Site-specific mass absorption cross-section and influence on black carbon at an urban site in the Mediterranean

Black carbon (BC) aerosols in urban areas affect human health and climate, leading to increased monitoring efforts. This study presents a comprehensive assessment of equivalent BC (eBC) in a Mediterranean urban environment (Rome, Italy). Harmonized protocols by ACTRIS and RI-URBANS were adopted to enhance data comparability. The study examined: (i) temporal eBC variability contributing to submicrometer aerosol mass; (ii) source apportionment into fossil fuel (eBCff) and biomass burning (eBCbb) using the “aethalometer model”; (iii) local Mass Absorption Cross-section (MACexp); (iv) eBC trend analysis with and without MAC corrections. Results indicate traffic emissions are the primary source, with eBCff approximately 70% of the total eBC mass. The findings underscore the importance of considering the local MAC to assess eBC: the difference between eBC estimated using MACexp (eBCMACjavax.xml.bind.JAXBElement@38139ab1 ) and uncorrected eBC varies with time. The decline in eBC inferred using a constant MAC (−10% yr−1, p<0.001) disappears when accounting for MAC variability, while MACexp itself decreases at an exceptional rate of −12% yr−1 (-1 m2g−1yr−1) . The intraannual eBCMACjavax.xml.bind.JAXBElement@1a709069-to-eBC ratio is on average close to 1, but it ranges from 0.7 in summer to 1.2 in winter. Temporal changes in meteorological conditions and emission patterns were found to influence both MACexp and BC aerosol properties. The eBCff -to -eBC ratio showed a significant positive trend (+0.01yr−1), linked to increased vehicle numbers and complemented by the eBCbb-to-eBC ratio exhibiting a similar but negative trend, associated with a +1.2 ∘C rise in temperature (+0.3∘Cyr−1). Accordingly, the observed decrease in MACexp is coherent with a negative trend in the absorption Å ngström exponent (AAE) (−0.018yr−1). These findings highlight BC dynamics in urban areas, aiding efforts to harmonize measurements and improve climate and air quality models, guiding public health strategies.