4 th Imeko TC19 Symposium on Environmental Instrumentation and Measurements Protecting Environment, Climate Changes and Pollution Control June 3-4, 2013, Lecce, Italy APPLICATION OF XPS SURFACE ANALYSIS FOR CHARACTERIZATION OF SIZE-SEGREGATED PARTICULATE MATTER FROM A URBAN BACKGROUND SITE IN LECCE M. R. Guascito 1 , P. Ielpo 2,3 , D. Cesari 3 , A. Genga 1 , C. Malitesta 1 , R.A. Picca 1 , D. Chirizzi 3-4 , D. Contini 3 1 Dipartimento di Scienze e Tecnologie Biologiche e Ambientali, Università del Salento, Lecce, 73100 2 Istituto di Ricerca Sulle Acque, CNR, Bari, 70132 3 Istituto di Scienze dell’Atmosfera e del Clima, CNR, Lecce, 73100 4 Dipartimento di Beni Culturali, Università del Salento, Lecce, 73100 e-mail: mrachele.guascito@unisalento.it Abstract: Application of X-ray Photoelectron Spectroscopy (XPS) to chemical surface analysis of Particulate Matter (PM) is not yet a routine method in aerosol characterisation. Nonetheless, in the last years the interest towards the potentialities of this technique for PM analysis has rapidly grown. Surface chemical composition plays an important role in determining the optical properties of aerosol and its reactivity. In this communication we present an XPS surface study of different size fractions of PM, suitably collected using a 10-stage MOUDI-II impactor. Results were compared with chemical analysis of water soluble ions and water soluble carbon in the bulk of the collected particles. Elemental % (conc. > 0.1-1%) surface chemical composition was determined for each size fraction with particular attention to S (SO 4 2- ), Na + , N (NH 4 + , NO 3 - , organic nitrogen) and Cl - . Detailed analysis of C1s XPS spectra allowed to distinguish oxygen-containing groups such as carbonylic, carboxylic and carbonate groups. Surface and bulk analyses relevant to size fractions characteristic of coarse and accumulation modes will be reported, considering also a particular case of sea spray accompanied to an intrusion of Saharan Dust. Keywords: MOUDI impactor, XPS, size segregated aerosol, particle surface composition. 1. INTRODUCTION Heterogeneous atmospheric reactions take place on the surface of particles. Therefore, the determination of surface composition of atmospheric aerosol is an extremely important task because the surface of particles contains chemical species responsible for particle growth as well as potentially toxic compounds [1,2]. Moreover, surface chemistry together with nanostructure are important features that may shed light on particulate origin, its reactivity and participation in heterogeneous reactions thus influencing the oxidation state, elemental composition, the formation of functional groups on the surface and the radiative properties that determine the climatic effects [3,4]. In particular, particle modification in terms of composition and structure may lead to multiple evolution pathways. There are many methods and instruments available for studying different properties of the surface of airborne particulate matter, ranging from morphology to elemental composition [5-9]. The use of X-ray Photoelectron Spectroscopy (XPS) for chemical surface analysis of airborne particulate is not a routine method in aerosol characterisation. However, in the last years there has been an increase of interest in applying this surface analytical technique to study the atmospheric particulate. There are several advantages related to the possibility of element identification, their distribution and quantification on particle surface, and of chemical speciation. In particular XPS can provide chemical (speciation) information on the surface of a solid without any particular sample preparation. Moreover, XPS is capable of analysing even insulating samples without any pre-treatment. Additionally, its sensitivity allows detection of surface elements at 0.1-1% atomic percentage (depending on the element) and distinction among the main oxygen- based carbon functional groups and as well as among different oxidation states for other species is also possible [1]. On the other hand, it should be pointed out that XPS allows the characterisation of a near surface region of ~ 10- 15 nm for solid materials. It is clear, however, that surface values slightly differ from bulk ones only for chemically homogeneous samples. More often it should be considered that values are averaged over escape depth of the photoelectrons. It is also worth mentioning the determination of particle surface composition as a function of the average aerodynamic diameters [10] in order to correlate the growth processes involved in the formation of the aerosol [11-13]. Moreover, the aerosol particle size distribution is one of the key variables involved in the analysis of radiative aerosol effects, and, numerous works have examined the spatio- temporal characteristics of particle size distributions with respect to the distribution of anthropogenic sources [14]. Actually, only a few, but significant works are based on XPS studies to correlate surface chemical composition of aerosol particles as sampled on filters and size-segregation thereby giving information on surface properties as function 68 ISBN:9788896515204 4th IMEKO TC19 SYMPOSIUM