7th Conf. on Atmospheric Chemistry


Trans-Pacific Transport of Black Carbon and Dust during the Cloud Indirect Forcing Experiment (CIFEX)

Odelle Lariviere, SIO/Univ. Of California, La Jolla, CA; and V. Ramanathan, G. R. Carmichael, G. Roberts, Y. Tang, and G. S. Mauger

The Cloud Indirect Forcing Experiment (CIFEX) took place in April 2004, during which time aerosols from two Asian dust storms arrived at the Northern California coast. Both airborne and ground-based measurements of the aerosol were made during these events. Airborne measurements included total number concentration, size distribution, scattering efficiency, and black carbon mass concentration. Ground station instruments measured black carbon concentration, number concentration, size distribution, and CCN efficiency. The CFORS (Chemical weather FORecast System) supplied forecasts for the horizontal and vertical dispersal of dust, sulfate, and black carbon (BC) across the Pacific Ocean and was used extensively in planning the flight tracks, as well as post-processing of the data.

Total mass concentration of aerosol layers observed during the flights agrees reasonably well with the CFORS model prediction for the combined mass concentration of dust, sulfate, BC, and organic carbon. BC typically accounts for about 3 percent of the total aerosol mass, so large errors in BC estimates will not affect the agreement significantly. The CFORS modeled BC transport across 130W is nearly twice that of the total United States BC emissions. Analysis of the CFORS transport and subsequent validation by ground measurements of BC compares well in the Western Pacific, but over-predicts BC in the Eastern Pacific by a factor of 3. Reducing the model output by a factor of 3 modifies the estimate of BC mass transport to approximately 25 to 80 percent of US direct emissions, still a substantial quantity. As the model does not include a parameter for wet removal of BC, the discrepancy between model-observation agreement in the West and East Pacific Ocean is a good indicator that BC acquires a hydrophilic coating during transport and is subject to wet removal. The rate of BC removal due to wet scavenging is currently not well parameterized in global climate models (GCM). The present results suggest that wet removal of BC is an important process for representations of BC atmospheric residence time and radiative forcing.


Session 3, Aerosol Measurements and Radiative Forcing Effects
Tuesday, 11 January 2005, 8:45 AM-12:00 PM

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