Direct radiative forcing during Sahara dust intrusions at a site in the Central Mediterranean: Anthropogenic particle contribution

Abstract

The clear-sky, instantaneous Direct Radiative Effect (DRE) by all and anthropogenic particles is calculated during selected dust intrusion events at a site in the Central Mediterranean, to evaluate the role of anthropogenic particle's radiative effects and obtain a better estimate of the DRE by desert dust. The Mediterranean basin is continuously influenced by long-range transported emissions from continental Europe and northern Africa. The clear-sky aerosol DRE is calculated by a two stream radiative transfer model in the solar (0.3–4 μm) and infrared (4–200 μm) spectral range, at the top of the atmosphere (ToA) and at the Earth's surface (sfc), during typical weak and strong dust intrusion events. Aerosol optical properties by AERONET sun-sky photometer measurements and aerosol vertical profiles by EARLINET lidar measurements, both performed at Lecce (40.33°N, 18.10° E) in the Central Mediterranean, are used to perform radiative transfer simulations. Instantaneous solar DREs are negative as a consequence of the cooling effect by aerosol particles and span the – (25–11) W m − 2 and – (53–25) W m − 2 range at the ToA and surface, respectively during the investigated dusty days. Infrared DREs offset the solar DRE from 8% up to 14% at the ToA and from 9% up to 22% at the surface. A methodology has been implemented to estimate the contribution of anthropogenic particles during dusty days since their contribution can be significant over the Central Mediterranean. We have found that the aerosol optical depth (AOD) by anthropogenic particles is from 35% up to 65% of the AOD by all particles. The solar DRE by anthropogenic particles represents from 44% up to 83% and from 28% up to 60% of the DRE by all particles, at the ToA and surface, respectively. If we assume that the DRE by natural and anthropogenic particles can be linearly added, the contribution of natural particles to the whole aerosol DRE is lower than 56% and 72% at the ToA and surface, respectively during the tested dusty days.