Characteristics of brown carbon in the urban Po Valley atmosphere

We investigate optical-microphysical-chemicalproperties of brown carbon (BrC) in the urban ambient atmosphereof the Po Valley. In situ ground measurements ofaerosol spectral optical properties, PM1 chemical composition(HR-ToF-AMS), and particle size distributions werecarried out in Bologna. BrC was identified through itswavelength dependence of light absorption at visible wavelengths,as indicated by the absorption Ångström exponent(AAE). We found that BrC occurs in particles with a narrowmonomodal 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, andSSA530 of 0.98 ± 0.01. We demonstrate that specific complexrefractive index values (k530 = 0.017 ± 0.001) are necessaryin addition to a proper particle size range to match thelarge AAEs measured for this BrC (AAE467-660 = 3.2 ± 0.9with values up to 5.3). In terms of consistency of these findingswith literature, this studyi. provides experimental evidence of the size distributionof BrC associated with the formation of secondaryaerosol;ii. shows that in the lower troposphere AAE increases withincreasing OA-to-BC ratios rather than with increasingOA - contributing to sky radiometer retrieval techniques(e.g., AERONET);iii. extends the dependence of AAE on BC-to-OA ratiospreviously observed in chamber experiments to ambientaerosol dominated by wood-burning emissions.These findings are expected to bear important implicationsfor atmospheric modeling studies and remote sensing observationsas regards the parametrization and identification ofBrC in the atmosphere.