83rd Annual

Thursday, 13 February 2003
Near real time measurement of sea-salt aerosol during the SEAS campaign: Comparison of emission based sodium detection with an aerosol volatility technique
A. J. Hynes, Univ. of Miami/RSMAS, Miami, FL; and P. Campuzano-Jost, H. Maring, C. D. Clark, D. S. Covert, S. Howell, V. Kapustin, A. Clarke, and E. S. Saltzman
Poster PDF (334.3 kB)
The effects of sea salt in direct scattering of solar radiation and its indirect effect via impact on cloud formation and cloud droplet size distribution are a topic of much current interest. Until recently sea salt aerosol was assumed to constitute a small fraction of the submicron aerosol in the marine boundary layer with sulfate being the dominant chemical constituent. In recent years this view has been challenged and it has been suggested that sea salt aerosol can provide the primary source of CCN, even under sulfate rich conditions. One of the problems in assessing the relative roles of sea salt and sulfate aerosol is related to the difficulty of measuring in-situ “chemically resolved” aerosol size distributions. In this work we report the first deployment of an emission based sodium aerosol detector, designed to chemically characterize size segregated marine aerosols on a near real-time basis. The basic principle of operation of the ASD is the volatilization of aerosol particles in a high temperature flame, atomization of the sodium salts to give sodium atoms and detection of the emission at 589.0 (D2 line) and 589.6 nm (D1 line) from thermally excited sodium atoms. The ASD consists of an aerosol sampling and injection system to introduce aerosol particles into the flame, a pre-mixed laminar hydrogen/air flame for volatilization of the aerosol and atomization of sodium salts, and PMTs (photomultiplier tubes) that detect the emission and associated electronics for data acquisition. It is possible to monitor size resolved aerosols by sampling through a differential mobility analyzer. Deployment occurred as part of the Shoreline Environment Aerosol Study (SEAS) from April 16 to May 1, 2000 at Bellows Air Force Base on the east side of Oahu, where the University of Hawaii Department of Oceanography maintains a tower for aerosol measurements. We operated the instrument in size unsegregated mode and made measurements which included two, extended continuous sampling periods, each of which lasted for twenty four hours. During this time, we compared the ASD with measurements that used aerosol volatility coupled with optical particle counting to infer sea-salt size distributions. We obtained reasonable agreement between the instruments when sampling in clean air, validating the ASD and demonstrating that aerosol volatility measurements can provide reliable sea salt distributions. The combination of these measurements indicated that seasalt was the dominant constituent of aerosol particles with diameters larger than 500 nm and that sulfate was the dominant constituent at smaller diameters during clean air sampling.

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