Light scattering by atmospheric aerosol particles affects Earth's energy balance and contributes to the negative radiative forcing of climate. The overall impact of aerosols on the radiative balance comprises the absorption and scattering of radiation in the solar spectrum (direct effect) as well as the modification of the optical properties and lifetime of clouds (indirect effect). The quantification of these effects pose one of the largest uncertainties on the calculation of the global radiative forcing [Intergovernmental Panel on Climate Change (IPCC), 1995]. The uncertainties are due to the complexity of the physical and chemical processes involving aerosol particles and due to the high temporal and spatial variability of their global and regional distribution. In this regard, measurements of aerosol optical properties are required on a regional scale, covering certain key areas.
 In the marine atmosphere, the typical background aerosol comprises primary produced sea salt particles and secondary produced particles consisting of non sea salt (nss) sulphate and organic compounds. The sea salt particles dominate the supermicrometer (d > 1 μm) part of the total aerosol size distribution spectrum whereas the nss sulphate particles fall within the submicrometer (d < 1 μm) range.
 Recent investigations however suggest that a fraction of the primary produced sea salt aerosol contributes significantly to the submicrometer portion of the particle size distribution [O'Dowd and Smith, 1993; O'Dowd et al., 1997]. In terms of radiative forcing of climate, the submicrometer size range is most relevant for light scattering because it comprises particles with diameters comparable to the wavelength of the visible solar radiation. Measurements of the size segregated chemical composition of aerosols over the Southern Ocean and the application of Mie scattering theory to this data set confirmed the important role of submicrometer sea salt. Sea salt was found to dominate both the chemical composition and the aerosol light scattering of sub-μm particles [Murphy et al., 1998].
 The relevance of sea salt aerosol for climatological processes stems from its radiative properties in conjunction with its production mechanism, which is highly wind speed-dependent. Climatic induced changes in wind speed pattern can affect the sea salt production and therefore alter the radiation balance over marine areas [Latham and Smith, 1990]. A competing process is the removal of sea salt aerosols by precipitation since precipitation patterns are also sensitive to climatic variations. In addition, sea salt contributes to the indirect radiation effect by serving as cloud condensation nuclei (CCN). A recent study by O'Dowd et al. [1999a] identifies sea salt aerosol as the primary source of CCN under high wind speed conditions in the marine boundary layer (MBL).
 Recently, the coastal zone was also identified as a strong source for secondary aerosol particles [O'Dowd et al., 1999b, 1998]. The observed nucleation and growth processes are related to enhanced biological emissions of the intertidal zone during exposure to the atmosphere. The newly formed particles have the potential to grow into radiatively active sizes and thus could alter the radiation balance, which is most sensitive to perturbations over the low-albedo oceans [O'Dowd, 2002].
 A unique platform for studying marine aerosols is the Global Atmospheric Watch (GAW) atmospheric research station at Mace Head, located on the west coast of Ireland (53° 19′ N, 9° 54′ W). The research station is situated at about sea level on a peninsula, which is surrounded by coastline and tidal areas except for a small sector ranging from 20° to 40°. The distance between the station and the shoreline is about 50 to 100 m in the westerly direction. A wind direction sector between 180° and 300° opens to the Atlantic Ocean and is associated with the advection of marine background air masses [Jennings et al., 1997]. A map detailing the coastal features around Mace Head is shown in Figure 1.
 As part of the “New Particle Formation and Fate in the Coastal Environment” (PARFORCE) study, an intensive measurement campaign was conducted in Mace Head in June 1999. During this campaign a broad range of physical and chemical aerosol properties as well as meteorological parameters were simultaneously measured [O'Dowd et al., 2002a]. Results are presented here from aerosol light scattering measurements in conjunction with Mie calculations of aerosol scattering coefficients. The contribution of background aerosols and local aerosol sources as well as the relative contribution of submicrometer and supermicrometer particles to the light scattering coefficient will be examined.