Atmospheric Environment 39 (2005) 1655–1668 Modal structure and spatial–temporal variations of urban and suburban aerosols in Helsinki—Finland Tareq Hussein a,Ã , Kaarle Ha¨meri a,b , Pasi P. Aalto a , Pentti Paatero a , Markku Kulmala a a Department of Physical Sciences, University of Helsinki, P.O. Box 64, Helsinki FIN-00014, Finland b Finnish Institute of Occupational Health, Topeliuksenkatu 41a A, Helsinki FIN-00250, Finland Received 17 March 2004; received in revised form 16 August 2004; accepted 10 November 2004 Abstract Particle number size distributions were measured in the urban and suburban atmosphere of Helsinki. In the absence of a direct traffic emission influence, the temporal variation of the aerosol particle number concentrations and their particle size distribution characteristics can be generalized within the Helsinki metropolitan region. In general, the particle number size distributions are characterized by three modes: nucleation (o25nm), Aitken (25–90nm), and accumulation (490nm). Under certain conditions such as overlapping between the nucleation and Aitken modes, it is possible to characterize the particle number size distributions by two modes: ultrafine mode (o100nm) and accumulation mode (4100 nm). Traffic combustions are considered as one of the major sources of ultrafine particles (UFP, D p o100nm) in the urban atmosphere. The total particle number concentration is highest in the urban centers (as high as 140000cm 3 , more than 90% is UFP), In general, 70–80% of the submicrometer particle number concentration is UFP in the suburban atmosphere. Close to major highways, the geometric mean diameters of the submicrometer modes are smaller and the total number concentrations can exceed 60000cm 3 (more than 60% is nucleation mode particles). According to the wind speed and temperature analysis, the submicrometer aerosol particles in the urban and suburban atmosphere consist of two components: UFP that is diluted with wind speeds and inversely proportional to the ambient temperature, and particles larger than 100 nm in diameter that is re-suspended with wind and proportional to the ambient temperature. The correlation analysis showed that UFP number concentrations are best correlated within the urban areas. Particles larger than 100 nm showed good correlation factors (about 0.80) within the Helsinki metropolitan area, which is an indication of similar kinds of aerosols such as regional transported particles. r 2005 Elsevier Ltd. All rights reserved. Keywords: Particle number size distribution; Air quality; Ultrafine particles; Traffic; Weather 1. Introduction Atmospheric aerosols in urban areas cause adverse health effects (e.g. Pope et al., 2002; Dockery and Pope, 1994) and loss of visibility (e.g. Finlayson-Pitts and Pitts, 2000). Heavily industrialized areas suffer from pollution smogs that are often related to coal burning and manufacture combustions, and nowadays traffic emissions also significantly affect the air quality in cities. The health problems related to atmospheric aerosols are believed to be due to particles with diameters smaller than 10 mm (PM 10 ), which can penetrate deep in the ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.11.031 Ã Corresponding author. Tel.: +358919150709; fax: +358919150717. E-mail address: tareq.hussein@helsinki.fi (T. Hussein).