Cloud processing of aerosols

If the cloud is raining, there are additional interactions between the raindrops and the aerosols both in and around clouds, leading to removal of material from the atmosphere. Finally, there are other processes that can occur around clouds that may lead to the formation of new particles. 

Finally, there are other processes that can occur around clouds that may lead to the formation of new particles. 

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Effect of cloud processing of aerosols in the remote marine atmosphere has been demonstrated in a series of field studies (Hoppel et al. 1986 Frick and Hoppel 1993). Figure 17.23 shows the formation of a second peak in the accumulation mode as an airmass is advected off North America to the Atlantic and the Pacific Oceans. Note that the two modes observed in the number distribution should not be confused with modes of the... 

The results obtained by Menon et al. (2003) revealed substantial differences in the use of physically substantiated prognostic schemes of how aerosol acts taking into account vertical velocity and empirical schemes based on diagnostic data on the vertical velocity at cloud bottom level. Prognostic schemes are characterized by the stronger variability of results compared with diagnostic ones because of differences in scheme of the interaction between the processes of aerosol activation and precipitation of drizzle when calculating N. 

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

FIGURE 17.21 Schematic of the cloud processing of an aerosol particle. 

Operator splitting methods require less computational resources but demand more thought. In general, stability is not guaranteed. However, operator splitting encourages modular models and allows the use of the best available numerical technique for each module. Thus advection, kinetics, cloud processes, and aerosol processes each reside in different modules. Techniques used for these subproblems will be outlined subsequently. For applications in other problems, the reader is referred to Oran and Boris (1987). 

Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

The condensed phases also are important to the physical processes of the atmosphere however, their role in climate poses an almost entirely open set of scientific questions. The highest sensitivity of physical processes to atmospheric composition lies within the process of cloud nucleation. In turn, the albedo (or reflectivity for solar light) of clouds is sensitive to the number population and properties of CCN (Twomey, 1977). At this time, it appears impossible to predict how much the temperature of the Earth might be expected to increase (or decrease in some places) due to known changes in the concentrations of gases because aerosol and cloud effects cannot yet be predicted. In addition, since secular trends in the appropriate aerosol properties are not monitored very extensively there is no way to know... 

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.)... 

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). 

Distortion of the particle size during the sampling process is a concern in the use of this probe on an aircraft. Compressional heating due to deceleration of the particles may distort the size distribution, because evaporation of water from aerosol particles reduces their diameters. Likewise, particle sizes can be reduced by use of a heater, incorporated into some models of this probe, to prevent icing when supercooled clouds are being flown through. One study (88) indicated that the probe heater removes most of the water from aerosol particles sampled at relative humidities of 95%. Thus, size distributions of aerosol particles measured with the probe heater on correspond to that of the dehydrated aerosol. These results were confirmed by a later study (90) in which size distributions of aerosols measured with a nonintrusive probe were compared to size distributions measured with a de-iced PCASP probe. Measurement of the aerosol size distribution with the probe heater on may be an advantage in certain studies. 

Understanding the physical and chemical properties of marine aerosol particles is crucial because of the role these particles play in a number of atmospheric processes. Marine aerosols affect climate directly through scattering and absorption of radiation and indirectly as they can act as cloud condensation nuclei and thus influence the albedo of clouds. In addition, marine aerosol particles play an important role in the cycling of various elements through the atmosphere. 

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). 

The interaction processes in the aerosol-cloud-radiation system that determine the indirect impact of aerosol on climate remain poorly studied, though they are an important factor in RF formation (the respective estimates vary between OWm-2 and —4.8Wm-2). The contribution to indirect climatic impact by aerosol due to lower-level stratus clouds is important since... 

To analyze the formation and variability of the indirect climatic impact of aerosol within the second field experiment on studies of aerosol (ACE-2) and the PACE program to substantiate parameterization of this impact, Menon et al. (2003) undertook a comparison of six 1-D numerical models of the processes in the aerosol-cloud-radiation system that determine the climatic impact of aerosol under... 

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