Aerosol Liquid Water Content

for example, the species participating in the reaction are all gases, then the activity can be replaced with the partial pressures and  

Equilibrium Constant for Ammonium Sulfate Formation Let us determine the equilibrium constant for the following reaction  

Water is an important component of atmospheric aerosols. Most of the water associated with atmospheric particles is chemically unbound (Pilinis et at. 1989). At very low relative 

FIGURE 10.4 Diameter change of (NH SC, NH4HSO4, and H2S04 particles as a function of relative humidity. Dp0 is the diameter of the particle at 0% RH. 

TABLE 10.1 Deliquescence Relative Humidities of Electrolyte Solutions at 298 K 

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The behavior in these two different regimes is depicted in Figure 10.23. The transition occurs at approximately 3.5 pgm3. At very low ammonia concentrations, sulfuric acid and bisulfate constitute the aerosol composition. As ammonia increases ammonium nitrate becomes a significant aerosol constituent. The aerosol liquid water content responds nonlincarly to these changes, reaching a minimum close to the transition between the two regimes. 

Assume extreme conditions (aerosol surface, aerosol liquid water content, aerosol organic carbon concentration) for these estimates and that the propionic acid vapor pressure is 0.005 atm. 

The behavior of inorganic salts when RH is decreased is different from that discussed in the Sections 9.2.2 and 9.2.3 (Figure 9.4). For example, for (NH4)2S04, as the RH decreases below 80% (the DRH of (NH4)2S04) the particle evaporates but not completely. The particle remains liquid until a RH of 37%, where crystallization finally occurs. This hysteresis phenomenon is characteristic of most salts. For such salts, knowledge of the RH alone is insufficient for calculation of the aerosol liquid water content in this regime. One needs to know the RH history of the particle. 

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.)... 

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. 

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). 

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)... 

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. 

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 radiative transfer model in Madronich (1987) permits the proper treatment of several cloud layers, each with height-dependent liquid water contents. The extinction coefficient of cloud water is parameterized as a function of the cloud water computed by the three-dimensional model based on a parametrization given by Slingo (1989). For the Madronich scheme used in WRF/Chem, the effective radius of the cloud droplets follows Jones et al. (1994). For aerosol particles, a constant extinction profile with an optical depth of 0.2 is applied. 

Figure 7 Vertical profiles of the liquid water content (in rn x ), pH, S(VI) (H2S04aq + HSO4 + SO ), and HN03° (HN03,jq + NO ) in the sea salt aerosol (in molm ) according to the one-dimensional MBL model Mistra-MPIC (source von Glasow and Sander, 2001).

Figure 25 Dependence of cloud-top albedo on cloud thickness and indicated values of cloud-droplet radius r and number concentration Liquid water content, 0.3 cm asymmetry parameter, 0.858 (Twomey, 1977 reproduced by permission of Twomey from Atmospheric Aerosols, 1977).

When fog is formed from water-saturated air, water droplets condense on aerosol particles. In addition to components of the aerosols, the fog droplets can absorb such gases as NOj, SO2, NH3, and HCl they form a favorable milieu for various oxidation processes, especially the formation of H2SO4. Fog droplets (10-50 urn in diameter) are much smaller than rain droplets the liquid water content of fog is often in the range of 1 x lO"" liter m air, so that... 

Fog droplets (10-50 m diameter) are formed in the water-saturated atmosphere (relative humidity = 100%) by condensation on aerosol particles (see Figure 5.2). The fog droplets absorb gases such as SO2, NH3, HCl, and NO. The water droplets are a favorable milieu for the oxidation of many reductants, above all, of SO2 to H2SO4. The liquid water content of a typical fog is often on the order of 10 liter water per m air. The concentrations of ions in fog droplets are often 10-50 times larger than those of rain (Figure 5.11). Clouds process substantial volumes of air and transfer gas and aerosols over large distances. On the other hand, fog droplets are important collectors of local pollutants in the proximity of the earth s surface. 

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