Aqueous phase photolysis of 4-nitrocatechol: Reaction kinetics, evolutions of chemical composition, light absorption and oxidation potential

4-nitrocatechol (4NC) is a model compound of nitrophenols, an important group pf light-absorbing species, a.k.a., brown carbon (BrC), that have significant impacts on air quality and radiation forcing. This work aims to elucidate the kinetics, chemical and optical properties as well as oxidation potential (OP) of 4NC during aqueous photolysis. Results show that hydroxyl radical (∙OH) initiated photo-oxidation has the fastest reaction rate (6.1 ×10 -5 s -1 ) followed by organic triplet state ( 3 C*) induced photolysis (3.8 ×10 -5 s -1 ) and direct photolysis (2.4 ×10 -5 s -1 ); the second-order rate constant of ∙OH with 4NC is determined to be 4.3 × 10 10 M -1 s -1 . Reaction pathways of 3 C*-initiated photolysis and ∙OH-initiated photolysis are largely different, with the former generating more oxygenated species, but their mass yields of products after 12 hours of irradiation are close (∼40 %). Aqueous ageing of 4NC overall leads to the decrease of light absorption, and the photo-bleaching rates are in an order of ∙OH (2.58 × 10 -5 s -1 ) > 3 C* (1.06 × 10 -5 s -1 ) > direct photolysis (5.83 × 10 -6 s -1 ) too; the second-order photo-bleaching rate constant of ∙OH with 4NC is 1.77 × 10 10 M -1 s -1 . On the other hand, aqueous ageing can also produce BrC species (not sufficient to offset the photo-bleaching effect) of which some nitrogen-containing organics are an important part, and 3 C*-photolysis has the strongest production ability. Furthermore, aqueous photolysis of 4NC can decrease its OP, especially the ∙OH-photolysis products are nearly non-toxic while those from 3 C*-photolysis still has significantly high OP at the end of oxidation. Finally, we estimated the atmospheric lifetimes of 4NC against ∙OH in different atmospheric environments, which range from a few seconds to tens of hours.