Aerosol light scattering measurements were carried out at the Global Atmosphere Watch (GAW) research station, Mace Head, on the west coast of Ireland. Analysis of ten years (2001-2010) of the aerosol light scattering coefficient (sigma(scat)) at 550nm reveals a strong seasonal trend with sigma(scat) values during the winter season being approximately three times that of the summer season. The sigma(scat) and the aerosol light backscattering coefficient (sigma(bscat)) for the winter (Low Biological Activity - LBA) and the summer (High Biological Activity - HBA) are dependent on the square of wind speed (U). Scattering properties for the LBA period are approximately twice those of the HBA period. 70% of the observed LBA-HBA scattering difference can be attributed to differences in refractive index alone, resulting from organic-matter enrichment during the HBA period.