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Volume 17, issue 1
Atmos. Chem. Phys., 17, 313–326, 2017
https://doi.org/10.5194/acp-17-313-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 17, 313–326, 2017
https://doi.org/10.5194/acp-17-313-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 05 Jan 2017

Research article | 05 Jan 2017

Characteristics of brown carbon in the urban Po Valley atmosphere

Francesca Costabile1, Stefania Gilardoni2, Francesca Barnaba1, Antonio Di Ianni1, Luca Di Liberto1, Davide Dionisi1, Maurizio Manigrasso3, Marco Paglione2, Vanes Poluzzi4, Matteo Rinaldi2, Maria Cristina Facchini2, and Gian Paolo Gobbi1 Francesca Costabile et al.
  • 1Institute for Atmospheric Sciences and Climate (ISAC), National Research Council (CNR), Rome, Italy
  • 2Institute for Atmospheric Sciences and Climate (ISAC), National Research Council (CNR), Bologna, Italy
  • 3INAIL, Rome, Italy
  • 4ARPA ER, Bologna, Italy

Abstract. We investigate optical–microphysical–chemical properties of brown carbon (BrC) in the urban ambient atmosphere of the Po Valley. In situ ground measurements of aerosol spectral optical properties, PM1 chemical composition (HR-ToF-AMS), and particle size distributions were carried out in Bologna. BrC was identified through its wavelength dependence of light absorption at visible wavelengths, as indicated by the absorption Ångström exponent (AAE). We found that BrC occurs in particles with a narrow monomodal size distribution peaking in the droplet mode, enriched in ammonium nitrate and poor in black carbon (BC), with a strong dependance on OA-to-BC ratios, and SSA530 of 0.98 ± 0.01. We demonstrate that specific complex refractive index values (k530 = 0.017 ± 0.001) are necessary in addition to a proper particle size range to match the large AAEs measured for this BrC (AAE467 − 660 = 3.2 ± 0.9 with values up to 5.3). In terms of consistency of these findings with literature, this study

i. provides experimental evidence of the size distribution of BrC associated with the formation of secondary aerosol;

ii. shows that in the lower troposphere AAE increases with increasing OA-to-BC ratios rather than with increasing OA – contributing to sky radiometer retrieval techniques (e.g., AERONET);

iii. extends the dependence of AAE on BC-to-OA ratios previously observed in chamber experiments to ambient aerosol dominated by wood-burning emissions.

These findings are expected to bear important implications for atmospheric modeling studies and remote sensing observations as regards the parametrization and identification of BrC in the atmosphere.

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We investigate the particle size distribution and spectral optical properties of brown carbon (BrC) associated with the formation of secondary aerosol in the ambient atmosphere and relate these properties to major aerosol chemical components. We found that BrC occurs in particles in the droplet mode size range, enriched in ammonium nitrate and poor in black carbon (BC), with a strong dependance on the organic aerosol to BC ratio.
We investigate the particle size distribution and spectral optical properties of brown carbon...
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