Three years of aerosol mass, black carbon and particle number concentrations at Montsec (southern Pyrenees, 1570 m a.s.l.)

Abstract. Time variation of mass particulate matter (PM₁ and PM_1&minus10), black carbon (BC) and number of particles (N₃: number of particles with an aerodynamic diameter higher than 3 nm, and N₁₀: higher than 10 nm) concentrations at the high-altitude site of Montsec (MSC) in the southern Pyrenees was interpreted for the period 2010–2012.

At MSC, PM₁₀ (12 μg m⁻³) and N₇ (2140 # cm⁻³) three-year arithmetic average concentrations were higher than those measured at other high-altitude sites in central Europe during the same period (PM₁₀: 3–9 μg m⁻³ and N: 634–2070 # cm⁻³). By contrast, BC concentrations at MSC (0.2 μg m⁻³) were equal to or even lower than those measured at these European sites (0.2–0.4 μg m⁻³). These differences were attributed to the higher relevance of Saharan dust transport and to the higher importance of the biogenic precursor emissions and new particle formation (NPF) processes, and to the lower influence of anthropogenic emissions at MSC.

The different time variation of PM and BC concentrations compared with that of N suggests that these aerosol parameters were governed by diverse factors at MSC. Both PM and BC concentrations showed marked differences for different meteorological scenarios, with enhanced concentrations under North African air outbreaks (PM_1&minus10: 13 μg m⁻³, PM₁: 8 μg m⁻³ and BC: 0.3 μg m⁻³) and low concentrations when Atlantic advections occurred (PM₁₋₁₀: 5 μg m⁻³, PM₁: 4 μg m⁻³ and BC: 0.1 μg m⁻³). PM and BC concentrations increased in summer, with a secondary maximum in early spring, and were at their lowest in winter, due to the contrasting origin of the air masses in the warmer seasons (spring and summer) and in the colder seasons (autumn and winter). The maximum in the warmer seasons was attributed to long-range transport processes that mask the breezes and regional transport breaking the daily cycles of these pollutants. By contrast, PM and BC concentrations showed clear diurnal cycles, with maxima at midday in the colder seasons. A statistically significant weekly variation was also obtained for the BC concentrations, displaying a progressive increase from Tuesday to Saturday, followed by a significant decrease on Sunday and Monday.

N concentrations depended more on local meteorological variables such as temperature and solar radiation intensity than on the origin of the air mass. Therefore, arithmetic averages as a function of meteorological episodes showed the highest concentrations of N during summer regional episodes (N₃: 4461 # cm⁻³ and N₇: 3021 # cm⁻³) and the lowest concentrations during winter regional scenarios (N₃: 2496 # cm⁻³ and N₇: 1073 # cm⁻³). This dependence on temperature and solar radiation also accounted for the marked diurnal cycle of N concentrations throughout the year, with a peak at midday and for the absence of a weekly pattern.

Measurements carried out at MSC enabled us to characterize the tropospheric background aerosols in the western Mediterranean basin (WMB). Our results highlight the importance of the NPF processes in southern Europe, underline the high contribution of long-range dust transport with respect to central Europe and its prevalence in elevated layers, and reveal that MSC is much less affected by anthropogenic emissions than other high-altitude sites in central Europe.