Abstract
ABSTRACTThis study focuses on the daily and seasonal variability of particle number size distributions and concentrations, performed at the Montseny (MSY) regional background station in the western Mediterranean from October 2010 to June 2011. Particle number concentrations at MSY were shown to be within range of various other sites across Europe reported in literature, but the seasonality of the particle number size distributions revealed significant differences. The Aitken mode is the dominant particle mode at MSY, with arithmetic mean concentrations of 1698 cm3, followed by the accumulation mode (877 cm−3) and the nucleation mode (246 cm−3). Concentrations showed a strong seasonal variability with large increases in particle number concentrations observed from the colder to warmer months. The modality of median size distributions was typically bimodal, except under polluted conditions when the size distribution was unimodal. During the colder months, the daily variation of particle number size distributions are strongly influenced by a diurnal breeze system, whereby the Aitken and accumulation modes vary similarly to PM1 and BC mass concentrations, with nocturnal minima and sharp day-time increases owing to the development of a diurnal mountain breeze. Under clean air conditions, high levels of nucleation and lower Aitken mode concentrations were measured, highlighting the importance of new particle formation as a source of particles in the absence of a significant condensation sink. During the warmer months, nucleation mode concentrations were observed to be relatively elevated both under polluted and clean conditions due to increased photochemical reactions, with enhanced subsequent growth owing to elevated concentrations of condensable organic vapours produced from biogenic volatile organic compounds, indicating that nucleation at MSY does not exclusively occur under clean air conditions. Finally, mixing of air masses between polluted and non-polluted boundary layer air, and brief changes in the air mass being sampled gave rise to unusual particle number size distributions, with specific cases of such behaviour discussed at length.
Highlights
The Western Mediterranean Basin (WMB) and Iberian Peninsula represent a very complex climate area owing to its geographical position
From October 2010 to June 2011, simultaneous measurements of particle number and size distribution of submicron particles (85% data coverage based on hourly resolution), particulate matter (PM) (92% data coverage) and Black Carbon (BC; 99% data coverage) mass concentrations, as well as gaseous pollutants (Â98% data coverage) were carried out at a regional background (RB) site at MSY (41819?N, 02821?E)
The measured particle number size distributions in this work are grouped in size bins, whereby the nucleation mode includes all particles of diameter between 9 and 20 nm, the Aitken mode includes particles of diameter 20Á100 nm and the accumulation mode 100Á825 nm
Summary
The Western Mediterranean Basin (WMB) and Iberian Peninsula represent a very complex climate area owing to its geographical position. Previous studies performed in this area have described the variability of particulate matter (PM) (Perez et al, 2008), the chemical speciation of PM (Cusack et al, 2012) and the aerosol optical properties of aerosols (Pandolfi et al, 2011). Sorribas et al (2011) published results on sub-micron particle size distributions for a rural background site in southern Spain, which was observed to be heavily influenced by marine, Saharan and continental air masses and, in some circumstances, emissions from a nearby industrial estate. The study of particle number concentration and size distributions in the RB of north-east (NE) Spain, until now, has been largely underdeveloped
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