LAUSR

Abstract Organic (OC) and Elemental Carbon (EC) are important components of atmospheric aerosol particles, playing a key role in climate system and potentially affecting human health. There is a lack of data reported for Southern Italy and this work aims to fill this gap, focusing the attention on the long-term trends of OC and EC concentrations in PM 2.5 and PM 10 , and on atmospheric processes and sources influencing seasonal variability. Measurements were taken at the Environmental-Climate Observatory of Lecce (SE Italy, 40°20′8″N-18°07′28″E, 37 m a.s.l.), regional station of the Global Atmosphere Watch program (GAW-WMO). Daily PM 10 and PM 2.5 samples were collected between July 2013 and July 2016. In addition, starting in December 2014, simultaneous equivalent Black Carbon (eBC) concentrations in PM 10 were measured using a Multi Angle Absorption Photometer. A subset of 722 PM samples (361 for each size fraction) was analysed by using a thermo-optical method with a Sunset Laboratory OC/EC analyser, to determine elemental and organic carbon concentrations. The average PM 10 and PM 2.5 concentrations were 28.8 μg/m 3 and 17.5 μg/m 3 . The average OC and EC concentrations in PM 10 were 5.4 μg/m 3 and 0.8 μg/m 3 , in PM 2.5 these were 4.7 μg/m 3 and 0.6 μg/m 3 . Carbonaceous content was larger during cold season with respect to warm season as well as secondary organic carbon (SOC) that was evaluated using the OC/EC minimum ratio method. SOC was mainly segregated in PM 2.5 and represented 53% - 75% of the total OC. A subset of EC data was compared with eBC measurements, showing a good correlation (R 2  = 0.80), however, eBC concentrations were higher than EC concentrations of an average factor of 1.95 (+/− 0.55 standard deviation). This could be explained by the presence of a contribution of Brown Carbon (BrC), for example from biomass burning, in eBC measurements. Weekly patterns showed a slight decrease of carbon content during weekends with respect to weekdays especially visible on eBC concentration due to the decrease of road traffic emissions. The daily patterns of hourly eBC concentrations showed a decrease in central diurnal hours, due to the cycle of planetary boundary-layer height, and concentrations peaks during rush hours due to road traffic emissions.