Sustainable Urban Air Quality Monitoring: Determination of Equivalent Black Carbon Concentrations and Mass Absorption Cross Sections (MAC) Using Multi-Wavelength Absorption Black Carbon Instrument (MABI) in Athens and Krakow

Black carbon is an important light-absorbing component of atmospheric particulate matter, with significant implications for urban air quality, climate forcing, human health, and sustainable urban development. Reliable monitoring of black carbon is therefore essential for assessing combustion-related pollution and supporting sustainable air quality management and emission-reduction strategies. This study evaluated the performance of the Multi-Wavelength Absorption Black Carbon Instrument for the offline determination of aerosol light absorption and equivalent black carbon under contrasting urban conditions. PM2.5 and PM10 samples were collected at urban-background sites in Krakow, Poland, and Athens, Greece, during winter and summer 2024, thereby covering heating and non-heating periods and a broad range of aerosol concentrations. Samples were collected in parallel on Quartz-fibre and Teflon filters. Their light absorption was measured offline using the Multi-Wavelength Absorption Black Carbon Instrument at 870 nm. Elemental carbon concentrations were independently determined on Quartz-fibre filters by thermo-optical transmittance using the EUSAAR_2 protocol, while continuous absorption measurements from an AE33 Aethalometer operated in Athens were used to assess the comparability of offline and online measurements. The consistency of two independent Multi-Wavelength Absorption Black Carbon Instruments operated in Krakow and Athens was also examined using a common set of filters. Strong linear relationships were observed between the offline absorption coefficients and elemental carbon concentrations, with coefficients of determination exceeding 0.90 for the investigated datasets. The offline measurements also reproduced the variability observed by the AE33, although they systematically yielded higher absorption coefficients than the continuous instrument. Quartz-fibre filters produced higher absorption values and higher apparent mass absorption cross-sections than Teflon filters, demonstrating a pronounced substrate-dependent effect. The differences in the quartz-based relationships between Krakow and Athens indicated that aerosol composition, source characteristics, ageing, and mixing state may also affect the relationship between optical absorption and elemental carbon concentration. These findings confirm that the instrument is suitable for offline and retrospective assessment of equivalent black carbon, particularly where continuous monitoring is unavailable. Reliable quantitative application requires filter-specific calibration and, where necessary, site-specific mass absorption cross-section values. By enabling the use of archived and routinely collected filter samples for black carbon assessment, this approach can complement continuous monitoring networks and support more accessible and sustainable air quality monitoring. Such measurements can contribute to the evaluation of pollution trends, emission-control measures, and evidence-based strategies aimed at improving urban environmental quality and advancing sustainability goals.