WO2016112943A1 - Ozone generator, no-to-no2 converter, process for converting no to no2 and use - Google Patents

Abstract

The invention relates to an ozone generator (100) for production of ozone from ambient air (1), to an NO-to-NO₂ converter (200) comprising the ozone generator (100), to a process for converting NO to NO₂ and to use of the ozone generator (100) or the NO-to-NO₂ converter (200). This ozone generator (100) has: – an ozone generation unit (15) for generation of ozone from the ambient air; – an oxidation unit (30) for oxidizing nitrogen monoxide formed in the ozone generation unit (15) to higher nitrogen oxides; and – a post-filtration unit (40) for at least partly filtering out the higher nitrogen oxides generated in the oxidation unit (15).

Description

"Ozone Generator, NO to N0 ₂ Converter, NO to N0 ₂ Conversion Method and Use"

description

The invention relates to an ozone generator, a NO to N02 converter with the ozone generator, a method for converting NO to NO2 and a use of the ozone generator and the NO to N02 converter.

The quality and composition of the air in our atmosphere is of particular importance to humans, as low concentrations of pollutants can already have a significant impact on health and the environment. Among the most critical substances for the current air quality are nitrogen oxides NO _x (ie NO and / or NO 2). The measurement of NO _{x also} plays an important role in science and is therefore widespread. In addition, such measurements are used in the industry in emissions but also in manufacturing processes and have also found wide application in medicine.

For the measurement of NO2, especially the differential optical absorption spectroscopy (DOAS) is suitable. The DOAS is described, for example, in Platt et al .: "Simultaneous measurements of atmospheric CH20, 03 and NO2 by differential optical absorption", J. Geophys. Res., 1979, 84, 6329-6335. The DOAS allows a direct and non-contact measurement of gases, especially NO2, in the atmosphere and is a very established and successful measurement method for atmospheric trace substances, especially in science. DOAS uses the characteristic wavelength-dependent absorption of light by molecules to quantitatively measure their concentration. It is exploited that the absorption spectrum of a substance represents a clear fingerprint. Due to their different absorption structure, it is possible to separate different molecules and to determine the respective concentrations separately. In order to obtain a sufficient sensitivity for the substances to be measured absorption paths of some 100 m to about 20 km are needed. These long light paths were originally realized between an artificial light source and a receiver. Alternatively, scattered sunlight was measured, which goes a long way to ground measurement. For many applications, such as typical environmental monitoring stations, mobile measurement vehicles, measurements in industrial plants or in indoor air measurements pollutant concentrations in one place are in demand. In order to realize the required long light path in a compact measuring cell of a few 10 cm to a few meters, the Cavity Enhanced (CE) - DOAS method was developed. It uses an optical resonator consisting of highly reflective mirrors to achieve absorption distances of 1 to 5 km in a compact measuring cell through multiple reflection. This allows the desired accuracy for the detection of trace gases in the ppb and even in the ppt (1 molecule per 10 ¹² air molecules) range. The same basic principle as in the CE-DOAS method is also used in the so-called BB-CEAS (broadband cavity enhanced absorption spectroscopy) method. If in the context of this description the CE-DOAS method is mentioned, then alternatively or additionally always the BB-CEAS method is meant.

For the monitoring of pollutants, the additional measurement of nitrogen monoxide (NO) makes sense, since NO and NO2 are usually in a variable chemical equilibrium. However, a direct measurement of NO by means of the CE-DOAS is not yet possible, since the absorption of NO in the deep ultraviolet is λ <240 nm, for the currently no suitable light sources and no mirror with sufficient reflection are available.

Therefore, it is proposed in the context of this invention to convert NO by the addition of ozone (O3) in NO2, which in turn can be measured with a N02 measuring instrument, for example with a DOAS or CE-DOAS instrument. In order to realize this procedure, it is advantageous if the admixed ozone is as free as possible of nitrogen oxides (NO _x , where x is in particular the number 1 or 2 represents), which can affect the measurement accuracy of the DOAS instrument unfavorable, and is present in the highest possible concentration. In conventional ozone generators, which are operated with ambient air, however, in addition to ozone and nitrogen oxides, in particular NO and NO2, produced. This is especially true for ozone generators that operate on the basis of high voltage. Thus, conventional ozone generators, which are operated with ambient air, hardly suitable for a precise meter, with the just the proportion of NO and / or NO2 in the air to be measured.

In conventional ozone generators operated with ambient air, the ozone production rate or the ozone concentration is too low to cause, on the one hand, rapid and complete conversion of NO to NO 2 and, on the other hand, to minimize the dilution of the measurement air. Should e.g. the O3 mixing ratio in the measuring air is 1 ppm, then the 03 generator must produce air with more than 10ppm O3 to get by with an air admixture of less than 10%. If a lower ozone concentration were used for the NO measurement, the necessary higher ozone addition would dilute the measuring air more strongly and thus reduce the measuring accuracy. With the same admixture, a longer reaction time for the conversion or conversion of NO to NO 2 would be necessary until a maximum conversion occurs, which then turns out to be somewhat lower. A longer reaction time also has the disadvantages of a large reaction volume and a time delay of the measurement. Since commercial ozone generators do not achieve sufficiently high ozone concentrations when operating with ambient air and because they greatly distort the measurement result by the generated nitrogen oxides, they can not readily for NO measurement by conversion with the ozone to NO2 and subsequent measurement by means of DOAS or other methods are used.

Although conventional ozone generators can be operated with pure oxygen (O2) instead of ambient air, thus avoiding the generation of nitrogen oxides. However, this is primarily useful only for scientific purposes, since the use of the consumption of oxygen additional costs and a high expenses arise. In particular, compressed gas cylinders are needed, which require safety precautions and must be changed regularly.

It is therefore an object of the present invention to simplify the accurate measurement of NO 2 and NO and to improve their accuracy. For this purpose, the invention provides a suitable ozone generator which can be operated with ambient air and generates an ozone-containing air largely free of NOx. In addition, the invention provides a NO to N02 converter that allows for indirect measurement of nitrogen monoxide content using a N02 meter, such as a DOAS instrument. In addition, the invention provides a related method and a use thereof.

In particular, the invention solves the underlying task thus by the subject matters of the independent claims. Advantageous embodiments are the subject of the dependent claims.

A first independent aspect for achieving the object relates to an ozone generator for generating ozone from ambient air, comprising:

 an ozone generating unit for generating ozone from the ambient air;

 an oxidation unit for oxidizing nitrogen monoxide produced in the ozone generating unit to higher nitrogen oxides; and

a post-filtering unit for at least partially or almost completely filtering out the higher nitrogen oxides produced in the oxidation unit.

The ozone generator may further include a pump for sucking the ambient air. The ozone generating unit can then generate ozone from the intake ambient air.

The ozone generating unit can, for example, by means of photolysis, convert at least some of the oxygen contained in the ambient air into ozone. For this purpose, the ozone generating unit may have, for example, a mercury vapor lamp. The oxidation unit may comprise a reaction zone or a reaction volume. In the oxidation unit, the NO produced in the ozone generating unit, in particular by a reaction of excited oxygen with nitrous oxide, can successively react with the ozone to produce higher nitrogen oxides such as NO 2, NO 3 and / or N 2 O 5. In the post-filtering unit, these higher nitrogen oxides are filtered out, so that the ozone-containing air generated by the ozone generator is advantageously substantially free of nitrogen oxides, ie not contaminated with nitrogen oxides, although the ozone generator is operated only with ambient air. In this way, the contamination of NO (and other NOx) can thus be significantly reduced. Consequently, the ozone generator according to the invention, in comparison to conventional ozone generators operated with ambient air, is suitable for an indirect measurement of the NO content with the aid of an NO 2 measuring device.

In a preferred embodiment of the ozone generator according to the invention, the post-filtration unit comprises a post-filter or a post-filter cartridge or a reaction volume with silica gel. Silica gel filters out or absorbs the higher nitrogen oxides such as NO2, NO3 and N2O5. Although the silica gel also absorbs ozone, but achieves saturation after a few hours. After that, ozone can pass the silica gel without any losses. Furthermore, the postfiltering unit may preferably comprise an aerosol postfilter. This advantageously prevents particles from escaping from the silica gel.

In a further preferred embodiment, the ozone generator furthermore has a prefiltration unit for removing reactive compounds or substances from the ambient air, in particular from the intake ambient air.

The prefiltration unit advantageously ensures that the introduction of nitrogen oxides into the ozone generator is avoided. Thereby, the generation of undesirable substances such as NO2 in the ozone generator or the ozone generating unit of the ozone generator can be reduced by the Prefilter unit removes reactive substances or compounds from the air. In particular, the prefiltration unit at least partially filters out nitrogen dioxide from the ambient air and / or prevents the penetration of particles.

In a preferred embodiment of the ozone generator according to the invention, the prefilter unit comprises a filter with activated carbon. This can be used to remove numerous reactive substances. Alternatively or additionally, the prefilter unit comprises a filter with silica gel. The silica gel filter is used to filter NO2, NO3 and N2O5. At the same time, the silica gel filter also dries the air. Further alternatively or additionally, the prefilter unit comprises an aerosol filter, i. a membrane filter or a depth filter. The aerosol filter prevents the penetration of particles into the ozone generator or into the ozone generating unit of the ozone generator. The aerosol filter may e.g. Teflon or partially or completely formed of Teflon. Preferably, the pore size of the aerosol filter is less than or equal to 5 μιη to filter out even correspondingly small aerosols. For the purposes of the present application, a filter can generally comprise or be a filter cartridge and / or a reaction volume.

In a further preferred embodiment of the ozone generator according to the invention, the ozone generating unit comprises an airtight container with an air inlet to the inlet of air, in particular from the Vorfilterungseinheit, and an air outlet to the outlet of ozone-enriched air, wherein in the airtight container, a UV lamp for emitting of UV radiation for a photolysis of oxygen molecules is arranged such that the UV lamp is flowed around by the air entering the container, in particular in direct or direct contact.

The airtight container of the ozone generating unit is preferably an airtight tube. The length of the container or tube is preferably in the range of 5 to 30 mm and the diameter of the container or tube is preferably in the range of 15 to 25 mm. The UV lamp is preferably a luminous tube, which is arranged along a longitudinal axis of the container or tube, in particular collinear to the tube, so that the air entering or entering the container is guided around the UV lamp close and completely enclosing. The ultraviolet lamp can emit ultraviolet (UV) radiation, ie, short wavelength radiation, which is capable of causing photolysis of oxygen molecules analogously to ozone generation in the stratosphere. The UV lamp may have, for example, a wavelength range of about 180 to 242 nm. Preferably, the UV lamp is a mercury vapor lamp (Hg lamp). For a Hg lamp, the 184.9 nm emission line can be used to photolyze oxygen, ie the conversion of oxygen to ozone by shortwave UV light.

The air inlet and the vent outlet of the container are preferably offset from one another on opposite sides of the container and along a longitudinal direction of the container or a longitudinal direction of the UV lamp. Preferably, the air inlet at a first axial end of the container or the UV lamp and the air outlet at a second axial end of the container or the UV lamp is arranged. Further preferably, the UV lamp is disposed between the air inlet and the air outlet. This ensures that the ambient air passing through the container is irradiated as effectively as possible by the UV lamp.

The air inlet and / or the air outlet may comprise a controllable valve with which the air flow flowing into the ozone generating unit and / or the air flow flowing out of the ozone generating unit can be regulated. Through the air outlet, the ozone-enriched air can flow out of the ozone generating unit and enter or flow into the oxidation unit.

The arrangement of the UV lamp in the interior of the container, ie in direct or direct contact with the air flowing into the container, the ozone generator according to the invention or the ozone generating unit advantageously has a compared to conventional ozone generators, for example compared to Ozone generators with an arranged next to or outside of quartz glass tubes mercury vapor lamp, higher ozone production or ozone production rate.

Especially for applications in which a high ozone production rate or a high ozone yield is desired or required, the above-described ozone generating unit, regardless of the oxidation unit and / or the Nachfilterungseinheit, compared to conventional ozone generators or ozone generating units advantageous properties. Therefore, the ozone generating unit may also constitute an independent aspect of this description.

In a further preferred embodiment of the ozone generator according to the invention, an inner wall of the container comprises a material or an inner wall of the container is formed partially or completely of a material which has a reflectivity for the ultraviolet radiation emitted by the UV lamp and suitable for photolysis of at least 0, 1 has.

The UV radiation reflective material on the inner wall of the container (hereinafter referred to briefly as reflective material) may also have a reflectivity of at least 0.2, preferably at least 0.4, more preferably at least 0.5, and most preferably of at least 0.75.

Under the inner wall of the container is in the context of the present application, an inner surface, i. a surface or surface facing the interior of the container. In comparison with the outer wall or outer surface of the container, the inner wall or inner surface of the container faces the UV lamp arranged in the container.

For example, the inner wall of the container may be partially or completely coated with the reflective material. But it is also possible that the entire container from the reflective material, such as aluminum, is formed. A suitable for the photolysis of oxygen molecules UV radiation is a radiation having a wavelength which is suitable to split oxygen molecules and thus leads to the formation of ozone upon irradiation of oxygen-enriched air. In principle, light with a wavelength of less than 242 nm is suitable for the photolysis of oxygen molecules. For example, the light of a mercury vapor lamp having a wavelength of 184.9 nm is suitable for photolysis.

Due to the additionally occurring rejections of the UV light on the inner surface of the container can be achieved with the ozone generator according to the invention advantageously a much higher efficiency of ozone generation compared to conventional ozone generators, for example ozone generators with a mercury vapor lamp, which is arranged next to or outside of quartz glass tubes , In the usual quartz glass tubes, only a very small part of the emitted mercury vapor lamp light is used to illuminate the oxygen-containing air in the quartz glass tube. This results in a much lower efficiency. By contrast, in the ozone generator according to the invention, even small UV lamps, e.g. Hg lamps with a length of 9 cm, to a saturation ozone concentration of over 350ppm with an air flow of less than 0.1 l / min.

In a further preferred embodiment of the ozone generator according to the invention, the UV lamp is a mercury vapor lamp. Thus, the 184.9 nm emission line of the mercury vapor lamp can be used for the photolysis of oxygen molecules and thus for ozone generation. In addition, the airtight container or the airtight tube on the inner wall of the container aluminum. For example, the inner wall of the container is completely or partially coated with aluminum. Preferably, the entire container is formed of aluminum. The reflectance of aluminum is particularly high for UV radiation. For example, the reflectivity of pure aluminum for the 184.9 nm emission line is about 0.8. Due to the additional reflections generated in the ozone generating unit, the efficiency of the ozone generator can be increased. In a further preferred embodiment, the ozone generator according to the invention further comprises a flow controller for setting or controlling the air flow through the ozone generating unit. The flow regulator is preferably a controllable valve with a flow meter. Preferably, the flow regulator is arranged between the prefiltration unit and the ozone generating unit of the ozone generator. With the air flow also the residence time of the air in the ozone generator or in the ozone generating unit is adjusted.

Preferably, the flow controller is configured such that the residence time of the air in the ozone generating unit is less than a photolysis equilibrium time. For example, the flow regulator is configured or the air flow adjusted so that the residence time of the air in the ozone generating unit is less than 99% of the photolysis equilibrium time.

The photolysis equilibration time is understood to be the time from which the ozone production rate and the ozone destruction rate for the air contained in the ozone generating unit are substantially equal. From the photolysis equilibration time, the ozone concentration no longer increases, i. the system is in photolytic equilibrium. In other words, the photolysis equilibration time is that time necessary to reach the maximum possible ozone concentration. It is a characteristic of the ozone generator and can e.g. be determined experimentally.

In order to achieve the highest possible ozone concentration, one would thus in principle choose the residence time greater than or equal to the photolysis equilibrium time. However, it has been found in the context of the present invention that some undesirable substances, especially N 2 O (nitrous oxide), can pass through the prefiltration unit. The N ₂ O content in the air is about 317 ppm. In particular, it has been found that N2O in the ozone generator is converted to NO via excited oxygen and then successively into NO2, NO3 and N2O5. This can lead to a disturbing N02 contamination of the measuring air and thus to measurement errors. For a typical Admixture of 1% - 0% of ozonated air to the measurement air may result in more than 10ppb of NC contamination. However, if the residence time of the air in the ozone generating unit is less than the photolysis equilibration time, the N2O conversion in relation to the O3 production can be kept as low as possible. In particular, the undesired formation of NO, and successive NO 2, in the ozone generator can be kept as low as possible or reduced in this way. If the air remained in the ozone generating unit for as long or longer than the photolysis equilibrium time, the N2O conversion would continue while the O3 concentration did not increase further.

A further independent aspect for achieving the object relates to a nitrogen monoxide-to-nitrogen dioxide converter (NO-to-N02 converter) for the indirect measurement of the nitrogen monoxide content of measuring air with the aid of a nitrogen dioxide measuring device, in particular with the aid of a DOAS or CE-DOAS -Instrumentes. The NO-to-NO 2 converter comprises an ozone generator according to the invention for generating ozone and a reaction unit for converting at least part of the nitrogen monoxide contained in the measurement air into nitrogen dioxide by means of the ozone generated by the ozone generator.

The reaction unit is preferably a reaction volume in which the measurement air is combined or mixed with the ozone generated by the ozone generator. The nitrogen monoxide contained in the measuring air reacts in this reaction unit with the ozone to form nitrogen dioxide and oxygen:

NO + O3 -> NO2 + O2.

The NO2 mixing ratio of the measurement air is then composed of the atmospheric NO 2 concentration and the concentration of the converted NO:

[NO2] = [Νθ2, atmosphere] + C- [NO Atmorphäre].

To determine the NO content or NO mixing ratio of the measuring air can first with the same or another N02 measuring device in a reference measurement, the atmospheric N0 ₂ content can be determined by a measurement without O3 addition. Subsequently, by means of the N0 ₂ measuring device, the NO ₂ content can be measured with Cb addition. By comparing the two measurements, the mixing ratio of the converted NO and thus essentially the mixing ratio of NO in the measurement air can be determined.

In a preferred embodiment of the NO-to-N0 ₂ converter, the reaction unit comprises a container through which the measurement air can flow, the volume of the container having a size such that the measurement air and the ozone generated by the ozone generator remain there for so long until the conversion of nitrogen monoxide contained in the measuring air to nitrogen dioxide has taken place substantially completely

The container is preferably inert, i. it is designed so that it does not participate in certain chemical processes, in particular reactions with NO2. For example, the container may consist entirely or partially of glass or Teflon. The container may e.g. be a hose. Over the length of the tube, the volume of the tube and thus the duration of the measurement air in the tube can be set or determined. By "substantially complete" it is meant in the sense of this specification that the conversion of the nitrogen monoxide contained in the measurement air to nitrogen dioxide has occurred at least 50%, preferably at least 90% and most preferably at least 95%. However, too long a residence time is also undesirable since NO2 can then oxidize to higher oxides. The optimal volume of the container or the optimal residence time of the measuring air in the container depends on the respective concentrations. Preferably, the volume of the container is such that the residence time of the measurement air in the container or in the reaction unit is about 5 to 15 seconds.

Another independent aspect for achieving the object relates to a method for converting nitrogen monoxide contained in measuring air to nitrogen dioxide for indirect measurement of the nitrogen monoxide content of the measuring air with the aid of a NO2 measuring instrument, in particular with the aid of a DOAS or CE-DOAS instrument. The method comprises the steps:

 - Providing an ozone generator according to the invention;

 - generating ozone by means of the ozone generator;

 Combining or mixing or mixing the measurement air and the ozone generated by the ozone generator in a reaction unit for converting at least part of the nitrogen monoxide contained in the measurement air into nitrogen dioxide.

Preferably, the measurement air and the ozone are left in the reaction unit until the conversion of the nitrogen monoxide contained in the measurement air into nitrogen dioxide has essentially taken place completely.

In a preferred embodiment of the method, the production of ozone by means of the ozone generator comprises adjusting the air flow through the ozone generator, in particular by means of a flow regulator, so that the residence time of the air in the ozone generator or in the ozone generating unit is less than a photolysis equilibrium time.

The method may further comprise one or more of the following steps:

Feeding the measuring air converted in the reaction unit into an NO 2 measuring cell, in particular a DOAS or CE-DOAS measuring cell, and measuring the nitrogen dioxide content of the converted measuring air by means of the NO 2 measuring cell;

 Determining or determining or calculating the nitrogen monoxide content of the measurement air by comparing the measured nitrogen dioxide content of the converted measurement air with a reference measured value.

The reference measurement results from a reference measurement, wherein the reference measurement is a measurement of the nitrogen dioxide content of the pure measurement air, ie the measurement air without adding ozone. In other words, the reference measurement is one Measurement of the measuring air without using the NO-to-N02 converter or ozone generator. By comparing the measured nitrogen dioxide content of the converted measuring air with the reference measurement or the reference measured value, the mixing ratio of the converted nitrogen monoxide can be determined. This mixing ratio essentially corresponds to the mixing ratio of nitrogen monoxide in the measuring air, ie the nitrogen monoxide content of the measuring air, divided by the conversion factor typical for the measuring instrument. Since the conversion factor is ideally close to 1, it can be neglected. Thus, the NO content of the measurement air can also be measured indirectly by means of an NO 2 measuring device, in particular by means of DOAS or CE-DOAS.

A further independent aspect for achieving the object relates to a use of the ozone generator according to the invention for the indirect measurement of the nitrogen monoxide content of measuring air with the aid of a nitrogen dioxide measuring device, in particular with the aid of a DOAS or CE-DOAS or with the aid of a DOAS or CE-DOAS measuring device ,

A further independent aspect for achieving the object relates to a use of the nitrogen monoxide to nitrogen dioxide converter according to the invention for the indirect measurement of the nitrogen monoxide content of measuring air with the aid of a nitrogen dioxide measuring device, in particular with the aid of a DOAS or CE-DOAS or with the aid of a DOAS or CE-DOAS measuring device.

For the above-mentioned further independent aspects and in particular for related preferred embodiments, the statements made above or below apply to the embodiments of the first aspect.

In the following, individual embodiments for solving the problem will be described by way of example with reference to the figures. In this case, the individual embodiments described have in part features that are not absolutely necessary in order to carry out the claimed subject matter, but which provide desired properties in certain applications. So should Embodiments are considered to be covered by the technical teaching described, which does not have all the features of the embodiments described below. Further, in order to avoid unnecessary repetition, certain features will be mentioned only with respect to each of the embodiments described below. It should be noted that the individual embodiments should therefore be considered not only in isolation but also in a synopsis. Based on this synopsis, those skilled in the art will recognize that individual embodiments may also be modified by incorporating one or more features of other embodiments. It should be understood that a systematic combination of the individual embodiments with single or multiple features described with respect to other embodiments may be desirable and useful, and therefore should be considered and also understood to be encompassed by the description.

Brief description of the drawings

Figure 1 shows a schematic drawing of an ozone generator according to a preferred embodiment;

Figure 2 shows a schematic drawing of an ozone generating unit of the ozone generator according to the invention according to a preferred embodiment;

FIG. 3 shows a schematic representation of the conversion of N 2 O in a photolytic ozone generator;

Figure 4 shows a schematic flow diagram for an ozone generator according to a preferred embodiment;

Figure 5 shows a schematic drawing of a NO to N02 converter according to a preferred embodiment with a downstream DOAS or BB-CEAS measurement;

FIG. 6 shows a simulated time profile of the ratio of

NO ₂ to NO x (NO + NO ₂ ) in an NO to NO ₂ converter according to the invention with an admixed ozone concentration of 1.5 ppm at the beginning of the NO to NO 2 core version (t = 0) for different NO starting concentrations NO initial concentration also corresponds to the initial concentration of NO2.

Detailed description of the drawings

The location selected in the present description, such as location. B. top, bottom, side, etc. are each related to the immediately described and illustrated figure and are mutatis mutandis to transfer to a new position to the new situation.

FIG. 1 shows a schematic drawing of an ozone generator 100 according to a preferred embodiment. The ozone generator 100 includes a pump 3 for sucking in ambient air 1, a pre-filtering unit 10 for pre-filtering the ambient air, a flow controller 13 for adjusting the air flow, an ozone generating unit 15 for generating ozone-containing air 50, an oxidation unit 30, and a post-filtering unit 40. The individual components are each connected to each other by means of a supply line or gas line 6.

In order to avoid generation of other undesirable substances in the ozone generating unit 15, the ozone generator. 100, the prefilter system 0 from an activated carbon filter 5, a silica gel filter 7 and a particulate filter 9, which removes reactive compounds from the air. In particular, the activated carbon filter 5 removes many reactive substances. Silica gel filter 7 filters out nitrogen oxides such as NO2, NO3 and N2O5. And the Aeorosol filter 9 acts as a membrane filter or depth filter the penetration of particles into the ozone generator 100 or in the ozone generating unit 15 or prevents penetration of particles into the ozone generator 100 and in the ozone generating unit 15th

The flow regulator 13 comprises a controllable valve 11 and a flow meter 12. Based on the flow of the air measured by the flow meter 12, the valve 11 can be opened or closed further to adjust the air flow to a predetermined value.

The ozone generating unit 15 comprises an airtight container or an airtight tube 17, wherein the container or the tube 17 is preferably formed wholly or partly of aluminum. In the airtight container 17, a UV lamp 25, preferably a mercury vapor lamp is arranged, which is fastened by means of a lamp holder 27 on the container 17. An opening of the container or tube 17 is mounted, i. in operational or operational condition, with a cap 19 hermetically sealed or sealed. The closure cap 19 must be sufficiently inert and is thus preferably made of Teflon. It is understood that a construction without cap 19 is possible. For example, an aluminum rod in which a cavity is drilled or rotated from one side or an aluminum-coated glass body may be used. The ozone generating unit 15 or the container 17 also has an air inlet or an air inlet opening 21 for receiving the pre-filtered by the Vorfilterungseinheit 10 ambient air 1 and an air outlet or an air outlet opening 23 to the outlet of the ozone-enriched air 50. For mounting or replacing the UV lamp 25, e.g. a compression fitting 26, 28 may be provided, which seals against the lamp holder 27.

The ozone generator 100 is operated with ambient air 1 and therefore does not require any consumption gases, ie there is no need for an oxygen gas cylinder, which must be connected to the ozone generator. This means a large cost saving and at the same time is an important advantage for the operation since the use of oxygen cylinders is often due to safety regulations is critical.

The ozone generator 100 is based, analogous to 0 ₃ generation in the stratosphere, on the photolysis of oxygen molecules by short-wave UV radiation. Preferably, the 184.9 nm emission line of a mercury vapor lamp 25 is used for this purpose. The mercury vapor lamp 25 is for this purpose in an air-tight aluminum tube 17, which has a defined air inlet 21 and a defined air outlet 23 installed. For a high efficiency, ie a high ozone generation rate or a high ozone concentration, it has been found that the material of the container or tube 17 is crucial. In order to achieve a high efficiency of the ozone generator, which is necessary for example for an indirect DOAS measurement of the NO concentration, the container, in particular the inner wall of the container, a material with a high reflectivity with respect to that of the UV lamp 25 to Having photolysis emitted radiation. Because of its good reflectivity at 184.9nm, above all, aluminum is the material for the container 17. In this case, the container or the tube 17 may be made entirely of aluminum or only partially. For example, only the inner wall of the container or tube 17 may be completely or partially coated with aluminum. In addition, it is important that the UV or Hg lamp 25 is arranged in the container 17 such that the air is passed around the mercury vapor lamp 25 closely and completely enclosing. In combination with the additional reflections on the aluminum, a significantly higher efficiency of ozone production compared to comparable ozone generators is achieved with a mercury vapor lamp.

ungefähr 300ppm. Im Hinblick auf eine maximale Ozonausbeute ist es prinzipiell sinnvoll, die Aufenthaltszeit der vorgefilterten Umgebungsluft 1 in der Ozonerzeugungseinheit 15 auf mindestens die Gleichgewichtszeit t_gg, d.h. die Zeit bis die maximal mögliche Ozonkonzentration erreicht wird, einzustellen. Wie aber bereits weiter oben ausgeführt wurde, hat sich im Rahmen der Erfindung herausgestellt, dass es vorteilhaft ist, die Aufenthaltszeit der vorgefilterten Umgebungsluft 1 in der Ozonerzeugungseinheit 15 kleiner als die Photolyse-Gleichgewichtszeit tgg zu wählen. Auf diese Weise kann die durch N₂0 hervorgerufene unerwünschte Entstehung von NO, und sukzessive NO2, im Ozongenerator 100 bzw. in der Ozonerzeugungseinheit 5 möglichst gering gehalten bzw. reduziert werden. Die Gleichgewichtszeit t_gg kann durch eine Charakterisierung des Ozongenerators 100 ermittelt werden, indem die erhaltene Ozonkonzentration in Abhängigkeit vom Luftfluss gemessen wird. Wird das Plateau der maximal erreichbaren Ozonkonzentration von ca. 300ppm bis 370ppm bei einer Verringerung des Flusses erreicht, so ist die Gleichgewichtszeit t_gg erreicht. Für den Fall, dass die Gleichgewichtszeit t_gg des Ozongenerators 100 bzw. der Ozonerzeugungseinheit 15 auf Grund einer zu hohen Ozonproduktionsrate zu gering für eine einfache praktische Anwendung ist, kann die Gleichgewichtszeit t_gg auch durch eine kleinere Quecksilberdampflampe oder eine geringere Lampenspannung vergrößert werden. The ozone production in the ozone generator 100 is limited by the photolysis of 0 ₃ . The main contributor to this is the emission line of the Hg lamp at 253.6 nm, which accounts for almost 100% of the lamp intensity. If air enters the ozone generator, then initially the ozone production rate is greater than the rate of ozone depletion and the ozone concentration increases. However, as the ozone concentration increases, the ozone depletion rate increases until it is as high as the ozone production rate. From this equilibrium time, the so-called photolysis equilibrium time t _gg , increases Ozone concentration is no longer on, the system is from this point in the photolytic equilibrium. The equilibrium concentration of ozone for a typical Pen-Ray mercury vapor lamp (with an intensity ratio of 184.9 nm and 253.6 nm emission lines of

about 300ppm. With regard to a maximum ozone yield, it makes sense in principle to set the residence time of the prefiltered ambient air 1 in the ozone generating unit 15 to at least the equilibrium time t _gg , ie the time until the maximum possible ozone concentration is reached. As has already been stated above, it has been found within the scope of the invention that it is advantageous to choose the residence time of the prefiltered ambient air 1 in the ozone generating unit 15 to be smaller than the photolysis equilibrium time tgg. In this way, the undesirable formation of NO, and successive NO _2, caused by N ₂ O can be kept as low as possible or reduced in the ozone generator 100 or in the ozone generating unit 5. The equilibrium time t _gg can be determined by characterizing the ozone generator 100 by measuring the ozone concentration obtained as a function of the air flow. If the plateau of the maximum achievable ozone concentration of about 300ppm to 370ppm is reached with a reduction of the flow, the equilibrium time t _{gg is} reached. In the event that the equilibration time t _{gg of} the ozone generator 100 or the ozone generating unit 15 is too low for a simple practical application due to a too high ozone production rate, the equilibrium time t _{gg can} also be increased by a smaller mercury vapor lamp or a lower lamp voltage.

2 shows a schematic drawing of an ozone generating unit 15 of the ozone generator 100 according to the invention according to a preferred embodiment. As in FIG. 1, the air-tight container 17, the UV lamp 25 connected to an electrical cable 29, the lamp holder 27 and the closure cap 19 are also shown in FIG. The air inlet 21 and the air outlet 23 are located on opposite sides of the container 17 and are also offset from one another in an axial direction of the container 17. The air inlet 21 is at a first axial end 17a of the container 17 or the UV lamp 25, while the air outlet 23 is arranged at a second axial end 17b of the container 17 or the UV lamp 25. The UV lamp 25 is thus arranged between the air inlet 21 and the air outlet 23. In this way it is achieved that the ambient air 1 passing through the container 17 is irradiated as effectively as possible by the UV lamp 25, whereby the photolysis of the oxygen-containing air and thus the ozone production or the efficiency of the ozone generator 100 can be increased. In addition, a flange 26 and a seal or a clamping ring 28, which is preferably formed of Teflon, for fastening and sealing of the UV lamp 25 inserted into the container 17 are also shown in FIG. The flange 26 and clamping ring 28 in particular form a compression fitting. The UV lamp 25 is designed as a light tube and arranged in the container or tube 17 such that a longitudinal axis of the arc tube is aligned parallel to a longitudinal axis of the container or tube 17. The UV lamp 25 extends substantially through the entire container or through the entire tube 17th

With the ozone generator 100 of the present invention and the ozone generating unit 15, it is possible to increase the ozone production rate by more than a factor of ten as compared with conventional ozone generators using Hg lamps.

As shown schematically in FIG. 3, it has been found in the context of the present invention that N 2 O (laughing gas) in an ozone generator or an ozone generating unit via excited oxygen, ie 0 ( ¹ D) from the 03 photolysis at 253.5 nm, in NO and then successively converted into NO2, NO3 and N2O5. For a typical admixture of 1% to 10% of ozonized air to the ambient air or measurement air, such reactions may result in an N02 contamination of more than 0ppb. Since such an impurity can lead to a considerable measurement error in an indirect NO measurement, the ozone generator 100 still has an oxidation unit or a reaction volume 30 and a post-filtering unit 40 (see FIG. 1). The NO produced from excited oxygen and nitrous oxide in the ozone generating unit 15 can thus be minimized. In the oxidation unit 30, at least part of the nitrogen monoxide produced by excited oxygen and nitrous oxide in the ozone generating unit 15 is oxidized to higher oxides such as NO 2, NO 3, N 2 O 5 (NO reacts very rapidly with the ozone, ie successively in less than about 2 seconds) higher oxides like

In the post-filtering unit 40, the resulting higher oxides can finally be filtered out (NO can hardly be filtered in comparison). For this purpose, the post-filtering unit 40 has a filter 42 with silica gel. The silica gel absorbs NO2 as well as NO3 and N2O5, but ozone can pass well. Although the silica gel also absorbs ozone, but reaches saturation after a few hours, i. E. it is in ozone saturation. In this saturated state of the silica gel, ozone can pass the silica gel without loss. In addition to or instead of silica gel, other NOx filters can be used. The post-filtering unit 40 also has an aerosol filter 44. The aerosol filter 44 reduces or prevents the escape of particles from the silica gel.

By means of the oxidation unit 30 and the post-filtering unit 40, the contamination of the ozone-containing air generated by the ozone generating unit 15 can be significantly reduced with NO and NO x, respectively, so that by means of the ozone generator 100 according to the invention an indirect NO measurement, e.g. with the help of a DOAS measuring device. For this purpose, the NO of the measuring air with the ozone to NO2 must be converted with the aid of a NO-to-N02 converter (see FIG. 5).

In order to keep the N20 conversion in relation to the 03 production as low as possible, it is advantageous if the residence time of the air in the ozone generator 100 or in the ozone generating unit 15 _falls below the equilibrium time t _gg . A corresponding adaptation can take place on the one hand by the adaptation of the power of the ozone generator 100 or of the ozone generating unit 15 and on the other hand by the variation of the air flow. For longer residence times, the N2O conversion will continue while the 03 concentration does not increase further. By this precise coordination, the undesired formation of NO, and successive NO 2, in the ozone generator 100 or in the ozone generating unit 15 can be reduced to such an extent that the NO produced by the ozone generator 100 has a concentration which is below the measurement accuracy for NO of the DOAS measuring device lies. The nitrogen oxide entry from the ozone generator 100 is <10 Oppt below the detection limit.

FIG. 4 shows a schematic flow diagram for an ozone generator 100 according to a preferred embodiment. As illustrated in the diagram of FIG. 4, the following processes or method steps take place in the ozone generator 100 for producing ozone:

I. suction of ambient air by means of a pump 3;

II. Removing numerous reactive substances from the intake ambient air by means of a filter cartridge 5 with activated carbon;

III. Filtering out higher nitrogen oxides such as NO 2, NO 3 and N 2 O 5 and / or drying the sucked ambient air by means of a filter cartridge 7 with silica gel;

IV. Filtering the sucked ambient air by means of an aerosol filter or

 Membrane filter or depth filter 9 for reducing or preventing the ingress of particles, in particular in the ozone generating unit 30 of the ozone generator 100;

V. adjusting the flow of intake ambient air by means of a flow regulator 13 comprising a valve 11 and a flow meter 12 to adjust the ozone concentration, and in particular the ozone admixture, to the measuring air of a DOAS meter; VI. Generating ozone by means of an ozone generating unit 15, in particular by means of a mercury vapor lamp 25 with an emission at 184.9 nm by the splitting of oxygen;

VII. Oxidizing at least a portion of the nitrogen monoxide produced by excited oxygen and nitrous oxide in the ozone generating unit 15 to higher oxides such as NO 2, NO 3, N 2 O 5 in an oxidation unit 30;

VIII. Filtering out the higher oxides by means of a post-filtration unit 40 having a filter 42 with silica gel;

IX. Reducing or preventing the escape of particles from the silica gel with the aid of an aerosol filter 44.

FIG. 5 shows a schematic drawing of a NO-to-NO 2 converter 200 according to a preferred embodiment with a downstream DOAS or BB-CEAS measurement. The NO-to-NO 2 converter 200 comprises an ozone generator 100 according to the invention which is capable of producing a sufficiently high ozone concentration with a negligible NO x impurity, and a reaction unit 130 in which the air to be measured, i. the measurement air whose NO content is to be measured, and the ozone-containing air of the ozone generator 100 are merged or mixed or mixed. As shown in Figure 5, this may e.g. be realized in a simple manner by means of supply lines 110 which are connected via a coupling element 115. In FIG. 5, φ03 denotes the flow of the ozone or the ozone-containing air generated by the ozone generator 100, φΜ the flow of the measurement air, and φθ the flow through an N02 or DOAS measuring device 150.

The reaction unit 130 comprises a reaction volume. The reaction volume is an inert container or simply a long tube, which ensures that the air mass is in the reaction unit 130 long enough before being fed into the DOAS measuring system 150, so that the necessary reaction NO + O 3 -> NO 2 as completely as possible.

However, a too long residence time of the mixture of measuring air and ozone is not desirable, since then NO2 can oxidize to higher oxides, which would falsify the subsequent measurement. The optimal residence time depends on the respective concentrations and is in the range of about 5 to 15 seconds. In the construction with a long tube as the reaction volume, the residence time of the measurement air or the ozone-containing air in the reaction unit 130 can be easily varied and adjusted by the length of the tube.

The reaction unit 130 or the reaction volume is adapted to the air flow and to the volume of the DOAS measuring cell. For example, in a DOAS measuring cell with a measuring cell volume of VDOAS = 0.23 l and a typical flow φ o of 2 l / min, the reaction volume VR is as follows:

_ ^p a ^A s + v _R

 Topt .No> N02 - 8 sec - - (1)

^V R = ^T Opt: NC N02 ^X <t> _c ~ = ° · ¹⁵¹ ( ² ) ·

VDOAS is the volume of the DOAS measuring cell and F is the volume flow of the air. The optimum reaction time To _P t: No-> N02 is calculated so that the maximum conversion in the middle of the measuring cell is achieved. This ensures that only minimal losses occur in the measuring cell. If the reaction volume is, for example, a tube with an inside diameter of 4 mm, the reaction volume calculated above corresponds to a length of the tube of 2 m.

In order to achieve a short reaction time, sufficient amount of O3 must be mixed with the NO-containing air. Here, an ozone mixture ratio of 1 -2 ppm and a reaction time Topt: No-> NO 2 of 8 seconds has proven to be optimal. Then there is the contribution of the loss reactions

N0 ₂ + 0 ₃ - »N0 ₃ + 0 ₂ N0 ₂ + N0 ₃ + M -> N ₂ 0 ₅ + M is relatively low and, depending on the NO and NO ₂ concentration, a conversion of at least 90% up to 98% can be achieved. In the above reaction equations M represents another collision partner which is necessary for reasons of energy conservation.

Shorter reaction times lead in most cases to a low degree of conversion and thus to an unknown error in the indirect measurement of the NO fraction of the measurement air by means of DOAS. Too long reaction times also result in a lower degree of conversion for most concentrations as the above-mentioned loss reactions become significant.

Figure 6 shows a simulated time course of the ratio of NO 2 to NO x (i.e., NO and NO 2) in an NO to NO 2 converter according to the present invention. The admixed ozone concentration at the beginning of the NO to N02 conversion, i. at time t = 0, is 1, 5ppm. In the diagram of FIG. 6, curves for different NO initial concentrations [NO] are plotted, the NO initial concentration also corresponding to the initial concentration of NO 2. It can be seen that NO is rapidly converted to NO 2 by the high ozone concentration for all curves shown. At Topt: No- »N02 = 8 sec, a plateau with the highest degree of conversion of up to 98% is achieved. Thereafter, the degree of conversion falls by loss reactions again. The time with the highest degree of conversion varies only very slightly from the NO initial concentration.

LIST OF REFERENCE NUMBERS

1 Unfiltered ambient air

 2 pre-filtered ambient air

 3 pump

 5 filter / filter cartridge with activated carbon

 6 supply line / gas line

7 filter / filter cartridge with silica gel as NOx filter Aerosol filter / membrane filter / depth filter / Teflon filter

Vorfilterungseinheit

 Valve

 flowmeter

 Flow Controllers

 Ozone generating unit

 Container / aluminum tube

a first axial end

b second axial end

 cap

 Air inlet / air inlet opening

 Air outlet / air outlet

 UV lamp / mercury vapor lamp

 Clamping ring / flange

 lamp holder

 Seal / clamping ring

 Oxidation unit / reaction volume

 postfilter

 Postfilter with silica gel as NOx filter

 Aerosol afterfilter / membrane filter / depth filter / Teflon filter

Ozone-containing air

0 ozone generator

0 supply line / gas line

5 coupling element

0 reaction unit / reaction volume

0 N02 meter

0 NO to N02 converter

Claims

claims 1 . Ozone generator (100) for generating ozone from ambient air (1), comprising:  - An ozone generating unit (15) for generating ozone from the ambient air;  - an oxidation unit (30) for oxidizing nitric oxide formed in the ozone generating unit (15) to higher nitrogen oxides; and  - A post-filtering unit (40) for at least partially filtering out the higher nitrogen oxides generated in the oxidation unit (15). 2. ozone generator (100) according to claim 1, wherein the post-filtering unit (40) comprises a post filter (42) with silica gel and preferably an aerosol post-filter (44). The ozone generator (100) of claim 1, further comprising:  - A prefilter unit (10) for removing reactive substances from the ambient air (1), wherein the prefilter unit (10) preferably a filter (5) with activated carbon, a filter (7) with silica gel and / or an aerosol filter (9). 4. ozone generator (100) according to any one of the preceding claims, wherein the ozone generating unit (15) has an airtight container (17) with an air inlet (21) for the inlet of air and an air outlet (23) to the outlet of ozone-enriched air (50) and wherein in the airtight container (17) a UV Lamp (25) for emitting UV radiation for a photolysis of oxygen molecules is arranged such that the UV lamp (25) is flowed around by the air entering the container (17). 5. ozone generator (100) according to claim 4, wherein an inner wall of the container (17) comprises a material which for the of the UV lamp (25) emitted and suitable for the photolysis UV radiation has a reflectance of at least 0.1 , 6. ozone generator (100) according to claim 4 or 5, wherein the UV lamp (25) is a mercury vapor lamp and wherein the airtight container (17) on the inner wall of the container (7) comprises aluminum. An ozone generator (100) according to any one of the preceding claims, further comprising a flow regulator (13) for adjusting the flow of air through the ozone producing unit (15). The ozone generator (100) of claim 7, wherein the flow controller (13) is configured such that the residence time of the air in the ozone generating unit (15) is less than a photolysis equilibrium time. 9. Nitric oxide-to-nitrogen dioxide converter (200) for the indirect measurement of the nitrogen monoxide content of measuring air by means of a nitrogen dioxide measuring device, comprising:  - An ozone generator (100) according to any one of claims 1 to 8 for the production of ozone;  - A reaction unit (130) for converting at least a portion of the nitrogen monoxide contained in the measurement air to nitrogen dioxide by means of the ozone generated by the ozone generator (100). 10. Nitrogen monoxide to nitrogen dioxide converter (200) according to claim 9, wherein the reaction unit (130) comprises a container through which the measuring air can flow, wherein the volume of the container has a size, so that the measuring air and the from Ozone Generator (100) ozone generated so long until the conversion of nitrogen monoxide contained in the measurement air to nitrogen dioxide has taken place substantially completely. 1 1. A method for converting nitrogen monoxide contained in measuring air into nitrogen dioxide for the indirect measurement of the nitrogen monoxide content of the measuring air with the aid of a nitrogen dioxide measuring device, comprising the steps:  - Providing an ozone generator (100) according to one of claims 1 to 8;  - generating ozone by means of the ozone generator (100);  - Merging the measurement air and the ozone generated by the ozone generator in a reaction unit (130) for converting at least a portion of the nitrogen contained in the measurement nitrogen to nitrogen dioxide. 12. The method of claim 1 1, wherein generating ozone by means of the ozone generator (100) comprises adjusting the air flow through the ozone generator (100) such that the residence time of the air in the ozone generating unit (5) of the ozone generator (100) becomes smaller is a photolysis equilibrium time. 13. Use of the ozone generator (100) according to one of claims 1 to 8 for the indirect measurement of the nitrogen monoxide content of measuring air with the aid of a nitrogen dioxide measuring device. 14. Use of the nitric oxide to nitrogen dioxide converter (200) according to claim 9 or 10 for the indirect measurement of the nitrogen monoxide content of measuring air with the aid of a nitrogen dioxide measuring device.