**1. Introduction**

220 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

Yu, J., Huang, X., Xu, J. & Hu, M. (2005). When aerosol sulfate goes up, so does oxalate: implication for the formation mechanisms of oxalate. *Environmental Science and* 

*Technology,* Vol. 39, No. 1, (January 2005), pp. 128–133, ISSN: 1520-5851

Atmospheric particles interact directly with solar radiation extinguishing part of it and decreasing the amount of radiation that reaches the Earth's surface. This effect produces a change in the local radiative balance. On the other hand, it also presents an indirect effect on the interaction with radiation because these particles are an important element in the formation and development of clouds influencing their optical properties and the length of residence.

There are studies that have focused primarily on understanding and explaining the role of atmospheric particles in the formation and evolution of clouds. They have shown enough information able to explain those processes in theory (e.g. Pruppacher and Klett, 1997). And they have been validated with experimental works (e.g. Twomey, 1991; Raga and Jonas, 1993 a, b).

However, other issues of importance that do not yet have much information are the processes that modify the properties of atmospheric particles interacting with the cloud and the effects of changes in the environment. Particles increase their average size in regions of high relative humidity (RH) near the clouds (Baumgardner et al, 1996; Baumgardner and Clarke, 1998). Other studies show that the clouds condensation nuclei (CCN) are relatively higher in regions where a cloud is evaporated compared with places without clouds (DeFelice and Saxena, 1994; DeFelice and Cheng, 1998; Naoki et al, 2001). Also, the composition of atmospheric particles may change resulting from chemical reaction in aqueous state (Hegg et al, 1980; O'Dowd et al, 2000, Alfonso and Raga, 2002). Aerosol particles used as CCN show an increase in size after the cloud drops are evaporated (Hobbs, 1993). Towmey (1974) and Albrecht (1989) showed that changes in particles concentrations

© 2012 Jiménez-Escalona and Peralta, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Jiménez-Escalona and Peralta,, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in an area influenced by polluted sources modify the local cloud albedo and its life time, inhibiting the rain process (Rosenfeld, 1999).

Interaction Between Aerosol Particles and Maritime Convective Clouds:

buoyancy forces. From now on, the cloud experiences a violent vertical development reaching the maximum height generated by the strong temperature gradient between the cloud core and the environment surrounds it. However, the growing process will be affected by dry air entrainment on the cloud's side walls, inhibiting the vertical development of

The air entrainment dilutes and evaporates droplets releasing aerosol particles, which served as CCN, back into the atmosphere and cooling the air surrounding. The process generates downward movements in the cloud and promotes a high turbulence that mixes

Atmospheric particles play a very important role in the climate system. Their effects on the direct radiative forcing scattering or absorbing sunlight, and facilitating indirectly the formation of clouds are a relevant object of study since there is no adequate knowledge of their significance on clouds creation. Particles radiative forcing is globally comparable to greenhouse gases, but in the opposite direction because it causes a cooling climate (Charlson et al, 1992). Coakley and Grams (1976) consider that particles between the range 0.05 < r < 1 μm may cause a cooling surface. Research has shown that the radiative forcing of atmospheric particles depends on their composition, size and altitude (Hansen et al, 1980; Pollack et al, 1981). So, it may be considered that a change in one or several properties on the

The indirect radiative forcing occurs when the aerosol particles are used as CCN creating

a. Radiative forcing induced by an increase of anthropogenic particles promoting a higher concentration of droplets that change the cloud's albedo (Twomey, 1974). This effect is

b. Radiative forcing caused by a higher concentration of anthropogenic particles, causing a decrease in the droplets diameter and more competition for the water vapor available in the atmosphere. This will reduce the precipitation efficiency and modify the cloud's residence time in the atmosphere (Albrecht, 1989). The event is known as cloud lifetime

East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System 2001 (EPIC 2001) was sponsored by The U.S. Climate Variability and Predictability Research Program (CLIVAR), which has the goal of providing the observational basis needed to improve the representation of certain key physical processes in coupled ocean atmosphere models. In addition to physical processes, EPIC 2001 research was directed toward a better understanding and simulation of the effects of short-term variability in the east Pacific on climate. This variability is particularly important in the region because conditions in the

convective cloud (Squires, 1958; Emanuel, 1982).

atmospheric particles might affect the local forcing.

cloud droplets. The effects are classified in two types:

or Albrecht effects.

**1.4. EPIC 2001** 

also known as the cloud's albedo or Twomey effects.

**1.3. Interaction of atmospheric particles with solar radiation** 

the air masses.

Measurements in ITCZ During the EPIC 2001 Project 223

So, there are many processes that change the properties of atmospheric particles and their interactions with clouds. This chapter is focused on identifying and assessing the main processes involved with particles in the vicinity of maritime convective clouds at the Inter-Tropical Convergence Zone (ITCZ).
