Chemical constituents in clouds and rainwater in the Puerto Rican rainforest: Potential sources and seasonal drivers

Cloud- and rain-water samples collected between 1984 and 2007 in the Luquillo Experimental Forest, Puerto Rico, were analyzed in order to understand the main processes and sources that control their chemistry. Three sites were used: El Verde Field Station (380 m asl), Bisley (361 m asl), and East Peak (1051 m asl). Bulk rainwater samples were collected from all sites, and cloud water was also collected on East Peak. All samples were analyzed for pH, conductivity, and concentrations of Cl−, SO42−, NO3−, NH4+, Na+, K+, Ca2+ and Mg2+. Similar patterns in overall chemistry were observed for both cloud- and rain-water samples. The majority of samples had low acidity (average pH of 4.4–5.0), similar to other remote sites. Sea salt (Na+ and Cl−) had a large influence on rain and cloud chemistry and accounted for approximately 70% of the total mass of solutes, followed by SO4=, which controls the acidity of the clouds and rainwater. Calcium accounted for 6–8% of the total cations and dominated neutralization processes. The highest concentrations of Ca2+ and NO3− in both cloud- and rain-water were observed in the summertime when large amounts of dust from the African continent reached the sites. Enrichment Factor and Principal Component Analyses showed that Na+, Cl−, and Mg2+ in the cloud- and rain-water were primarily of marine origin, while most of the Ca2+ was from crustal sources; and NO3− was predominantly anthropogenic, presumably from both local and long-range sources. In general, the results of this study suggested that cloud- and rain-water chemistry in northeastern Puerto Rico is strongly influenced by natural and marine sources rather than local anthropogenic sources. The pollutant species in the samples were mainly derived from long distance transport.