Source apportionment of size-segregated atmospheric particles based on the major water-soluble components in Lecce (Italy)

Abstract

Atmospheric aerosols have potential effects on human health, on the radiation balance, on climate, and on visibility. The understanding of these effects requires detailed knowledge of aerosol composition and size distributions and of how the different sources contribute to particles of different sizes. In this work, aerosol samples were collected using a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI). Measurements were taken between February and October 2011 in an urban background site near Lecce (Apulia region, southeast of Italy). Samples were analysed to evaluate the concentrations of water-soluble ions (SO42−, NO3−, NH4+, Cl−, Na+, K+, Mg2+ and Ca2+) and of water-soluble organic and inorganic carbon. The aerosols were characterised by two modes, an accumulation mode having a mass median diameter (MMD) of 0.35±0.02μm, representing 51±4% of the aerosols and a coarse mode (MMD=4.5±0.4μm), representing 49±4% of the aerosols. The data were used to estimate the losses in the impactor by comparison with a low-volume sampler. The average loss in the MOUDI-collected aerosol was 19±2%, and the largest loss was observed for NO3− (35±10%). Significant losses were observed for Ca2+ (16±5%), SO42− (19±5%) and K+ (10±4%), whereas the losses for Na+ and Mg2+ were negligible. Size-segregated source apportionment was performed using Positive Matrix Factorization (PMF), which was applied separately to the coarse (size interval 1–18μm) and accumulation (size interval 0.056–1μm) modes. The PMF model was able to reasonably reconstruct the concentration in each size-range. The uncertainties in the source apportionment due to impactor losses were evaluated. In the accumulation mode, it was not possible to distinguish the traffic contribution from other combustion sources. In the coarse mode, it was not possible to efficiently separate nitrate from the contribution of crustal/resuspension origin.