Atmospheric Environment 231 (2020) 117533 Available online 22 April 2020 1352-2310/© 2020 Elsevier Ltd. All rights reserved. Aerosol absorption over the Aegean Sea under northern summer winds Georgia Methymaki a, * , Elissavet Bossioli a , John Kalogiros b , Giorgos Kouvarakis c , Nikolaos Mihalopoulos b, c , Athanasios Nenes d, e , Maria Tombrou a a Department of Physics, National and Kapodistrian University of Athens, Athens, 15772, Greece b Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Athens, 15236, Greece c Department of Chemistry, University of Crete, Heraklion, 700 13, Greece d Laboratory of Atmospheric Processes and Their Impacts, School of Architecture, Civil and Environmental Engineering, � Ecole Polytechnique F� ed� erale de Lausanne, Lausanne, 1015, Switzerland e Institute for Chemical Engineering Sciences, Foundation of Research and Technology Hellas, Patras, 26504, Greece HIGHLIGHTS � The forcing of BC, dust, and sea salt absorption under Etesians is estimated at 1.2, 0.1 and nearly zero W m 2 , accordingly. � BC absorption reduces the SW↓ by 5.5 W m -2 near the surface and augments the LW↑ by 0.3 W m -2 at the TOA. � BC absorption reduces the cloud water mixing ratio on average by 10% (semi-direct effect). � BC absorption infuences all physical and dynamical heating processes producing heating rates up to 0.2 K day 1 . � The mean daily temperature increases by up to 0.8 K near the surface. A R T I C L E INFO Keywords: Aerosol absorption Black carbon absorption WRF-Chem Direct effect Semi-direct effect heating rate Mediterranean Aerosol-radiation interaction ABSTRACT In this modelling study, the absorption infuence on radiation, apart from scattering, is studied above the Aegean Sea (Eastern Mediterranean) under a typical warm 13-day period with northern winds, transporting polluted air masses. The simulated (WRF-Chem) forcing caused by the total absorption is estimated along with black carbon (BC), dust, and sea salt contributions, 1.3, 1.2, 0.1 and nearly zero W m 2 , accordingly. As dust and sea salt infuence is negligible, the main focus is on BC. BC absorption reduces downward shortwave irradiance reaching the ground by up to 5.2 W m 2 and the upward part by up to 1.7 W m 2 . The downward and the upward longwave irradiances are augmented by up to 2.3 and 1.2 W m 2 , accordingly. Even though the cloud formation is not favoured during the study period, BC absorption reduces overall the cloud water mixing ratio by 10% (semi-direct effect). However, during specifc days and over limited cloudy areas, the semi-direct effect reduces low level clouds up to 20% while in case of higher clouds the reduction reaches up to ~29%. In order to examine the physical mechanisms below semi-direct effect, all modelled heating rates are analysed. Radiation direct absorption increases the air temperature with a rate up to 0.2 K day 1 , with an exception inside the surface layer, where unexpectedly longwave cooling prevails. The heating of the surface layer is mainly attributed to the advection process, as more heated air masses are transported over the Aegean Sea. 1. Introduction Atmospheric aerosols are the most uncertain driver of global climate change (IPCC et al., 2013). The particles scatter and absorb solar and terrestrial radiation (Angstrom, 1929; Coakley et al., 1983; Charlson et al., 1992; Ramanathan and Feng, 2009; Boucher et al., 2013), the so called direct effect. Through the aerosol absorption, the temperature, the atmospheric stability, and the near-cloud relative humidity are locally altered and consequently modify the cloudiness, well known as the semi-direct effect (Hansen et al., 1997; Ackerman et al., 2000; Jacobson, 2002). Concurrently, aerosols serve as cloud condensation or ice nuclei (indirect effect) (Gunn and Phillips, 1957; Twomey, 1972; Altaratz et al., 2014), thus indirectly cool the planet by increasing its albedo, the frst indirect effect (Twomey, 1974, 1977), and differentiate * Corresponding author. E-mail address: gmethymaki@phys.uoa.gr (G. Methymaki). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: http://www.elsevier.com/locate/atmosenv https://doi.org/10.1016/j.atmosenv.2020.117533 Received 13 November 2019; Received in revised form 2 March 2020; Accepted 17 April 2020