Influence of clouds on the photochemical degradation in aqueous phase - I: Suitability of metamitron as actinometer

In the past detailed investigations of photochemical degradation reactions of pesticides
in aqueous solution were performed[1]. Necessary parameters to determine the
degradation rate constant, kp, are well known and models are available and
elaborated[2). Hence, with respect to the importance of this degradation pathway in the
environment, determination of the quantum yield in aqueous solution as a main
parameter is regulated by law in the authorization process of pesticides. However, on
the one hand, due to the comparability of the photochemical degradation with respect to
latitude and time and, on the other hand, due to the uncertainty with respect to cloudy
scenarios[31, lifetimes are calculated usually for cloud free and clear days almost always
in summer using quantum yields and uv-spectra obtained in the laboratory. [2]
Furthermore, experiments to investigate the decrease in kp of organic compounds such
as pesticides in aqueous solution by clouds (i.e. diurnal variations) and the comparison
with suitable models were seldom performed.
Within a project to investigate the influence of clouds on the photochemical degradation
in aqueous phase at highly different sites in Brazil and Germany with respect to
meteorology (and therefore environmental conditions) a suitable model compound to be
used as an actinometer was necessary. In addition, for reasons of practical relevance
and to be able to transfer results, the actinometer had to be a pesticide.
A method was developed using the herbicide metamitron[4] as a superb actinometer due
to fundamental chemical, photochemical and analytical properties in the aqueous phase.
New results with respect to the photochemical reaction and properties obtained in the
laboratory and the application of metamitron as actinometer are presented on the poster