Monthly-averaged anthropogenic aerosol direct radiative forcing over the Mediterranean from AERONET derived aerosol properties

The all-sky direct radiative effect by anthropogenic aerosol (DREa) is calculated in the solar (0.3-4 mu m) and infrared (4-200 mu m) spectral ranges for six Mediterranean sites. The sites are differently affected by pollution and together reflect typical aerosol impacts that are expected over land sites of the central Mediterranean basin. Central to the simulations are aerosol optical properties from AERONET sun-/sky-photometer statistics for the year 2003. A discussion on the variability of the overall (natural+anthropogenic) aerosol properties with site location is provided. Supplementary data include MODIS satellite sensor based solar surface albedos, ISCCP products for high- mid- and low cloud cover and estimates for the anthropogenic aerosol fraction from global modelling. Since anthropogenic aerosol particles are considered to be smaller than 1 mu m in size, mainly the solar radiation transfer is affected with impacts only during sun-light hours. At all sites the (daily average) solar DRE sub(a) is negative all year round at the top of the atmosphere (ToA). Hence, anthropogenic particles produce over land sites of the central Mediterranean a significant cooling effect. Monthly DRE sub(a) values vary from site to site and are seasonal dependent as a consequence of the seasonal dependence of available sun-light and microphysical aerosol properties. At the ToA the monthly average DRE sub(a) is − (4 plus or minus 1) W m super(− 2) during spring-summer (SS, April-September) and − (2 plus or minus 1) W m super(− 2) during autumn-winter (AW, October-March) at the polluted sites. In contrast, it varies between − (3 plus or minus 1) W m super(− 2) and − (1 plus or minus 1) W m super(− 2) on SS and AW, respectively at the less polluted site. Due to atmospheric absorption the DRE sub(a) at the surface is larger than at the ToA. At the surface the monthly average DRE sub(a) varies between the most and the least polluted site between − (7 plus or minus 1) W m super(− 2) and − (4 plus or minus 1) W m super(− 2) during SS, and between − (4 plus or minus 3) W m super(− 2) and − (1 plus or minus 1) W m super(− 2) during AW. The DRE sub(a) at infrared wavelengths is positive but negligible, especially at the ToA (