Ultrafine, solid particles and Black Carbon Near Real Time assessment and critical properties at a traffic Hotspot MI-TRAP monitoring station in Athens

Air quality in urban areas and other pollution hotspots, where transport emissions strongly influence human exposure, remains a complex environmental challenge and a major public health concern. The revised EU Air Quality Directive reflects a growing consensus that mass-based metrics, such as PM₁₀ and PM₂.₅, are not sufficient to fully capture the impact of air pollution on human health. As a result, increasing attention is being directed toward emerging pollutants such as black carbon (BC) and ultrafine particles (UFPs). These pollutants are closely associated with combustion processes, especially traffic emissions, and have been strongly linked to adverse health outcomes.

Within this context, the MI-TRAP project aims to improve understanding of these pollutants after emission, through the establishment of a network of 30 monitoring stations across 10 European cities. As part of MI-TRAP, a monitoring station was deployed at a traffic hotspot in Athens (Apr-Nov, 2025). The station was equipped with high time-resolution instrumentation, including an Aethalometer (Magee AE31, 7λ), a Mobility Particle Size Spectrometer (MPSS, TSI), an Optical Particle Sizer (OPS, GRIMM), and a Condensation Particle Counter (CPC, TSI), enabling measurements of aerosol absorption coefficient (b_abs) and black carbon concentration, particle number and mass size distributions, and total particle number concentration (NC), respectively. To better characterize the link between tailpipe emissions and ambient measurements, a catalytic stripper (Catalytic Instruments) was installed upstream of the AE31, MPSS, and CPC (to enable monitoring of the solid particles), together with an automatic valve, allowing alternating measurements between ambient and heated sampling conditions. These measurements were coupled with an in-house traffic counting and fleet recognition tool.

The results revealed that b_abs were strongly correlated with the total number concentration of UFPs with sizes above 20nm (NC_20nm), whereas a weaker correlation was observed between b_abs (mean b_abs,880,_amb = 13 ± 10 Mm^-1 with more than 90% attributed to fossil-fuel combustion) and PM₂.₅ mass (mean PM_2.5 = 9.4 μg/m³).  The diurnal cycle of b_abs exhibited a clear peak during the early-morning traffic hours, coinciding with the peak in NC_>20nm. In contrast, NC_<20nm showed a peak around noon, reflecting the influence of photochemical processes. After passing through the CS, the aerosol exhibited a clear shift in its NSD toward smaller diameters, accompanied by a significant reduction in NC in the 10–300 nm size range; the remaining solid particle number concentration accounted for approximately 60% of the ambient level (mean NC_total,amb = 2.6*10⁴ ± 1.3*10⁴ cm^-3). In terms of particle volume, the average reduction after heating was about 50%. The impact of the CS on aerosol absorption was less pronounced than that on particle number.