| Summary: | In this paper, we present aerosol number concentration and number size distribution measurements in the urban area of Helsinki, Finland. The outdoor air measurements were performed over several years between 1999 and 2003 giving a rather good overview of the general properties of fine aerosols as well as their daily, weekly and seasonal characteristics. The aerosol number concentrations in urban areas were significantly higher in winter as compared with those in summer. The morning rush hour during workdays was observed as an average peak concentration of more than 4 x 104 cm–3, but no clear afternoon/evening peak was observed. Typical number size distributions were calculated for the winter and summer seasons separately. During the winter the morning peak in the size distribution in urban sites was found always between 10 and 50 nm in particle diameter. These particles were probably from local traffic emissions. The summer size distributions showed that the ultrafine (below 100 nm) size mode had often two separate peaks, one below 20 nm and the other at about 50 nm. The indoor air measurements have been done during several intensive field studies in various locations around Helsinki. The simultaneous indoor and outdoor aerosol size distribution measurements showed that a good quality filter can reduce the concentrations by a factor of about 10 and therefore influence the human exposure to urban particulate air pollution significantly.
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