Joint analysis of continental and regional background environments in the western Mediterranean: PM1 and PM10 concentrations and composition

The complete chemical composition of atmospheric particulate matter (PM1 and PM10) from a continental (Montsec, MSC, 1570 m a.s.l.) and a regional (Montseny, MSY, 720 m a.s.l) background site in the western Mediterranean Basin (WMB) were jointly studied for the first time over a relatively long-term period (January 2010-March 2013). Differences in average PMX concentration and composition between both sites were attributed to distance to anthropogenic sources, altitude, and different influence of atmospheric episodes. All these factors result in a continental-to-regional background increase of 4.0 μg m-3 for PM10 and 1.1 μg m-3 for PM1 in the WMB. This increase is mainly constituted by organic matter, sulfate, nitrate, and sea salt. However, higher mineral matter concentrations were measured at the continental background site owing to the higher influence of long-range transport of dust and dust resuspension. Seasonal variations of aerosol chemical components were attributed to evolution of the planetary boundary layer (PBL) height throughout the year, variations in the air mass origin, and differences in meteorology. During warmer months, weak pressure gradients and elevated insolation generate recirculation of air masses and enhance the development of the PBL, causing the aging of aerosols and incrementing pollutant concentrations over a large area in the WMB, including the continental background. This is reflected in a more similar relative composition and absolute concentrations of continental and regional background aerosols. Nevertheless, during colder months the thermal inversions and the lower vertical development of the PBL leave MSC in the free troposphere most of the time, whereas MSY is more influenced by regional pollutants accumulated under winter anticyclonic conditions. This results in much lower concentrations of PMX components at the continental background site with respect to those at the regional background site. The influence of certain atmospheric episodes caused different impacts at regional and continental scales. When long-range transport from central and eastern Europe and from north Africa occurs, the continental background site is frequently more influenced, thus indicating a preferential transport of pollutants at high altitude layers. Conversely, the regional background site was more influenced by regional processes. Continental and regional aerosol chemical composition from the WMB revealed (a) high relevance of African dust transport and regional dust resuspension; (b) low biomass burning contribution; (c) high organic matter contribution; (d) low summer nitrate concentrations; and (e) high aerosol homogenization in summer.This study was supported by the Ministry of Economy and Competitiveness and FEDER funds under the PRISMA (CGL2012-39623-C02-1) and CARIATI (CGL2008- 06294/CLI) projects, and by the Generalitat de Catalunya (AGAUR 2009 SGR8 and the DGQA). The research received funding from the European Union Seventh Framework Programme (FP7/ 2007- 2013) ACTRIS under grant agreement no. 262254. The authors would like to extend their gratitude to the personnel from the COU and the OAdM. We would also like to express our gratitude to the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model, and boundary layer height calculation, used in this publication.Peer reviewe