Snow covers are very sensitive to contamination from soot agglomerates derived from vehicles. A spectroradiometric system covering a wavelength from 300 to 2500 nm with variable resolution (from 2.2 to 7.0 nm) was used to characterize the effect of soot derived from a diesel vehicle whose exhaust stream was oriented towards a limited snowed area. The vehicle was previously tested in a rolling test bench where particle number emissions and size distributions were measured, and fractal analysis of particle microscopic images was made after collecting individual agglomerates by means of an electrostatizing sampler. Finally, the experimental results were compared to modelled results of contaminated snow spectral albedo obtained with a snow radiative transfer model developed by our research group (OptiPar) and with other models. Both experimental and modelled results show that increasingly accumulated soot mass reduces the snow albedo with a constant rate of around 0.03 units per mg/kg, with a predominant effect on the UV-VIS range. Based on the small size of the primary particles (around 25 nm), the Rayleigh-Debye-Gans approximation, further corrected to account for the effect of multiple scattering within the agglomerates, was revealed as an appropriate technique in the model.

Soot aerosols produced during incomplete combustion processes are agglomerates composed of quasi-spherical carbonaceous primary particles with markedly different arrangements (shape, size, and internal structure) and varying compositions. Although they have been considered to be one of the main contributors to climate change, it remains difficult to quantify their global warming potential (GWP) precisely. Exact knowledge of the optical properties of soot agglomerates (black carbon -BC) would be necessary to fairly establish a CO2 equivalent GWP. BC warming effects depend on their size and shape, and despite their limited residence time in the atmosphere, their high radiative forcing would lead to GWP ~ 2000. The quantification of BC concentrations in the atmosphere is often based on optical measurements, which usually lack accuracy. Our calculations on the direct radiative forcing of BC showed that the irregularity of the agglomerate, and their size markedly affect the cooling/heating capacity due to extreme variations of its absorbing and scattering characteristics. In addition, the estimation of the incremental radiative forcing of aerosols is challenging since it is highly dependent on local parameters such as cloudiness, surface albedo, aerosol concentration, etc., which are highly variable worldwide. Based on that, we do not recommend using a unique CO2 equivalent GWP for BC.