Heterogeneous ozonolysis of pirimicarb and isopropalin: mechanism of ozone-induced N-dealkylation and carbonylation reactions

Abstract

Environmental contextPesticides emitted to the atmosphere can undergo extensive chemical transformations through reaction with atmospheric oxidants. Understanding the atmospheric lifetime and degradation mechanism of typical pesticides is very important to health and environmental risk assessments. We investigate the degradation products, lifetimes and reaction mechanisms of two representative pesticides oxidised by ozone in order to understand the environmental behaviours of these pesticides and their analogues. AbstractN,N-Dialkyl-substituted pyrimidine and N,N-dialkylaniline are basic structures for many pesticides. In this study, the heterogeneous reactions of O3 with pirimicarb and isopropalin adsorbed on silica particles are investigated. The N-dealkylation and carbonylation of the N,N-dialkyl group have been observed as the important reaction pathways for both pirimicarb and isopropalin. The measured effective rate constants for pirimicarb and isopropalin under room temperature (298 ± 2 K) are 1.45 × 10–18 (s.d. ±0.17) and 2.70 × 10–19 cm3 molecules–1 s–1 (±0.27). The corresponding half-life for the particulate pirimicarb and isopropalin are 5.6 and 30 days, assuming an average tropospheric ozone concentration of 40 ppbv. Detailed reaction mechanisms are proposed for the first time based on the density functional theory calculations. In addition, the transformation of –NO2 into –NHOH is observed in the ozonolysis of isopropalin, which has not been reported in previous studies.