Cloud processing

Liou, K.-N, Radiation and Cloud Processes in the Atmosphere Theory, Observation, and Modeling, Oxford Univ. Press, New York, 1992. 

Van Dingenen, R., F. Raes, and N. R. Jensen, Evidence for Anthropogenic Impact on Number Concentration and Sulfate Content of Cloud-Processed Aerosol Particles over the North Atlantic, J. Geophys. Res., 100, 21057-21067 (1995). 

Rosenfeld D, Lahav R, Khain A, Pinsky M (2002) The Role of Sea Spray in Cleansing Air Pollution Over Ocean via Cloud Processes. Science 297 1667... 

Chemical oxidation reactions and radical-induced hydrophobic-to-hydrophilic aging processes tend to increase the water solubility of OAs and, therefore, are thought to enhance the activity of atmospheric OAs as cloud condensation nuclei (CCN). As discussed by Gysel et al. (2004), at 75-90% of relative humidity (RH) the inorganic fraction dominates the water uptake (59-80%). Nevertheless, under the same conditions of RH, between 20% and 40% of total particulate water is associated with water-soluble organic matter. More data concerning the multiphase aerosol and cloud processes, as well as the chemical reactivity of carbonaceous aerosol components, have been compiled in the reviews of Jacobson et al. (2000), Kanakidou et al. (2005), and Poschl (2005) (and references therein). 

Spokes, L.J., Jickells, T.D. and Lim, B. (1994) Solubilisation of lattice bound trace metals by cloud processing - a laboratory study. Geochim. Cosmochim. Acta, 58, 3281-3287. 

The composition of liquid-water clouds and processes responsible for this composition are of obvious current interest in conjunction with the so-called acid precipitation phenomenon since clouds constitute the immediate precursor of precipitation. Additionally, cloud composition is of interest because impaction of cloud droplets on surfaces may directly deliver dissolved substances onto natural or artificial materials. In-cloud processes also influence clear-air composition since dissolved substances resulting from such reactions are released into clear air as gases or aerosol particles upon cloud evaporation. It is thus desired to gain enhanced description of the composition of clouds and the mecha-... 

Chloride ions from supermicron sea-salt particles are replaced by sulfate, nitrate and minor contributions of oxalate, malonate and succinate. The principal mechanisms causing accumulation of sulfate in sea-salt particles are cloud processing and, to a lesser degree, heterogeneous reactions taking place in deliquescent particles. Mechanisms for the chloride replacement by nitrate are not clear [54]. 

Environmental monitoring has also taken advantage of acoustic levitation for the investigation of physico-chemical processes relevant to the troposphere — mainly at temperatures below 0°C. Gas-liquid transfer of H2O2 from the gas phase to the levitated droplet was studied from in situ chemiluminescence measurements. Also, freezing of stably positioned droplets was observed by means of a microscope and a video camera, and the usefulness of this technique for simulation and investigation of cloud processes thus demonstrated. Ex situ microanalysis of sub-microlitre droplets by the use of an optical fibre luminometer also proved an effective means for investigating important physicochemical processes at the micro scale [100]. 

Table 2.5 Treatments of aerosol-cloud interactions and cloud processes of on-line models... 

Feedback mechanisms are considered to be important for a coupling strategy. This is especially relevant for aerosols. Aerosol forcing mechanisms influence radiative and optical properties as well as cloud processes, leading for instance to changes of precipitation and circulation. 

I am just wondering about your statement that aerosols may have an effect on climate only during periods of strong volcanic activity by increasing the albedo. I am no expert in cloud physics, but I am wondering if increased aerosol burnings in the troposphere could affect cloud processes and thereby have an impact on climate. 

Sulfate aerosols are important for cloud processes and climate because they act, like sea salt aerosol, as CCN. The total number and size distribution of CCN determine the microphysical and, therefore, also radiative properties of clouds. In Section 4.02.4.4 we already discussed the possible involvement of iodine in seeding the formation of sulfate CCN. DMS is the most important precursor for SO2 and sulfate aerosol in the remote MBL. A possible link between iodine and sulfur chemistry was investigated by Chatfield and Crutzen (1990), using a reaction rate coefficient of the lO -p DMS reaction available at that time. They concluded that at that rate this reaction could play a significant role for the oxidation of DMS. In an alternative scenario with a slower rate coefficient, they found the iodine and sulfur cycles to be decoupled, in better agreement with field observations. Later the reaction turned out to be even slower than their lower limit (DeMore et al., 1997 Knight and Crowley, 2001). 

Rosenfeld D., Lahav R., Khain A., and Pinsky M. (2002) The role of sea spray in cleansing air pollution over ocean via cloud processes. Science 297, 1667—1670. 

In-cloud processes are a second major class of chemical transformations of aerosol particles (cf. Section 4.04.7.3). Clouds are technically aerosols, a special class in which the particles consist mainly of liquid or solid water and the gas phase generally exhibits slight supersaturation with respect to the condensed phase, i.e., an RH slightly greater than 100%. Clouds form in the atmosphere mainly as a consequence of air parcels being cooled below the dew-point of water, the temperature at which water vapor in a given air parcel is saturated with respect to the liquid. Generally as an air parcel rises to lower pressure. 

Demott P. (2002) Laboratory studies of cirrus cloud processes. In Cirrus (eds. D. K. Lynch, K. Sassen, D. O C. Starr, and G. Stephens). Oxford University Press, Oxford, pp. 102-135. 

Ciais P. and Jouzel J. (1994) Deuterium and Oxygen 18 in precipitation an isotopic model including mixed cloud processes. J. Geophys. Res. 99, 16793-16803. 

Rain clouds process a considerable volume of air over relatively large distances and thus are able to absorb gases and aerosols from a large region. Because fog is formed in the lower air masses, fog droplets are efficient collectors of pollutants close to the earth s surface. The influence of local emissions (such as NH3 in agricultural regions or HCl near refuse incinerators) is reflected in the fog composition. 

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. 

Kuo-Nan liou Radiation and cloud processes in tiie atmosphere... 

Mixed-phase cloud processes A variety of mixed-phase and ice cloud models exist, describing the homogeneous and heterogeneous formation of water droplets and ice crystals. The implications of aerosol particles on mixed-phase clouds may be evaluated if their ice nucleating properties are known. 

Choularton, T.W. and K.N. Bower The role of clouds processing in the relationship between wet deposited sulphur and sulphur dioxide emissions, Water, Air, and Soil Pollut. Focus, 1 (2001), 365-372. 

Finally, HC1 is soluble in water, and can therefore be removed from the atmosphere in cloud processes. 

Tuck, A.F, T. Davies, S.J. Hovde, M. Noguer-Alba, D.W. Fahey, S.R. Kawa, K.K. Kelly, D.M. Murphy, M.H. Proffitt, J.J. Margitan, M. Loewenstein, J.R. Podolske, S.E. Strahan, and K.R. Chan, Polar stratospheric cloud processed air and potential vorticity in the northern hemisphere lower stratosphere at mid-latitudes during winter. J Geophys Res 97, 7883, 1992. 

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