OH Radical Reactivity of Airborne Terbuthylazine Adsorbed on Inert Aerosol

Abstract

The reaction of terbuthylazine (TBA) with OH radicals was investigated in an aerosol smog chamber (2400 L). TBA was adsorbed well below a monolayer on silicon dioxide as inert carrier. OH radicals were produced from hydrogen peroxide, ozone, and nitrogen dioxide. The OH concentrations span a region of 10 5 e c OH /cm -3 e 10 7 and were monitored by the consumption of butane, 2,2dimethylbutane, pentafluorobenzene, 2,2,4,4-tetramethylbutane, and toluene as reference compounds. The OH rate constant of TBA obtained from nine smog chamber runs at T ) 300 K on the aerosol is k OH (TBA) ) (1.1 ( 0.2) × 10 -11 cm 3 s -1 . Beside the OH reaction, a loss path not initiated by OH radicals was found (k ) (1.9 ( 0.5) × 10 -5 s -1 ), which can be explained only in part by loss processes observed in the dark. Using the OH rate constant determined on the aerosol as an estimate for the corresponding OH reaction in the environment, a half-life of about 1 day (with OH concentrations of 5-10 × 10 5 cm -3 ) is obtained. Using the UV spectrum of TBA in hexane or in aqueous solution and known sunlight intensities a negligible loss by direct irradiation can be estimated. Furthermore, the ozone reactivity at T ) 300 K was measured to be negligible, too (k O3 (TBA) e 5 × 10 -19 cm 3 s -1 ). Hence, abiotic degradation of TBA in the environment (and assumed for all other chlorotriazines) is mainly attributed to OH radicals in the atmosphere. The main product of the OH radical reaction found on the aerosol is the corresponding acetyl-TBA, in addition to the des-alkylation product desethyl-TBA. τ )1

Heterogeneous OH Oxidation, Shielding Effects, and Implications for the Atmospheric Fate of Terbuthylazine and Other Pesticides

“…We have used the data of TBA decay rates with OH radicals generated by photolysis of O 3 or H 2 O 2 . 22 In the experiments of Pflieger et al,…”

“…20,21 Oxidation of TBA yields acetyl and desethyl products which have the potential to be toxic as well. 22 It is therefore necessary to understand the lifetime and chemical transformation of TBA in the atmosphere.…”

“…23 The heterogeneous reactivity of TBA with OH radicals has so far been investigated in three different peer-reviewed studies, two studies by Palm et al, 22,24 and a study by Pflieger et al 25 Palm et al 22 determined an effective second-order rate coefficient between gas-phase OH radicals and TBA adsorbed on silica particles (Aerosil 200) as 1. . This high value was confirmed by a second study by Palm et al 24 However, Pflieger et al 25 determined a rate coefficient which was 2 orders of magnitude lower.…”

“…The study of Palm et al 22 was performed in an aerosol smog chamber, and it was reported that TBA was adsorbed on silica particles (Aerosil 200) at a concentration of less than one monolayer, as shown in Figure 1. In reality, these silica particles formed agglomerates as shown in the electron microscope image of such particles in Figure S1 of the Supporting Information, SI.…”

“…However, note that a background was subtracted for the data of Palm et al 22 which corresponds to a rate constant for the loss processes not initialized by OH radicals, such as evaporation of TBA. 22 For the Pflieger data, the silica particles and pesticide coating on the flow tube walls were treated as a quasi-homogeneously mixed thin film, with an identical concentration of TBA in each layer of the bulk. Note that in reality the bulk is an agglomerate of silica particles deposited in a flow tube (multilayers of silica particles).…”