Cloud and aerosol particles

For incident unpolarized radiation, a complete analysis leads to 

At the other extreme the particle is large compared with the wavelength (a A) and the concepts employed in geometric optics and Fraunhofer diffraction theory can be used to advantage. Let a beam of parallel radiation be incident on any surface element of the spherical particle, and require the width D of the beam to be much larger than X and much smaller than a i.e., X D a. In geometric optics such a beam is called a ray. 

If incidence is grazing, Fraunhofer diffraction will occur around the edge of the particle. Because D X, the diffracted radiation will be concentrated in a very narrow cone, the axis of which is along the direction of incidence. On the other hand, if the point of incidence is not near the edge of the particle, diffraction is unimportant and only refraction and reflection in accordance with Snell s laws [Eqs. (1.6.4) and (1.6.6)] with certain modifications need be considered. 

In practice a few corrections to this procedure are required. The simple theory fails at a focus or focal line, and phase shifts occur throughout the process. If these phase shifts do not average out (i.e., if the coherence of all rays is not completely destroyed), interference between outgoing rays will disallow a simple addition of outgoing fluxes. In order to minimize this complication it is necessary to introduce the further requirement that a( r — 1) X, where is the real part of the refractive index. 

Cloud particles are basically dielectrics and thus tend to be rather poor reflectors. Hence, even in the absence of strong absorption, only the lower values of j will contribute significantly to the scattered radiation field. From the figure it would appear that the component j = I plus Fraunhofer diffraction contribute almost all the radiation at small scattering angles (forward directions), and that the components j =0,2 contribute most of the scattered radiation at other angles. It 

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Van Dingenen, R., F. Raes, and N. R. Jensen, Evidence for Anthropogenic Impact on Number Concentration and Sulfate Content of Cloud-Processed Aerosol Particles over the North Atlantic, J. Geophys. Res., 100, 21057-21067 (1995). 

Particle Measurements. A variety of instruments is available for measuring the number density and size distribution of particles sampled from airborne platforms. This discussion is restricted to instruments that measure particles smaller than 50 xm (cloud droplets and aerosol particles) because these particles are of most interest to atmospheric chemists. 

Figure 5.2. Various interactions that determine the composition of a water droplet in the atmosphere (e.g., cloud, fog). Aerosol particles, which to a large extent consist of (NH4)2S04 and NH4NO3, can form the nuclei for the condensation of liquid water. Various gases can become absorbed into the aqueous phase. The atmosphere is an oxidative environment the water phase, often assisted by light, promotes oxidation reactions, for example, the oxidation of SO2 to H2SO4 and of organic matter to CO2. NH3 neutralizes mineral acids and buffers the solution phase.

While the product is in the aerosol can or nebuliser vial, the dosage form that the patient experiences is a dynamic aerosol cloud. For example, the quality of the cloud from a DPI is often dependent on the airflow through the device, and a MDI cloud rapidly evaporates as it travels through the air. These factors affect the particle size distribution in the aerosol cloud, and the particle size is the critical parameter for lung deposition. 

The first challenge concerns the involvement of multiple phases in wet deposition. Not only does one deal with the three usual phases (gas, aerosol, and aqueous), but the aqueous phase can be present in several forms (cloudwater, rain, snow, ice crystals, sleet, hail, etc.), all of which have a size resolution. To complicate matters even further, different processes operate inside a cloud, and others below it. Our goal will initially be to create a mathematical framework for this rather complicated picture. To simplify things as much as possible we consider a warm raining cloud without the complications of ice and snow. There are four media or phases present, namely, air, cloud droplets, aerosol particles, and rain droplets. A given species may exist in each of these phases for example, nitrate may exist in air as nitric acid vapor, dissolved in rain and cloud droplets as nitrate, and in various salts in the aerosol phase. Nonvolatile species like metals exist only in droplets and aerosols, while gases like HCHO exist only in the gas phase and the droplets. The size distribution of cloud droplets, rain droplets, and aerosols provides an additional complication. Let us initially neglect this feature. For a species i, one needs to describe mathematically its concentration in air C(,air, cloudwater C,[C 0ud, rainwater C .rain, and the aerosol phase Qpan- We assume that all concentrations are expressed as moles of i per volume of air (e.g., mol m 3 of air). These concentrations will be a function of the location (x,y,z) and time and can be described by the atmospheric diffusion equation... 

As should be evident, this survey has dealt on the whole with idealized models of aerosols and aerosol particles which are different from the complex aerosols of practical interest in such fields as air pollution, cloud physics, aerosol therapeutics, military science, industrial technology, etc. Indeed this complexity presents great difficulties in establishing a science of aerosols. 

Fig. 3.8.2 Single scattering functions for a water cloud and an aerosol haze, each associated with a real refractive index n = 1.33. The cloud and haze particle size distributions peak at particle radii a 4.0 and a 0.05 pm, respectively. Computations were made for a wavelength k = 0.8189 pm (after Hansen, 1969).

Table 7-2 includes most of the main gaseous constituents of the troposphere with observed concentrations. In addition to gaseous species, the condensed phases of the atmosphere (i.e. aerosol particles and clouds) contain numerous other species. The physical characteristics and transformations of the aerosol state will be discussed later in Section 7.10. The list of major gaseous species can be organized in several different ways. In the table, it is in order of decreasing concentration. We can see that there are five approximate categories based simply on concentration ... 

Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

Chemical interactions also occur in the condensed phases. Some of these are expected to be quite complex, e.g., the reactions of free radicals on the surfaces of or within aerosol particles. Simpler sorts of interactions also exist. Perhaps the best understood is the acid-base relationship of NH3 with strong acids in aerosol particles and in liquid water (see Chapter 16). Often, the main strong acid in the atmosphere is H2SO4, and one may consider the nature of the system consisting of H2O (liquid), NH3, H2SO4, and CO2 under realistic atmospheric conditions. Carbon dioxide is not usually important to the acidity of atmospheric liquid water (Charlson and Rodhe, 1982) the dominant effects are due to NH3 and H2SO4. The sensitivity the pH of cloud (or rainwater produced from it) to NH3 and... 

We will illustrate the necessity of including solute from CCN by a simple calculation, recalling that pH = 5.6 is the supposed equilibrium value for water in contact with 300 ppm of CO2. (That calculation will appear later.) In clean, marine air, the concentration of submicrometer aerosol particles (by far the most numerous) is small, say 0.25 pg m . It is known from measurements that the molecular form is often NH4HSO4, and we assume it is all dissolved in 0.125 g/m of liquid water in a cloud - which is typical for fair-weather marine clouds. Thus the average concentration of sulfate ion [SO4 ], mol/L, is... 

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