Experimental evaluation of particle-based tracers for indoor ventilation performance assessment

Reliable evaluation of ventilation performance and age of air is essential for improving indoor environmental quality and reducing airborne infection risks. Traditionally, tracer gas methods using CO2 have been employed, but these experiments require specialized equipment, are relatively costly, and cannot represent particle removal processes by devices such as air cleaners. As a potential alternative, this study experimentally investigates the applicability of aerosol particles as tracers for measuring ventilation and age of air in indoor environments. Experiments were conducted in both a controlled laboratory and field settings, where artificially generated droplets of different solution compositions were sprayed and their concentration decay was monitored with a particle counter. Several solutions, including tap water, calcium carbonate solution, and mixtures containing glycerin, were compared to identify suitable tracer properties. The influence of gravitational settling on indoor particle concentration and its impact on ventilation rate estimation were analyzed, and comparative measurements using CO2 gas as a reference tracer were also performed. The results highlight three main findings. First, particles containing glycerin and other impurities are effective for generating sufficiently high and stable tracer concentrations. Second, differences between gas- and particle-based experiments can be largely explained by gravitational settling, which becomes more significant with particle diameter. Third, focusing on particles in the 0.3-0.5 μm size range yields ventilation and age of air values that correspond well with those obtained from CO2 experiments, while larger particles tend to result in measurement differences due to deposition losses. These results indicate that particle tracers can be applied as practical substitutes for gas tracers and also allow evaluation of equivalent ventilation performance of air-cleaning devices by considering particle removal efficiency.