Simultaneous temporal, spatial and size-resolved measurements of aerosol particles in closed indoor environments applying mobile filters in various use-cases

This paper focuses on simultaneous, time-and space-resolved measurements of particle size distributions in three different closed indoor environments (small office room, elementary-school classroom, and seminar room) applying mobile air filters in four scenarios (decay curves, filtra-tion while people are present, a temporal strong point source, impact of filter orientation & cross-flow ventilation). The experiments reveal, that mobile indoor air filters, equipped with high-performance filter media (HEPA -quality), remove particles in the investigated rooms in rele-vant submicron size classes (x < 1 mu m) efficiently and uniformly over time. For the description of the local decrease in particle concentration a simple mathematical model based on a transient continuous stirred tank reactor was applied. The local decay curves obtained in the different room-types were compared to simulated ones assuming ideal mixing of the indoor air. The real-room scenarios show a slower particle decay than the predicted ones assuming ideal mixing of the indoor air. The experiments reported in this contribution demonstrate, that indoor air filters, operated with a filtration rate of 3.5 h(-1) and positioned correctly, are capable of lowering the particle concentration in all relevant size classes in real-world closed indoor envi-ronments slowly over time (e.g. a reduction in particle concentration of 50 % after 30 min in a classroom w/o particle sources). In the investigated set-ups, at filtration rates above 9 h(-1), the filters' performance is close to cross-flow window-ventilation. The experiments reveal, that mo-bile air filters cannot avoid close distance transmission of submicron aerosol particles from one person to another. Therefore, they do not replace any of the well-known methods to avoid aerosol-driven infection (like wearing an efficient face mask correctly, limiting the number of people and time of stay in closed indoor environments, frequent ventilation). Mobile air filter devices may represent an additional component in an entire prevention strategy, especially when rooms cannot be ventilated regularly, efficiently or the constellation of people in groups changes frequently (e.g. waiting areas).