Fig. 1.

Distributions of aliphatic, aromatic, and carbonyl carbon content in the four quadrants defined by the first two PCs, based on 80 group 1 DOM samples in total. “High” and “low” in the following are relative terms and specific to each type of carbon. Quadrant a is low content of both aliphatic and aromatic carbon and high content of carbonyl carbon. Quadrant b is low aromatic carbon and high carbonyl and aliphatic carbon content. Quadrant c is low content of carbonyl and aliphatic carbon and high content of aromatic carbon. Quadrant d is high content of aliphatic and aromatic carbon and low content of carbonyl carbon. In each quadrant, the percentages indicate how many DOM samples belong to the different types of environmental DOM sources.