Organic aerosols in the Caribbean trade winds: A natural source?

Mass concentrations of carbonaceous and inorganic components of submicron (aerodynamic diameters < 0.6 μm) aerosol particles were measured at Cape San Juan, a coastal site located on the extreme northeastern tip of Puerto Rico. Additionally, carbonaceous aerosols and condensation nuclei (CN) were measured offshore during a limited shipboard sampling experiment at an Atlantic Ocean site away from the coast. Both sites were exposed to trade winds during the sampling periods. Under these conditions no obvious upwind anthropogenic sources should impact the sites. From the analyses of collected filter samples, mass concentrations of major inorganic anions and filterable particulate organic aerosol material (OC p ) were determined. The derived OC p concentrations may represent lower limits because of the uncertainty in the collection of semivolatile organic material lost from the particles during sampling. Analyses of Cape San Juan samples show that (1) mass concentrations of submicron OC p (average about 390 ng m −3 ) exceed sulfate concentrations (average about 270 ng m −3 ); (2) this organic aerosol material is water soluble (and the particles act as effective cloud condensation nuclei); and (3) primary combustion aerosol does not appear to be a major contributor to OC p . The fact that OC p concentrations measured at the coastal site are similar to OC p concentrations (330–400 ng m −3 ) measured at an Atlantic Ocean site removed from the coast suggests that a substantial fraction of the OC p 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.