Air quality in a bus depot and a way of improving it: effect of using air purifiers

This study assessed airborne particulate matter and black carbon concentrations and their distribution inside a restricted bus depot over two campaigns. Particles with a diameter <2.5 μm were evenly distributed across the depot, influenced by limited bus activity and the depot's spacious layout with three entrances and one exit, facilitating particle dispersion. Their average baseline concentration was 25.2 μg m^-3, 4.6 times higher than outdoor levels, primarily driven by bus emissions and maintenance activities. Number concentrations of particles smaller than 0.3 μm (0.01-0.3 μm) averaged 1.3 × 10³ particles cm^-3, while larger particles (0.3-10 μm) averaged 33 × 10⁰ particles cm^-3. Black carbon averaged 1.3 μg m^-3. Concentration peaks occurred from 23:00-9:00 and 16:00-18:00 due to bus activities, maintenance, and soil resuspension. The impact of air purifiers on air quality was also investigated focusing on their location, number, and air volume flow for optimal results. APs operating at half air volume flow and placed within 6 m of the measurement equipment achieved reductions of up to 45.2 % for PM_2.5 and 73.6 % for particles sized 0.3-10 μm. However, air purifiers were much less effective for particles <0.3 μm. Crossed airflows and higher air volume flow decreased effectiveness due to turbulence and particle resuspension. This study underscores the need for optimized air purifiers placement, air volume flow settings, and operational strategies to mitigate air pollution in (semi-)closed transport environments like bus depots, improving air quality and health for passengers and workers.