From air quality to climate: Impact of aerosol sources on optical properties at urban, regional and continental levels in the north-western Mediterranean

Further research is needed to reduce the existing uncertainties on the effect that specific aerosol sources have on radiative forcing, thus supporting the assessment of future mitigation strategies which should be focused on both air quality and climate, and not acting separately. This study presents a new approach aimed at quantifying the mass scattering and absorption efficiencies (MSE and MAE) of different aerosol sources at urban (Barcelona-BCN), regional (Montseny-MSY) and remote (Montsec-MSA) background sites in the northwestern (NW) Mediterranean. An analysis of source apportionment to the measured scattering and absorption coefficients was performed by means of a multilinear regression (MLR) model during 2010–2014 at BCN and MSY and during 2011–2014 at MSA. The source contributions to PM10 mass, identified by means of the Positive Matrix Factorization (PMF) model, were used as dependent variables in the MLR model in order to take into account the internal mixing state of atmospheric aerosols. Seven aerosol sources were obtained at MSA and MSY and 8 sources at BCN. Mineral, Aged marine, Ammonium sulfate, Ammonium nitrate and V-Ni bearing sources were common at the three sites. Traffic, Industrial/metallurgy and Road-resuspension were isolated at BCN, whereas Industrial/Traffic and Aged organics were solely identified at MSY and MSA. The highest MSE were found for Ammonium sulfate (4.5 and 10.7 m2 g−1), Ammonium nitrate (8.8 and 7.8 m2 g−1) and V-Ni (8 and 3.5 m2 g−1) at MSY and MSA respectively, dominating the scattering throughout the year with marked seasonal trends. V-Ni bearing, originated mainly from shipping emissions, simultaneously contributed to both scattering and absorption being the second most efficient absorptive source in BCN (0.9 m2 g−1). The Traffic source at BCN and the equivalent Industrial/Traffic at MSY and MSA mainly governed the light absorption and exhibited the highest MAE (1.7, 0.9 and 0.2 m2 g−1, respectively). Sources predominantly composed by fine and relatively dark particles such as Industrial/Traffic, Aged organics and V-Ni were simultaneously characterized with low single scattering albedo (SSA) and high scattering Angstrom exponent (SAE). Conversely, Mineral and Aged marine showed the highest SAE and the lowest SSA, being scattering the dominant process in the light extinction. The good agreement between modeled and measured optical properties allowed for the reconstruction of scattering, absorption and SSA time series by means of the PMF-MLR technique for the period 2004–2014 at MSY. Significant decreasing trends were found for the modeled scattering and absorption (−4.6 and −4.1 % y−1) coefficients. Interestingly, the observed reduction in the SSA (−0.11 % y−1) might suggests a less effectiveness of the air quality strategies focused on reducing pollutants containing black carbon (BC) particles, which highly contribute to light absorption and thus climate warming.

This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness) and FEDER funds under the PRISMA project (CGL2012-39623-C02/00), by the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and by the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA). This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654109. Marco Pandolfi is funded by a Ramón y Cajal Fellowship (RYC-2013-14036) awarded by the MINECO. The authors would like to express their gratitude to D. C. Carslaw and K. Ropkins for providing the OpenAir software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012).

Peer reviewed