorter lived of two short lived radionuclides will increase more rapidly initially, causing the ratio of the two radionuclides of different half-lives to change with time until equilibrium is reached. Therefore, the time since the air moved from a lower altitude, the speed of the upward motion, and the altitude from which the air originated could be calculated from the concentrations and concentration ratios of cosmic ray radionuclides of different half-lives. Vertical motions of different speeds could be studied since several cosmic ray radionuclides of different half-lives are present in the atmosphere (Table I). Many other radionuclides are produced by cosmic rays in the atmosphere, but they have not yet been detected. Some of these with half-lives of a few minutes could serve as tracers of very short term processes such as post-nucleation scavenging. 

Equilibrium vapor pressure is the vapor pressure of a system in which two or more phases or a substance coexist in equilibrium. In meteorology, the reference is to water substance, unless otherwise specified, If the system consists of moist air in equilibrium with a plane surface of pure water or ice, the more specialized term saturation vapor pressure is usually employed, in which case, the vapor pressure is a function of temperature only. In the atmosphere, the system is complicated by the presence of impurities in liquid or solid water substance (see also Raoult s Law), drops or ice crystals or both, existing as aerosols and, in general, the problem becomes one of nucleation. For example, the difference in vapor pressure over supercooled water... 

Yu, F., and R. Turco, Aerosol formation via ion-mediated nucleation in the lower atmosphere, Geophys. 

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