Aerosol-fog interactions ATOC 5600 Physics and Chemistry of Clouds & Aerosols
Aerosol-fog interactions
Intricate relationships exist between aerosols and fog. Fog may appear under the influence of particular aerosol characteristics such as number concentration and chemical composition. Fog is more likely to form in an environment with large concentrations of aerosols characterized by a low activation supersaturation (level of supersaturation at which aerosol particles spontaneously grow to become cloud drops). Although in a large number of fogs, the distinction between unactivated and activated droplets is not as straightforward as for other clouds types (Hudson, 1980). At the same time, fog has an influence on aerosols. Indeed, aerosol chemical composition can be modified through aqueous-phase chemical reactions taking place inside fog droplets. Furthermore, fogwater chemistry is strongly influenced by dynamical processes in fogs (Bott and Carmichael, 1993). Also, aerosols are removed from the atmospere by the wet deposition process (fog droplets settling on the ground surface).
Aerosol
fog interactions
As mentioned previously, aerosols have an influence on the attenuation of solar and infrared radiation. Different heating/cooling rates are obtained for different aerosol concentration and composition. Thus, the formation and dissipation of a fog layer depends on the aerosol type characterizing a particular environment.
Also, aerosols can act as Cloud Condensation Nuclei (CCN). Consequently, the properties of aerosols in the ambient air play an important role in fog onset as the activation of fog droplets is a function of these properties. First, the number of aerosols will influence the number of activated fog droplets, thus determining the fog's liquid water content and visibility. Also, the activation of fog droplets occurs at different supersaturations for different chemical composition of aerosols. Hanël (1976, 1981) showed that for a given dry particle radius, the levels of supersaturations required for droplet activation are functions of the soluble substance (chemical composition) as well as on the fraction of soluble to insoluble masses of the particle. This phenomenon is expressed by the Köhler curves (Fig.5). The activation of particles occurs at the supersaturation corresponding to the maximum of the curves.
Figure 5. Köhler curves for NaCl and (NH4)2SO4 particles for various dry diameters (Dd).
As stated earlier, fogs are also composed of non-activated (haze) particles. The relative humidity at which dry aerosols start absorbing water (deliquesence relative humidity) is a function of the aerosol composition (Seinfeld and Pandis, 1998, p.507). Thus the concentration of haze particles depends on the composition of ambient aerosols. Also, some highly soluble gases are able to dissolve into an aerosol even for relative humidities lower than 100%. Consequently, the aerosols are able to take up even more water as the solute concentration increases. By this process, particles grow to droplet-sizes even before the traditional activation of particles has occurred (Kulmala et al., 1997). These highly soluble gases are generally abundant in polluted air. This effect may contribute to higher occurrences of fogs in polluted air.
Thus it can be seen that the properties of aerosols (size and chemical composition) have a strong impact on the occurrences of fog. Moreover, aerosols will also have an impact on the subsequent evolution of the fog layer. Indeed, since aerosol properties influence the size distribution of fog droplets, the average sedimentation velocity within the fog layer will be a function of these properties. Thus the liquid water content of the fog is also a strong function of aerosol properties as the sedimentation flux is the major factor determining the loss of condensed water in the air during the night.
Bott (1991) performed a numerical investigation of the influences of aerosol properties on fog. He used a 1D radiation fog model with detailed microphysics, along with aerosol size distributions and properties typical for urban, rural and maritime environment, to show that aerosols have a direct influence on the life-cycle of a fog layer. His results indicate that the fog layer formation is delayed for smaller aerosol concentrations (Fig. 6). Thus the case with a high concentration of aerosols yields the highest vertical extent of the fog and the highest fog water content. Supersaturations within the fog is also strongly dependent on aerosol properties. The higher the particle concentration, the lower supersaturations are, although large supersaturations can result from mixing processes (Gerber, 1991). Droplet size distributions are bi-modal for the three aerosol models used, with the local minima in droplet concentration located at 5 microns. Similar features have been observed by Frank et al. (1998) in Po Valley (Italy) fogs. The main difference between urban and other aerosol models is in droplet concentrations for the submicron-size drops. The concentration of unactivated drops is roughly an order of magnitude higher for the urban aersols compared to the rural aerosols. The same is observed for rural aerosols compared to maritime aerosols.
Figure 6. Time after sunset at which a liquid water content of 0.05 gm-3 first appears in simulations performed with urban, rural and maritime aerosols. Data from Bott (1991).
Fog
The previous section described the effect of aerosols on the formation and life-cycle on a fog layer. But the presence of fog droplets act to modify the characteristics of aerosols. Aqueous-phase chemical reactions occur in fog droplets thus modifying the properties of the aerosols. Generally speaking, some aerosols are activated and grow into fog droplets during the formation of a fog layer. Subsequently, they are delivered back to the atmosphere as the fog dissipates. But during the fog lifetime, the droplets scavenge water-soluble trace gases leading to an increase in size and solubility of the particles emerging from the evaporating fog droplets. In areas where new aerosol particle production is small, this processing of aerosols by fogs has an influence on the characteristics of the subsequent formation of another fog layer. It should be mentioned that these fog droplet-aerosol interactions are droplet size-dependent, further complicating the thorough description of the microstructure of a fog layer.
Also, the deposition of droplets on the surface is believed to be the most important process in the removal of some chemical species near the surface. Experiments have shown that as much as 500-2000 mgm-2 of sulfate, 2500-6500 mgm-2 of nitrate and 2000-3500 mgm-2 of ammonium can be removed from the atmosphere during a typical fog episode (Lillis et al., 1999).
This processing of aerosols by fogs act to modify the size distribution of the aerosol population. Figure 7 shows aerosol size distributions observed during clear and foggy conditions. It is observed that there is a marked decrease of aerosol concentration for all the particle size modes. The larger particles may be activated to become fog droplets and subsequently be removed from the atmosphere through the wet deposition process. The drop in small particles may be attributed to the coagulation and diffusion of these small particles onto fog droplets (Yuskiewicz et al., 1998).
Figure 7. Mean aerosol size distributions observed in the Po Valley (Italy) in clear conditions and in fog. Data from Yuskiewicz et al. (1998).
This brief discussion on aerosol-fog interactions suggest that dynamical and chemical mechanisms act to modify the characteristics of a fog layer. The interactions between aerosol populations and fog droplet populations are complex and much remains to be thoroughly described. Advances in measuring techniques and modeling capabilities should yield further insights into to these dynamical-chemical feedback mechanisms.
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