JP4543186B1 - Nitrogen dioxide photolysis converter and nitrogen oxide concentration measuring apparatus equipped with nitrogen dioxide photolysis converter - Google Patents

Abstract

Nitrogen dioxide photolysis converter and nitrogen dioxide photolysis converter having high conversion efficiency for converting nitrogen dioxide (NO ₂ ) to nitric oxide (NO) and being stable and less interference due to decomposition of other nitrogen compounds A chemiluminescence nitrogen oxide concentration measuring apparatus is provided.
A nitrogen dioxide converter main body for introducing a sample gas to be measured and an ultraviolet light source provided in the nitrogen dioxide converter main body, irradiated with ultraviolet light from the ultraviolet light source, and nitrogen dioxide contained in the sample gas Is a nitrogen dioxide photolysis converter used in a nitrogen dioxide measuring device that measures nitrogen dioxide concentration in the sample gas by photodegrading and converting to nitric oxide, wherein the ultraviolet light source is intermittently lit a plurality of times An ultraviolet light emitting diode is used, and each of the plurality of ultraviolet light emitting diodes has a peak wavelength of 355 nm to 410 nm.
[Selection] Figure 2

Description

TECHNICAL FIELD The present invention relates to a nitrogen dioxide photolysis converter and a nitrogen oxide concentration measuring apparatus equipped with a nitrogen dioxide photolysis converter, and more particularly, to monitor atmospheric pollution, measure atmospheric environment, and measure nitrogen oxide (NO _x ) concentration in the atmosphere and automobiles. exhaust gas concentration of NO _x secondary, especially nitrogen dioxide photolysis converter and nitrogen photolytic converter dioxide which is suitable for measuring the chemiluminescence method to detect which is the main component nitrogen dioxide of the NO _x to (NO ₂₎ in the chemiluminescence intensity of relates nitrogen dioxide (NO ₂₎ the nitrogen oxide concentration measurement device, such as a concentration with a.

Nitrogen oxides (NO _x ) in the atmosphere are not only harmful to the human body, but also generate ozone (O ₃ ) in the troposphere by photochemical reactions in the atmosphere. Tropospheric ozone is not only badly affected by humans and plants as a major component of photochemical smog, but is also considered to affect climate change as a powerful greenhouse gas. NO _x is a raw material for nitric acid (HNO ₃ ), which is an acid rain-causing substance. In recent years, an increasing trend of tropospheric ozone has been shown, and there is concern about an increase in nitric acid in precipitation. Since NO _x (nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ )) plays a decisive role in the production of ozone and nitric acid, its measurement is important for air environment conservation.

Nitrogen oxide (NO _x ) is released into the atmosphere mainly in the form of nitric oxide (NO) in the exhaust of automobiles and factories, etc., but it is converted into nitrogen dioxide (NO ₂ ) by chemical reaction in the atmosphere. it is converted, which is the main component usually NO ₂ is NO _x. However, the photochemical reaction in the NO and NO ₂ is the day, is converted to quickly cross with a time constant of several minutes, the measurement of the NO _x it is important to measure both simultaneously.

Air pollution monitoring, and nitrogen oxides in the atmosphere (NO _x) concentration and automotive nitrogen oxides in the exhaust gas secondary of (NO _x) chemiluminescence method as a method for measuring the concentration are widely used for air quality measurement . In this chemiluminescence method, a sample gas collected from the atmosphere or exhaust gas and ozone (O ₃ ) are brought into contact with each other inside a reaction tank which is a measuring unit of a nitrogen oxide concentration measuring device, and nitrogen monoxide ( This is a method for measuring the concentration of NO in a sample gas by detecting the intensity of chemiluminescence generated when a chemical reaction occurs between NO) and O ₃ with a photoelectric detector (photosensor).

Nitrogen oxides (NO _x ) contained in the ambient air and automobile exhaust gas are composed of nitric oxide (NO) and nitrogen dioxide (NO ₂ ), but between NO ₂ and ozone (O ₃ ). in since there is no chemiluminescent reaction, nO ₂ can not be measured by direct chemiluminescence. Therefore, a chemiluminescent nitrogen oxide concentration measuring device is provided with a sample gas introduction channel for introducing a sample gas collected from the atmosphere or exhaust gas into the reaction tank of the measurement unit so that the concentration of NO ₂ can be measured. A branch is made, one of which passes through the nitrogen dioxide converter. And NO ₂ is also measured by the chemiluminescence method by reducing and converting NO ₂ in the sample gas into NO while the sample gas passes through the nitrogen dioxide converter.

In the above-described nitrogen oxide concentration measuring apparatus, measurement is performed by alternately switching the flow path that passes through the nitrogen dioxide converter and the flow path that does not pass through. The measured value obtained when passing through the nitrogen dioxide converter is the concentration of nitrogen oxide (NO _x ), which is a combination of nitrogen dioxide (NO ₂ ) and nitrogen monoxide (NO), and when not passing through the nitrogen dioxide converter. A concentration of NO is obtained. The concentration of NO ₂ is obtained by subtracting the concentration of NO from the concentration of NO _x . Thus, a nitrogen oxide concentration measurement device according to chemiluminescence method, using nitrogen dioxide converter for converting NO ₂ to NO in order to measure the concentration of NO _2. (Patent Document 1)

Most conventional nitrogen oxide concentration measuring devices using chemiluminescence methods use a molybdenum (Mo) catalyst or carbon as a nitrogen dioxide converter for converting nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO). In this nitrogen dioxide converter, NO ₂ is converted to NO by a heated Mo catalyst or a chemical reaction on the carbon surface.

In the nitrogen oxide concentration measuring apparatus provided with the nitrogen dioxide converter incorporating the above-described molybdenum (Mo) catalyst or the like, the conversion efficiency for converting nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO) is as high as 95% or more. However, in this nitrogen oxide concentration measuring device, other nitrogen compounds in the atmosphere, such as nitrogen trioxide (NO ₃ ) and dinitrogen pentoxide (N ₂ O ₅ ), nitrous acid (HONO), nitric acid, etc. are obtained by using a Mo catalyst or the like. Since (HNO ₃ ), organic nitric acid, and the like are also converted to NO, an error (interference) in which the measured values of nitrogen oxide (NO _x ) and nitrogen dioxide (NO ₂ ) concentrations become excessive occurs, so that accurate NO _x and Measurement of NO ₂ concentration was difficult.

On the other hand, a nitrogen dioxide photolysis converter that converts nitrogen dioxide (NO ₂ ) into nitrogen monoxide (NO) with ultraviolet rays (UV) has recently been developed as a nitrogen dioxide converter used in a chemiluminescent nitrogen oxide concentration measuring apparatus. ing. In this nitrogen dioxide photolysis converter, a UV light source is attached to one side surface of a cell made of glass into which a sample gas collected from the atmosphere or exhaust gas is introduced, and the sample gas is irradiated with UV, so that NO therein ₂ is photodegraded to NO.
NO ₂ + hν → NO + O (Formula 1)

Ultraviolet (UV) lamps, which are often used as light sources in this nitrogen dioxide photolysis converter, do not have wavelength selectivity, and most of the light output is suitable for photolysis of nitrogen dioxide (NO ₂ ). UV or visible light other than the wavelength. Therefore, it is necessary to use a UV lamp with low efficiency and high light output. However, the UV intensity at a wavelength suitable for photodecomposition of NO ₂ is still insufficient, and conversion that converts NO ₂ into nitric oxide (NO). It has been difficult to increase efficiency.

In a nitrogen dioxide photolysis converter that uses this ultraviolet (UV) lamp as a light source, the temperature inside the nitrogen dioxide photolysis converter rises due to the heat generated by the UV lamp. When the temperature inside the nitrogen dioxide photolysis converter rises to 40 ° C. or higher, other nitrogen compounds such as organic nitric acid contained in the sample gas are also thermally decomposed into nitrogen dioxide (NO ₂ ), so that nitrogen oxides (NO _x ) And nitrogen dioxide (NO ₂ ) concentration measurement values become excessively large (interference). Since powerful cooling is required to keep the temperature rise low, a nitrogen dioxide photolytic converter using such a UV lamp is not practical and has not been used outside research.

Further, in the nitrogen dioxide photolysis converter using this ultraviolet (UV) lamp, since the UV lamp does not have wavelength selectivity, when the sample gas is directly irradiated with the light from the UV lamp, the trioxide in the sample gas. Other nitrogen compounds such as nitrogen (NO ₃ ), dinitrogen pentoxide (N ₂ O ₅ ), nitrous acid (HONO), and nitric acid (HNO ₃ ) are photolyzed and converted into nitric oxide (NO), causing interference. In order to prevent this, it is necessary to provide an optical filter that does not pass light of a necessary wavelength between the UV lamp and the glass cell into which the sample gas is introduced. However, the conversion efficiency for converting nitrogen dioxide (NO ₂ ) to nitric oxide (NO) is reduced due to UV attenuation by the optical filter. In addition, since strong UV light is directly applied to the optical filter, its deterioration is fast and long-term performance cannot be maintained. From these facts, a nitrogen dioxide photolytic converter using a UV lamp has not been practical.

In recent years, a nitrogen dioxide photolytic converter using an ultraviolet (UV) light emitting diode as a light source has been developed. However, a nitrogen dioxide photolytic converter using a UV light emitting diode as a light source is not practical for the following reasons.
(1) Since a normal UV light emitting diode has a small light output, it is difficult to obtain sufficient UV intensity even if a large number of UV light emitting diodes are integrated and used as a light source.
(2) Since the emission wavelength range of the UV light emitting diode includes a wavelength range that photodecomposes other nitrogen compounds, it is necessary to provide an optical filter. For UV attenuation by the optical filter, nitrogen dioxide ( The conversion efficiency for converting NO ₂ ) to nitric oxide (NO) is low.
(3) By integrating and lighting a large number of UV light-emitting diodes, the temperature of the UV light-emitting diode elements rises and deteriorates quickly, and the optical filter is directly irradiated with strong UV light, so the deterioration is also quick.

JP 2002-148193 A

An object of the present invention is to increase the temperature by which the interference due to the conversion of other nitrogen compounds into nitric oxide (NO) or nitrogen dioxide (NO ₂ ) can be minimized in the measurement of nitrogen oxide concentration by the chemiluminescence method. The present invention is to develop and provide a nitrogen dioxide photolysis converter using a light source having a small wavelength and high wavelength selectivity, and to provide a nitrogen oxide concentration measuring apparatus equipped with this nitrogen dioxide photolysis converter.

The object of the present invention is to develop and provide a nitrogen dioxide photolysis converter having high conversion efficiency for converting nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO) in the measurement of nitrogen oxide concentration by chemiluminescence. An object of the present invention is to provide a nitrogen oxide concentration measuring device provided with this nitrogen dioxide photolysis converter.

The object of the present invention is to maintain long-term performance such as high conversion efficiency for converting nitrogen dioxide (NO ₂ ) to nitric oxide (NO) in the measurement of nitrogen oxide concentration by chemiluminescence. The object is to develop and provide a nitrogen dioxide photolytic converter and to provide a nitrogen oxide concentration measuring apparatus equipped with the nitrogen dioxide photolytic converter.

In order to achieve the above object, the present invention comprises a nitrogen dioxide converter main body for introducing a sample gas to be measured, and an ultraviolet light source provided in the nitrogen dioxide converter main body, and the sample is irradiated with ultraviolet light from the ultraviolet light source. A nitrogen dioxide photolysis converter for use in a nitrogen dioxide measuring apparatus for measuring nitrogen dioxide concentration in the sample gas by photolyzing nitrogen dioxide contained in the gas and converting it into nitrogen monoxide, wherein the ultraviolet light source Is composed of a plurality of ultraviolet light-emitting diodes, and each of the plurality of ultraviolet light-emitting diodes has a peak wavelength of 355 nm to 410 nm and a wavelength at which 80% or more of the energy of the emitted light is in the wavelength range of 355 nm to 420 nm. The ultraviolet light emission intensity is 50 mW or more, and the plurality of ultraviolet light emitting diodes are A plurality of light emitting diodes constituting the first and second light source portions are provided as first and second light source portions on one side surface of the nitrogen converter body and on the opposite side surface thereof, respectively. The first and second light source sections are alternately irradiated with UV light alternately from two different directions, or alternately turned on / off on each side of the first and second light source sections. Thus, a nitrogen dioxide photolysis converter is provided , which is divided into a plurality of groups and lights up intermittently for each group .

Further, the present invention includes a plurality of light emitting diodes constituting the first and second light source unit, alternately UV from two different directions from the first and second light source section of the is uniformly irradiated Thus, a nitrogen dioxide photolysis converter is provided , which is divided into a plurality of groups and lights up intermittently for each group .

The present invention also provides a nitrogen dioxide photolytic converter, wherein the plurality of ultraviolet light-emitting diodes are always lit by any one of the opposing ultraviolet light-emitting diodes .

The present invention also provides a nitrogen dioxide photolytic converter characterized in that a reflecting portion is provided on the outer periphery of a cylindrical glass cell having a sample gas inlet and a sample gas outlet .

  In order to achieve the above object, the present invention provides a nitrogen oxide concentration measuring apparatus comprising the nitrogen dioxide photolysis converter.

According to the present invention, in the measurement of nitrogen oxide concentration by the chemiluminescence method, performance such as high conversion efficiency for converting nitrogen dioxide (NO ₂ ) to nitric oxide (NO) is stably maintained over a long period of time. A nitrogen dioxide photolysis converter is developed and provided, and a nitrogen oxide concentration measuring device including the nitrogen dioxide photolysis converter is obtained.

It is the schematic which shows the structure of the nitrogen oxide concentration measuring apparatus provided with the nitrogen dioxide photolysis converter which concerns on embodiment of this invention. It is a top view which shows the structure of the nitrogen dioxide photolysis converter which concerns on embodiment of this invention. It is a perspective schematic diagram showing the composition of the nitrogen dioxide photolysis converter concerning an embodiment of the invention. It is a top view of the 1st light source part attached to the converter main body (glass cell) in the nitrogen dioxide photolytic converter which concerns on embodiment of this invention. It is a block diagram which shows lighting and extinguishing of the high output ultraviolet light-emitting diode arrange | positioned in the 1st light source part in the nitrogen dioxide photolysis converter which concerns on embodiment of this invention. It is a block diagram which shows lighting and extinguishing of the high output ultraviolet light-emitting diode arrange | positioned at the 2nd light source part in the nitrogen dioxide photolysis converter which concerns on embodiment of this invention. In nitrogen dioxide photolysis converter according to an embodiment of the present invention, it is a graph showing the variation due to nitrogen dioxide (NO ₂₎ Conversion efficiency (%) of the cumulative operating time (day). Nitrogen dioxide (NO ₂ ) measurement using the nitrogen dioxide photolysis converter according to the embodiment of the present invention, and NO ₂ measurement using a nitrogen dioxide converter incorporating a molybdenum (Mo) catalyst, which is a conventional technique, in Japan Standard Time is a graph showing the relationship between errors due to interference of the time (hour) and other nitrogen compounds (ppbv, it converted to NO ₂ concentration).

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an embodiment of a nitrogen oxide concentration measuring apparatus equipped with a nitrogen dioxide photolytic converter according to the present invention. In the nitrogen oxide concentration measuring apparatus A of FIG. 1, the nitrogen dioxide photolysis converter C according to the present embodiment is disposed between the three-way solenoid valve SV1 and the three-way solenoid valve SV2.

  A sample gas (in the present embodiment, air in the atmosphere) from the sample gas channel 1 is introduced into the three-way solenoid valve SV1. The sample gas branched from the three-way solenoid valve SV1 under the control of the calculation / control unit 8 passes through the sample gas upper channel (bypass channel) 2 when measuring nitric oxide (NO), and the three-way solenoid valve. The sample is introduced into the reaction vessel 5 through the sample gas channel 6 via SV2.

When measuring nitric oxide (NO) and nitrogen dioxide (NO ₂ ) at the same time, the three-way solenoid valves SV1 and SV2 are switched, and the sample gas (air) passes through the sample gas lower flow path 3 and the nitrogen dioxide photolysis converter C To be introduced. The sample gas introduced into the nitrogen dioxide photolysis converter C enters the three-way solenoid valve SV2, and is introduced into the reaction vessel 5 through the sample gas flow path 6.

The ozone generator 4 introduces external air, dehumidifies the air, and generates ozone (O ₃ ) from the oxygen contained in the air by silent discharge or ultraviolet irradiation. This O ₃ is introduced into the reaction vessel 5 of the measuring section and mixed with the sample gas.

The reaction tank 5 is evacuated by a vacuum pump 9 and kept at a low pressure. In the reaction tank 5, nitric oxide (NO) in the sample gas and O ₃ react to generate chemiluminescence, and the intensity thereof is measured by a photoelectric detector (photosensor) 7. A signal proportional to the chemiluminescence intensity from the photoelectric detector (photosensor) 7 is arithmetically processed by the arithmetic / control unit 8 to generate nitrogen monoxide (NO), nitrogen dioxide (NO ₂ ), and nitrogen oxide in the sample gas. The concentration of (NO _x ) is quantified.

  Here, the nitrogen dioxide photolytic converter C according to the present embodiment will be described. FIG. 2 is a plan view of the nitrogen dioxide photolysis converter C, and FIG. 3 is a schematic perspective view showing the configuration of the nitrogen dioxide photolysis converter C.

  As shown in FIG. 2, the nitrogen dioxide photolytic converter C has a cylindrical cell 21 that is a nitrogen dioxide converter main body at the center, and cell holding rings 22 a and 22 b, brackets 23 a, 23b, radiation fins 24a and 24b, LED substrates (also used as cell holding plates) 25a and 25b, and fans 26a and 26b. A different-diameter union elbow 31 serving as a sample gas inlet is attached to the upper left portion of the cylindrical cell 21, and a different-diameter union elbow 32 serving as a sample gas outlet is attached to the upper right portion.

  The left and right side surfaces of the cylindrical cell 21 are held by LED substrates 25a and 25b that also serve as cell holding rings 22a and 22b and a cell holding plate. LED boards 25a and 25b are fixed by brackets 23a and 23b. Further, heat radiation fins 24a and 24b and fans 26a and 26b are attached to the LED boards 25a and 25b for heat radiation.

  As shown in FIG. 3, the nitrogen dioxide photolytic converter C configured as described above has a configuration in which the LED substrate 25 a and the LED substrate 25 b are provided to face both the left and right side surfaces of the cylindrical cell 21.

  In the present embodiment, the cylindrical cell 21 is made of glass, has a length of 180 mm, and an outer diameter of 57 mm. A cylindrical reflector 21 having a thickness of 0.2 mm and having a single-sided mirror gloss is wound around the outer circumference of the cylindrical cell 21 to reflect ultraviolet rays from the high-power ultraviolet light-emitting diode.

  FIG. 4 shows an LED substrate 25a as a first light source unit installed on the left side of the cylindrical cell 21 of the nitrogen dioxide photolytic converter C according to the embodiment of the present invention. In the present embodiment, twelve high-power ultraviolet light-emitting diodes LED1 are arranged in four upper and lower stages, two in the uppermost stage, four in the second and third stages, and two in the lowermost stage on the LED substrate 25a. Has been placed.

  An LED substrate 25b installed as a second light source unit on the right side of the converter body 21 so as to face the LED substrate 25a as the first light source unit has the same configuration as 25a and includes 12 high-power ultraviolet light-emitting diodes. The LED 2 is arranged point-symmetrically with the LED substrate 25a.

  FIG. 5 is an operation explanatory diagram of the first light source unit of the nitrogen dioxide photolytic converter C according to the embodiment of the present invention. Among the twelve high-power ultraviolet light emitting diodes LED1 arranged on the LED substrate 25a constituting the first light source unit, six LEDs 1a indicated by black circles and six LEDs 1b indicated by white circles have a certain time (for example, In this embodiment, the light is alternately turned on every 1 second). That is, as shown in FIG. 5, when the lighting group LED 1a is lit, the extinguishing group LED 1b is extinguished. Thus, the lighting group LED1a, which is half of the twelve high-power ultraviolet light-emitting diodes LED1, is turned on, and the light-off group LED1b, which is half, is turned off.

  FIG. 6 is an operation explanatory diagram of the second light source unit. As in FIG. 5, the six LEDs 2a indicated by black circles and the six LEDs 2b indicated by white circles are alternately lit at regular intervals. That is, as shown in FIG. 6, when the lighting group LED 2a is lit, the unlit group LED 2b is unlit. Thus, half of the twelve high-power ultraviolet light emitting diodes LED2 are turned on, and half of the turn-off group LEDs 2b are turned off.

  That is, in the present embodiment, LED 1a and LED 2a indicated by black circles in FIGS. 5 and 6 constitute a group of one ultraviolet light emitting diode, and LEDs 1b and LED 2b indicated by white circles constitute a group of other ultraviolet light emitting diodes. . That is, the plurality of ultraviolet light emitting diodes (LED 1a, LED 2a, LED 1b, LED 2b) provided in the nitrogen dioxide photolytic converter C are divided into a plurality of groups.

In the present embodiment, LED 1a and LED 2b constituting one ultraviolet light emitting diode group and LED 1b and LED 2a constituting another ultraviolet light emitting diode group are arranged to face each other. Ultraviolet rays are irradiated almost uniformly from two different directions (opposite directions) from the one light source unit and the second light source unit. However, the embodiment of the present invention is not limited to this. For example, the LED 1a and the LED 2a constituting one group of ultraviolet light emitting diodes are arranged in the LED 25a which is the first light source unit, and other ultraviolet light emission is performed. LED1b and LED2 b constituting a group of diodes arranged LED25b a second light source unit, for turning on and off every each group, i.e., such that turning on and off alternately at the right and left side of the cylindrical cell 21 It may be configured.

  Further, in the present embodiment, LED 1a and LED 2a constituting one ultraviolet light emitting diode group indicated by black circles in FIGS. 5 and 6 and LED 1b and LED 2b constituting other ultraviolet light emitting diode groups indicated by white circles are arranged. It is not necessary to turn on and off at the same time, that is, it is not necessary to synchronize the blinking patterns. Even if the blinking patterns of each group are shifted, there is a part in which the blinking patterns overlap in each group. It suffices if it is turned on.

  In short, in the present embodiment, each high-power ultraviolet light-emitting diode (LED1 and LED2) provided in the nitrogen dioxide photolytic converter C is divided into a plurality of groups, and lighting and extinguishing are repeated for each group. In other words, by intermittently lighting, the temperature of the high-power ultraviolet light-emitting diode element is prevented from rising to prevent the ultraviolet light-emitting diode element from deteriorating, and the temperature inside the converter is prevented from rising to 40 ° C. or higher. Prevents thermal decomposition.

  Further, since at least one of the plurality of groups is always lit, the conversion efficiency from nitrogen dioxide to nitrogen monoxide in the nitrogen dioxide photolysis converter C is kept high.

  As described above, the sample gas introduced into the nitrogen dioxide photolysis converter C is irradiated with ultraviolet rays from the high-power ultraviolet light-emitting diodes LED1 and LED2 of the light source unit arranged facing both sides of the cylindrical cell 21 of the converter body. Irradiated with (UV), nitrogen dioxide in the sample gas is reduced and converted to nitric oxide.

FIG. 7 shows the efficiency (%) of converting the nitrogen dioxide (NO ₂ ) of the photolysis converter into nitrogen monoxide (NO) obtained by the continuous operation test of the nitrogen dioxide photolysis converter C according to the embodiment of the present invention. It is a graph which shows the change by integration operation time (days) of).

  As shown in FIG. 7, the conversion efficiency decreases by several percent immediately after the start of operation, but the conversion efficiency is almost within 93% ± 1% for 140 days or more thereafter, and the high conversion efficiency is stably maintained. Is shown. Although there are two portions with no data in FIG. 7, the operation is continuously performed only by measuring the conversion efficiency.

  The specifications of the high-power ultraviolet light-emitting diodes LED1 and LED2 employed in the present embodiment will be described. LED1 and LED2 are the product name “UV chip type LED” and the model name “NCSU34A (T)” manufactured by Nichia Corporation. Initial electrical and optical characteristics are standard, with a peak wavelength of 385 nm, a spectral half width of 10 nm, and a minimum optical output of 310 mW. Regarding reliability, the lifetime during continuous operation is 500 hours.

The emission wavelength region of the ultraviolet light-emitting diode used in this embodiment is preferably such that the peak wavelength is in the range of 355 nm to 410 nm, and 80% or more of the emitted light energy is in the range of 355 nm to 415 nm. When the peak wavelength exceeds 410 nm, nitrogen dioxide (NO ₂ ) is not decomposed. When the peak wavelength is less than 355 nm, photolysis of nitrous acid (HONO) to nitric oxide (NO) occurs, and the measured NO ₂ value is excessive. This is because the reduction conversion efficiency from nitrogen dioxide to nitrogen monoxide is reduced.

  In order to obtain a sufficient UV intensity, the emission intensity of the ultraviolet light emitting diode used in this embodiment is preferably 50 mW or more.

  In the present embodiment, the high-power ultraviolet light-emitting diodes (LED1 and LED2) of the nitrogen dioxide photolytic converter C are divided into a plurality of groups, and are repeatedly turned on and off, that is, intermittently lit. This intermittent lighting prevents the temperature increase of the high-power ultraviolet light-emitting diode element, and has the effect of extending the period during which performance can be maintained.

  As described above, the period during which the ultraviolet light emission intensity of the high-power ultraviolet light-emitting diode used is maintained is 500 hours as a standard in the case of continuous lighting. As shown in FIG. 7, in the continuous operation test, the conversion efficiency is stable for 140 days, far exceeding 500 hours, and 3300 hours or more. In another test, the period during which the conversion efficiency is maintained is 6000 hours or more, and it has been demonstrated that the performance of the photolytic converter C can be maintained over a long time that is 10 times longer than that in continuous operation.

FIG. 8 shows the nitrogen dioxide (NO ₂ ) concentration by chemiluminescence using the nitrogen dioxide photolysis converter C according to the embodiment of the present invention and the nitrogen dioxide converter incorporating the molybdenum (Mo) catalyst according to the prior art. In the measured experiment, the time in Japan standard time (hour) and the amount of nitrogen dioxide (NO ₂ ) interference (ppbv), that is, the error due to interference caused by the conversion of other nitrogen compounds by the nitrogen dioxide converter is converted into NO ₂ concentration. It is a graph which shows the relationship with measured quantity. A curve 7A in FIG. 8 shows the result of measurement using the nitrogen dioxide photolytic converter C according to one embodiment of the present invention, and a curve 7B shows the result of measurement using the nitrogen dioxide converter incorporating the MO catalyst.

In the nitrogen dioxide photolysis converter C, the error due to interference is approximately 0.5 ppb or less, and can be ignored in consideration of data variation. On the other hand, in a nitrogen dioxide converter incorporating a Mo catalyst, the error due to interference reaches 4 ppb in the daytime. The reason why the error due to interference increases in the converter with a built-in Mo catalyst is that the concentration of nitrogen compounds that cause interference such as nitric acid (HNO ₃ ) and organic nitric acid increases in the daytime due to photochemical reactions in the atmosphere. .

DESCRIPTION OF SYMBOLS 1 ... Sample gas flow path, 2 ... Sample gas upper flow path, 3 ... Sample gas lower flow path, 4 ... Ozone generator, 5 ... Reaction tank, 6 ... Sample gas flow path, 7 ... Photoelectric detector (light sensor) 8 ... Calculation / control unit, 9 ... vacuum pump, 21 ... cylindrical cell, 22a ... cell holding ring, 22b ... cell holding ring, 23a ... bracket, 23b ... bracket, 24a ... radiation fin, 24b ... radiation fin, 25a ... LED substrate (cell holding plate), 25b ... LED substrate (cell holding plate), 26a ... Fan, 26b ... Fan, 31 ... Different diameter union elbow, 32 ... Different diameter union elbow, C ... Nitrogen dioxide photolytic converter, LED1 ... High output ultraviolet light emitting diode of the first light source unit, LED 1a... High output ultraviolet light emitting diode (lighting group) of the first light source unit, LED 1b. LED2 ... High output ultraviolet light emitting diode of the second light source unit, LED2a ... High output ultraviolet light emitting diode (lighting group) of the second light source unit, LED2b ... High output ultraviolet light of the second light source unit Light emitting diode (light-off group), SV1 ... three-way solenoid valve, SV2 ... three-way solenoid valve.

Claims (5)

It consists of a nitrogen dioxide converter body that introduces a sample gas to be measured, and an ultraviolet light source provided in the nitrogen dioxide converter body, and irradiates ultraviolet light from the ultraviolet light source to photolyze nitrogen dioxide contained in the sample gas. A nitrogen dioxide photolysis converter used in a nitrogen dioxide measuring device for measuring nitrogen dioxide concentration in the sample gas by converting into nitrogen monoxide,
The ultraviolet light source is composed of a plurality of ultraviolet light emitting diodes, and each of the plurality of ultraviolet light emitting diodes has a peak wavelength of 355 nm to 410 nm, and 80% or more of the energy of emitted light is within a wavelength range of 355 nm to 420 nm. The ultraviolet light emission intensity is 50 mW or more, and the plurality of ultraviolet light emitting diodes are provided on the one side surface and the opposite side surface of the nitrogen dioxide converter body, respectively. The plurality of light-emitting diodes constituting the first and second light source units are arranged so as to face each other, and the ultraviolet rays are alternately uniform from two different directions from the first and second light source units. Each of the light sources is illuminated or turned on / off alternately on each side of the first and second light source units. Divided into groups of intermittently nitrogen dioxide photolysis converter, characterized in that the lighting for each group. 2. The nitrogen dioxide optical converter according to claim 1, wherein the plurality of light emitting diodes constituting the first and second light source units alternately emit ultraviolet rays from two different directions from the first and second light source units. A nitrogen dioxide photolytic converter characterized by being divided into a plurality of groups so as to be irradiated uniformly and intermittently lighting each group . 3. The nitrogen dioxide photolytic converter according to claim 1, wherein any one of the opposed ultraviolet light emitting diodes is always lit in the plurality of ultraviolet light emitting diodes. 4. Decomposition converter. The nitrogen dioxide photolytic converter according to any one of claims 1 to 3, wherein a reflecting portion is provided on an outer periphery of a cylindrical glass cell having a sample gas inlet and a sample gas outlet. . A nitrogen oxide concentration measuring apparatus comprising the nitrogen dioxide photolytic converter according to any one of claims 1 to 4 .

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