OH radical reactivity of airborne terbuthylazine adsorbed on inert aerosol

The reaction of terbuthylazine (TBA) with OH radicals was investigated in an aerosol smog chamber (2400 L). TBA was adsorbed well below a monalayer on silicon dioxide as inert carrier. OH radicals were produced from hydrogen peroxide, ozone, and nitrogen dioxide. The OH concentrations span a region of 10⁵ ≤ c(OH)/cm^-3 ≤ 10⁷ and were monitored by the consumption of butane, 2,2-dimethylbutene, 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) x 10^-11 cm³ s^-1. Beside the OH reaction, a loss path not initiated by OH radicals was found (k = (1.9 ± 0.5) x 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 x 10⁵ cm^-3) is obtained. Using the UV spectrum of TBA in hexene or in aqueous solution and known sunlight intensities e 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) ≤ 5 x 10^-19 cm³ 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.