Direct radiative effects by anthropogenic particles at a pollutedsite: Rome (Italy)

The direct radiative effect (DRE) by all (anthropogenic plus natural) and anthropogenic aerosols is calculated at the solar (0.3–4 μm) and infrared (4–200 μm) spectral range to better address the annual cycle of the anthropogenic aerosol impact at a site (Rome, Italy) significantly affected by pollution. Aerosol optical and microphysical properties from 2003 AERONET Sun/sky-photometer measurements and solar surface albedos based on MODIS satellite sensor data constitute the necessary input to radiative transfer simulations. Clear- and all-sky conditions are investigated by adopting ISCCP monthly products for high-, mid-, and low-cloud cover. It is shown that monthly mean values of aerosol optical depths by anthropogenic particles (AODa) are on average more than 50% of the corresponding all-aerosol-optical-depth (AOD) monthly means. In particular, the AODa/AOD ratio that varies within the (0.51–0.83) on autumn-winter (AW, October-March), varies within the (0.50–0.71) range on spring-summer (SS, April-September) as a consequence of the larger contribution of natural particles on SS. The surface (sfc), all-sky DRE by anthropogenic particles that is negative all year round at solar wavelengths, represents on average 60% and 51% of the all-sky sfc-DRE by all aerosols on AW and SS, respectively. The all-sky atmospheric forcing by anthropogenic particles (AFa) that is positive all year round, is little dependent on seasons: it varies within the (1.0–4.1)W/m2 and (2.0–4.2)W/m2 range on AW and SS, respectively. Conversely, the all-sky AF by all aerosols is characterized by a marked seasonality. As a consequence, the atmospheric forcing by anthropogenic particles that on average is 50% of the AF value on AW, decreases down to 36% of the AF value on SS. Infrared aerosol DREs that are positive all year round are significantly smaller than the corresponding absolute values of solar DREs. Clouds decrease on average ToA- and sfc-DRE absolute values by anthropogenic particles of 36% and 23%, respectively and are quite responsible of the seasonal dependence of aerosol forcing efficiencies by all and anthropogenic aerosols.