Seasonal Variation of the Aerosol Light Scattering Coefficient in Marine Air of the Northeast Atlantic

Abstract

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 (𝜎scat), for clean marine air masses, is observed with a high 𝜎scat value, [average (geometric mean)] of 35.3 Mm−1 (29.5 Mm−1), in January and a low 𝜎scat value of 13.7 Mm−1 (10.2 Mm−1), in July. This near threefold increase in the 𝜎scat 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 Ångström exponent (Å) values (>1.2) and the percentage occurrence of lower 𝜎scat values (5–15 Mm−1) in the summer season. 𝜎scat and wind-speed have a high positive correlation coefficient of 0.88 whereas Å and wind-speed have a negative correlation coefficient of −0.89. Å values during the summer months indicate the dominance of sub-𝜇m particles thus indicating the contribution of non-sea-salt sulphate and organics towards the 𝜎scat as these species show an enhanced concentration during the summer months.