Aerosol condensation processes

It is well established that in non-arid regions, precipitation is the primary means by which contaminating aerosols are removed from the atmosphere. Many chemical, physical, and meteorological parameters affect the micro, meso, and synoptic scale processes through which precipitation transports radioactive aerosols from atmosphere to ground. These parameters include the radioactivity component of the natural aerosols, the processes by which water vapor condenses and grows to raindrops, and the incorporation of the radioactive aerosol into the precipitation. Thus, the prediction of specific deposition from fundamental considerations has proved to be difficult because of the many uncertainties yet prevalent in these processes. Many attempts have been made to evaluate the deposition of these aerosols by empirical studies. 

It should be kept in mind that not all the atmospheric aerosol is available for the condensation process. In fact, it is only a small fraction of the total. As might be expected from reference to Fig. 14.2, the largest (and most soluble) nuclei are activated preferentially. Thus utilization of a given size of nuclei for condensation depends to a large extent on the degree of supersaturation present, and in the atmosphere this, in turn, depends on the rate of cooling of the air. 

It has been reported (1) that the recovery of pyrolysis liquids is not a simple condensation process, because their rapid indirect cooling is leading to the formation of stable aerosols and micron-size droplets, which are usually entrained in the gaseous stream, thus avoiding capture. Therefore, impingement and coalescence of the pyrolysis vapors is considered an essential feature in any liquids recovery process. 

Table II lists slope and intercept values for the linear-regression equations and Gaussian statistics, both for the full data set and for subsets categorized by various sampling, meteorological, or oceanographic conditions. The overall statistics for the cruise (Case I in Table II) indicate that the geometric mean salt aerosol concentration was 11.5 3.0 jug/SCM, and the arithmetic mean wind speed was 9.8 3.9 m/s. Generally, data are included in the table to show that condensation processes, hysteresis effects, and advection impose difficulties when trying to match the time series of local wind speed and salt aerosol concentrations.

Condensation processes may play a decisive roll by incorporating salt aerosol into larger droplets. Under these conditions the upper size limit of particles being sampled will determine the observed salt load. Thus, we... 

A simple physical relationship does not exist between synoptic measurements of wind speed and sea-salt aerosol concentrations in the marine atmosphere because of advection, hysteresis, condensation processes, and the varying stability of the marine boundary layer. In the region of the South Atlantic Ocean discussed in this chapter, the low correlation between the time series for sea-salt aerosol concentration and local wind speed is attributed to the high variability of the effects just mentioned. Removing the temporal constraint by ordering both data sets results in an extremely high (r = 0.99) correlation coefficient. This result provides promise for the... 

Monodisperse aerosols are almost always used to calibrate the instruments described previously. They are also important for perfomiance studies of gas-cleaning devices and in investigations of fundamental aerosol behavior such as light scattering. Aerosols composed of uniform particles can be produced by condensation processes or by atomization of liquids. 

The Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic hosts the leading Aerosol Laboratory in the country. The main effort is aimed at experimental study of individual parts of the condensation process (nucleation, condensation and evaporation, heat and mass transfer) as well as at more complex phenomena such as gas-phase synthesis of nano-particles, and combustion or atmospheric aerosols. 

The following sections describe, in turn, particulate size distributions, coagulation and condensation processes, production mechanisms, chemical composition, removal from the atmosphere, and the tropospheric budget of the aerosol. 

Because the detection of these particles is based on the condensation of a liquid, usually either water or butanol, surface effects come into play. The condensation process is the transfer of excess material in one of both phases gas or aerosol and is driven by evaporation/subUmation and condensation, which take place in parallel. Is the surface of an aerosol particle curved molecules can enter the gas phase more easily and the interaction between absorbed molecules is less in strength (Seinfeld and Pandis 2006). Therefore the saturation above a curved, i.e. smaller particle is larger and particles tend to evaporate. This effect is called Kelvin effect after Lord Kelvin, who figured out and explained the effect about 150 years ago. The second effect of relevance is the solution effect. Two different compounds with the potential to get dissolved in each other as for instance a salt in water, will stick to each other even at subsaturation because the evaporation/sublimation is drastically reduced (Raoult effect) (Friedlander 2000). 

Condensation processes are especially suitable for the cleaning of low flow highly concentrated streams of exhaust gas. The entire waste gas stream is cooled below the dew point of the vapors contained therein, so that these can condense on the surface of the heat exchanger (partial condensation). Theoretically, the achievable recovery rates depend only on the initial concentration, the purification temperature and the vapor pressure of the condensables at that temperature. In practice however, flow velocities, temperature profiles, the geometry of the equipment, etc. play decisive roles, as effects such as mist formation (aerosols), uneven flow in the condensers and uncontrolled ice formation interfere with the process of condensation and prevent an equilibrium concentration from being reached at the low temperatures. 

Approaching the formation of colloids from the other end of the size range involves one of several growth mechanisms. Such processes are commonly employed for the production of dispersions and aerosols, and less commonly in the production of emulsions. Typical examples of important condensation processes include fog formation (both water and chemical), silver halide emulsions (really dispersions) for use in photographic products, crystallization processes, colloidal silica, latex polymers, etc. 

Aerosols composed of solid particles suspended in a gas are commonly referred to as dust or smoke, the exact terminology usually depending on the size and sedimentation rate of the particles, or the method of aerosol formation. In some situations, aerosols formed through dispersion processes are termed dusts while those arising from condensation processes are called smokes. Alternatively, some prefer to label as dusts aerosols of sufficient particle size to have relatively rapid (e.g., noticeable over a short time span) sedimentation rates in air, while smokes would be of smaller, lighter particles. Regardless of the terminology employed, it is clear that solid aerosols constitute a very important, and usually undesirable, component of many modern processes. 

In the coarse particle mode, practically all aerosol particles at relative humidities below 100% originate from meehanical processes. Most of the particles originate from condensation processes occurring in the atmosphere. Coarse particles are produced from natural and/or man-made (anthropogenic) mechanical processes. The origin, behaviour and removal processes of fine particles are almost entirely independent of the coarse particles. 

The aerosol particles are formed either by coagulation and condensation processes or by gas-to-particle conversion. Analytically ... 

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. 

Condensation is the result of collisions between a gaseous molecule and an existing aerosol droplet when supersaturation exists. Condensation occurs at much lower values of supersaturation than nucleation. Thus, when particles already exist in sufficient quantities, condensation will be the dominant process occurring to relieve the supersaturated condition of the vapor-phase material. 

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