Mechanism and kinetic study of the reaction of benzoic acid with OH, NO3 and SO4− radicals in the atmosphere

Abstract

Benzoic acid (BA) is one of the most common organic acids in the Earth’s atmosphere and an important component of atmospheric aerosol particles. The reaction mechanism of OH, NO₃ and SO₄⁻ radicals with BA in atmospheric water droplets and that of OH radicals with BA in the atmosphere were studied in this paper. The results show that in atmospheric water droplets the potential barriers of the elementary addition reactions of BA with OH radicals are lower than those of elementary abstraction reactions, and the potential barriers of OH-initiated reactions are less than for NO₃ and SO₄⁻ reactions. The initiation reactions of OH radicals and BA are exothermic, but the abstraction reactions of NO₃ and SO₄⁻ are endothermic processes. Among the products, 6-hydroxybenzoic acid (6-HBA) and 4,6-dihydroxybenzoic acid (4,6-DHBA) are the most stable, while 3-hydroxybenzoic acid (3-HBA) and 3,5-dihydroxybenzoic acid (3,5-DHBA) are much less stable and, thus, much less abundant compared to 6-HBA and 4,6-DHBA. The initiation and subsequent degradation of BA with OH radicals in the gas phase were calculated. The products of addition and abstraction reactions of BA with OH radicals can be further oxidized and degraded by O₂/NO. According to the results of kinetic calculations, the total reaction rate constant of OH radicals with BA at 298.15 K in atmospheric water droplets is 2.35 × 10⁻¹¹ cm³ per molecule per s. The relationship between reaction rate constants, temperature and altitude were also investigated and discussed in the present study.