Dissociation constants of relevant secondary organic aerosol components in the atmosphere

Abstract

The presented studies focus on measuring the determination of the acidity constant (pK_a) of relevant secondary organic aerosol components. For our research, we selected important oxidation products (mainly carboxylic acids) of the most abundant terpene compounds, such as α-pinene, β-pinene, β-caryophyllene, and δ-3-carene. The research covered the synthesis and determination of the acidity constant of selected compounds. We used three methods to measure the acidity constant, i.e., ¹H NMR titration, pH-metric titration, Bates-Schwarzenbach spectrophotometric method. Moreover, the pK_a values were calculated with Marvin 21.17.0 software to compare the experimentally derived values with those calculated from the chemical structure. pK_a values measured with ¹H NMR titration ranged from 3.51 ± 0.01 for terebic acid to 5.18 ± 0.06 for β-norcaryophyllonic acid. Moreover, the data determined by the ¹H NMR method revealed a good correlation with the data obtained with the commonly used potentiometric and UV-spectroscopic methods (R² = 0.92). In contrast, the comparison with in silico results exhibits a relatively low correlation (R²_Marvin = 0.66). We found that most of the values calculated with the Marvin Program are lower than experimental values obtained with pH-metric titration with an average difference of 0.44 pKa units. For di- and tricarboxylic acids, we obtained two and three pK_a values, respectively. A good correlation with the literature values was observed, for example, Howell and Fisher (1958) used pH-metric titration and measured pK_a1 and pK_a2 to be 4.48 and 5.48, while our results are 4.24 ± 0.10 and 5.40 ± 0.02, respectively.

Keywords: Acidic constant; Atmospheric aerosol; Physicochemical properties of aerosol; Terpene oxidation products.