Aerosol light scattering measurements were carried out using a TSI 3563 Nephelometer at the Mace Head Atmospheric Research Station, on the west coast of Ireland from year 2001–2010. A strong seasonal trend in the aerosol light scattering coefficient at 550？nm ( ), for clean marine air masses, is observed with a high value, [average (geometric mean)] of 35.3？Mm？1 (29.5？Mm？1), in January and a low value of 13.7？Mm？1 (10.2？Mm？1), in July. This near threefold increase in the value during the winter season is because of the large contribution of wind-speed generated sea-salt particles in the marine boundary layer. A high positive correlation coefficient of 0.82 was found between the percentage occurrence of relatively large ？ngstr？m exponent (？) values (>1.2) and the percentage occurrence of lower values (5–15？Mm？1) in the summer season. and wind-speed have a high positive correlation coefficient of 0.88 whereas ？ and wind-speed have a negative correlation coefficient of ？0.89. ？ values during the summer months indicate the dominance of sub- m particles thus indicating the contribution of non-sea-salt sulphate and organics towards the as these species show an enhanced concentration during the summer months. 1. Introduction Knowledge of the light scattering properties of atmospheric aerosol particles is of vital importance in estimating the radiative forcing of climate and in global radiation budget studies. Uncertainties in the global aerosol direct and indirect effects are nearly the same or twice as much as the magnitude of the effect itself as estimated by the Intergovernmental Panel on Climate Change (IPCC) in its 4th assessment report [1]. Several-long term measurements of key aerosol properties (e.g., aerosol light scattering and aerosol light absorption coefficient, condensation nuclei number concentration, etc.) at various measurement stations across the globe, for example [2–6], have been reported. Such long-term measurements of key aerosol properties at worldwide locations will be effective in reducing the uncertainties associated with aerosol direct and indirect effects. The aerosol light scattering coefficient, an extensive optical property, can yield crucial information about the aerosol size distribution and composition if its wavelength dependence from which the ？ngstr？m exponent—an intensive optical property (independent of particle number concentration) is derived [7]. In the marine environment, the main constituent of aerosol particles is wind speed-generated sea salt [8, 9]. Non-sea salt (nss) sulphate and organics also can dominate the
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