Measurement of Total Reactive Nitrogen, NOy, together with NO2, NO, and O3 via Cavity Ring-down Spectroscopy

• US 20150377850A1
• Filed: 05/27/2015
• Published: 12/31/2015
• Est. Priority Date: 06/26/2014
• Status: Active Grant

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: The United States of America As Represented By The Secretary of Agriculture
• Inventors: Brown, Steven S., Dub, William P., Wild, Robert J.
• Granted Patent: US 9,804,138 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01N 33/0016 by regulating a physical variable, e.g. pressure, temperature
• CPC Class Codes: G01N 33/0037 for NOx
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Abstract

A sensitive, compact detector measures total reactive nitrogen (NO_y), as well as NO₂, NO, and O₃. In all channels, NO₂ is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 405 nm. Ambient O₃ is converted to NO₂ in excess NO for the O₃ measurement channel. Likewise, ambient NO is converted to NO₂ in excess O₃. Ambient NO_y is thermally dissociated at 700C to form NO₂ or NO in a heated quartz inlet. Any NO present in ambient air or formed from thermal dissociation of other reactive nitrogen compounds is converted to NO₂ in excess O₃ after the thermal converter. The precision and accuracy of this instrument make it a versatile alternative to standard chemiluminescence-based NO_y instruments.

Claims

1. An instrument for measuring at least reactive nitrogen (NO_y) content in a gas sample, comprising:

an inlet for receiving the gas sample;

a heater, coupled to the inlet receiving the gas sample and heating the gas sample, decomposing reactive nitrogen in the gas sample into nitrogen oxides to output a decomposed gas sample;

a first reactor, coupled to the heater and an ozone supply, receiving the decomposed gas sample and ozone and reacting nitrogen oxide in the decomposed gas sample into nitrogen dioxide to produce a decomposed and reacted gas sample; and

a first cavity ring-down spectrometer, coupled to the first reactor and receiving the decomposed and reacted gas sample, measuring the nitrogen dioxide content of the decomposed reacted gas sample, and outputting a first output signal proportional to the nitrogen dioxide content in the decomposed reacted gas sample;

wherein the nitrogen dioxide content in the decomposed reacted gas sample is proportional to the reactive nitrogen content (NO_y) in the gas sample and the output signal is proportional to the reactive nitrogen content (NO_y) in the gas sample.

2. The instrument of claim 1, further comprising:

a data processing system, receiving the first output signal and calculating reactive nitrogen content in the gas sample as a function of the first output signal.

3. The instrument of claim 2, wherein the gas sample comprises an atmospheric air sample.

4. The instrument of claim 3, wherein the heater heats the gas sample to substantially 650°

C. to 750°

C. to decompose reactive nitrogen NO_y into nitrogen into nitrogen oxides NO and NO₂.

5. The instrument of claim 4, wherein the heater comprises a quartz tube wrapped with nichrome wire.

6. The instrument of claim 5, wherein the first cavity ring-down spectrometer comprises an elongated optical cavity with minors at each end, and a substantially 405 nm laser beam traversing the optical cavity between the minors, and a photomultiplier tube located at one end of the optical cavity, such that when the laser beam is turned off, measurement is made by the photomultiplier tube of how long it takes for the light to decay to 1/e of its initial intensity to calculate the concentration of the nitrogen oxide in the optical cavity.

7. The instrument of claim 1, further comprising an instrument for measuring ambient nitrogen dioxide in the gas sample, comprising:

an inlet for receiving the gas sample;

a second cavity ring-down spectrometer, coupled to the inlet and receiving the gas sample, measuring the nitrogen dioxide content of the gas sample, and outputting an second output signal proportional to the ambient nitrogen dioxide content gas sample.

8. The instrument of claim 7, further comprising an instrument for measuring ambient nitrogen oxides (NO_x) content in the gas sample, comprising:

an inlet for receiving the gas sample;

a second reactor, coupled to the inlet and to the ozone supply, receiving the gas sample and ozone and reacting nitrogen oxide (NO) in the gas sample into nitrogen dioxide (NO₂) to produce a second reacted gas sample; and

a third cavity ring-down spectrometer, coupled to the second reactor and receiving the second reacted gas sample, measuring the nitrogen dioxide content of the second reacted gas sample, and outputting an output signal proportional to the nitrogen dioxide content in the second reacted gas sample;

wherein the nitrogen dioxide content in the second reacted gas sample is proportional to the ambient nitrogen oxides (NO_x) content in the gas sample and the third output signal is proportional to the ambient nitrogen oxides (NO_x) content in the gas sample.

9. The instrument of claim 8, wherein the data processing system subtracts a measured ambient nitrogen dioxide content value as measured by the instrument for measuring ambient nitrogen dioxide from a measured ambient nitrogen oxides (NO_x) content value as measured by the instrument for measuring ambient nitrogen oxides (NO_x) to calculate nitrogen oxide (NO) in the gas sample.

10. The instrument of claim 8, wherein the data processing system subtracts a measured ambient nitrogen oxides (NO_x) content value as measured by the instrument for measuring ambient nitrogen oxides (NO_x) from an reactive nitrogen content (NO_y) value as measured by the instrument for measuring at least reactive nitrogen (NO_y), to calculate oxidized reactive nitrogen (NO_z) in the gas sample.

11. The instrument of claim 8, further comprising an instrument for measuring ozone in the gas sample, comprising:

an inlet for receiving the gas sample;

a third reactor, coupled to the inlet and an nitrogen oxide (NO) supply, receiving the gas sample and nitrogen oxide and reacting ozone in the gas sample with nitrogen oxide into nitrogen dioxide to produce a third reacted gas sample; and

a fourth cavity ring-down spectrometer, coupled to the third reactor and receiving the third reacted gas sample, measuring the nitrogen dioxide content of the third reacted gas sample, and outputting an output signal proportional to the nitrogen dioxide content in the third reacted gas sample;

wherein the nitrogen dioxide content in the third reacted gas sample is proportional to the ozone content in the gas sample and the output signal is proportional to the ozone content in the gas sample.

12. The instrument of claim 11, further comprising:

a laser, providing a substantially 405 nm laser beam;

an optical splitter, for splitting the substantially 405 nm laser beam into four beams, such that each of the four laser beams are fed to a corresponding one of the first cavity ring-down spectrometer. the second cavity ring-down spectrometer, the third cavity ring-down spectrometer, and the fourth cavity ring-down spectrometer.

13. A method of measuring at least reactive nitrogen content in a gas sample, comprising the steps of:

receiving the gas sample in an inlet;

heating the gas sample in a heater coupled to the inlet, to decompose reactive nitrogen in the gas sample into nitrogen oxides outputting a decomposed gas sample;

reacting the decomposed gas sample with ozone in a first reactor coupled to the heater and an ozone supply, to react nitrogen oxide in the decomposed gas sample with the ozone, into nitrogen dioxide to produce a decomposed and reacted gas sample;

measuring nitrogen dioxide content in the decomposed and reacted gas sample in a cavity first ring-down spectrometer coupled to the reactor; and

outputting from the first cavity ring-down spectrometer, an output signal proportional to the nitrogen dioxide content in the decomposed reacted gas sample;

wherein the nitrogen dioxide content in the decomposed reacted gas sample is proportional to the reactive nitrogen content in the gas sample and the output signal is proportional to the reactive nitrogen content in the gas sample.

14. The method of claim 13, wherein the gas sample comprises an atmospheric air sample.

15. The method of claim 14, wherein the step of heating the gas sample comprises the step of heating the gas sample to substantially 650°

C. to 750°

C. to decompose reactive nitrogen NO_y into nitrogen into nitrogen oxides NO and NO₂.

16. The method of claim 15, wherein the heater comprises a quartz tube wrapped with nichrome wire.

17. The method of claim 16, wherein the cavity ring-down spectrometer comprises an elongated optical cavity with minors at each end, and a substantially 405 nm laser beam traversing the optical cavity between the minors, and a photomultiplier tube located at one end of the optical cavity, wherein the step of measuring nitrogen dioxide content in the decomposed and reacted gas sample comprises the steps of:

measuring with photomultiplier tube of how long it takes for the light to decay to 1/e of its initial intensity when the laser beam is turned off; and

calculating concentration of the nitrogen oxide in the optical cavity as a function of how long it takes for the light to decay to 1/e of its initial intensity when the laser beam is turned off.

18. The method of claim 13, further comprising the steps of measuring ambient nitrogen dioxide in the gas sample, comprising:

receiving the gas sample in an inlet;

measuring, in a second cavity ring-down spectrometer coupled to the inlet and receiving the gas sample, the nitrogen dioxide content of the gas sample; and

outputting an second output signal proportional to the ambient nitrogen dioxide content gas sample.

19. The method of claim 18, further comprising the steps of measuring ambient nitrogen oxides (NO_x) content in the gas sample, comprising:

receiving the gas sample in an inlet;

reacting in a second reactor coupled to the inlet and to the ozone supply, the gas sample and ozone and reacting nitrogen oxide (NO) in the gas sample into nitrogen dioxide (NO₂) to produce a second reacted gas sample; and

measuring, in a third cavity ring-down spectrometer coupled to the second reactor and receiving the second reacted gas sample, the nitrogen dioxide content of the second reacted gas sample; and

outputting an output signal proportional to the nitrogen dioxide content in the second reacted gas sample;

wherein the nitrogen dioxide content in the second reacted gas sample is proportional to the ambient nitrogen oxides (NO_x) content in the gas sample and the third output signal is proportional to the ambient nitrogen oxides (NO_x) content in the gas sample.

20. The method of claim 19, further comprising the steps of:

subtracting, in a data processing system, a measured ambient nitrogen dioxide content value from a measured ambient nitrogen oxides (NO_x) content value to calculate nitrogen oxide (NO) in the gas sample.

21. The method of claim 19, further comprising the steps of:

subtracting in a data processing system, a measured ambient nitrogen oxides (NO_x) content vale from a measured reactive nitrogen content (NO_y) in the gas sample to calculate oxidized reactive nitrogen (NO_z) in the gas sample.

22. The method of claim 19, further comprising the step of measuring ozone in the gas sample, comprising:

receiving the gas sample in an inlet;

reacting in a third reactor coupled to the inlet and an nitrogen oxide (NO) supply, ozone in the gas sample with nitrogen oxide into nitrogen dioxide to produce a third reacted gas sample;

measuring in a fourth cavity ring-down spectrometer coupled to the third reactor and receiving the third reacted gas sample, the nitrogen dioxide content of the third reacted gas sample; and

outputting an output signal proportional to the nitrogen dioxide content in the third reacted gas sample;

wherein the nitrogen dioxide content in the third reacted gas sample is proportional to the ozone content in the gas sample and the output signal is proportional to the ozone content in the gas sample.

23. The method of claim 22 further comprising:

generating, using a laser, a substantially 405 nm laser beam; and

splitting, with an optical splitter, the substantially 405 nm laser beam into four beams, such that each of the four laser beams are fed to a corresponding one of the first cavity ring-down spectrometer. the second cavity ring-down spectrometer, the third cavity ring-down spectrometer, and the fourth cavity ring-down spectrometer.