ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-4-391-2004 Urban aerosol number size distributions Hussein T. 1 Puustinen A. 1 Aalto P. P. 1 Mäkelä J. M. 2 Hämeri K. 1 3 Kulmala M. 1 Department of Physical Sciences, University of Helsinki, P.O. Box 64, FIN-00014, UHEL, Finland Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland Physics Department, Finnish Institute of Occupational Health, Topeliuksenkatu 41a A, FIN-00250 Helsinki, Finland 01 03 2004 4 2 391 411 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/4/391/2004/acp-4-391-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/4/391/2004/acp-4-391-2004.pdf

Aerosol number size distributions have been measured since 5 May 1997 in Helsinki, Finland. The presented aerosol data represents size distributions within the particle diameter size range 8-400nm during the period from May 1997 to March 2003. The daily, monthly and annual patterns of the aerosol particle number concentrations were investigated. The temporal variation of the particle number concentration showed close correlations with traffic activities. The highest total number concentrations were observed during workdays; especially on Fridays, and the lowest concentrations occurred during weekends; especially Sundays. Seasonally, the highest total number concentrations were observed during winter and spring and lower concentrations were observed during June and July. More than 80% of the number size distributions had three modes: nucleation mode (<IMG WIDTH="36" HEIGHT="30" ALIGN="MIDDLE" BORDER="0" src="acp-4-391-img1.gif" ALT="$D_{p}{&lt;}$">30nm), Aitken mode (20-100nm) and accumulation mode (<IMG WIDTH="36" HEIGHT="30" ALIGN="MIDDLE" BORDER="0" src="acp-4-391-img2.gif" ALT="$D_{p}{&gt;}$">90nm). Less than 20% of the number size distributions had either two modes or consisted of more than three modes. Two different measurement sites were used; in the first (Siltavuori, 5.5.1997-5.3.2001), the arithmetic means of the particle number concentrations were 7000cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-391-img3.gif" ALT="$^{-3}$">, 6500cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-391-img3.gif" ALT="$^{-3}$">, and 1000cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-391-img3.gif" ALT="$^{-3}$"> respectively for nucleation, Aitken, and accumulation modes. In the second site (Kumpula, 6.3.2001-28.2.2003) they were 5500cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-391-img3.gif" ALT="$^{-3}$">, 4000cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-391-img3.gif" ALT="$^{-3}$">, and 1000cm<IMG WIDTH="20" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" src="acp-4-391-img3.gif" ALT="$^{-3}$">. The total number concentration in nucleation and Aitken modes were usually significantly higher during workdays than during weekends. The temporal variations in the accumulation mode were less pronounced. The lower concentrations at Kumpula were mainly due to building construction and also the slight overall decreasing trend during these years. During the site changing a period of simultaneous measurements over two weeks were performed showing nice correlation at both sites.