**3.3. Mechanisms of interaction between particles and clouds**

The mechanisms that modify the properties of atmospheric particles are varied and have different efficiencies depending on the size of the particles. The size distribution and the area plots of particles in the study showed four patterns that may be associated with processes of interaction between particles and clouds:


Charts on figure 3 illustrate the general features that are described as follows:

1. Vertical transport with mixing and dilution

This cloud processing mechanism, discussed in detail by Flossman (1998), transports particles from cloud base to upper regions of the cloud where they eventually are detrained, either at cloud top edges or by mixing with ambient aerosols at detrainment level. PSD signatures take one of two forms. If RH at the point of measurement is higher than RH at cloud base, then PSD exhibits a tail at larger sizes exceeding that of the cloud base PSD (Fig. 3, pattern A1). Other studies have shown the correlation between RH and changes in particle size near cloud boundaries (e.g., Baumgardner and Clarke, 1998). This, or the particles have mixed with air close of to the same RH as at the cloud base so that the resulting PSD is one that has approximately the same shape as at cloud base, but with lower concentrations as a result of the dilution with ambient air (Fig. 3, pattern A2).

2. Aqueous phase oxidation of aerosol precursors

In-cloud oxidation of dissolved species is a process that increases the mass of aerosol particles and may change their composition (Hegg and Hobbs, 1982; Leaitch, 1996; O'Dowd et al., 2000). The likely precursor gas in the EPIC research region is SO2, which evolves from dimethyl sulfide produced by phytoplankton or from anthropogenic sources, as discussed below. The PSD pattern produced by this process will be indistinguishable from pattern A unless additional information is known about the aerosol chemistry. As discussed in section 2.4, measurements were not made for particle composition, but the average refractive index of particles could be estimated. A comparison of the average refractive index at cloud base with the near-cloud and far-cloud values at higher altitudes suggest changes in particle composition, as shown in Fig. 3 where the cloud base refractive index is near that of sea salt (1.54), while the near-cloud value at 2500 m is closer to that of ammonium sulfate (1.48). The observed differences are based on a technique that has a large amount of uncertainty and is used qualitatively in the present study as an indicator of composition change.

3. Droplet coalescence

230 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

particles that are on base of the cloud (Wang and Pruppacher, 1977).

**3.3. Mechanisms of interaction between particles and clouds** 

processes of interaction between particles and clouds:

1. Vertical transport with mixing and dilution

2. Aqueous phase oxidation of aerosol precursors

3. Droplet coalescence (> 1 micron) 4. Removal by precipitation

2. Aqueous phase oxidation of aerosol precursors (≤ 1 micron)

The removal of particles by precipitation is a cleaning process from the atmosphere. This mechanism helps to maintain a balance between sources and sinks of particles. The precipitation removes mechanically particles by inertial collection and transportation of raindrops, and also removes the nuclei when the drops become rain. However, it depends on the size of the interstitial space of the particles, related to the size of the drop. Experimental studies of Chate et al, (2003) demonstrated that this mechanism is more efficient on particles in the range of coarse mode (> 1 μm). Other studies have also shown that removal by inertial collection and transportation only affects to a small percentage of

The mechanisms that modify the properties of atmospheric particles are varied and have different efficiencies depending on the size of the particles. The size distribution and the area plots of particles in the study showed four patterns that may be associated with

This cloud processing mechanism, discussed in detail by Flossman (1998), transports particles from cloud base to upper regions of the cloud where they eventually are detrained, either at cloud top edges or by mixing with ambient aerosols at detrainment level. PSD signatures take one of two forms. If RH at the point of measurement is higher than RH at cloud base, then PSD exhibits a tail at larger sizes exceeding that of the cloud base PSD (Fig. 3, pattern A1). Other studies have shown the correlation between RH and changes in particle size near cloud boundaries (e.g., Baumgardner and Clarke, 1998). This, or the particles have mixed with air close of to the same RH as at the cloud base so that the resulting PSD is one that has approximately the same shape as at cloud base, but with lower

In-cloud oxidation of dissolved species is a process that increases the mass of aerosol particles and may change their composition (Hegg and Hobbs, 1982; Leaitch, 1996; O'Dowd et al., 2000). The likely precursor gas in the EPIC research region is SO2, which evolves from dimethyl sulfide produced by phytoplankton or from anthropogenic sources, as discussed below. The PSD pattern produced by this process will be indistinguishable from pattern A unless additional information is known about the aerosol chemistry. As discussed in section

1. Vertical transport with mixing and dilution with minimum changes in the size

Charts on figure 3 illustrate the general features that are described as follows:

concentrations as a result of the dilution with ambient air (Fig. 3, pattern A2).

d. Mechanical removal

Coalescence decreases the number concentration of particles while shifting the mass to large sizes. Each coalescence event decreases the number of original CCN by one and the resulting mass is the sum of the two nuclei. If nuclei are of different composition, then this process also changes the chemistry of the particle contained in the resulting drop. The large particle mode, with a peak between 5 – 6 m, seen in Fig. 3 indicates coalescence, since neither the cloud base nor far-cloud PSD have particles in this size range.

4. Removal by precipitation

Precipitation removes particles mechanically by inertial or nucleation scavenging when cloud droplets become raindrops. Mechanical scavenging depends on the size of interstitial aerosol in relation to the raindrop size. Experimental results (Wang and Pruppacher, 1977) suggest that only a few percent of the interstitial and sub-cloud particles are removed by this mechanism and this is not considered as a major factor here. The majority of aerosols removed by precipitation will be those that are in cloud droplets growing by condensation and coalescence to precipitable sizes. Figure 3 illustrates this process where PSDs at the cloud base level are quite different depending upon whether the measurements were made at the actual cloud base or in the far-cloud air. The far-cloud PSD has particles of supermicron sizes, but such particles are noticeably missing at the cloud base. In this particular case, the cloud base measurements were made after the cloud had formed and the supermicron particles had been activated and grew quickly to droplet sizes that could coalesce and precipitate.
