Cloud samples particle size distribution

The Particle Size Distribution of Nuclear Cloud Samples... 

Gravitational sedimentation causes a change in the particle size distribution anywhere in and below the cloud compared with the size distribution at stabilization time. Thus, to reconstruct the size distribution at stabilization time, corrections must be applied to the size distributions measured in the samples. These corrections were calculated by assuming Stokesian settling modified by a drag slip correction. It was assumed further that at stabilization time the cloud was axially symmetric and consisted of spherical particles. Wind and diffusion effects were neglected. 

The over-all distribution function consists of a linear combination of two lognormal functions. This is based on the observation that size distribution from very early aerial clouds samples from subsurface detonations are described accurately by the lognormal form of distribution. (This is shown below in connection with subsurface detonation analyses.) It is also supported by the work of particle analysts in industry, who find that particle population produced by crushing or grinding are described by lognormal distributions. 

As in the case of the land surface burst, complete characterization of the particle population requires only that particle mass, a volatile species, and a refractory species distribution with particle size be determined. All other isotopic distributions may be deduced from the istotope partition calculations described above. In the subsurface detonation, the earliest aerial cloud sample was obtained in the cloud 15 minutes after detonation. The early sample was, therefore, completely representative of the aerial cloud particle population. In Figure 5 the results of the size analysis on a weight basis are shown. Included for comparison is a size distribution for the early, local fallout material. The local fallout population and the aerial cloud population are separated completely from the time of their formation. 

Integration of the specific activities over the particle size yields a much lower amount of 90Sr than of 147Pm produced, even if estimates from analyses of particles greater than 100/x are included (7). Such a result suggests that the sample was not representative of the true radioactive cloud distribution. 

The size distributions of the particles in cloud samples from three coral surface bursts and one silicate surface burst were determined by optical and electron microscopy. These distributions were approximately lognormal below about 3/x, but followed an inverse power law between 3 and ca. 60 or 70p. The exponent was not determined unequivocally, but it has a value between 3 and 4.5. Above 70fi the size frequency curve drops off rather sharply as a result of particles having been lost from the cloud by sedimentation. The effect of sedimentation was investigated theoretically. Correction factors to the size distribution were calculated as a function of particle size, and theoretical cutoff sizes were determined. The correction to the size frequency curve is less than 5% below about 70but it rises rather rapidly above this size. The corrections allow the correlation of the experimentally determined size distributions of the samples with those of the clouds, assuming cloud homogeneity. 

Specifically, the calculations had as their goal the computation of (dZi/dz) as a function of particle size for clouds of different heights at various altitudes and sampling times, including the parameters applicable to the samples analyzed. A detailed exposition of the theory and its limitations is presented in the Appendix. The values of (dzjbz) are divided point by point into the measured size distribution—i.e., f(a,z,t)— to arrive at the size distribution at stabilization time—i.e.y f[a,z(a,z,t),0], according to Equation 2. An additional output of the calculations are the cutoff diameters (smallest and largest diameters) in the samples. 

Figure 8. Size distribution corrections as a function of particle size for samples taken at 15,000 meters altitude at various times after cloud stabilization...

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. 

Distortion of the particle size during the sampling process is a concern in the use of this probe on an aircraft. Compressional heating due to deceleration of the particles may distort the size distribution, because evaporation of water from aerosol particles reduces their diameters. Likewise, particle sizes can be reduced by use of a heater, incorporated into some models of this probe, to prevent icing when supercooled clouds are being flown through. One study (88) indicated that the probe heater removes most of the water from aerosol particles sampled at relative humidities of 95%. Thus, size distributions of aerosol particles measured with the probe heater on correspond to that of the dehydrated aerosol. These results were confirmed by a later study (90) in which size distributions of aerosols measured with a nonintrusive probe were compared to size distributions measured with a de-iced PCASP probe. Measurement of the aerosol size distribution with the probe heater on may be an advantage in certain studies. 

Fig. 7-8. Influence on cloud nuclei formation of the mass fraction e (water-soluble material/particle dry mass). Left Critical supersaturation of aerosol particles as a function of particle dry radius. Right Cloud nuclei spectra calculated for e = 0.1 and 1 on the basis of two size distributions each for continental and maritime aerosols (solid and dashed curves, respectively). [Adapted from Junge and McLaren (1971).] The curves for the maritime cloud nuclei spectra are displaced downward from the original data to normalize the total number density to 300 cm-3 instead of 600 cm-3 used originally. The curves for e = 1 give qualitatively the cumulative aerosol size distributions starting from larger toward smaller particles (sk = 10 4 corresponds to r0 0.26 p.m, sk = 3 x 10 3 to rs 0.025 Atn). Similar results were subsequently obtained by Fitzgerald (1973, 1974). The hatched areas indicate the ranges of cloud nuclei concentrations observed in cloud diffusion chambers with material sampled mainly by aircraft [see the summary of data by Junge and McLaren (1971)] the bar represents the maximum number density of cloud nuclei observed by Twomey (1963) in Australia.

The CCN behavior of ambient particles can be measured by drawing an air sample into an instrument in which the particles are subjected to a known supersaturation, a so-called CCN counter (Nenes et al. 2001). If the size distribution and chemical composition of the ambient particles are simultaneously measured, then the measured CCN behavior can be compared to that predicted by Kohler theory on the basis of their size and composition. Such a comparison can be termed a CCN closure, that is, an assessment of the extent to which measured CCN activation can be predicted theoretically [see, for example, VanReken et al. (2003), Ghan et al. (2006), and Rissman et al. (2006)]. The next level of evaluation is an aerosol-cloud drop closure, in which a cloud parcel model, which predicts cloud drop concentration using observed ambient aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against direct airborne measurements of cloud droplet number concentration as a function of altitude above cloud base. The predicted activation behavior can also be evaluated by independent measurements by a CCN instrument on board the aircraft. Such an aerosol-cloud drop closure was carried out by Conant et al. (2004) for warm cumulus clouds in Florida. 

Figure 1 depicts the SEM of cloud substance from guava juice under various processing conditions. The cloud surface of the fresh sample (unheated and not pressurized) was irregular and with unique particle size (Fig. 1, A B). The particle distribution and appearance of pressurized juice (6,000 atm, 25 °C, 15 min) were similar to the fresh sample (Fig. 1, C D). Takahashi et al (1993) have reported that the soluble solid particle distribution in citrus juice does not change after high pressure treatment. However, the cloud surface of pasteurized juice was observed to be greatly different from the pressurized juice (Fig. 1, E F) due to coagulation of the small particles.

The distribution of the volatile species, the refractory species, and mass within the particle population can be determined by analysis of size separated fractions of early samples from the aerial cloud and close-in fallout. 

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