Our study shows that across ASEAN 50 cities, these model results reveal that 39 % of observed low-visibility days can be explained by either fossil fuel burning or biomass burning emissions alone, a further 20 % by fossil fuel burning alone, a further 8 % by biomass burning alone, and a further 5 % by a combination of fossil fuel burning and biomass burning. The remaining 28 % of observed low-visibility days remains unexplained, likely due to emissions sources that have not been accounted for.