Table 1.

Schematic showing examples of contaminants that are transformed via direct and/or indirect photochemical reactions (photoreactions). Changes in UV radiation, linked to changes in stratospheric ozone, as well as climate, modulate the balance between direct and indirect photoreactions

Direct photoreactions	Both	Indirect photoreactions
Photosensitive pesticides. UV-B induced, direct photoreactions dominate. UV-absorbing films can be used to reduce rates of photodegradation.3,207	Organophosphoro-thionate pesticides. Fenitrothion undergoes direct photodegradation while diazinon is degraded by the indirect, nitrate-sensitised pathway.191	POPs, PBDEs, and other biorefractory chemicals. Susceptible to indirect photoreactions.78,123,158
Carbonyl compounds (especially aromatic ketones). Absorb in the UV-B region; triplet states participate in H-atom abstraction and electron transfer and also initiate indirect photoreactions.123	Antibiotics. Direct pathway is dominant for Cipro and indirect pathway for others.111	Antibiotics. Indirect pathway is usually dominant.111,123
	Lampricide. Direct and indirect photoreactions influence fate.119,120	Nanosilver. CDOM-sensitised photoreactions reduce ionic silver to nanosilver.88
	Graphene oxide. See ref. 88 and 89.	UV filters. Only indirect photoreactions are important.164
	Oil spills. Combination of indirect and direct photodegradation at surfaces, coupled with photofacilitated biodegradation.82,205	Microplastics. Are produced by indirect photoreactions7 (also see ref. 6).
	Pathogenic bacteria, viruses, and protozoans. Undergo direct (endogenous) and indirect (exogenous) photoinactivation133 (see Fig. 8).

^aPPPs, persistent organic pollutants; PBDEs, polybrominated diphenyl ethers.