Abstract
Abstract. Real-time measurements of inorganic (sulfate, nitrate, ammonium, chloride and black carbon (BC)) and organic submicron aerosols (particles with an aerodynamic diameter of less than 1 μm) 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 online and offline 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 reservoir layers at relatively high altitudes. The combination of all these atmospheric processes results in a high variability of PM1 components, with poorly defined daily patterns, except for the organic aerosols (OA). OA was mostly composed (up to 90%) of oxygenated organic aerosol (OOA), split in two types: semivolatile (SV-OOA) and low-volatility (LV-OOA), the rest being hydrocarbon-like OA (HOA). The marked diurnal cycles of OA components regardless of the air mass origin indicates that they 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. OA was also mainly composed (71%) of OOA, with contributions from HOA (5%) and biomass burning OA (BBOA; 24%). Moreover, in winter sporadic long-range transport from mainland Europe is observed. 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 and processes of SOA formation at remote sites.
Highlights
Earth’s climate system is modulated by atmospheric aerosols
In order to establish the consistency of the different measurements during this study, the sum of the aerosol chemical speciation monitor (ACSM) species and the black carbon (BC) mass concentrations was compared with the co-located PM1 and light scattering measurements (Fig. 2)
A similar seasonal pattern has been described at other high altitude sites in Europe (e.g., Carbone et al, 2014; Cozic et al, 2008; Freney et al, 2011; Tositti et al, 2013), which are associated with differences in the air mass origin from summer to winter, and to variations in the planetary boundary layer (PBL) height
Summary
Earth’s climate system is modulated by atmospheric aerosols. Submicron particles (< 1 μm in aerodynamic diameter) play a dominant role in both cloud formation and scattering or absorbing solar radiation (IPCC, 2013). The complexity of aerosol sources and processes results in an uncertainty in the radiative forcing of climate. Aerosol optical properties are connected to direct and indirect climate forcing effects, and they are dependent on particle composition. Aerosol composition may provide valuable information on aerosol sources and processes. Long-term measurements of PM1 chemical composition are needed to better understand aerosol sources, to quantify their lifetime in the atmosphere and to constrain the uncertainties of their climatic influence
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