Unveiling the Mechanism for the Photochemistry and Photodegradation of Vanillin

Phototransformation of Vanillin in Artificial Snow by Direct Photolysis and Mediated by Nitrite

The photodegradation of vanillin, as a proxy of methoxyphenols emitted by biomass burning, was investigated in artificial snow at 243 K and in liquid water at room temperature. Nitrite (NO₂^-) was used as a photosensitizer of reactive oxygen and nitrogen species under UVA light, because of its key photochemical role in snowpacks and atmospheric ice/waters. In snow and in the absence of NO₂^-, slow direct photolysis of vanillin was observed due to back-reactions taking place in the quasi-liquid layer at the ice-grain surface. The addition of NO₂^- made the photodegradation of vanillin faster, because of the important contribution of photoproduced reactive nitrogen species in vanillin phototransformation. These species triggered both nitration and oligomerization of vanillin in irradiated snow, as the identified vanillin by-products showed. Conversely, in liquid water, direct photolysis was the main photodegradation pathway of vanillin, even in the presence of NO₂^-, which had negligible effects on vanillin photodegradation. The results outline the different role of iced and liquid water in the photochemical fate of vanillin in different environmental compartments.

Detection of excited triplet species from photolysis of carbonyls: Direct evidence for single oxygen formation in atmospheric environment

Excited triplet species play an important role in the photolytic formation of ¹O₂ from carbonyls, but the related mechanism is still uncertain, due to lack of direct evidence. In this study, steady-state and transient photolysis of eleven carbonyls to produce ¹O₂ was investigated. Dicarbonyl displayed greater ¹O₂ production ability than monocarbonyl, while dicarbonyl containing both ketone and carboxyl groups connected by CC bond (i.e., pyruvic acid (PA)) showed the highest ¹O₂ steady-state concentration ([¹O₂]_SS). For the first time, the production of ³PA* from PA with narrow energy gap was confirmed by laser flash photolysis technique and the second-order decay rate constant of ³PA* was 2.78 × 10⁷ M^-1 s^-1. Quenching results verified the dominant contribution of ³PA* to ¹O₂ production from PA. Addition of inorganic salt or increase in solution pH showed negligible effect on ³PA*, but significantly decreased the [¹O₂]_SS of PA by up to two orders of magnitude, due to reduction of hydrate content. Photolysis of methylglyoxal and dimethylamine mixture led to higher content of excited triplet species at pH ≈ 11 and remarkably enhanced [¹O₂]_SS, which was 2.3 times of that from PA and dimethylamine mixture. These findings provide direct evidence for the contribution of transient species from carbonyls or their product to ¹O₂ formation in atmospheric environment.