Long-term observations of aerosol optical properties at three GAW regional sites in the Central Mediterranean

In this work we present the results of the four-years (2015-2018) data-analysis of aerosols optical extensive properties such as scattering (?) and backscattering coefficients (?), scattering Ångström exponent (SAE), single scattering albedo (SSA), and asymmetry parameter (g). They were measured at three GAW (Global Atmosphere Watch-WMO) regional observatories in the South of Italy: CGR (Capo Granitola), LMT (Lamezia Terme) and ECO (Lecce). The aim of this work is to characterize the optical properties of the aerosols, in terms of scattering, absorption and radiative forcing, to study their relationship with the equivalent black carbon (eBC) mass concentration, meteorological parameters and to evaluate their variability in the Central Mediterranean area. From the mean values of SAE (1.73 ECO, 1.93 LMT) and SSA (0.81 ECO, 0.78 LMT) observations can be argued that ECO and LMT are mainly influenced by ultrafine particles, while CGR, with mean values of SAE and SSA of 1.15 and 0.87, respectively, is characterized by natural sources, mainly marine. In all stations, g is very similar, 0.68 at CGR and 0.63 at LMT, with the greatest value (0.70) at ECO. The aerosol optical properties, at the three stations, are significantly influenced by the meteorological conditions. The daily pattern for eBC concentration at ECO and LMT is essentially influenced by local activities, namely due to vehicular traffic, and, for LMT, to local sea-land breeze circulation. Wood burning is the main source contribute to eBC concentration in the remote site of CGR. Aerosols optical properties were analysed in the cold (from October to March) and warm (from April to September) period in the three observatories to highlight the behaviour of optical parameters as a response to changing black carbon concentration, especially, from local sources. As expected, three observatories exhibited low values of SSA, during the cold season, 0.87 at CGR, 0.78 at LMT and 0.80 at ECO, which confirm an increment of industrial/traffic and wood combustion contributions at the three observatories. SSA values are very similar in warm and cold period, especially in the middle part of the day. A persistent and important results in the findings of this study is that aerosol optical properties vary widely from station to station throughout the central Mediterranean basin. Thus, the wide spatial and temporal variability of aerosol characteristics in the basin need additional investigations to study the relationship between particle size distribution, optical parameters and local sources contribution at the three sites. Further, our results show a negative radiative forcing of aerosols, with mean values of -58.8 at CGR, -45.4 at LMT and - 55.9 at ECO, according also to several studies in central Mediterranean area.