The effect of different transport modes on urban PM 10 levels in two European cities László Makra a, ⁎, Ioana Ionel b , Zoltán Csépe a , István Matyasovszky c , Nicolae Lontis b , Francisc Popescu b , Zoltán Sümeghy a a Department of Climatology and Landscape Ecology, University of Szeged, HU-6701 Szeged, P.O. Box 653, Hungary b Department of Mechanical Machines, “Politehnica” University, RO-300222 Timişoara, Bv. Mihai Viteazu, No. 1, Romania c Department of Meteorology, Eötvös Loránd University, H-1117 Budapest, Pázmány Péter Street 1/A, Hungary HIGHLIGHTS • The 3D delimination of the clusters by the function “convhull” is a novel approach. • For Bucharest, the most relevant source areas of PM10 transport are Central Europe with the Western Mediterranean. • For Szeged, Southern and Central Europe are the most important sources of long-range transport of PM10. • Occasional North-African-origin dust over Romania and Hungary is also detected, respectively. • A statistical procedure is developed in order to separate medium- and long-range PM10 transport for both cities. abstract article info Article history: Received 12 March 2013 Accepted 8 April 2013 Available online xxxx Keywords: PM 10 transport Backward trajectories Cluster analysis Mahalanobis metric Separation of medium- and long-range PM 10 transport The aim of the study is to identify transport patterns that may have an important influence on PM 10 levels in two European cities, namely Szeged in East-Central Europe and Bucharest in Eastern Europe. 4-Day, 6-hourly three-dimensional (3D) backward trajectories arriving at these locations at 1200 GMT are computed using the HYSPLIT model over a 5-year period from 2004 to 2008. A k-means clustering algorithm using the Mahalanobis metric is applied in order to develop trajectory types. Two statistical indices are used to evaluate and compare exceedances of critical daily PM 10 levels corresponding to the trajectory clusters. For Bucharest, the major PM 10 transport can be clearly associated with air masses arriving from Central and Southern Europe, as well as the Western Mediterranean. Occasional North African dust intrusions over Romania are also found. For Szeged, Southern Europe with North Africa, Central Europe and Eastern Europe with regions over the West Siberian Plain are the most important sources of PM 10 . The occasional appearance of North- African-origin dust over Hungary is also detected. A statistical procedure is developed in order to separate medium- and long-range PM 10 transport for both cities. Considering the 500 m arrival height, long-range transport plays a higher role in the measured PM 10 concentration both for non-rainy and rainy days for Bucharest and Szeged, respectively. © 2013 Elsevier B.V. All rights reserved. 1. Introduction PM 10 is a measure of particles in the atmosphere with a diameter of less than or equal to a nominal 10 μm. The 24-h limit value for PM 10 (50 μg·m -3 ) is frequently exceeded in the urban environment. The short- and long-term human exposure to high particulate matter concentrations observed in urban environment increases the risk of respiratory (Schindler et al., 2009) and cardiovascular (Feng and Yang, 2012) diseases. For Bucharest, the predicted average gain in life expectancy for people of 30 years of age for a decrease in average annual PM2.5 level from 38.2 μg·m -3 (2004–2006) to 10 μg·m -3 (World Health Organization, 2000) is 22.1 months (Medina et al., 2004). For Szeged, PM 10 counts a major pollutant influencing respira- tory diseases (Matyasovszky et al., 2011); furthermore, a set of ex- planatory variables including PM 10 indicates a strong association with allergic asthma emergency room visits (Makra et al., 2012). Therefore, studying potential key regions and long-range transport effects on urban PM 10 levels is of great importance. Several authors have published backward trajectory modeling re- sults to help detect the long-range transport of pollutant air masses that may have an impact on local PM 10 levels (Salvador et al., 2008), to better describe the related tropospheric circulations (Jorba et al., 2004) or to characterize and identify spatial and temporal trends of pollutants (Coury and Dillner, 2007). However, single backward trajectories generally applied to detect key regions of extreme PM episodes for given sites (Hongisto and Sofiev, 2004) are not suitable Science of the Total Environment 458-460 (2013) 36–46 ⁎ Corresponding author. Tel.: +36 62 544 856; fax: +36 62 544 624. E-mail addresses: makra@geo.u-szeged.hu (L. Makra), ionel_monica@hotmail.com (I. Ionel), h480623@stud.u-szeged.hu (Z. Csépe), MATYA@ludens.elte.hu (I. Matyasovszky), lontis_nicolae@yahoo.com (N. Lontis), ingfrancisc@gmx.net (F. Popescu), sumeghy@geo.u-szeged.hu (Z. Sümeghy). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.04.021 Contents lists available at SciVerse ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv