Development of particle number size distribution near a major road in Helsinki during an episodic inversion situation

• Published in: Atmospheric environment (1994) Vol. 41; no. 8; pp. 1759 - 1767
• Main Authors: Kerminen, Veli-Matti, Pakkanen, Tuomo A., Mäkelä, Timo, Hillamo, Risto E., Sillanpää, Markus, Rönkkö, Topi, Virtanen, Annele, Keskinen, Jorma, Pirjola, Liisa, Hussein, Tareq, Hämeri, Kaarle
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.03.2007; Elsevier Science
• Subjects: Analysis methods; Applied sciences; Atmospheric pollution; Coagulation; Condensation; Dispersion; Exact sciences and technology; Particle number; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Traffic exhaust; Europe; Finland; Finland, Etelae-Suomi, Helsinki; Traffic exhaust; Condensation; Dispersion; Coagulation; Particle number; Wind velocity; Environmental factor; Motor vehicle; Urban area; Ultrafine particle; Spatial variation; Submicron particle; Road side; Winter; Anthropogenic factor; Atmospheric condition; Pollutant behavior; Temperature reversal; Particle size distribution; Flocculation; Pollution source; Aerosols; Air pollution; Road traffic; Concentration distribution
• ISSN: 1352-2310; 1873-2844
• DOI: 10.1016/j.atmosenv.2006.10.026

The influence of traffic on urban air quality is highest at low wind speeds and the presence of a temperature inversion. By relying on detailed aerosol measurements conducted simultaneously at two distances close to a major road, we studied one such episode encountered in Helsinki, Finland, during the wintertime. The observed episode was characterized by exceptionally weak dilution of traffic emissions, with particle number concentration decreasing by no more than 10–30% between 9 and 65 m distances from the road. During the nighttime with relatively minor traffic flow, dilution and particle growth by vapor condensation were found to be the dominant processes in this road-to-ambient evolution stage. The latter process shifted a significant fraction of nucleation mode particles to sizes >30 nm diameter, modifying thereby the shape of the particle number size distribution. During the rush hours in the morning, particle number concentrations were elevated by approximately an order of magnitude compared with nighttime, such that also the self-coagulation of nucleation mode particles became important. Our study demonstrates that under suitable meteorological conditions (low wind speeds coupled with temperature inversions), traffic emissions are able to affect submicron particle number concentrations over large areas around major roads and may be a dominant source of ultrafine particles in the urban atmosphere. Under conditions characterized by exceptionally slow mixing, simultaneous processing of ultrafine (nucleation and Aitken mode) particles by dilution, self- and inter-modal coagulation, as well as by condensation and evaporation seriously questions the applicability of particle number emission factors, derived from the measurements at few tens of meters from the roadside.