**3. Particles processing by clouds**

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 the shapes of the distributions of sizes.

Interaction Between Aerosol Particles and Maritime Convective Clouds:

**3.2. Interaction between particles and clouds** 

a. Vertical distribution

troposphere (i.e., Flossmann, 1998).

c. Collision-coalescence of drops

2000).

b. Mass incorporation into the drop by diffusion

particle can increase its size to double in about 400 seconds.

changes the chemical composition of the resulting drop.

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

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

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,

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

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

Measurements in ITCZ During the EPIC 2001 Project 229
