1. Introduction
Pesticides are widely used in agriculture in order to improve the efficiency of food production. About 5 million tons of pesticides are used world-wide annually (OECD, 2003).
A disadvantage of the large pesticide use is its potential impact on the environment by toxic effects. Therefore, the interest in the distribution in the environment and the chemical reactions increases constantly. The pesticides are semivolatile compounds. They are aerosol-borne in the troposphere to a major portion. The variety of phase-transfer processes (such as absorption and adsorption) has a big influence on chemical and biological transformation and on the dispersion of the pesticides. Each process must be quantified individually in order to understand the relevant processes for the degradation of a certain pesticide. The most important degradation path of the pesticides is the chemical reaction with OH-radicals in the atmosphere. Therefore, the reaction with OH-radicals is an important process with respect to the regulations of the authorities on persistence in the environment.
There are various substances which are highly toxic and harmful for the human health and for the environment, and 12 most harmful substances were included in the Stockholm convention on persistent organic pollutants (POPs): Aldrin, Chlordane, dichlorodiphenyltrichloroethane (DDT), Dieldrin, Endrin, Heptachlor, Mirex, Toxaphene, polychlorobiphenyls (PCBs), hexachlorobenzene, dioxins and furans (EU, 2004). Aldrin was chosen for the experiments of the present work. The degradation in the atmosphere and its products have been investigated actively in the 70s of the past century. The aim of this work is to investigate Aldrin with a novel experimental and mathematical approach.
It turned out in the present study that the smog chamber method for examining the kinetics of the experiments needs to be interpreted in a new way. Agglomerated particles of fused silica (Aerosil 380, DEGUSSA) served as model particles, were coated with the test substance, exposed to OH radicals in the chamber and analyzed for the test substance. It now appears that migration of the pesticides within the particles has to be taken into account after an improved understanding of the transport processes in the porous particles.
This work attempts to explain the observations. The diffusion equation, coupled with chemical reaction, was used to simulate the degradation behavior of the substance by the reaction with OH-radicals.
This work delivers:
structure parameters of the agglomerates
degradation experiments of Aldrin
degradation products
develops a diffusion model with chemical reaction
fits the experimental results and obtains the rate constant The chapters of this work are given as follows:
In chapter 2, previous work on photochemical experiments on Aldrin will be described, and the role of the aerosols in the atmosphere will be explained.
In chapter 3, the experimental facility is introduced. The smog chamber was constructed to fit into a refrigerated laboratory. The powder coating, production of the agglomerates, sampling and analysis technology are explained in this chapter. Measurements of temperature gradients in the chamber will be explained in detail.
In chapter 4 the production and measurement of OH radicals will be explained. A gas chromatograph with a preconcentration device for gas samples and an ozone analyzer were connected with the chamber and were used for analysis of the gas phase. Four reference hydrocarbons were used to calculate the OH exposure from their degradation rate.
In chapter 5, the analysis of Aldrin and its reaction products will be explained. The aerosol density was measured and interpolated for the aerosol sample used for the concentration analysis.
In chapter 6 the theoretical basis of the diffusion model is introduced. The diffusion processes are briefly explained. The influence of the particle structure is also given. The individual parts of the model are explained in connection with the physical processes.
In chapter 7, some relationships will be presented for the calculation of the OH rate constant, the lifetime and the long-range transport of substances occurring in the atmosphere.
The results of this research are presented in the following chapters and will be divided into two sections – experimental and theoretical results.
In chapter 8, the temperature gradients in the chamber, the concentration of the aerosol and the concentration of the substance on the aerosol will be presented. On the basis of
Introduction 3
experiments, a coating of the powder by the substance with a defined surface coverage is given.
The agglomerates were characterized by diameter and inner structure. The diameter was measured by an electrostatic particle sizer. The structure was examined by ion etching and scanning electron microscopy (SEM). The experiments and the strategy of product identification are introduced.
In chapter 9 the theoretical results, obtained from the fitting procedure, are presented. The relations between the fitted values will be explained. The lifetime was calculated by fitting an appropriate function to the experimental points. The OH rate constant was then calculated from known OH concentrations.
The reactivity of Aldrin with ozone was investigated in chapter 10. The time profiles of Aldrin and Dieldrin were compared at different concentrations of ozone.
In chapter 11, the reaction products are identified. For this purpose, separate experiments with Aldrin-coated microballoons were made, and the products were analyzed by GC-MS.
In chapter 12, the experimental and theoretical results will be discussed.
In chapter 13, some conclusions will be given.