Indoor and outdoor particle size characterization at a family house in Espoo–Finland

Aerosol particle number size distributions (3–400 nm) were measured for three weeks both indoors and outdoors in a family house with natural ventilation in Espoo, Finland. We investigated the indoor-to-outdoor relationship of aerosol particles and analyzed the effects of indoor activities on the particle number size distributions. We also estimated the decay rate of indoor aerosol particle number concentrations. As expected, in the absence of indoor sources of aerosol particles the indoor particles originated from outdoors and the number concentrations followed similar patterns as those outdoors. The maximum penetration was found for particles between 100 and 400 nm in diameter. The mean value of the I/O values was 0.36 for ultrafine particles (UFP diameter <100 nm) and 0.60 for particles larger than 100 nm in diameter. Because of the penetration and deposition processes of aerosol particles, the modal structure of indoor particle number size distributions had larger geometric mean diameters and significantly lower number concentrations of UFP than those outdoors. The natural ventilation did not provide a stable and controlled indoor-to-outdoor relationship of aerosol particles and it caused variable I/O values and time-lags (10–45 min). In the presence of indoor activities, the indoor particle number concentrations cannot be directly estimated from the outdoor number concentrations only. The loss rate of UFP in the indoor air ranged from 1 h−1 for 10 nm particles to 0.1 h−1 for 100 nm particles in diameter. The quantitative and qualitative results presented in the current study are building and condition specific. However, the results provide a better understanding of the particle number size distribution characterizations indoors, especially during different indoor activities.