ORGANIC AEROSOLS IN THE CARIBBEAN TRADE WINDS - A NATURAL SOURCE

Mass concentrations of carbonaceous and inorganic components of submic ron (aerodynamic diameters < 0.6 mu m) aerosol particles were measured at Cape San Juan, a coastal site located on the extreme northeastern tip of Puerto Rico. Additionally, carbonaceous aerosols and condensati on nuclei (CN) were measured offshore during a limited shipboard sampl ing experiment at an Atlantic Ocean site away from the coast. Both sit es were exposed to trade winds during the sampling periods. Under thes e conditions no obvious upwind anthropogenic sources should impact the sites. From the analyses of collected filter samples, mass concentrat ions of major inorganic anions and filterable particulate organic aero sol material (OCP) were determined. The derived OCP concentrations may represent lower limits because of the uncertainty in the collection o f semivolatile organic material lost from the particles during samplin g. Analyses of Cape San Juan samples show that (1) mass concentrations of submicron OCP (average al-rout 390 ng m(-3)) exceed sulfate concen trations (average about 270 ng m(-3)); (2) this organic aerosol materi al is water soluble (and the particles act as effective cloud condensa tion nuclei); and (3) primary combustion aerosol does not appear to be a major contributor to OCP. The fact that OCP concentrations measured at the coastal site are similar to OCP concentrations (330 - 400 ng m (-3)) measured at an Atlantic Ocean site removed from the coast sugges ts that a substantial fraction of the OCP in the Caribbean trade winds may be associated with natural oceanic emissions. Results imply that background organic marine aerosol should be included in estimating the indirect forcing of climate by anthropogenic sulfate aerosol.