POTENTIAL ORIGIN OF ORGANIC CLOUD CONDENSATION NUCLEI OBSERVED AT MARINE SITE

Observations by Novakov and Penner [1993] (hereinafter, NP) suggest th at organic aerosol particles may make up a significant portion of clou d condensation nuclei (CCN), comparable perhaps to the sulfate aerosol contribution to CCN. Because organic CCN would influence the albedo a nd radiative properties of clouds and play a role in climate change, i t is important to identify the sources of these aerosols. In their res earch, Novakov and Penner showed that particles larger than 0.05 mu m could act as CCN at supersaturation ratios of 0.5% water vapor. Here w e use model simulations to investigate the origins of the nucleation m ode aerosol (0.03 < d(p) < 0.1 mu m) observed at a marine site in Puer to Rico by NP. NP measured the size segregated mass and composition of the atmospheric aerosol at this site and found that organic aerosol m ass dominated the total mass in the nucleation mode. The presence of o rganics in this size range may be the result of nucleation of a gas ph ase organic and/or condensational growth. Therefore identifying source s of the gas phase precursors is important to determining the origins of the organic CCN. Back trajectory analysis (J. Merrill, private comm unication, 1994) shows that the air mass that Novakov and Penner studi ed was without continental contact for 8-12 days prior to being sample d. This implicates two possible sources for the gas phase organic: bio genic emissions from the Puerto Rican forest upwind of the mountain sa mpling site and emissions from the ocean surface. In this study both t he forest and ocean source scenarios are explored. Using input paramet ers taken from the literature, it is shown that an assumed organic vap or flux for the forest scenario, very similar to the measured biogenic fluxes reported by Zimmerman et al. [1988], generated size distributi ons similar to the observed aerosol. In contrast, literature estimates for the n-alkane vapor flux from the ocean are not consistent with th e observed size distribution at the mountain site.