Improving air quality in a metro platform by using air purifiers

• PM 2.5 peaked during high train frequency and ventilation shutdown. • PM 2.5 distribution varied across the platform due to ventilation inflow. • With ventilation off, APs reduced PM (29%), BC (24.3%), PN 0.3-10 (24.4%). • Best results were obtained when APs and ventilation were both on. This study assessed air quality on the platforms of a Metropolitano de Lisboa station and the impact of air purifiers (APs). It evaluated particle mass, number, and black carbon concentrations and their spatiotemporal distribution. PM 2.5 showed a clear diurnal pattern, peaking during high train frequency and inactive mechanical ventilation, with uneven platform distribution due to ventilation inflow location. Median PM 2.5 concentration without APs was >7.5 times higher than outdoors (49.1 vs. 6.3 μg m -3 ), indicating dominant internal sources. Median particle number concentrations for 0.01–0.3 and 0.3–10 μm particles were 4830 and 55 particles cm -3 , respectively, and median black carbon concentration was 3.4 μg m -3 . The particle size distribution (0.01–10 μm) showed clear modes at 86.6, 550, and 750 nm. The efficiency of APs under different platform ventilation conditions was evaluated using a controlled design. Although results may vary with ventilation, day, and time, PM 2.5 , particle number (>0.3 μm), and black carbon concentrations were reduced by 29%, 24.4%, and 24.3%, respectively, when only APs operated. Their impact was distance-dependent, with mass reductions up to 78% near the units, as measured by low-cost PM sensors. The greatest reductions in particle mass, number (>0.3 μm), and black carbon occurred when APs operated with mechanical ventilation compared to both systems off (34%, 31.3%, and 56.4%). Overall, the study demonstrates that APs are especially useful when platform ventilation is inactive or for localized control, guiding targeted mitigation strategies in metro stations.