The effect of probe choice and solution conditions on the apparent photoreactivity of dissolved organic matter

Abstract

Excited triplet states of dissolved organic matter (³DOM) are quantified directly with the species-specific probes trans,trans-hexadienoic acid (HDA) and 2,4,6-trimethylphenol (TMP), and indirectly with the singlet oxygen (¹O₂) probe furfuryl alcohol (FFA). Although previous work suggests that these probe compounds may be sensitive to solution conditions, including dissolved organic carbon concentration ([DOC]) and pH, and may quantify different ³DOM subpopulations, the probes have not been systematically compared. Therefore, we quantify the apparent photoreactivity of diverse environmental waters using HDA, TMP, and FFA. By conducting experiments under ambient [DOC] and pH, with standardized [DOC] and pH, and with solid phase extraction isolates, we demonstrate that much of the apparent dissimilarity in photochemical measurements is attributable to solution conditions, rather than intrinsic differences in ³DOM production. In general, apparent quantum yields (Φ_¹O2 ≥ Φ_³DOM,TMP ≫ Φ_³DOM,HDA) and pseudo-steady state concentrations ([¹O₂]_ss > [³DOM]_ss,TMP > [³DOM]_ss,HDA) show consistent relationships in all waters under standardized conditions. However, intrinsic differences in ³DOM photoreactivity are apparent between DOM from diverse sources, as seen in the higher Φ_¹O2 and lower Φ_³DOM,TMP of wastewater effluents compared with oligotrophic lakes. Additionally, while conflicting trends in photoreactivity are observed under ambient conditions, all probes observe quantum yields increasing from surface wetlands to terrestrially influenced waters to oligotrophic lakes under standardized conditions. This work elucidates how probe selection and solution conditions influence the apparent photoreactivity of environmental waters and confirms that ³DOM or ¹O₂ probes cannot be used interchangeably in waters that vary in [DOC], pH, or DOM source.