83rd Annual

Thursday, 13 February 2003
E. D. Nilsson, Stockholm University, Stockholm, Sweden; and M. Mårtensson, S. van Ekeren, G. de Leeuw, M. Moerman, C. O'Dowd, R. Flanagan, and M. Geever
There is a source of aerosol particles from the ocean because small droplets are ejected into the atmosphere when bubbles burst at the sea surface in white caps. This source plays a role for the sea-air flux of water, sea-salt and pollutants. It may be very important for the climate since sea-salt particles are very good cloud condensation nuclei and hence can control the cloud albedo and lifetime. The sea salt aerosol has an influence on the atmospheric sulfur cycle and the climate effects of the natural or anthropogenic sulfate aerosol. In addition, sea salt is an important tracer in the climate records of Arctic and Antarctic snow and ice cores. Besides sea-salt, the primary marine aerosol has an organic component, which results both from natural (biogenic) and man-made sources, dissolved in the surface water or as a thin film on the water surface. However, there are large uncertainties in the existing estimates and parameterizations of the primary marine aerosol source. The sub-micrometer part of this aerosol source has almost been entirely neglected, and very little is known about the size and composition of the organic component. In order to better understand the primary marine aerosol source, to achieve an improved size resolved and chemically specific parameterization, and to provide better estimates of the primary marine aerosol source, we have used the eddy correlation technique, and achieved the first direct measurements of this source flux. Measurements have been made over the Barents Sea and the Norwegian sea during the AOE in 1996 (Nilsson et al., 2001, JGR Atmospheres, 106, 32,139-32,154.); over the Pacific Ocean during the RED experiment in 2001; and by the Atlantic coast at Mace Head, Ireland, during the first QUEST experiment in 2002. The eddy correlation measurements utilize an ultrasonic anemometer and at sea a full motion package with inclinometers and accelerometers. The system includes also a TSI Condensation Particle Counter with a lower size cut at 10 nm diameter and an Optical Particle Counter (OPC) resulting in a size resolved flux up to several micrometer diameter. During the last campaign, two eddy systems have been run in parallel, one of them equipped with a volatility system in front of the OPC in order to achieve not only a size resolved flux, but also its chemical identity: sea-salt or organics. We have found that the source flux is a strong function of wind speed, which confirms previous assumptions. The aerosol number flux was larger than expected because it included a large fraction of sub micrometer particles. Two modes are present: one centered around 100 nanometer in diameter; one at a few micrometers. Complementing laboratory experiments have resulted in a first size resolved source parameterization. The data sets from 2001 and 2002 are now being analyzed in order to examine how the source flux is dependent on size, wind speed, stability etc., and how much sea-salt and organics contributes, respectively. Measurements in open sea and at the coast, measurements in the Arctic and in tropical waters will be compared and contrasted.

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