Application of trajectory clustering for determining the source regions of secondary inorganic aerosols measured at K-puszta background monitoring station, Hungary Zita Ferenczi 1 , Kornélia Imre 2 & László Bozó 1 1 Hungarian Meteorological Service P.O.Box 39, H-1675 Budapest, Hungary, 2 MTA-PE Air Chemistry Research Group, Egyetem ut 10, H-8200, Veszprém ferenczi.z@met.hu Abstract Understanding the formation process of atmospheric particles is vital be- cause of the significant impact of particulate matter on human health and climate change. Atmospheric particles can be formed by nucleation process via a number of different mechanisms, such as binary nucleation (involving H2SO4 and water va- pour), ternary nucleation (involving NH3, H2SO4 and water vapour) and ion-induced nucleation for charged particles, depending on the environmental conditions. Parti- cle formation increases the total number concentration of ambient submicron parti- cles and contributes thereby to climate forcing. The transformation processes of new particle formation (NPF) and secondary organic aerosol have been studied. It was found that gaseous sulphuric acid, ammonia, and organic compounds are important precursors to NPF events and H2SO4-NH3-H2O ternary nucleation is one of the im- portant mechanisms. Using cluster analysis on the backward trajectories makes it possible to identify the most relevant types of air mass transport routes, and the directions from where precursor gases are transported. The influence of synoptic- scale atmospheric transport patterns on observed levels of sulphate, nitrate and am- monium has been examined. 1. Introduction Atmospheric particulate matter (PM) can have both a natural and an anthropogenic origin that influences its composition and size. The effect of local meteorological conditions and in addition, long-range transport can have a significant influence on PM concentration levels recorded at a specific site (Abdalmogith and Harrison, 2005), but this is not currently well documented in many European geographical areas. Secondary inorganic aerosols (SIA) – SO4 2- , NO3 - , NH4 + – constitute a dom- inant part of particulate matter in Europe. They are called secondary because they are not emitted directly into the atmosphere but produced as a result of chemical reactions concerning sulphur dioxides, nitrogen oxides and ammonia. These gases are emitted by transport, industry and agriculture. During the last decades their emissions decreased considerably in Europe and this resulted in lower ambient con- centrations for SIA. New particle formation and subsequent growth of atmospheric particles have been observed in different environments: from sub-arctic Lapland and remote boreal for- est to urban and suburban environments. Results showed that H2SO4 is the key com- pound affecting the NPF. However, before NPF, sulphuric acid clusters have to be