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 (eBC ff ) and biomass burning (eBC bb ) using the “aethalometer model”; (iii) local Mass Absorption Cross-section (MAC exp ); (iv) eBC trend analysis with and without MAC corrections. Results indicate traffic emissions are the primary source, with eBC ff 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 MAC exp (eBC MAC exp ) 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 MAC exp itself decreases at an exceptional rate of − 12 % yr −1 (-1 m 2 g − 1 y r − 1 ) . The intraannual eBC MAC exp -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 MAC exp and BC aerosol properties. The eBC ff -to -eBC ratio showed a significant positive trend ( + 0 . 01 y r − 1 ), linked to increased vehicle numbers and complemented by the eBC bb -to-eBC ratio exhibiting a similar but negative trend, associated with a +1.2 ∘ C rise in temperature ( + 0 . 3 ∘ C yr − 1 ). Accordingly, the observed decrease in MAC exp is coherent with a negative trend in the absorption Å ngström exponent (AAE) ( − 0 . 018 y r − 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. • Equivalent black carbon (eBC) in Rome primarily driven by traffic emissions. • Nominal MAC biases eBC; site-specific MAC (MAC e x p ) significantly influences eBC. • Local factors (meteorology, emissions) strongly affect eBC and MAC from daily to yearly scales. • Winter-summer eBC ratio rises from 2.17 to 2.76 when MAC variability is included. • Variable MAC removes the apparent −10% y r − 1 eBC decline caused by absorption changes.