Potential origin of organic cloud condensation nuclei observed at marine site

Observations by Novakov and Penner [1993] (hereinafter, NP) suggest that organic aerosol particles may make up a significant portion of cloud condensation nuclei (CCN), comparable perhaps to the sulfate aerosol contribution to CCN. Because organic CCN would influence the albedo and radiative properties of clouds and play a role in climate change, it is important to identify the sources of these aerosols. In their research, 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 we use model simulations to investigate the origins of the nucleation mode aerosol (0.03 < d(p) < 0.1 mu m) observed at a marine site in Puerto Rico by NP. NP measured the size segregated mass and composition of the atmospheric aerosol at this site and found that organic aerosol mass dominated the total mass in the nucleation mode. The presence of organics in this size range may be the result of nucleation of a gas phase organic and/or condensational growth. Therefore identifying sources of the gas phase precursors is important to determining the origins of the organic CCN. Back trajectory analysis (J. Merrill, private communication, 1994) shows that the air mass that Novakov and Penner studied was without continental contact for 8-12 days prior to being sampled. This implicates two possible sources for the gas phase organic: biogenic emissions from the Puerto Rican forest upwind of the mountain sampling site and emissions from the ocean surface. In this study both the forest and ocean source scenarios are explored. Using input parameters taken from the literature, it is shown that an assumed organic vapor flux for the forest scenario, very similar to the measured biogenic fluxes reported by Zimmerman et al. [1988], generated size distributions similar to the observed aerosol. In contrast, literature estimates for the n-alkane vapor flux from the ocean are not consistent with the observed size distribution at the mountain site.