Articles | Volume 14, issue 22
Atmos. Chem. Phys., 14, 12109–12132, 2014
Atmos. Chem. Phys., 14, 12109–12132, 2014

Research article 18 Nov 2014

Research article | 18 Nov 2014

Measurements of the aerosol chemical composition and mixing state in the Po Valley using multiple spectroscopic techniques

S. Decesari1, J. Allan2, C. Plass-Duelmer3, B. J. Williams4,5, M. Paglione1, M. C. Facchini1, C. O'Dowd6, R. M. Harrison7,9, J. K. Gietl3,7, H. Coe2, L. Giulianelli1, G. P. Gobbi1, C. Lanconelli1, C. Carbone1, D. Worsnop4, A. T. Lambe4, A. T. Ahern4,**, F. Moretti8, E. Tagliavini8, T. Elste3, S. Gilge3, Y. Zhang5, and M. Dall'Osto6,* S. Decesari et al.
  • 1Institute of Atmospheric Sciences and Climate of the National Research Council of Italy (ISAC–CNR), Bologna, Italy
  • 2School of Earth, Atmospheric and Environmental Science, University of Manchester, Manchester, UK
  • 3Deutscher Wetterdienst (DWD), Meteorological Observatory, Hohenpeissenberg, Germany
  • 4Aerodyne Research, Inc., Billerica, MA, USA
  • 5Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, USA
  • 6School of Physics, National University of Ireland, Galway, Ireland
  • 7School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK
  • 8Centro Interdipartimentale di Ricerca per le Scienze Ambientali, University of Bologna, Bologna, Italy
  • 9Department of Environmental Sciences/Center of Excellence in Environmental Studies, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia
  • *now at: Institut de Ciències del Mar, CSIC, Barcelona, Spain
  • **now at: Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA

Abstract. The use of co-located multiple spectroscopic techniques can provide detailed information on the atmospheric processes regulating aerosol chemical composition and mixing state. So far, field campaigns heavily equipped with aerosol mass spectrometers have been carried out mainly in large conurbations and in areas directly affected by their outflow, whereas lesser efforts have been dedicated to continental areas characterised by a less dense urbanisation. We present here the results obtained at a background site in the Po Valley, Italy, in summer 2009. For the first time in Europe, six state-of-the-art spectrometric techniques were used in parallel: aerosol time-of-flight mass spectrometer (ATOFMS), two aerosol mass spectrometers (high-resolution time-of-flight aerosol mass spectrometer – HR-ToF-AMS and soot particle aerosol mass spectrometer – SP-AMS), thermal desorption aerosol gas chromatography (TAG), chemical ionisation mass spectrometry (CIMS) and (offline) proton nuclear magnetic resonance (1H-NMR) spectroscopy. The results indicate that, under high-pressure conditions, atmospheric stratification at night and early morning hours led to the accumulation of aerosols produced by anthropogenic sources distributed over the Po Valley plain. Such aerosols include primary components such as black carbon (BC), secondary semivolatile compounds such as ammonium nitrate and amines and a class of monocarboxylic acids which correspond to the AMS cooking organic aerosol (COA) already identified in urban areas. In daytime, the entrainment of aged air masses in the mixing layer is responsible for the accumulation of low-volatility oxygenated organic aerosol (LV-OOA) and also for the recycling of non-volatile primary species such as black carbon. According to organic aerosol source apportionment, anthropogenic aerosols accumulating in the lower layers overnight accounted for 38% of organic aerosol mass on average, another 21% was accounted for by aerosols recirculated in residual layers but still originating in northern Italy, while a substantial fraction (41%) was due to the most aged aerosols imported from transalpine areas. The different meteorological regimes also affected the BC mixing state: in periods of enhanced stagnation and recirculation of pollutants, the number fraction of the BC-containing particles determined by ATOFMS was 75% of the total, while in the days of enhanced ventilation of the planetary boundary layer (PBL), such fraction was significantly lower (50%) because of the relative greater influence of non-BC-containing aerosol local sources in the Po Valley. Overall, a full internal mixing between BC and the non-refractory aerosol chemical components was not observed during the experiment in this environment.

Short summary
We made use of multiple spectrometric techniques for characterizing the aerosol chemical composition and mixing in the Po Valley in the summer. The oxygenated organic aerosol (OOA) concentrations were correlated with simple tracers for recirculated planetary boundary layer air. A full internal mixing between black carbon (BC) and the non-refractory aerosol components was never observed. Local sources in the Po Valley were responsible for the production of organic particles unmixed with BC.
Final-revised paper