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Aerosol production in the surf zone and effects on IR extinction

Author: Neele, F.P. · Leeuw, G. de · Eijk, A.M.J. van · Vignati, E. · Hill, M.K. · Smith, M.H.
Type:article
Date:1998
Publisher: NATO RTO
Place: Neuilly-sur-Seine
Institution: TNO Fysisch en Elektronisch Laboratorium
Source:RTO Meeting Proceedings - E-O Propagation, Signature and System Performance Under Adverse Meterological Conditions Considering Out-of-Area Operations - Papers presented at the Sensors & Electronics Technology Panel Symposium, Naples, Italy, 16-19 March 1998, 24/1-24/11
Identifier: 95120
Report number: RTO-MP-1 AC/323(SET)TP/2
Keywords: Physics · Aerosol fluxes · Aerosol production · Breaking Waves · Coastal zone · Atmospheric transmission · Electro-optical sytems · Infrared radiation · Marine environments · Extinction

Abstract

The aerosol production in the surfzone, as determined from measurements, at two sites along the Califomia coast is presented. The data used were collected during three EOPACE (Electro-Optical Propagation Assessment in Coastal Environments) measurement campaigns in 1996 and 7997. Particle counters were deployed at both the end and the base of two piers which extended into the ocean, beyond the surf zone. For winds from the sea, a clear increase in aerosol concentration was measured, between the particle counters at the end and the base of the piers. Aerosol concentrations were measured at the base of the piers at three heights, which allowed for the estimation of the aerosol production in the surf zone. Taking into account the different whitecap ratios, the surf zone aerosol source function derived from the data compares well with previously reported open-ocean source functions, in agreement with the common bubble (film and jet drops) origin. Wind speeds measured during the experiments were up to about 9 m/s; therefore, the source function presented here applies to the bubble part of the source spectrum only. Infra-red extinction coefficients were computed from the aerosol concentrations, using a Mie scattering code. Extinction values may be up to two orders of magnitude larger than in the unperturbed oceanic air mass; the vertical gradients in extinction are also much stronger than those reported for open-ocean conditions. A simple aerosol dispersion model, using observed surf zone aerosol production rates, predicts that air masses up to several km from the surf zone may be significantly affected by the surf-produced aerosol. For winds from land, this proves the importance of the surfzone in assessing the performance of electro-optical systems in coastal areas.