Article pubs.acs.org/est
Heterogeneous Oxidation of Terbuthylazine by “Dark” OH Radicals under Simulated Atmospheric Conditions in a Flow Tube Maryline Pflieger,*,§ Anne Monod, and Henri Wortham Aix Marseille Université, CNRS, LCE, FRE 3416, Case Courrier 29, 3 Place Victor Hugo, 13331 Marseille Cedex 03, France S Supporting Information *
ABSTRACT: In order to investigate the heterogeneous oxidation kinetics of the herbicide terbuthylazine (TERB), a stable and reproducible generation system of “dark” hydroxyl radical in the gas phase was developed and optimized using a PTR-MS. TERB was adsorbed on silica particles, which were coated on the walls of a flow tube. The hydroxyl radical was produced in the dark through the ozonolysis of 2,3-dimethyl-2-butene (DMB). The radical concentration was determined applying two different methods of calculation based on the monitoring of (i) a gaseous compound used as a tracer, m-xylene; (ii) one of the OH radical precursors, DMB. The obtained gaseous OH radical concentration in the reactor was (9.0 ± 4.0) × 107 radical cm−3. Exposing TERB to the oxidant for 1−14 h, a heterogeneous kinetic constant of kOH = (1.5 ± 0.8) × 10−13 cm3 molecule−1 s−1 was found at 26 °C and RH < 1%. As a result, the heterogeneous oxidation of TERB by OH radicals seems to be much slower (by a factor of 63) when the organic compound is present in the particulate phase than when it reacts in homogeneous gas phase.
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INTRODUCTION
Several parameters can influence heterogeneous kinetics such as the concentration on the solid support, the number of organic compounds simultaneously adsorbed as well as the physicochemical characteristics of the solid support.10−16 Oxidation kinetics are commonly studied using a reference compound, whose kinetic rate constant (toward OH radicals) is well-known. The determination of the radical concentration is then not required. In the case of heterogeneous reactivity, both the reference compound and the compound under study have to be adsorbed on the same substrate. This leads to one major limitation: since the nature of the solid surface may influence the reactivity, the kinetic constant of the reference molecule has to be known for the solid support used. Moreover, the reference molecule and the compound under study should have a degradation rate in the same order of magnitude. It seems difficult to find such a reference bringing all together these characteristics. As a result, relative rate method cannot be applied and absolute kinetic experiments have to be carried out. This implies that the OH radical concentrations have to be quantified. It is then required to build up stringent techniques of generation and measurement of hydroxyl radical. In this study, the heterogeneous oxidation of terbuthylazine (TERB) was investigated regarding OH radical. TERB is a widely used herbicide allowed in Europe mainly on maize culture.17 According to its low vapor pressure (0.15 mPa, Supporting Information (SI) Table S1) as well as to field
Semi volatile organic compounds (SVOC) can be directly emitted in the atmosphere (polycyclic aromatic hydrocarbons (PAH), pesticides, etc.) or produced in situ according to various reaction mechanisms such as oxidation processes of VOC (volatile organic compounds). Because of their volatility properties (e.g., vapor pressure), SVOC split into the different atmospheric phases.1,2 As one fraction remains in the gas phase, another one reaches the particulate phase by nucleation or adsorption on preexisting particles. These processes lead to the formation of secondary organic aerosols (SOA). The reactivity of SVOC has been mainly studied in the gaseous state. However, to understand their atmospheric behavior and environmental fate, their particulate fraction has also to be studied. Thus the investigation of the heterogeneous reactivity of SVOC is currently an emergent area of research in atmospheric chemistry. As hydroxyl radical is the main oxidant of organic compounds in the atmosphere,3,4 it is necessary to develop experimental setups making it possible to investigate heterogeneous oxidation kinetic of SVOC with this radical. The laboratory techniques of OH radical’s generation are usually based on the photolysis of a precursor compound.5,6 Because OH radical has a very short lifetime (
