Environmental implications of oxalic and malonic acids with tropospheric oxidants

Abstract

Dicarboxylic acids (DCAs) are major players in the formation of secondary organic aerosols (SOAs) and climate change. DCAs have potential impact on human health and environmental issues ranging from local scale to global scale participate mainly in the cloud condensation. In this context, oxalic acid (OA) and malonic acid (MA) are the most dominant DCAs in the atmosphere. A full atmospheric degradation mechanism of OA and MA with the most reactive tropospheric oxidants, namely, OH, Cl and NO₃ radicals, were studied using M06-2X, ωB97XD/cc-pVTZ and 6-311++G(2df,2p) level of theories. To evaluate the atmospheric influence, this study enables us to deep investigation of fate of OA and MA with respect to the mentioned radicals and their subsequent secondary reactions. The latter result in the formation of carbon dioxide (CO₂), formic acid (HCOOH), which contributes to the formation of SOA and climate change. The reaction mechanism in this study was initiated through H-abstraction reaction, followed by dehydrogenation and decarboxylation reaction of both DCAs. The rate coefficients of OA, MA with OH, Cl and NO₃ radicals are determined theoretically using variational transition state theory (VTST) with Eckart tunnelling method in the temperature range of 278–1000 K. At 298 K, the rate coefficient of OA with OH, Cl and NO₃ are 2.48 × 10⁻¹⁵, 2.37 × 10⁻²⁰, 6.16 × 10⁻²³ in cm³ molecule⁻¹ s⁻¹, whereas MA with OH, Cl and NO₃ are 9.76 × 10⁻¹⁴, 1.01 × 10⁻¹² and 5.89 × 10⁻¹⁸ in cm³ molecule⁻¹ s⁻¹, respectively. Our present results shed light on the atmospheric implications of two DCAs and provide the significant insight for the atmospheric pathways.