Articles | Volume 16, issue 17
Atmos. Chem. Phys., 16, 10879–10897, 2016

Special issue: BACCHUS – Impact of Biogenic versus Anthropogenic emissions...

Special issue: The Pan European Gas-Aerosols Climate Interaction Study...

Atmos. Chem. Phys., 16, 10879–10897, 2016

Research article 01 Sep 2016

Research article | 01 Sep 2016

Size-resolved aerosol composition at an urban and a rural site in the Po Valley in summertime: implications for secondary aerosol formation

Silvia Sandrini1, Dominik van Pinxteren2, Lara Giulianelli1, Hartmut Herrmann2, Laurent Poulain2, Maria Cristina Facchini1, Stefania Gilardoni1, Matteo Rinaldi1, Marco Paglione1, Barbara J. Turpin3, Francesca Pollini1, Silvia Bucci4,5, Nicola Zanca1, and Stefano Decesari1 Silvia Sandrini et al.
  • 1Institute for Atmospheric Sciences and Climate (ISAC), National Research Council (CNR), Bologna, 40129, Italy
  • 2Leibniz-Institut für Troposphärenforschung (TROPOS), Leipzig, 04318, Germany
  • 3Environmental Science and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, NC 27599-7431, USA
  • 4Institute for Atmospheric Sciences and Climate (ISAC), National Research Council (CNR), Rome, 00133, Italy
  • 5Dept. of Physics, Ferrara University, Ferrara, 44121, Italy

Abstract. The aerosol size-segregated chemical composition was analyzed at an urban (Bologna) and a rural (San Pietro Capofiume) site in the Po Valley, Italy, during June and July 2012, by ion-chromatography (major water-soluble ions and organic acids) and evolved gas analysis (total and water-soluble carbon), to investigate sources and mechanisms of secondary aerosol formation during the summer. A significant enhancement of secondary organic and inorganic aerosol mass was observed under anticyclonic conditions with recirculation of planetary boundary layer air but with substantial differences between the urban and the rural site. The data analysis, including a principal component analysis (PCA) on the size-resolved dataset of chemical concentrations, indicated that the photochemical oxidation of inorganic and organic gaseous precursors was an important mechanism of secondary aerosol formation at both sites. In addition, at the rural site a second formation process, explaining the largest fraction (22 %) of the total variance, was active at nighttime, especially under stagnant conditions. Nocturnal chemistry in the rural Po Valley was associated with the formation of ammonium nitrate in large accumulation-mode (0.42–1.2 µm) aerosols favored by local thermodynamic conditions (higher relative humidity and lower temperature compared to the urban site). Nocturnal concentrations of fine nitrate were, in fact, on average 5 times higher at the rural site than in Bologna. The water uptake by this highly hygroscopic compound under high RH conditions provided the medium for increased nocturnal aerosol uptake of water-soluble organic gases and possibly also for aqueous chemistry, as revealed by the shifting of peak concentrations of secondary compounds (water-soluble organic carbon (WSOC) and sulfate) toward the large accumulation mode (0.42–1.2 µm). Contrarily, the diurnal production of WSOC (proxy for secondary organic aerosol) by photochemistry was similar at the two sites but mostly affected the small accumulation mode of particles (0.14–0.42 µm) in Bologna, while a shift to larger accumulation mode was observed at the rural site. A significant increment in carbonaceous aerosol concentration (for both WSOC and water-insoluble carbon) at the urban site was recorded mainly in the quasi-ultrafine fraction (size range 0.05–0.14 µm), indicating a direct influence of traffic emissions on the mass concentrations of this range of particles.

Short summary
This paper deals with impactor measurements performed in the summer 2012 during the EU project PEGASOS campaign in the Po Valley, at an urban and a rural site. The paper tries to disentangle the effects of weather anomalies (temporal and spatial) from those of diverse emissions (NH3) and chemical processes on the formation of secondary aerosols in the region, with special focus on nocturnal ammonium nitrate formation and its implications (aqueous formation of secondary organic aerosol).
Final-revised paper