Long Range Transport of Amazon Aerosol in the Free Troposphere: Influence of Amazon Combustion Aerosol on CCN in the Pacific Marine Boundary Layer

The Pacific Atmosphere Sulfur Experiment (PASE, Sept. 2007) on the NCAR C-130 aircraft was based in the Equatorial Pacific to explore the remote marine sulfur cycle. We investigated sources that control particle number and cloud condensation nuclei, CCN, in the clean marine boundary layer (MBL). Earlier studies here demonstrated particle number above the MBL was dominated by natural production of new volatile particles in cloud outflow. However, during PASE we also found coated refractory aerosol (non-volatile at 350C) aloft were linearly related to ozone concentrations and were effective CCN at 0.2% supersaturation. These aerosol had larger diameters than naturally produced volatile aerosol and trajectory analysis traced them back to deep convection in biomass burning haze over the Amazon basin over 10,000km away. These refractory soot and/or organic aerosol appear to be detrained from deep convective clouds after the near- source scavenging of larger sizes that dominate the smoke/haze aerosol mass. Following transport and once mixed into MBL they were found to account for as much as 30% of the CCN at the value of 0.2%S (a typical value for small trade-wind cumulus clouds). Hence, cloud-scavenged combustion derived aerosol, too small to be detectable optically in satellite plumes, appears to provide seed nuclei for CCN in the remote marine boundary layer. This acts over hemispheric scales for this region and presumably elsewhere. Hence; various mechanisms including convective scavenging, long range transport, particle production aloft, entrainment into the MBL, boundary layer nucleation and sea-salt production all need to be considered in modeling the MBL CCN population.