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

Abstract 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 ( Y TAOH ), by fitting data using linear regression we obtained the relationship Y TAOH  = (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 Y TAOH  = (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 H 2 O 2 ) 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, k TA, OH  = (4.0 ± 0.1) × 10 9  M −1  s −1 in aerated solution, at pH 6.0 and 293 ± 2 K.