Modal structure and spatial–temporal variations of urban and suburban aerosols in Helsinki—Finland

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 (<25 nm), Aitken (25–90 nm), and accumulation (>90 nm). 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 (<100 nm) and accumulation mode (>100 nm). Traffic combustions are considered as one of the major sources of ultrafine particles (UFP, ) in the urban atmosphere. The total particle number concentration is highest in the urban centers (as high as 140 000 cm−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 60 000 cm−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.