Cloud liquid water content

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.)... 

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. 

Cloud liquid water content (LWC) is measured by a CO2 laser... 

The characterization of the factors which control the accuracy, precision, and validity of measurements made in a simulation facility for studying in-cloud chemical processes was described. An analysis of a large number of experimental data collected under widely varying conditions was performed. Cloud liquid water content, an observable principally dependent on cooling rate and reaction time, was found to be the most influential of the physical factors controlling the resultant chemistry. In order to precisely control and reproduce the physical conditions in the simulation facility, standard operating procedures and computer control were instituted. This method reduced the uncertainty of the SO2 to sulfate transformation rate by a factor of 4.4. 

Potential gas-phase organic acid concentrations [P (/tg/m )] were then calculated for two assumed values of cloud liquid water content (L, g H20/m air) using the equation... 

Several areas in which chemical measurement technologies have become available and/or refined for airborne applications have been reviewed in this paper. It is a selective review and many important meteorological and cloud physics measurement capabilities of relevance to atmospheric chemistry and acid deposition (e.g., measurement of cloud liquid water content) have been ignored. In particular, we have not discussed particle size spectra measurements for various atmospheric condensed phases (aerosols, cloud droplets and precipitation). Further improvements in chemical measurement technologies can be anticipated especially in the areas of free radicals, oxidants, organics, and S02 and N02 at very low levels. Nevertheless, major incremental improvements in the understanding of acid deposition processes can be anticipated from the continuing airborne application of the techniques described in this review. 

The profiles of the actinic flux are computed at each grid point of the model domain. To determine the absorption and scattering cross sections needed by the radiative transfer model, predicted values of temperature, ozone, and cloud liquid water content are used below the upper boundary of WRF. Above the upper boundary of WRF, fixed typical temperamre and ozone profiles are used to determine the absorption and scattering cross sections. These ozone profiles are scaled... 

The aqueous-phase fraction of A is plotted as function of the Henry s law constant and the cloud liquid water content in Figure 7.3 for a range of expected liquid water content values. For species with Henry s law constants smaller than 400Matm-1 less than 1% of their mass is dissolved in the aqueous phase inside a cloud. Such species include 03, NO, NO2, and all the atmospheric hydrocarbons. A significant fraction (more than 10%) of a species resides in the aqueous phase in the atmosphere only if its Henry s law constant exceeds 5000Matm-1. 

FIGURE 7.3 Aqueous fraction of a species as a function of the cloud liquid water content and the species Henry s law constant. 

FIGURE 7.10 Equilibrium fraction of total ammonia in the aqueous phase as a function of pH and cloud liquid water content at 298 K. 

Partitioning of Nitric Acid inside a Cloud Calculate the fraction of nitric acid that will exist in the aqueous phase inside a cloud as a function of the cloud liquid water content (L = 0.001-lg m-3) and pH. What does one expect in a typical cloud with liquid water content in the 0.1-1 gm 3 and pH in the 2-7 ranges ... 

For pH values below 4 most of the formic acid remains in the cloud interstitial air and only a small fraction (less than 10%) dissolves in the cloud water. For pH values above 7 practically all the available formic acid is transferred into the aqueous phase and only traces remain in the interstitial air. In the intermediate pH regime from 4 to 7 the formic acid equilibrium partitioning varies considerably depending on the cloud liquid water content and pH. 

R = 0.082 atm L K-1 mol 1, and Tis in K. Note that the S02 oxidation rate given by (7.77) is independent of the S02 concentration and depends only on the mixing ratio of A, the cloud liquid water content, and the temperature. Equation (7.77) should be used only if A exists in both gas and aqueous phases and Henry s law equilibrium is satisfied by both S(IV) and A. If the two species do not satisfy Henry s law, (7.76) can still be used. 

FIGURE 7.16 Rate of aqueous-phase oxidation of S(IV) by ozone (30 ppb) and hydrogen peroxide (I ppb), as a function of solution pH at 298 K. Gas-aqueous equilibria aie assumed for all reagents. R/ so. represents the aqueous phase reaction rate per ppb of gas-phase SOj. R/L represents rate of reaction referred to gas-phase SO2 pressure per (gm 3) of cloud liquid water content. 

Note that for an open system, the aqueous phase concentration of H202 is independent of the cloud liquid water content. For a closed system, the total concentration of H202 per liter (physical volume) of air, [H202]total, will be equal to the initial amount of H202 or ... 

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