Chemical Characterization of Cloud Water at the East Peak, Puerto Rico, during the Rain In Cumulus over the Ocean Experiment (RICO)

As part of the Rain In Cumulus over the Ocean Experiment (RICO) (http://www.ofps.ucar.edu/rico), we present results on the chemical characterization of cloud water samples collected at the East Peak in the Caribbean National Forest of Puerto Rico. The main objective of this study was to determine the concentrations of water-soluble ions (acetate, formate, oxalate, methane sulfonic acid, chloride, nitrate, sulfate, ammonium, calcium, hydrogen, magnesium, potassium and sodium) in cloud water samples under the influence of the tropical trade winds.

Cloud water samples were collected during January 2005 using a single-stage Caltech Active Strand Cloudwater Collector (ss-CASCC). The analysis of the water-soluble ions was performed using Ion Chromatography (IC). pH and conductivity measurements were also performed.

Preliminary results showed that the predominant inorganic species are: sodium (455 ueq/L) and chloride (427 ueq/L), followed by magnesium (103.1 ueq/L), sulfate (59.6 ueq/L) and nitrate (24.2 ueq/L). The main source of the ions analyzed was the ocean. The chemical composition of the cloud water changed when air masses from the North American continent reached the peak. In these cases, the highest concentrations of sulfate (98 ueq/L) and nitrate (85 ueq/L) were measured. The conductivity and pH values measured at the laboratory ranged from 40 to 94 uS/cm and from 6.5 to 8.2, respectively. However, lower pH values were measured at the site (4.8 to 6.4), immediately after the collection of the sample. A significant reduction in hydrogen concentrations was observed between pH measurements at the field (0.4 to 17.4 ueq/L H+) and at the laboratory (0.007 to 0.18 ueq/L H+). These losses of hydrogen probably occurred during the storage time due to the losses of organic acids. In-cloud scavenging efficiencies were also determined using aerosol samples collected in our upwind background sampling site (CSJ). These results will also be presented at the meeting.