decomposition of ozone
When ozone is produced it will decay rapidly, because ozone is an instable compound with a relatively short half-life. The half-life of ozone in water is a lot shorter than in air (see table 1). Ozone decays in water under drinking water conditions (pH: 6-8,5), partly in reactive OH-radicals. Therefor, the assessment of an ozone process always involves the reactions of two species: ozone and OH-radicals. When these OH-radicals are in the dominant particles in the solution, it is called an advanced oxidation process (AOP). The decay of ozone in OH-radicals in natural waters is characterized by a fast initial decrease of ozone, followed by a second phase in which ozone decreases by first order kinetics . Dependent on the quality of the water, the half-life of ozone is in the range of seconds to hours. Factors influencing the decomposition of ozone in water are temperature, pH, environment and concentrations of dissolved matter and UV light. Here, the main influence factors for ozone decomposition will be discussed.
Temperature has an important influence on the half-life of ozone. Table 1 shows the half-life of ozone in air and water. In water the half-life of ozone is much shorter than in air, in other words ozone decomposes faster in water . The solubility of ozone decreases at higher temperatures and is less stable. On the other hand, the reaction speed increases with a factor 2 or 3 per 10 °C [5,6]. Principally, ozone dissolved in water cannot be applied when temperatures are above 40 °C, because at this temperature the half-life of ozone is very short.
As mentioned above, ozone decomposes partly in OH-radicals. When the pH value increases, the formation of OH-radicals increases. In a solution with a high pH value, there are more hydroxide ions present, see formulas below. These hydroxide ions act as an initiator for the decay of ozone:
1 O3 + OH- ? HO2- + O2 2 O3 + HO2- ? •OH + O2 •- + O2
The radicals that are produced during reaction 2 can introduce other reactions with ozone, causing more OH-radicals to be formed.
In addition the pH influences acid/base equilibriums of some compounds and also the reaction speed of ozone. This applies also to the reaction with scavenger CO32-, which is also pH dependant (Pka HCO32-/CO32- = 10,3).
Figure 1 shows that the decay of ozone in a basic environment is much faster than in an acid environment.
Figure 1: effect of the pH on the decay of ozone (T = 15 °C)
3. Dissolved solids concentration
Dissolved ozone can react with a variety of matter, such as organic compounds, viruses, bacteria, etc. As a result, ozone decomposes to other matter; see figure 2. This figure illustrates that the half-life of ozone in distilled water is much shorter, compared to tap-water.
Figure 2: Ozone decomposition in different types of water at 20 °C. 1 = double-distilled water; 2 = distilled water; 3 = tap water; 4 = groundwater of low hardness; 5 = filtered water from Lake Zurich (Switzerland); 6 = filtered water from the Bodensee (Switzerland)
Ozone decomposes in water in OH-radicals. Dependent on the nature of the dissolved matter, these can accelerate (chain-reaction) or slow down the decay of ozone. Substances that accelerate this reaction are called promoters. Inhibitors are substances that slow down the reaction.
When water is ozonized, one often uses the term 'scavenging capacity'. Scavengers are entities that react with OH-radicals and slow down the chain-reaction. The scavenging capacity can be defined as follows :
kOH-DOC[DOC] + kOH-HCO3-[HCO3-] + kOH-CO32-[CO32-]
4. Carbonate and bicarbonate
Scavengers slow down the chain-reaction. This is because after the reaction of scavengers with OH-radicals, the reaction products do not react with ozone any further. Carbonate is a scavenger with a strong effect. The addition of carbonate (CO32-) can increase the half-life of ozone [5,6]. The effect on the reaction speed is highest at low concentrations. Above 2 mmol-1 for ozonisation and 3 mmol l-1 for advanced oxidation process (AOP), the decrease in the reaction speed is negligible .
When a solution mainly undergoes indirect reactions (with OH-radicals), for instance in a solution with a high pH value or an AOP-process, the presence of scavengers is undesired. The scavengers react very fast with OH-radicals and lower the oxidation capacity. For this kind of processes a low scavenging capacity is required.
Carbonate (CO32-) ions are a much stronger scavengers than bicarbonate (HCO32-) ions (reaction speed CO32-: k = 4,2 * 108 M-1s-1 and reaction speed HCO3-: k = 1.5 * 107 M-1s-1). That is why in an ozone process under drinking water conditions, the bicarbonate concentration is less important . Figure 3 illustrates the relation of the carbonate ratio, bicarbonate ratio and pH.
Natural Organic Material
Natural organic material (NOM) exists in every kind of natural water and is often measured as dissolved organic carbon (DOC). NOM reduces the quality of the water with regard to color and odor. Ozone can be used in water treatment, for the reduction of the concentration of NOM. The concentration of NOM in natural waters can vary from 0,2 – 10 mg l-1 . The influence of NOM on ozone is twofold. Dependent on the type of NOM, it can be oxidized directly by NOM. This is the case for compounds which easily react with ozone, such as double bonds, activated aromatic compounds, deprotonated amines and sulphide . On the other hand, OH-radicals can react with NOM (indirect reaction) and act as a promoter or as a scavenger. In natural waters, it is difficult to determine the stability of ozone as a result of the indefinite effect of NOM. That means it is not possible to estimate the fraction that accelerates or slows down the reaction
I was born in Amsterdam on December, 3, 1933, the son of Anna Gurk and Jozef Crutzen. I have one sister who still lives in Amsterdam with her family. My mother's parents moved to the industrial Ruhr region in Germany from East Prussia towards the end of the last century. They were of mixed German and Polish origin. In 1929 at the age of 17, my mother, moved to Amsterdam to work as a housekeeper. There she met my father. He came from Vaals, a little town in the southeastern corner of the Netherlands, Bordering Belgium and Germany and very close to the historical German city of Aachen. He died in 1977. He had relatives in the Netherlands, Germany and Belgium. Thus, from both parents I inherited a cosmopolitan view of the world. My mother, now 84 years old, still lives in Amsterdam, mentally very alert, but since a few months ago, wheelchair-bound. Despite having worked in several countries outside The Netherlands since 1958, I have remained a Dutch citizen.
In May, 1940, The Netherlands were overrun by the German army. In September of the same year I entered elementary school, "de grote school" (the big school), as it was popularly called. My six years of elementary school largely overlapped with the 2nd World War. Our school class had to move between different premises in Amsterdam after the German army had confiscated our original school building. The last months of the war, between the fall of 1944 and Liberation Day on May, 5, 1945, were particularly horrible. During the cold "hongerwinter" (winter of famine) of 1944-1945, there was a severe lack of food and heating fuels. Also water for drinking, cooking and washing was available only in limited quantities for a few hours per day, causing poor hygienic conditions. Many died of hunger and disease, including several of my schoolmates. Some relief came at the beginning of 1945 when the Swedish Red Cross dropped food supplies on parachutes from airplanes. To welcome them we waved our red, white and blue Dutch flags in the streets. I had of course not the slightest idea how important Sweden would become later in my life. We only had a few hours of school each week, but because of special help from one of the teachers, I was allowed together with two other schoolmates to continue to the next and final class of elementary school; unfortunately, all the others lost a year. More or less normal school education only became possible again with the start of the new school year in the fall of 1945.
In 1946, after a successful entrance exam, I entered the "Hogere Burgerschool" (HBS), "Higher Citizen School", a 5 year long middle school, which prepared for University entrance. I finished this school in June, 1951, with natural sciences as my focal subjects. However, we all also had to become proficient in 3 foreign languages: French, English and German. I got considerable help in learning languages from my parents: German from my mother, French from my father. During those years, chemistry definitely was not one of my favourite subjects. They were mathematics and physics, but I also did very well in the three foreign languages. During my school years I spent considerable time with a variety of sports: football, bicycling, and my greatest passion, long distance skating on the Dutch canals and lakes. I also played chess, which in the Netherlands is ranked as a "denksport" (thought sport). I read widely about travels in distant lands, about astronomy, as well as about bridges and tunnels. Unfortunately, because of a heavy fever, my grades in the final exam of the HBS were not good enough to qualify for a university study stipend, which was very hard to obtain at that time, only 6 years after the end of the 2nd world war and a few years after the end of colonial war in Indonesia, which had been a large drain on Dutch resources. As I did not want to be a further financial burden on my parents for another 4 years or more (my father, a waiter, was often unemployed; my mother worked in the kitchen of a hospital), I chose to attend the Middelbare Technische School (MTS), middle technical school, now called the higher technical school (HTS), to train as a civil engineer. Although the MTS took 3 years, the second year was a practical year during which I earned a modest salary, enough to live on for about 2 years. From the summer of 1954 until February, 1958, with a 21-month interruption for compulsory military service in The Netherlands, I worked at the Bridge Construction Bureau of the City of Amsterdam. In the meanwhile, on a vacation trip in Switzerland, I met a sweet girl, Terttu Soininen, a student of Finnish history and literature at the University of Helsinki. A few years later I was able to entice her to marry me. What a great choice I made! She has been the center of a happy family; without her support, I would never have been able to devote so much of my time to studies and science. After our marriage in February, 1958, we settled in G?vle, a little town about 200 km north of Stockholm, where I had found a job in a building construction bureau. In December of that same year our daughter Ilona was born. In March, 1964, she got a little sister, Sylvia. Ilona is a registered nurse. Her son Jamie Paul is 12 years old. Sylvia is a marketing assistant in M?nchen, Germany. All were present in Stockholm, Uppsala and G?vle during the Nobel week. We had a happy and unforgettable time.
All this time I had longed for an academic career. One day, at the beginning of 1958, I saw an advertisement in a Swedish newspaper from the Department of Meteorology of Stockholm H?gskola (from 1961, Stockholm University) announcing an opening for a computer programmer. Although I had not the slightest experience in this subject, I applied for the job and had the great luck to be chosen from among many candidates. On July 1, 1959, we moved to Stockholm and I started with my second profession. At that time the Meteorology Institute of Stockholm University (MISU) and the associated International Meteorological Institute (IMI) were at the forefront of meteorological research and many top researchers worked in Stockholm for extended periods. Only about a year earlier the founder of the institutes, Prof. Gustav Rossby, one of the greatest meteorologists ever, had died suddenly and was succeeded by Dr. Bert Bolin, another famous meteorologist, now "retired" as director of the Intergovernmental Panel on Climate Change (IPCC). At that time Stockholm University housed the fastest computers in the world (BESK and its successor FACIT).
With the exception of participation in a field campaign in northern Sweden, led by Dr. Georg Witt to measure the properties of noctilucent clouds, which appear during summer at about 85 km altitude in the coldest parts of atmosphere, and some programming work related to this, I was until about 1966 mainly involved in various meteorological projects, especially helping to build and run some of the first numerical (barotropic) weather prediction models. I also programmed a model of a tropical cyclone for a good friend, Hilding Sundquist, now a professor at MISU. At that time programming was a special art. Advanced general computer languages, such as Algol or Fortran, had not been developed, so that all programmes had to be written in specific machine code. One also had to make sure that all operations yielded numbers in the range -1 < x < 1, which meant that one had to scale all equations to stay within these limits; otherwise the computations would yield wrong results.
The great advantage of being at a university department was that I got the opportunity to follow some of the lecture courses that were offered at the university. By 1963 I could thus fulfill the requirement for the filosofie kandidat (corresponding to a Master of Science) degree, combining the subjects mathematics, mathematical statistics, and meteorology. Unfortunately, I could include neither physics nor chemistry in my formal education, because this would have required my participation in time consuming laboratory excercises. In this way I became a pure theoretician. I have, however, always felt close to experimental work, which I have strongly supported during my later years as director of research at the National Center of Atmospheric Research (NCAR) in Boulder, Colorado (1977-1980) and at the Max-Planck-Institute for Chemistry in Mainz, Germany (since 1980).
Being employed at the meteorological research institute, it was quite natural to take a meteorological topic for my filosofie licentiat thesis (comparable to a Ph.D. thesis). Building on my earlier experience further development of a numerical model of a tropical cyclone had been proposed to me. However, around 1965 I was given the task of helping a scientist from the U.S. to develop a numerical model of the oxygen allotrope distribution in the stratosphere, mesosphere and lower thermosphere. This project got me highly interested in the photochemistry of atmospheric ozone and I started an intensive study of the scientific literature. This gave me an understanding of the status of scientific knowledge about stratospheric chemistry by the latter half of the 1960's, thus setting the "initial conditions" for my scientific career. Instead of the initially proposed research project, I preferred research on stratospheric chemistry, which was generously accepted. At that time the main topics of research at the Meteorological Institute at the University of Stockholm were dynamics, cloud physics, the carbon cycle, studies of the chemical composition of rainwater, and especially the "acid rain" problem which was largely "discovered" at MISU through the work of Svante Od?n and Erik Eriksson. Several researchers at MISU, among them Prof. Bolin and my good friend and fellow student Henning Rodhe, now Professor in Chemical Meteorology at MISU, got heavily involved in the issue which drew considerable political interest at the first United Nation Conference on the Environment in Stockholm in 1972. However, I wanted to do pure science related to natural processes and therefore I picked stratospheric ozone as my subject, without the slightest anticipation of what lay ahead. In this choice of research topic I was left totally free. I can not overstate how I value the generosity and confidence which were conveyed to me by my supervisors Prof. Georg Witt, an expert on the aeronomy of the upper atmosphere, and the head of MISU Prof. Bert Bolin. They were always extremely helpful and showed great interest in the progress of my research.
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