**3.2. Interaction between particles and clouds**

There are four main processes involved in the interaction particles-clouds:

a. Vertical distribution

228 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

If a mass of moist air is forced to raise it creates a cloud, either by buoyancy promoted by surface heating or by mechanical means, such as climbing up a slope pushed by the wind. When the mass of air containing water vapor is cooled by adiabatic ascent, the vapor pressure increases until it reaches the complete saturation (RH = 100%). The increase in RH results in the formation of cloud droplets, because some particles act as CCN. The particles grow by molecular diffusion to the drop and form a solution. New droplets can interact with interstitial particles in the cloud or collide and coalesce with other droplets changing its composition. If the water droplet evaporates, then releases an aerosol particle with different mass and composition than the original particle. The particle is physical, chemical and hygroscopic different. According to the Köhler curve, a particle increasing its mass may trigger saturation values lower than a smaller particle (Pruppacher and Klett, 1997). In other words, the atmospheric particles that are processed by clouds acquire properties to become more efficient CCN. In addition, the size distribution of particles is modified and can directly influence the evolution of the cloud. The processes change the physical and chemical characteristics of particles, but their concentrations can be identified by analyzing

Atmospheric particles have different origins; some are from natural sources such as oceans, volcanoes, soil, pollen, forest fires, and so on. And human activities also generate particles that reach the atmosphere. Motor vehicles, power generation, industrial boilers and incineration of solid waste are some sources of anthropogenic particles. The diversity of emissions presents a wide range sizes, concentrations and compositions of particles. And their size range spans several orders of magnitude, ranging from nanometers to hundreds of micrometers. Similarly, the concentrations might be from 1.07 to 1.0-6 particles per cubic centimeter. In addition, the particles in the atmosphere undergo processes that transform their physical and chemical properties. Therefore the study of atmospheric particles is complicated. However, an important tool in particles analysis is the construction of size distribution graphs. These charts provide relevant information such as the nature of the particles (i.e., maritime, continental, urban, or rural zones), because each place has a kind of

In the study we use particle size distributions from inside, outside and away from the clouds to identify and analyze possible changes of particles properties interacting with cloud droplets. Particle counters at the C130 aircraft provided the information. The instruments measure aerosol particles and cloud droplets in different size ranges and cover

The PCASP dehydrates particles reducing the relative humidity below 30%, but the FSSP measure them at ambient relative humidity, so we calculate the dry particle diameter from both FSSP, based on the Tang's theory ( 1976) and Tang and Munkelwitz (1977), to obtain a

**3. Particles processing by clouds** 

the shapes of the distributions of sizes.

**3.1. Particles sizes distribution** 

specific particle sources.

a wide range of sizes (~ 0.1 to 50 microns).

size distribution of dry particles.

The classic physics model for developing convective clouds indicates that the aerosol particles are incorporated from the base of the cloud. Some particles form droplets that grow vertically while being transported by updrafts currents generated by latent heat during a phase change from vapor to liquid. A few drops reach the top of the cloud, where updrafts currents lose strength by neutral stability between the cloud and the environment. At this point, the interaction of clouds with dry air dilutes and evaporates drops. In this mixing and evaporation zone of droplets is where the particles, used as CCN to form cloud droplets, are released back into the upper top of the cloud reaching the high troposphere and in some cases of deep convection may lead them to the lower stratosphere. Some researchers have shown that this mechanism is the main transport of particles from the boundary layer to free troposphere (i.e., Flossmann, 1998).

b. Mass incorporation into the drop by diffusion

A particle in a high relative humidity environment will grow by diffusion and condensation of vapor molecules producing a cloud droplet. The particle can be diluted to form a solution within the droplet. In that case, if the droplet is in an atmosphere of various gases that can be absorbed by the same specie (i.e., SO2 in marine clouds) there is an increase in the mass concentration of solute within the droplet changing its physical properties (mass increase). A rapid change in pH also transform the chemical properties, resulting in the dissolution of species in a solution (Hegg and Hobbs, 1982; Leaitch, 1996; Leaitch et al, 1986, O'Dowd et al, 2000).

When a particle is in high relative humidity (~ 80%) environment, it becomes an effective site for oxidizing species in aqueous phase (Chameides and Stelson, 1992). For example, SO2 dissolved in a particle can react with ozone and hydrogen peroxide. In an acid particle (H2SO4) with low pH, the oxidant is hydrogen peroxide, but for particles with high pH (i.e., [NH4]2SO4), the oxidant will be ozone. The first reaction is more important in maritime areas, because SO2 is abundant from dimethyl sulphide emissions produced by phytoplankton. O'Dowd et al, (2000) estimate that under mass incorporation conditions a particle can increase its size to double in about 400 seconds.

c. Collision-coalescence of drops

When the droplets have certain size, they grow more efficiently by collision-coalescence. The collision and coalescence among cloud droplets is mainly governed by gravitational effects, so large droplets fall faster than small ones. This process produces a decrease in the concentration of drops, but form larger particles and evaporate the droplet mass. Each collision-coalescence between two original CCN becomes in to one drop, which has a mass equal to the sum of the two nuclei. If the original CCN have a different composition, it also changes the chemical composition of the resulting drop.

## d. Mechanical removal

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

Interaction Between Aerosol Particles and Maritime Convective Clouds:

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

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

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

Five flights and ten cloud systems were selected for analysis based on a visual evaluation of the records made with the forward- and side-looking video cameras on the aircraft. The criteria was that no other clouds could be seen within around 10 km on either side of a cloud line, such that far-cloud samples represent "ambient" aerosols, i.e., lacking any recently

We studied the data from flights 7, 9, 12, 13 and 17. The flights were conducted in convective clouds by passing through the clouds at different levels (1000, 2500, 4200, and 6000 meters). Moreover, we passed through the cloud base (300 meters) and at surface level (30 meters).

used qualitatively in the present study as an indicator of composition change.

neither the cloud base nor far-cloud PSD have particles in this size range.

3. Droplet coalescence

4. Removal by precipitation

and precipitate.

**4. Analysis and discussion** 

processed particles by clouds.

**4.1. Time series to identify clouds** 

Measurements in ITCZ During the EPIC 2001 Project 231
