Fig. 4

Network structure of Earth’s atmospheric reaction system. Panel A shows a minimal reaction system comprising of two reactions. Panel B illustrates a representation with two types of reaction nodes (photochemical and non-photochemical). Panel C shows the most common projection to a simple graph. Panel D displays a higher-order network representation (Battiston et al. 2020) where the molecular species involved in one reaction form a hyperedge—a generalization of edges to represent the interactions of an arbitrary number of elements. Panel E shows the atmospheric reaction network of Earth (data from Ref. Solé and Munteanu (2004)) in a representation similar to panel B