The impact of uncertainty in black carbon's refractive index on simulated optical depth and radiative forcing

Ruth A. R. Digby, Knut von Salzen, Adam H. Monahan, Nathan P. Gillett, and Jiangnan Li

Data sets

POLDER Data Release POLDER Data Release https://www.grasp-open.com/products/polder-data-release/

Global fire emissions estimates during 1997-2016 (https://www.geo.vu.nl/~gwerf/GFED/GFED4/) G. R. van der Werf et al. https://doi.org/10.5194/essd-9-697-2017

MISR Level 3 Component Global Aerosol product in netCDF format covering a month V004 NASA/LARC/SD/ASDC https://doi.org/10.5067/Terra/MISR/MIL3MAEN_L3.004

CEDS v. 2021.04.21 gridded emissions data (Version v_2021_02_05) P. O'Rourke et al. https://doi.org/10.5281/zenodo.4741285

BCRI simulation data R. Digby https://crd-data-donnees-rdc.ec.gc.ca/CCCMA/publications/2025_Digby_black_carbon_refractive_index/

Model code and software

digbyr/publication-scripts-bcri: revised submission (version v2.0) R. Digby https://doi.org/10.5281/zenodo.14715011

CanESM: The Canadian Earth System Model Canadian Centre for Climate Modelling and Analysis https://gitlab.com/cccma/canesm/

matplotlib/matplotlib: REL: v3.7.1 (v3.7.1) T. Caswell et al. https://doi.org/10.5281/zenodo.7697899

Short summary

The refractive index of black carbon (BCRI), which determines how much energy black carbon absorbs and scatters, is difficult to measure, and different climate models use different values. We show that varying the BCRI across commonly used values can increase absorbing aerosol optical depth by 42 % and the warming effect from interactions between black carbon and radiation by 47 %, an appreciable fraction of the overall spread between models reported in recent literature assessments.