Impact of aerosol particle sources on optical properties in urban, regional and remote areas in the north-western Mediterranean

Abstract. Further research is needed to reduce the existing uncertainties on the effect that specific aerosol particle sources have on light extinction and consequently on climate. This study presents a new approach that aims to quantify the mass scattering and absorption efficiencies (MSEs and MAEs) of different aerosol sources at urban (Barcelona – BCN), regional (Montseny – MSY) and remote (Montsec – MSA) background sites in the north-western (NW) Mediterranean. An analysis of source apportionment to the measured multi-wavelength light scattering (σ_sp) and absorption (σ_ap) coefficients was performed by means of a multilinear regression (MLR) model for the periods 2009–2014, 2010–2014 and 2011–2014 at BCN, MSY and MSA respectively. The source contributions to PM₁₀ mass concentration, identified by means of the positive matrix factorization (PMF) model, were used as dependent variables in the MLR model. With this approach we addressed both the effect that aerosol sources have on air quality and their potential effect on light extinction through the determination of their MSEs and MAEs. An advantage of the presented approach is that the calculated MSEs and MAEs take into account the internal mixing of atmospheric particles.

Seven aerosol sources were identified at MSA and MSY, and eight sources at BCN. Mineral, aged marine, secondary sulfate, secondary nitrate and V-Ni bearing sources were common at the three sites. Traffic, industrial/metallurgy and road dust resuspension sources were isolated at BCN, whereas mixed industrial/traffic and aged organics sources were identified at MSY and MSA. The highest MSEs were observed for secondary sulfate (4.5 and 10.7 m² g⁻¹, at MSY and MSA), secondary nitrate (8.8 and 7.8 m² g⁻¹) and V-Ni bearing source (8 and 3.5 m² g⁻¹). These sources dominated the scattering throughout the year with marked seasonal trends. The V-Ni bearing source, originating mainly from shipping in the area under study, simultaneously contributed to both σ_sp and σ_ap, being the second most efficient light-absorbing source in BCN (MAE = 0.9 m² g⁻¹). The traffic source at BCN and the industrial/traffic at MSY exhibited the highest MAEs (1.7 and 0.9 m² g⁻¹). These sources were major contributors to σ_ap at BCN and MSY; however at MSA, secondary nitrate exerted the highest influence on σ_ap (MAE = 0.4 m² g⁻¹). The sources which were predominantly composed of fine and relatively dark particles, such as industrial/traffic, aged organics and V-Ni, were simultaneously characterized by low single scattering albedo (SSA) and a high scattering Ångström exponent (SAE). Conversely, mineral and aged marine showed the lowest SAE and the highest SSA, being scattering the dominant process in the light extinction. The good agreement found between modelled and measured particle optical properties allowed the reconstruction of σ_sp and σ_ap long-term series over the period 2004–2014 at MSY. Significant decreasing trends were found for the modelled σ_sp and σ_ap (−4.6 and −4.1 % yr⁻¹).