Measurement of total reactive nitrogen, NOy, together with NO2, NO, and O3via cavity ring-down spectroscopy
Inventors
Brown, Steven S. • Dubé, William P. • Wild, Robert J.
Assignees
United States Department of Commerce • Government of the United States of America
Publication Number
US-9804138-B2
Publication Date
2017-10-31
Expiration Date
2035-05-27
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Abstract
A sensitive, compact detector measures total reactive nitrogen (NOy), 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 NOy is thermally dissociated at 700 C 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 NOy instruments.
Core Innovation
The invention provides a new method and apparatus for detection of total reactive nitrogen (NOy) as part of a compact system that simultaneously measures NO, NO2, NOy, and O3 based on cavity ring-down spectroscopy (CRDS). The system directly detects NO2 using laser diode based CRDS at 405 nm, and converts other species through specific reactions to enable their quantification by detecting NO2 formed either by thermal dissociation or chemical conversion in various channels.
The problem addressed by this invention relates to difficulties and limitations in accurately measuring NOy and related nitrogen oxide species in the atmosphere. Prior art methods commonly use catalytic converters and chemiluminescence detection which suffer from issues such as catalytic deterioration, incomplete understanding of chemical processes, inlet losses, calibration difficulties, and poor accuracy especially for NOx and NOy. There is a need for a compact, robust, accurate, and efficient instrument to measure NOy, NO2, NO, and O3 simultaneously with improved precision and reduced maintenance requirements.
The inventive apparatus uses four parallel channels driven by a laser diode for CRDS at 405 nm to measure NO, NO2, NOy, and O3. The NOy channel employs a heated quartz inlet at substantially 650° C. to 750° C. to thermally dissociate all reactive nitrogen compounds to NOx species, which are then converted to NO2 by reaction with excess ozone prior to CRDS measurement. Other channels convert NO and O3 via reactions with ozone or NO respectively to NO2 before measurement. The use of thermal dissociation combined with ozone conversion and CRDS detection provides greater accuracy and robustness versus prior catalytic converter based chemiluminescence instruments.
Claims Coverage
The patent presents four independent claims covering aspects of a multi-channel instrument and method for simultaneous measurement of NOy, NO2, NOx, and O3 content in gas samples using cavity ring-down spectroscopy technology.
Simultaneous multi-channel CRDS instrument with beam division
The instrument includes a laser producing a primary beam that is divided into at least four laser beams, each directed to a separate channel measuring NOy, NO2, NOx, and O3 respectively, enabling simultaneous measurements.
Thermal decomposition and ozone conversion for NOy measurement
The NOy channel includes a heater that thermally decomposes reactive nitrogen compounds in the gas sample into NOx species and a reactor that converts NO to NO2 by reaction with ozone before CRDS measurement. The nitrogen dioxide measured is proportional to total reactive nitrogen content.
Direct CRDS measurement of ambient NO2
The NO2 channel measures nitrogen dioxide content directly by CRDS without thermal or chemical conversion, providing a baseline measurement of ambient NO2.
Conversion of NO to NO2 by ozone reaction for NOx measurement
The NOx channel utilizes a reactor that reacts nitric oxide in the gas sample with ozone to convert it to nitrogen dioxide before CRDS measurement, where the measured NO2 content corresponds to ambient NOx (NO + NO2).
Conversion of ozone to NO2 by reaction with nitric oxide for ozone measurement
The O3 channel uses a reactor supplied with nitric oxide to convert ozone present in the gas sample to nitrogen dioxide before CRDS detection, measuring NO2 proportional to ozone content plus ambient NO2.
Data processing system for calculating species concentrations
Associated data processing receives output signals from each channel and subtracts ambient NO2 from NOx and O3 channels to calculate concentrations of NO and O3, and subtracts NOx from NOy to calculate oxidized reactive nitrogen (NOz).
Quartz tube heater wrapped with nichrome wire
The heater for thermal decomposition of NOy comprises a quartz tube wrapped with nichrome wire to achieve operating temperatures of 650° C. to 750° C.
The claims collectively define an integrated instrument and method employing thermal decomposition and ozone conversion coupled with multi-channel cavity ring-down spectroscopy for simultaneous, precise measurement of atmospheric reactive nitrogen species and ozone, incorporating specific reactors, heaters, optical cavities, and data processing for species quantification.
Stated Advantages
Lower power, size, weight, and vacuum requirements compared to chemiluminescence-based instruments.
Improved accuracy and precision in measuring NOy, NOx, NO2, and O3 simultaneously.
Reduced maintenance due to absence of catalytic converters that deteriorate over time.
Compact and robust design enabling use across various measurement platforms including moving vehicles and aircraft.
Elimination of uncertainties related to inlet losses and chemical interference through thermal dissociation and controlled ozone reactions.
Documented Applications
Measurement and study of atmospheric reactive nitrogen compounds (NOy), nitrogen oxides (NO, NO2, NOx), and ozone (O3) in ambient air.
Air pollution monitoring related to combustion emissions from sources such as power plants and automobiles.
Deployment in mobile platforms including instrumented vehicles and aircraft for atmospheric measurements.
Use in field campaigns investigating atmospheric chemistry, such as the Uintah Basin Winter Ozone Study and the Southeast Oxidant and Aerosol Study, to assess ozone formation and nitrogen species dynamics.
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