1 P2.19 CHEMICAL CHARACTERIZATION OF CLOUD WATER AT THE EAST PEAK, PUERTO RICO, DURING THE R AIN I N C UMULUS OVER THE O CEAN EXPERIMENT (RICO) Adriana Gioda, Olga L. Mayol-Bracero*, Ana Rodríguez, Flavia Morales-García, Ricardo Morales Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, PR, USA Jeff Collett Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA Lorenza Emblico, Stefano Decesari Institute of Atmospheric Science and Climate-C.N.R., Bologna, Italy 1. INTRODUCTION Aerosols interact with radiation and play an important role in climatic processes. They influence the Earth’s radiative balance directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei (CCN) affecting cloud droplets and cloud albedo (NRC, 1996). Some aerosol particles in the presence of supersaturated water vapor act as cloud condensation nuclei (CCN), which are responsible for the formation of cloud and fog droplets. The constituents in the liquid phase of clouds come from species in aerosol particles scavenged by the droplets, from the dissolution of trace gases within the droplets, or as products of the aqueous phase reaction of precursor species (Fuzzi, 2001). Clouds can accumulate large concentrations of different species in a small volume of cloud water. The concentration in cloud water of a particular species depends on its concentration in the air and on the efficiency by which it is scavenged. Scavenging theories have been used to evaluate the transfer from air to cloud water of a specific species. The first studies of scavenging efficiencies were focused on inorganic constituents, mainly sulfate (Hegg et al., 1984) followed by nitrate and ammonium (Kasper-Giebl et al., 1999). Studies showed that organic aerosols could contribute significantly to the CCN fraction and that they may be present in certain environments in concentrations similar to or even higher than sulfate aerosol (Novakov and Penner, 1993). Nevertheless, few studies have examined scavenging efficiencies to determine organic partitioning between aerosols and cloud water (Facchini et al., 1999: Limbeck and Puxbaum, 2000). Although these studies were not performed in the tropics and/or with marine organic aerosols, they have clearly shown that the organic aerosol composition should be taken into account in determining the hygroscopic behavior and CCN ability of atmospheric aerosols. This project is part of the international project R ain I n C umulus over the O cean Experiment (RICO) (http://www.ofps.ucar.edu/rico) that seeks to understand the properties of trade wind cumulus with emphasis on the importance of precipitation. It took place during the months of December 2004 and January 2005. Here we determined the inorganic composition of cloud water collected at the East Peak, located at the Caribbean National Forest (the only tropical rainforest that is part of the United States Forest Service) in Puerto Rico. Some measurements of aerosol collected at our reference site (Cape San Juan) were also considered to determine scavenging efficiencies. 2. METHODOLOGY 2.1. Sampling Site Cloud water samples were collected at the East Peak (EP), in the Caribbean National Rainforest, Puerto Rico (see Figure 1). East Peak is located ~35 km from San Juan and ~20 km from the South Atlantic Ocean at 1020 m.a.s.l. in a tropical rainforest. The area is a humid mountaintop with frequent and abundant cloud coverage fed by high altitude trade winds. The average yearly precipitation and temperature are about 4000 mm per year and 22.2 o C, respectively. Cape San Juan (CSJ), located at the most northeastern part of Puerto Rico, is our reference site used to collect aerosol samples. _______________________________________ Corresponding author address: Olga L. Mayol-Bracero, Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, PR, USA, PO Box 21910, San Juan, PR 00931-1910 USA, omayol@adam.uprr.pr