Experimental studies of the heterogeneous kinetics of the OH radical with terbuthylazine adsorbed on self-synthesized silica particles
2 Institute of Ecological Chemistry, University of Lüneburg
P 2.4 in Research in its Prime: First Results of Ongoing Research
Atmospheric lifetimes of pesticides are limited by reactions with OH, which have been studied for a long time. Most studies were focused on the gas phase and only very few experiments on semivolatile pesticides to investigate their OH reactions with aerosol-borne compounds. In the present study we reinvestigated the reaction of gas-phase OH with aerosol-borne terbuthylazine (TBA), studied by Palm et al. (1997) [1] on Aerosil 200 (commercial SiO2 particles with 10 nm diameter), on much larger (diameter around 160 nm) self-synthesized SiO2 particles (diameter around 160 nm) in an aerosol smog chamber. The particles were solvent-coated at submonolayer-thickness with the pesticide and then dispersed in water and sprayed into the chamber, where they were exposed to OH radicals, generated by photochemical degradation of methyl nitrite. Hourly samples of the particles were taken on filters, extracted by CH2Cl2 and analyzed by GC-FID. Surprisingly, the dependence of the decay rate on OH-levels was not linear but approached a limiting value at high OH. We explain this behaviour by a Langmuir-Hinshelwood mechanism, where a reversible adsorption of OH is followed by the surface-reaction of OH with the adsorbed pesticide. This has serious consequences for the atmospheric behaviour of pesticides: In contrast to our inert SiO2 particles the tropospheric aerosol is highly reactive against OH. This lowers the steady-state level of OH on the aerosol particles and leads to an increased lifetime of the pesticide and a long-range transport. Our study is the first one to show this effect for OH radicals. The effect needs to be considered in the Stockholm convention on persistent organic pollutants (POP-convention).
1. Palm W-U, Elend M, Krüger H-U, Zetzsch C (1997); Environ Sci Technol. 31: 3389–3396