Fig. 4: Number of different CHON− chromophore subgroups classified according to O/N ratios in their formulas.

a, c Number of different CHON− chromophore subgroups in HULIS-BrC and WI-BrC fractions before (2015) the “coal-to-gas” conversion measure. b, d Similar to (a, c), but after (2019) the “coal-to-gas” conversion measure.