Predicting Reactive Intermediate Quantum Yields from Dissolved Organic Matter Photolysis Using Optical Properties and Antioxidant Capacity

The antioxidant capacity
and formation of photochemically produced
reactive intermediates (RI) was studied for water samples collected
from the Florida Everglades with different spatial (marsh versus estuarine)
and temporal (wet versus dry season) characteristics. Measured RI
included triplet excited states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and the
hydroxyl radical (•OH). Single and multiple linear
regression modeling were performed using a broad range of extrinsic
(to predict RI formation rates, RRI) and
intrinsic (to predict RI quantum yields, ΦRI) parameters.
Multiple linear regression models consistently led to better predictions
of RRI and ΦRI for our
data set but poor prediction of ΦRI for a previously
published data set, probably because the
predictors are intercorrelated (Pearson’s r > 0.5). Single linear regression models were built with data
compiled
from previously published studies (n ≈ 120)
in which E2:E3, S, and ΦRI values
were measured, which revealed a high degree of similarity between
RI-optical property relationships across DOM samples of diverse sources.
This study reveals that •OH formation is, in general,
decoupled from 3DOM* and 1O2 formation,
providing supporting evidence that 3DOM* is not a •OH precursor. Finally, ΦRI for 1O2 and 3DOM* correlated negatively with
antioxidant activity (a surrogate for electron donating capacity)
for the collected samples, which is consistent with intramolecular
oxidation of DOM moieties by 3DOM*.