Long-term real-time chemical characterization of submicron aerosols at Montsec (Southern Pyrenees, 1570 m a.s.l.)

Abstract

Abstract. Real-time measurements of inorganic (sulfate, nitrate, ammonium, chloride and black carbon (BC)) and organic submicron aerosols from a continental background site (Montsec, MSC, 1570 m a.s.l.) in the Western Mediterranean Basin (WMB) were conducted for 10 months (July 2011–April 2012). An Aerosol Chemical Speciation Monitor (ACSM) was co-located with other on-line and off-line PM1 measurements. Analyses of the hourly, diurnal, and seasonal variations are presented here, for the first time for this region. Seasonal trends in PM1 components are attributed to variations in: evolution of the planetary boundary layer (PBL) height, air mass origin, and meteorological conditions. In summer, the higher temperature and solar radiation increases convection, enhancing the growth of the PBL and the transport of anthropogenic pollutants towards high altitude sites. Furthermore, the regional recirculation of air masses over the WMB creates a continuous increase in the background concentrations of PM1 components and causes the formation of reserve strata at relatively high altitudes. Sporadically, MSC is affected by air masses from North Africa. The combination of all these atmospheric processes at local, regional and continental scales results in a high variability of PM1 components, with poorly defined daily patterns, except for the organic aerosols (OA). OA was mostly oxygenated organic aerosol (OOA), with two different types: semi-volatile (SV-OOA) and low-volatile (LV-OOA), and both showed marked diurnal cycles regardless of the air mass origin, especially SV-OOA. This different diurnal variation compared to inorganic aerosols suggested that OA components at MSC are not only associated with anthropogenic and long-range-transported secondary OA (SOA), but also with recently-produced biogenic SOA. Very different conditions drive the aerosol phenomenology in winter at MSC. The thermal inversions and the lower vertical development of the PBL leave MSC in the free troposphere most of the day, being affected by PBL air masses only after midday, when the mountain breezes transport emissions from the adjacent valleys and plains to the top of the mountain. This results in clear diurnal patterns of both organic and inorganic concentrations. Moreover, in winter sporadic long-range transport from mainland Europe is observed and leads to less marked diurnal patterns. The results obtained in the present study highlight the importance of SOA formation processes at a remote site such as MSC, especially in summer. Additional research is needed to characterize the sources of SOA at remote sites.