Laboratory studies of atmospheric aqueous-phase free-radical chemistry: kinetic and spectroscopic studies of reactions of NO3 and SO4– radicals with aromatic compounds

Abstract

Recent results from kinetic and spectroscopic laboratory studies of reactions of free radicals such as NO₃ and SO₄^– with aromatic compounds in aqueous solution at T= 298 K are presented. Three different experimental approaches, (i) laser photolysis–visible long-path laser absorption, (ii) laser photolysis–UV long-path laser absorption and (iii) time-resolved broadband diode-array spectroscopy, have been used to investigate the kinetics of reactions of NO₃ and SO₄^– with aromatics, as well as the formation of oxidation intermediates.

For the reactions of NO₃ with benzene (1), anisole (2), p-xylene (3), p-cresol (4), toluene (5), mesitylene (6) and 1,4-dimethoxybenzene (7), rate coefficients of k₁=(4.0 ± 0.6)× 10⁸ l mol^–1 s^–1, k₂=(1.0 ± 0.4)× 10⁹ l mol^–1 s^–1, k₃=(1.6 ± 0.1)× 10⁹ l mol^–1 s^–1, k₄=(8.4 ± 2.3)× 10⁸ l mol^–1 s^–1, k₅=(1.2 ± 0.3)× 10⁹ l mol^–1 s^–1, k₆=(1.3 ± 0.3)× 10⁹ l mol^–1 s^–1 and k₇=(1.0 ± 0.3)× 10⁹ l mol^–1 s^–1 were obtained. For the corresponding reactions of SO₄^– with benzene (8), toluene (9), p-xylene (10), p-cresol (11) and mesitylene (12), rate coefficients of k₈=(6.4 ± 2.5)× 10⁸ l mol^–1 s^–1, k₉=(1.3 ± 0.6)× 10⁹ l mol^–1 s^–1, k₁₀=(2.7 ± 0.9)× 10⁹ l mol^–1 s^–1, k₁₁=(2.8 ± 0.8)× 10⁹ l mol^–1 s^–1 and k₁₂=(1.3 ± 0.4)× 10⁹ l mol^–1 s^–1 were determined. Reactivity correlations for both radical species with the aromatic compounds are presented and potential applications of such correlations are discussed. In addition, the effect of ionic strength in the reactions of NO₃ with benzene and toluene has been investigated. Transient intermediates have been spectroscopically identified in the reactions of the sulfate radical anion in aerated as well as in oxygen-free solutions. The nature of these intermediates is discussed in view of the existing literature. Finally, possible impacts of free-radical reactions with aromatic compounds on the current understanding of chemical conversion processes within tropospheric multiphase systems are considered.