Method for measuring concentration of nitrogen dioxide in air by single-wavelength laser induced fluorescence method, and apparatus for measuring concentration of nitrogen dioxide by the method

• US 20040262501A1
• Filed: 05/04/2004
• Published: 12/30/2004
• Est. Priority Date: 11/05/2001
• Status: Active Grant

• Current Assignee: Japan Science & Technology Trading Co. Limited
• Original Assignee: Japan Science & Technology Trading Co. Limited
• Inventors: Kajii, Yoshizumi
• Granted Patent: US 7,295,319 B2
• Time in Patent Office: Days
• US Class Current: 250/227.24
• CPC Class Codes: G01N 21/6402 Atomic fluorescence; Laser induced fluorescence
• CPC Class Codes: Y10T 436/17 Nitrogen containing

Abstract

A method of and apparatus for determining a concentration of nitrogen dioxide that is contained as a trace constituent in atmospheric air utilizes a single-wavelength laser induced fluorescence technique whereby a nitrogen dioxide concentration as lows as at a pptv level can be determined conveniently and accurately. The determination apparatus comprises a pulsed laser light source section (1), a photoexciting section (3) for exciting atmospheric air by passing therethrough a pulsed laser light (2) from the pulsed laser light source section (1), a light lead-in unit (4) for guiding the pulsed laser light (2) into the photoexciting section (3), an atmosphere conducting section (7) through which each of a test atmosphere (5) and a reference atmosphere (6) having nitrogen dioxide removed from the test atmosphere (5) is conducted selectively, a light detecting section (8) for measuring the intensity of a light emission generated in the photoexciting section (3), and a timing control section 9 whereby the time sequence of operations of the pulsed laser light source section (1) and the light detecting section (8) is controlled and detection signals issued from the latter are processed to display a result of the processing. Each of the test atmosphere (5) and the reference atmosphere (6) having nitrogen dioxide removed from the test atmosphere (5) is irradiated with a single wavelength laser light (2) to measure the intensity of a light emitted from the test atmosphere (5) and the intensity of a light emitted from the reference atmosphere (6), and the intensity of the light emitted from the reference atmosphere (6) is subtracted from the intensity of the light emitted from the test atmosphere (5) to determine the concentration of nitrogen dioxide in the test atmosphere. The reference atmosphere (6) is formed by passing atmospheric air (20) through a diffusion scrubber (19) having its tube wall coated with powdery titanium oxide (23).

Claims

1. An atmospheric nitrogen dioxide concentration determining method by a single-wavelength laser induced fluorescence technique, characterized in that it comprises the steps of:

sampling a test atmosphere from an ambient atmosphere containing nitrogen dioxide of which a concentration is as low as in the pptv order of magnitude and providing a reference atmosphere from the test atmosphere by removing nitrogen dioxide therefrom;

irradiating each of the test atmosphere and the reference atmosphere with a single-wavelength laser light having an energy of exciting a nitrogen dioxide molecule from its ground state ²A₁ to its excited state ²B₂;

measuring intensity of a light emitted from the test atmosphere and intensity of a light emitted from the reference atmosphere; and

subtracting the intensity of the light emitted from the reference atmosphere from the intensity of the light emitted from the test atmosphere to determine the concentration of nitrogen dioxide in the test atmosphere which is as low as in the pptv order of magnitude.

2. An atmospheric nitrogen dioxide concentration determining method by single-wavelength laser induced fluorescence technique as set forth in claim 1, characterized in that said reference atmosphere having nitrogen dioxide removed is provided by passing said test atmosphere through a diffusion scrubber in the form of a tube having a tube wall coated with powdery titanium oxide.

3. An atmospheric nitrogen dioxide concentration determining method by a single-wavelength laser induced fluorescence technique as set forth in claim 1, characterized in that said intensity of the fluorescence light emitted from the test atmosphere and said intensity of the fluorescence light emitted from the reference atmosphere are each measured in a vacuum of a degree of vacuum at which a radiative relaxation phenomenon that nitrogen dioxide molecules excited by the laser light are relaxed upon radiating a fluorescent light is maximized in occurrence.

19. An atmospheric nitrogen dioxide concentration determining method as set forth in claim 1, characterized in that said single-wavelength laser light having an energy of exciting a nitrogen dioxide molecule from its ground state ²A₁ to its excited state ²B₂ is a visible light having a wavelength of 532 nm.

4. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique, characterized in that it comprises:

a pulsed single-wavelength laser light source means for generating a pulsed laser light of a wavelength having an energy of exciting a nitrogen dioxide molecule from its ground state ²A₁ to its excited state ²B₂;

a photoexciting means for passing the pulsed laser light from the pulsed single-wavelength laser light source means through atmospheric air to excite the atmospheric air;

a light lead-in means for guiding the pulsed laser light into said photoexciting means;

an atmosphere conducting means for guiding each of a test atmosphere and a reference atmosphere having nitrogen dioxide removed from the test atmosphere by passing the test atmosphere through a diffusion scrubber in the form of a tube having a tube wall coated with powdery titanium dioxide, selectively to said photoexciting means;

a light detecting means for measuring intensity of a light emission produced in said photoexciting means;

and a control means for controlling the time sequence of operations of said pulsed laser light source means and said light detecting means and also processing detection signals issued from said light detection means to display a processing result,

whereby a concentration of nitrogen dioxide in said test atmosphere is determined which is as low as in the pptv order of magnitude.

5. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 4, characterized in that said pulsed laser light source means includes an Nd:

YAG laser unit and a second harmonic generator and is adapted to emit a pulsed laser light having a wavelength of 532 nm.

6. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 4, characterized in that said photoexciting means includes a photoexciting cell, and a light incident and a light exit tube connected to side walls of said photoexciting cell so that they extend coaxially for passing the pulsed laser light through said photoexciting means to excite the atmosphere therein.

7. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 6, characterized in that each of said light incident and exit tubes is sealed hermetically at its outer end with an inclined light transmissive window and is provided with a plurality of baffle plates to prevent scattered lights of the passing laser light from entering said photoexciting cell.

8. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 4, characterized in that said atmosphere conducting means includes an atmosphere selective supply for supplying said photoexciting cell with one of said test atmosphere and said reference atmosphere selectively, and an air exhauster for venting the atmosphere supplied to said photoexciting cell, whereby said test atmosphere and said reference atmosphere when selected are formed to flow as a laminar flow.

9. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 8, characterized in that said atmosphere selective supply and said air exhauster are connected to the side walls of said photoexciting cell so that said laminar flow of the atmosphere flows orthogonally to the optical path of said pulsed laser light.

10. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 8, characterized in that said atmosphere selective supply comprises a blank tube for supply of said test atmosphere, a diffusion scrubber for supply of said reference atmosphere, an atmosphere inlet port to which said blank tube and said diffusion scrubber are connected in parallel through their respective first open ends for introducing the atmospheric air into said blank tube and said diffusion scrubber, respectively, and a three-way solenoid valve disposed between respective second open ends of said blank tube and said diffusion scrubber and connected to said photoexciting cell for supplying the latter with one of said test and reference atmospheres selectively.

11. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 10, characterized in that said diffusion scrubber comprises a glass tube having an inner wall coated with powdery titanium oxide.

12. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 8, characterized in that said atmosphere selective supply includes an orifice disposed directly adjacent to said photoexciting cell whereby venting resistance can be increased to create a pressure at which the intensity of fluorescence is maximized.

13. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 12, characterized in that said orifice has a orifice diameter of 0.3 mm.

14. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 4, characterized in that said light detecting means comprises a light condensing mirror and convex lenses for collecting light emitted from the atmosphere in a region where said pulsed laser light and said laminar flow intersect orthogonally, a photon detector for detecting an intensity of the fluorescence light collected, and an optical filter disposed directly in front of said photon detector.

15. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 14, characterized in that said light detecting means is connected to a side wall of said photoexciting cell so that its optical axis intersects orthogonally with the optical path of said pulsed laser light and said laminar flow.

16. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 4, characterized in that said control means includes a system clock generator for generating system clocks on the basis of which a timing is taken for said pulsed laser light source to issue the pulses laser light wherein on the basis of said timing a timing is taken for said light detecting means to measure a value of each of said test and reference atmospheres, whereby the concentration of nitrogen dioxide in said test atmosphere is determined by processing the measured value of said test atmosphere and the measured value of said reference atmosphere and displayed.

17. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 16, characterized in that said control means is adapted to accumulate such measured values of said test atmosphere and measured values of said reference atmosphere which measured values for each of the said test and reference atmospheres are derived by a plurality of measurements whereby the concentration of nitrogen dioxide in said test atmosphere is determined by processing the measured values of said test atmosphere and the measured values of said reference atmosphere accumulated and is displayed.

18. An atmospheric nitrogen dioxide concentration determining apparatus implementing a single-wavelength laser induced fluorescence technique as set forth in claim 4, characterized in that said light intensity is detected by said photon detector by way of counting photons.