Performance and selectivity of the terephthalic acid probe for OH as a function of temperature, pH and composition of atmospherically relevant aqueous media

In the present study, we investigated the performance and selectivity of terephthalic acid (TA) as a trapping molecule for the laboratory quantification of photogenerated OH in synthetic water (where the organic and inorganic concentration is perfectly controlled), adopted as a proxy of natural atmospheric water (i.e. cloud droplets). TA reacts with OH to yield 2-hydroxyterephthalic acid (TAOH). First, we focused our investigation on the reactivity of TA as a function of pH and temperature of laboratory-made solutions, by using nitrate and hydrogen peroxide as photochemical OH sources. For the pH dependence of the TAOH formation yield (YTAOH), by fitting data using linear regression we obtained the relationship YTAOH = (0.0248 ± 0.0059)pH + (0.046 ± 0.035) at T = 288 K, in the pH range 3.9–7.5. For the temperature dependence we got YTAOH = (0.0059 ± 0.0011)T − (1.50 ± 0.31), between 278 and 303 K at pH 5.4. Furthermore, the performance and selectivity of TA were assessed in the presence of different photochemical OH sources (nitrate and H2O2) and scavengers (alcohols and carboxylic acids) at variable concentrations. These species are naturally present in the atmospheric aqueous phase. From these experiments we obtained the second-order reaction rate constant between TA and OH, kTA,OH = (4.0 ± 0.1) × 109 M−1 s−1 in aerated solution, at pH 6.0 and 293 ± 2 K.