Characteristics of brown carbon in the urban Po Valley atmosphere

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.