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 550nm (sigma(cat)), for clean marine air masses, is observed with a high sigma(cat) value, [average (geometric mean)] of 35.3Mm(-1) (29.5Mm(-1)), in January and a low sigma(cat) value of 13.7Mm(-1) (10.2Mm(-1)), in July. This near threefold increase in the sigma(cat) 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 angstrom ngstrom exponent (angstrom) values (> 1.2) and the percentage occurrence of lower sigma(cat) values (5-15Mm(-1)) in the summer season. sigma(cat) and wind-speed have a high positive correlation coefficient of 0.88 whereas angstrom and wind-speed have a negative correlation coefficient of -0.89. angstrom values during the summer months indicate the dominance of sub-mu m particles thus indicating the contribution of non-sea-salt sulphate and organics towards the sigma(cat) as these species show an enhanced concentration during the summer months