Direct and indirect radiative effects of sea-salt aerosols over Arabian Sea

Abstract

Estimation of the indirect radiative effect of aerosols requires an understanding of the role of aerosols in influencing cloud properties. Several investigations have focused on the determination of the indirect effect, but most of them were confined to the anthropogenic (manmade) sulphate aerosols. Studies on the indirect effect of natural aerosols (such as sea salt) are rather few. In this article, a simple approach has been used to determine the indirect effect of sea-salt aerosols over the Arabian Sea for different seasons using long-term data available from ship-borne and island-based observations in the past. We demonstrate that the indirect radiative effect of sea-salt (natural) aerosols (at the top of the atmosphere) is as large as –7 ± 4 Wm^–2 when compared to the direct radiative effect of –2 ± 1 Wm^–2, and hence cannot be ignored. These values are larger than the anthropogenic aerosol forcing (~ 5.0 ± 2.5 Wm^–2) reported over this region. The high variability in indirect effect from – 4 Wm^–2 to around –18 Wm^–2 brings out the importance of natural aerosols in this region. The study also demonstrates the important role of wind speed on aerosol characteristics and hence its impact on direct and indirect radiative effects. The magnitude of indirect radiative effect (and uncertainty) is severalfold more than the direct radiative effect of sea-salt aerosols.