Monthly-averaged anthropogenic aerosol direct radiative forcing over the Mediterranean based on AERONET aerosol properties

Abstract

The all-sky direct radiative effect by anthropogenic aerosol (DRE_a) is calculated in the solar (0.3–4 μm) and infrared (4–200 μm) spectral ranges for six Mediterranean sites. The sites are differently affected by pollution and together reflect typical aerosol impacts that are expected over land and coastal 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 aerosol models. Since anthropogenic aerosol particles are considered to be smaller than 1 μm in size, mainly the solar radiation transfer is affected with impacts only during sun-light hours. At all sites the (daily average) solar DRE_a is negative all year round at the top of the atmosphere (ToA). Hence, anthropogenic particles produce over coastal and land sites of the central Mediterranean a significant cooling effect. Monthly DRE_a values vary from site to site and are seasonally dependent as a consequence of the seasonal dependence of available sun-light and microphysical aerosol properties. At the ToA the monthly average DRE_a is −(4±1) W m^−2 during spring-summer (SS, April–September) and −(2±1) W m^−2 during autumn-winter (AW, October–March) at the polluted sites. In contrast, it varies between −(3±1) W m^−2 and −(1±1) W m^−2 on SS and AW, respectively at the less polluted site. Due to atmospheric absorption the DRE_a at the surface is larger than at the ToA. At the surface the monthly average DRE_a varies between the most and the least polluted site between −(7±1) W m^−2 and −(4±1) W m^−2 during SS, and between −(4±3) W m^−2 and −(1±1) W m^−2 during AW. The DRE_a at infrared wavelengths is positive but negligible, especially at the ToA (<0.3 W m^−2). The average of DRE_a monthly-means referring to all sites has allowed getting a ToA- and sfc-DRE_a yearly-mean value of −(3±2) and −(5±3) W m^−2, respectively at solar wavelengths. Last data, even if refer to a particular year, indicate that the radiative energy-balance of Central Mediterranean land and coastal sites is quite affected by anthropogenic particles.