Black Carbon Aerosol in Rome (Italy): Inference of a Long-Term (2001-2017) Record and Related Trends from AERONET Sun-Photometry Data

Surface concentration of black carbon (BC) is a key factor for the understanding of the impact of anthropogenic pollutants on human health. The majority of Italian cities lack long-term measurements of BC concentrations since such a metric is not regulated by EU legislation. This work attempts a long-term (2001-2017) inference of equivalent black carbon (eBC) concentrations in the city of Rome (Italy) based on sun-photometry data. To this end, aerosol light absorption coefficients at the surface are inferred from the "columnar" aerosol aerosol light absorption coefficient records from the Rome Tor Vergata AERONET sun-photometer. The main focus of this work is to rescale aerosol light absorption columnar data (AERONET) to ground-level BC data. This is done by using values of mixing layer height (MLH) derived from ceilometer measurements and then by converting the absorption into eBC mass concentration through a mass-to-absorption conversion factor, the Mass Absorption Efficiency (MAE). The final aim is to obtain relevant data representative of the BC aerosol at the surface (i.e., in-situ)-so within the MLH- and then to infer a long-term record of "surface" equivalent black carbon mass concentration in Rome. To evaluate the accuracy of this procedure, we compared the AERONET-based results to in-situ measurements of aerosol light absorption coefficients (alpha(abs)) collected during some intensive field campaigns performed in Rome between 2010 and 2017. This analysis shows that different measurement methods, local emissions, and atmospheric conditions (MLH, residual layers) are some of the most important factors influencing differences between inferred and measured alpha(abs). As a general result, "inferred" and "measured" alpha(abs) resulted to reach quite a good correlation (up to r = 0.73) after a screening procedure that excludes one of the major cause of discrepancy between AERONET inferred and in-situ measured alpha(abs): the presence of highly absorbing aerosol layers at high altitude (e.g., dust), which frequently affects the Mediterranean site of Rome. Long-term trends of "inferred" alpha(abs), eBC, and of the major optical variables that control aerosol's direct radiative forcing (extinction aerosol optical depth, AOD(EXT), absorption aerosol optical depth, AOD(ABS), and single scattering albedo, SSA) have been estimated. The Mann-Kendall statistical test associated with Sen's slope was used to test the data for long-term trends. These show a negative trend for both AOD(EXT) (-0.047/decade) and AOD(ABS), (-0.007/decade). The latter converts into a negative trend for the alpha(abs) of -5.9 Mm(-1)/decade and for eBC mass concentration of -0.76 mu g/m(3)/decade. A positive trend is found for SSA (+0.014/decade), indicating that contribution of absorption to extinction is decreasing faster than that of scattering. These long-term trends are consistent with those of other air pollutant concentrations (i.e., PM2.5 and CO) in the Rome area. Despite some limitations, findings of this study fill a current lack in BC observations and may bear useful implications with regard to the improvement of our understanding of the impact of BC on air quality and climate in this Mediterranean urban region.