Liquid water content

When considering environment it generally becomes difficult since actual service conditions are most of the time unpredictable. As an example, there is a systematic difference in the frequency distributions of liquid water content in rain. It appears that the areas most likely to have high values of liquid water are where there is a plentiful supply of moisture and a high instability in the atmosphere. The lowest values of liquid water are obtained from the climatic areas of light continuous rains such as that found along the northwest coast of the United States. 

We see from this diagram that partial pressures of H2O at ordinary conditions range from very small values to perhaps 30 or 40 mbar. This corresponds to a mass concentration range up to about 25 g H20/m. In typical clouds, relatively little of this is in the condensed phase. Liquid water contents in the wettest of cumulus clouds are around a few grams per cubic meter ordinary mid-latitude stratus clouds have 0.3-1 g/m. ... 

Fig. 16-4 pH sensitivity to SO4- and NH4. Model calculations of expected pH of cloud water or rainwater for cloud liquid water content of 0.5 g/m. 100 pptv SO2, 330 ppmv CO2, and NO3. The abscissa shows the assumed input of aerosol sulfate in fig/m and the ordinate shows the calculated equilibrium pH. Each line corresponds to the indicated amoimt of total NH3 + NH4 in imits of fig/m of cloudy air. Solid lines are at 278 K, dashed ones are at 298 K. The familiar shape of titration curves is evident, with a steep drop in pH as the anion concentration increases due to increased input of H2SO4. (From Charlson, R. J., C. H. Twohy and P. K. Quinn, Physical Influences of Altitude on the Chemical Properties of Clouds and of Water Deposited from the Atmosphere." NATO Advanced Research Workshop Acid Deposition Processes at High Elevation Sites, Sept. 1986. Edinburgh, Scotland.)... 

K2. Keily, D. P., Measurement of drop size distribution and liquid water content in natural clouds, Dept, of Meteorol., Mass. Inst, of Tech., Contr. No. AF19(628)-259, NASA Rept. No. N64-30005 (1964). 

In the atmosphere, suspended aqueous solutions are present in the form of aerosols, clouds, fogs, and rain. However, these have different liquid water contents (i.e., grams of H20(l) per cubic meter of air). As discussed in detail in Chapter 9, fine particles (< 2-yxm diameter) emitted directly into the air or formed by chemical reactions can remain suspended for long periods of time. Many of these particles contain water, either in the form of dilute aqueous solutions or as thin films covering an insoluble core as much as 50% of the mass may be liquid water. Since the total particulate mass in this size range per cubic meter of air can be as high as 10 4 g m-3 or more, the liquid water content due to these small particles is also of this order of magnitude. 

Clouds, fogs, and rain, however, have much greater liquid water contents and thus have the potential for contributing more to atmospheric aqueous-phase oxidations. Clouds typically have liquid water contents of the order of 1 g m-3, with droplet diameters of the order of 5-50 yxm the number concentration and size distribution depend on the type of cloud. Fogs, on the... 

The liquid water content of an air mass plays a role in determining the oxidation rate of SOz in aqueous atmospheric droplets. This can be seen from the expression developed in Box 8.2 for the rate of oxidation of S02 (in % h l) in the liquid phase. 

The expression for the rate of oxidation in % h 1 in the liquid phase can be developed from a knowledge of the gas- and aqueous-phase reactant concentrations, the solution rate constant, the Henry s law constants (Table 8.1), and the liquid water content of air. In 1 m3 of air, the rate of formation of S(VI) in the aqueous phase is given by... 

The reason that the aqueous-phase concentrations in fogs can be so high is related in part to the liquid water content (LWC), which is a major difference between clouds and fogs. The liquid water content for fogs is typically of the order of 0.1 g m-3 of air, whereas that for clouds is about an order of magnitude higher. This small LWC in fogs corresponds to increased solute concentrations. 

It was assumed that there were no limitations on the rates of oxidation due to mass transport as discussed in detail by Schwartz and Freiberg (1981), this assumption is justified except for very large droplets (> 10 yarn) and high pollutant concentrations (e.g., 03 at 0.5 ppm) where the aqueous-phase reactions are very fast. It was also assumed that the aqueous phase present in the atmosphere was a cloud with a liquid water content (V) of 1 g m-3 of air. As seen earlier, the latter factor is important in the aqueous-phase rates of conversion of S(IV) thus the actual concentrations of iron, manganese, and so on in the liquid phase and hence the kinetics of the reactions depend on the liquid water content. 

The uptake of water with increasing RH causes an increase in both mass and radius and a decrease in refractive index the net effect of all these factors is an increase in light scattering. This can be seen in Fig. 9.27, where the light scattering coefficient, bxai 6sp, for some ambient aerosol particles measured using an integrating nephelometer was found to increase with the liquid water content of the aerosols. 

Effect of aerosol particles on cloud drop number concentrations and size distributions Clouds and fogs are characterized by their droplet size distribution as well as their liquid water content. Fog droplets typically have radii in the range from a few /an to 30-40 /an and liquid water contents in the range of 0.05-0.1 g m" Clouds generally have droplet radii from 5 /an up to 100 /im, with typical liquid water contents of 0.05-2.5 gin"5 (e.g., see Stephens, 1978, 1979). For a description of cloud types, mechanisms of formation, and characteristics, see Wallace and Hobbs (1977), Pruppacher (1986), Cotton and Anthes (1989), Heyms-field (1993), and Pruppacher and Klett (1997). 

Another piece of evidence for anthropogenic emissions leading to increased CCN and hence effects on cloud properties such as albedo and extent is found in ship tracks. These are lines of clouds that trace ship movements, either in initially cloud-free regions (Conover, 1966 Platnick and Twomey et al., 1994) or superimposed on preexisting clouds (Coakley et al., 1987). Emissions associated with the ship exhausts serve as CCN. This allows clouds to form where the background CCN concentration is too small for cloud formation. Alternatively, the CCN can modify existing cloud properties in the exhaust plume by changing the number and size distribution of the cloud droplets as well as the liquid water content (e.g., Ferek et al., 1998). 

FIGURE 14-44 Effect of ship emissions on (a) cloud number concentration, N, (b) effective cloud droplet radius, / cM, (c) cloud liquid water content, LWC, and (d, e) down- and upwelling radiation at (d) 744 nm and (e) 2.2 /im (adapted from King el al., 1993). 

The operating principle of the CSIRO (Australian Commonwealth Scientific and Industrial Research Organization) King probe (Particle Measuring Systems Inc., Boulder, Colorado) is similar in concept to that of the Johnson-Williams probe. The King probe measures the amount of power necessary to maintain a heated wire at a constant temperature, whereas the Johnson-Williams probe measures the change in resistance due to cooling of the wire by water evaporation. The probe consists of a heated coil of wire that is maintained at a constant temperature. The amount of excess power required to maintain the wire at this temperature when it is impacted by water droplets is measured and is proportional to the cloud liquid water content. The nominal response time of the instrument is 0.05 s, and it has an accuracy of 20%. This instrument uses less power than a Johnson-Williams probe, an important consideration in aircraft applications. 

A third commonly used method for determining cloud liquid water content is integration of the droplet size spectrum as measured by a PMS FSSP probe. Estimates of cloud liquid water content using this technique are subject to large errors due to uncertainties in determining the number concentrations of droplets in the largest size ranges. 

The first indirect climatic effect of aerosol (the Twomey effect) is based on the assumption that with a constant equivalent liquid water content (LWC) of clouds an increase in atmospheric aerosol number density (and, hence, concentration of CCN)... 

Ionic liquid Water content of IL/wt% IL content of water/ppm... 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

Liquid-water clouds (5) represent a potentially important medium for atmospheric chemical reactions in view of their high liquid water content [104 to 105 times that associated with clear-air aerosol (6)] and high state of dispersion (typical drop radius 10 pm). Clouds are quite prevalent in the atmosphere (fractional global coverage 50%) and persistent (lifetimes of a few tenths of an hour to several hours). The presence of liquid water also contributes to thermochemical driving force for production of the highly soluble sulfuric and nitric acids. 

The right-hand ordinate of Figure 6 expresses the reaction rate as percent of gas-phase SO2 oxidized per hour, per unit liquid water content of the cloud. Oxidation rates in these units may be compared to clear-air oxidation rates (of order 1% h l), although this comparison should be tempered by the small fraction of the boundary layer that is occupied by clouds. 

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