Liquid concentration averaging, cloud

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... 

For the worst case evaluation, the weight of flammable material in a vapor cloud is assumed to be the total quantity of the substance that could be released from a vessel or pipeline. For liquids, this assumption infers that the liquid is above its atmospheric-pressure boiling point and that 100% flashing to vapor occurs. Also, the entire quantity of vapor is assumed to have concentrations between the lower and upper flammability limits and that the entire quantity explodes, with an energy-conversion efficiency e of 0.10. For mixtures, the weight-average heat of combustion would be used in the above equation. 

Since 5 is measured in percent supersaturation, 5 = 1 corresponds to 1% supersaturation, and c is the CCN concentration activated at 1% supersaturation, k is an empirical parameter. Approximate values of c and k determined in different environments have been given in Table 17.4. The number of CCN that are activated to cloud droplets increases as the peak supersaturation in the air increases. Based on (24.44), the larger the value of c, the larger the ultimate droplet concentration. Since the liquid water contents of continental and marine clouds do not differ greatly, the average size of droplets in continental clouds should be less than that in marine clouds. Marine cumulus clouds have a median droplet concentration of about 45 cm-3 and a median droplet diameter of about 30 pm continental cumulus clouds have a median droplet concentration of about 230cm-3 and a median diameter in the vicinity of 10 pm (Hobbs 1993). 

Thomerson and Billings [87] describe field tests in which chlorine was released at up to 70kg min from three 1-ton containers. Typical wind velocities were about 9 m s . Relative humidity was very low, the test site being located in the Nevada desert. It is noteworthy that the temperature of the spilled liquid stabilized at about -50 C, well below the boiling point of —34°C. The wind subcooled the liquid, which was collected in a well-insulated pan, and approximately 50% of the chlorine vaporized during a test. The use of downwind water sprays in these tests reduced the concentration of chlorine in the air by an average of 31%. This was attributed to the induction of dilution air by transfer of momentum from the spray. As noted above, the spray also forced the vapor cloud lower, so that the concentration of chlorine at 1.5 m elevation was actually higher for a distance of 230 m fi om the point of release. In these tests, portable fire water monitors performed relatively poorly. 

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