Impact of Air Filter Cleaning in the Subway Ventilation System on Pollutant Distribution at Different Stations: A Case Study of Xi’an

This study aims to address the lack of systematic research on how air filter cleaning affects pollutant distribution in subway stations, an urgent issue for ensuring indoor air quality in enclosed underground transit environments. Taking a representative subway line in Xi’an, China, as the research object, we monitored and analyzed pollutant concentrations (including particulate matter and microorganisms) at air supply outlets (near, semi-far, and far from the air filter) and different locations in a single station hall before and after filter cleaning, with the goal of clarifying the cleaning effect and its variation with distance from the air supply. The results show that, before cleaning, pollutant distribution exhibits distinct spatial characteristics: PM10 concentration is 16.7% higher at outlets closer to the filter, while PM2.5 and PM1.0 concentrations are 22.4% and 19.7% higher at distant outlets, respectively. After cleaning, all pollutants are significantly reduced: PM10 at near outlets decreases by 25.9%, PM2.5 and PM1.0 at far outlets drop by 44.2% and 42.7%, respectively, and the total numbers of bacteria and fungi in the air supply meet national hygiene standards. A key novel finding is that cleaning has differentiated effects on particle sizes across locations; it is more effective for larger particles at near outlets and for smaller particles at far outlets. This research not only quantifies the purification effect of filter cleaning but also reveals the spatial variation law of its impact, providing practical guidance for optimizing subway ventilation system maintenance (e.g., tailored cleaning strategies for different locations) and ensuring passenger health.