Fig. 3: Van Krevelen diagram for CHON chromophores assigned in ESI- mode before (2015) and after (2019) the “coal-to-gas” conversion measure.

CHON− chromophores in a, b HULIS-BrC and c, d WI-BrC fractions. The area of the circles is proportional to the fourth root of the intensity of individual chromophores, and the insert plot represents the chromophores identified only before or after the measure. The color bar denotes the aromaticity equivalent (black with XC < 2.5, red with 2.5 ≤ XC < 2.7, and blue with XC ≥ 2.7).