Single-scattering albedo and iterated refractive indices of fresh and aged black and brown carbon particles emitted from burning peat and boreal forest floor surface in chamber experiments

It has been predicted that boreal forest and peatland fires will increase in the future as the climate warms. It is important for climate models that the optical properties of the aerosols emitted during these fires are well described.  Laboratory experiments were carried out to study biomass burning (BB) aerosol emissions and their photochemical and dark aging to investigate (1) the emission factors of BrC (and BC) from different combustion processes, (2) chemical and optical properties of the emissions and (3) how different atmospheric aging conditions affect these properties. The experiments were performed at the ILMARI-facility of the UEF (www.uef.fi/ilmari) in November–December 2023. Details of the experiments were presented by Mukherjee et al. (2025) who also presented absorption properties of water-soluble and methanol-soluble organic carbon (WSOC and MSOC, respectively) analyzed from filter samples. In the present work we will present the optical properties of BB emissions from burning peat and dry and moist boreal forest floor surface (BFS) samples at high time resolution. Light scattering coefficient was measured with a 3-wl nephelometer, absorption with a 7-wl Aethalometer and a 3-wl photoacoustic spectrometer, and particle number size distributions (PNSD) with an SMPS. A Mie code was used for calculating scattering and absorption coefficients from the PNSDs by varying real and imaginary refractive indices (n_r and n_i, respectively), until the measured and modeled scattering and absorption agree within 1%. The flaming BB emissions were dark with single-scattering albedo (SSA) varying between 0.25 and 0.6. The darkest aerosols with SSA ≈ 0.30 ± 0.05 were measured from flaming dry BFS and the highest SSA > 0.95 from aged peat fires. Fitting lines with the n_r vs SSA show that the real refractive indices can be estimated from a logarithmic function n_r(450) = 0.191ln(SSA) + 1.792, r² = 0.402; n_r(525) = 0.218ln(SSA) + 1.773, r² = 0.524; n_r(635) = 0.295ln(SSA) + 1.862, r² = 0.734 and the imaginary refractive indices from polynomials: n_i(450) = -4.50SSA³ + 10.35SSA² - 8.05SSA + 2.17, r² = 0.97; n_i(525) = -2.91SSA³ + 6.79SSA² -  5.40SSA + 1.51, r² = 0.98; n_i (635) = -1.65SSA³ + 3.85SSA² - 3.16SSA + 0.96, r² = 0.98. The next steps are to calculate the mass absorption cross sections (MAC) of BC and BrC by combining the optical data with the soot particle aerosol mass  spectrometer (SP-AMS) data and the absorption coeffcients and refractive indices of WSOC measured with an online UV-IR spectrometer connected to a liquid-waveguide capillary cell (LWCC) and a particle-into-liquid sampler (PILS).

Reference

Mukherjee, A. et al.: Brown carbon emissions from laboratory combustion of Eurasian arctic-boreal and South African savanna biomass, Atmos. Chem. Phys., 25, 16747–16774, 2025

Acknowledgement

 This work was supported by the Research Council of Finland via the project “Black and Brown Carbon in the Atmosphere and the Cryosphere” (BBrCAC) (decision number 341271)