Indoor and outdoor air ions and aerosol particles in the urban atmosphere of Helsinki : characteristics, sources and formation

We measured air ion size distributions with an air ion spectrometer in the size range of 0.34-40.3 nm both indoors (in July) and outdoors (in August) in Helsinki, Finland in 2004. At the same time we measured particle number concentrations and size distributions with two condensation particle counters (indoors) and differential mobility particle sizer (outdoors). Our main focus was to study new-particle formation in an urban site. We observed new-particle formation indoors almost every day, even many times a day, and four times outdoors. Indoors, the observed growth rates were 2.3-4.9 nm h -1 for 1.3-3-nm ions, 6.5-8.7 nm h -1 for 3-7-nm ions and 5.1-8.7 nm h -1 for 7-20-nm ions. Outdoor ions (3-7 nm) grew at rates as large as 15.4 nm h -1 . Outdoor ion and particle number concentrations were dependent on the wind direction, whereas indoor concentrations were dependent on ventilation conditions. Secondary particle formation and growth affected concentrations both indoors and outdoors. We concluded, based on our measurement results and simulated penetration of outdoor particles through the ventilation system, that we had indoor sources for secondary particles.