Objectives The influence of traffic and meteorological conditions on aerosol characteristics outdoors, the relationship between indoor and outdoor aerosol particles, and the pollutant transport indoors by means of a mechanical ventilation system were studied.

Methods Indoor and outdoor concentrations of fine-particle numbers were measured during the summer (15 May-30 June 2000) in one office located in the basement of a building in Helsinki, Finland. The total number concentration was measured with a condensation particle counter, and the particle number size distribution (7-600 nm) was measured with a differential mobility particle sizer. The size distribution (0.3-25 μm) of the indoor particle numbers was periodically measured with a laser particle counter.

Results Meteorological conditions, especially wind direction, had the greatest effect on the total number conceration and the size distribution of aerosol particles outdoors. The outdoor number concentration of ultrafine particles (diameter <100 nm) was strongly dependent on traffic density. The temporal variations in the indoor number concentration of ultrafine and fine particles (7-600 nm) closely followed the corresponding temporal variations outdoors. The building ventilation system was the main factor transporting aerosols indoors. The mean penetration factor was 0.41 (SD 0.11) for the nucleation mode (7-25 nm), 0.74 (SD 0.09) for the Aitken mode (25-100 nm), and 0.87 (SD 0.06) for the accumulation mode (100-600 nm).

Conclusions The ultrafine particles were bimodal with a nucleation mode (particle diameter <25 nm) and an Aitken mode (25 nm <particle diameter <100 nm). An accumulation mode was oberved with a particle diameter larger than 100 nm. The patterns of fine-particulate air pollution inside an office can be largely estimated on the basis of the outdoor aerosol characterization and the mechanical ventilation system.

Key terms atmospheric aerosols; Helsinki; indoor; mechanical ventilation; outdoor; particle size; size distribution of particle numbers; ultrafine particle; urban aerosol