NASA - National Aeronautics and Space Administration

Instrument: Reactive Nitrogen

Principal Investigator: David W. Fahey

Organization:
NOAA Aeronomy Laboratory
325 Broadway,R/E/AL6
Boulder, CO 80303

Principle of Operation:
The instrument is designed to measure nitric oxide (NO) and the sum of reactive nitrogen oxides (NOy). Species included in NOy are NO, NO2, HNO3, N2O5 and ClONO2. NO is measured by detecting light from the chemiluminescent reaction between reagent ozone and NO in the ambient sample. NOy is reduced to NO by catalytic reduction on a gold surface with carbon monoxide (CO) acting as a reducing agent. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses. Two reaction vessels are incorporated in the instrument to allow for simultaneous measurement of NO and NOy. Ca1ibration with NO or NO2 is made by standard addition several times during a flight. The baseline of each measurement is determined in part by the addition of synthetic air that contains no reactive nitrogen. The difference between the sample flow velocity in the inlet opening and the aircraft velocity cause aerosol particles in the atmosphere to be oversampled. For sizes below 5 micrometers in diameter, this feature assists in the identification of aerosol particles that contain NOy.

Accuracy: < 20% plus precision
Detection limit: < 0.1 ppbv NOy, ~0.02 ppbv NO
Response time: 1 sec
Location on the ER2: Lower Q-bay rack

Reference: D.W. Fahey et al., In situ aerosol measurements of total reactive nitrogen, total water, and aerosol in a polar stratospheric cloud in the Antarctic, J. Geophys. Res. 94 11-99-11315, 1959.

The NOAA NOyO₃ 4-channel chemiluminescence (CL) instrument will provide in-situ measurements of nitric oxide (NO), nitrogen dioxide (NO₂), total reactive nitrogen oxides (NO_y), and ozone (O₃) on the NASA DC-8 during the FIREX-AQ project. Different versions of this instrument have flown on the NASA DC-8 and NOAA WP-3D research aircraft on field projects since 1995. It provides fast-response, specific, high precision, and calibrated measurements of nitrogen oxides and ozone at a spatial resolution of better than 100m at typical DC-8 research flight speeds. Detection is based on the gas-phase CL reaction of NO with O₃ at low pressure, resulting in photoemission from electronically excited NO₂. Photons are detected and quantified using pulse counting techniques, providing ~5 to 10 part-per-trillion by volume (pptv) precision at 1 Hz data rates. One detector of the integrated 4-channel instrument is used to measure ambient NO directly, a second detector is equipped with a UV-LED converter to photodissociate ambient NO₂ to NO, and a third detector is equipped with a heated gold catalyst to reduce ambient NOy species to NO. Reagent ozone is added to these sample streams to drive the CL reactions with NO. Ambient O₃ is detected in the fourth channel by adding reagent NO.

The NCAR NOxyO3 instrument is a 4-channel chemiluminescence instrument for the measurement of NO, NO2, NOy, and O3. NOx (NO and NO2) is critical to fast chemical processes controlling radical chemistry and O3 production. Total reactive nitrogen (NOy = NO + NO2 + HNO3 + PANs + other organic nitrates + HO2NO2 + HONO + NO3 + 2*N2O5 + particulate NO3- + …) is a useful tracer for characterizing air masses since it has a tendency to be conserved during airmass aging, as NOx is oxidized to other NOy species.

NOx (NO and NO2), NOy (total reactive nitrogen), and O3 are measured using the NCAR 4-channel chemiluminescence instrument, previously flown on the NASA WB-57F and the NCAR C130. NO is measured via addition of reagent O3 to the sample flow to generate the chemiluminescent reaction producing excited NO2, which is detected by photon counting with a dry-ice cooled photomultiplier tube. NO2 is measured as NO following photolytic conversion of NO2, with a time response of about 3 sec due to the residence time in the photolysis cell. NO is measured with an identical time response due to use of a matching volume. NOy is measured via Au-catalyzed conversion of reactive nitrogen species to NO, in the presence of CO, with a time response of slightly better than 1 sec. O3 is measured using the same chemiluminescent reaction but with the addition of reagent NO to the sample flow. Time response for the ozone measurement is slightly better than 1 s.

The NOy instrument has three independent chemiluminescence detectors for simultaneous measurements of NOy, NO2, and NO. Each detector utilizes the reaction between NO in the sample with reagent O3. The NO/O3 reaction produces excited state NO2 which emits light of near 1µ m wavelength. Emitted photons are detected with a cooled photomultiplier tube.

Because NOy species other than NO do not respond in the chemiluminescence detector, NOy component species are reduced to NO by catalytic reduction on a gold surface with carbon monoxide (CO) acting as a reducing agent. Conversion efficiencies are > 90% at surface temperatures of 300°C. An NO signal representing NOy is then detected by chemiluminescence in the detector module. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses. NO2 is photolytically converted to NO in a glass cell in the presence of intense UV light between 300 and 400 nm. The conversion fraction is > 50% for a residence time of 1 s. The chemiluminescence detector detects NO as well as the additional NO from NO2. The third channel measures NO directly by passing the ambient sample through the detector module.

The response of each detector is checked several times in flight by standard addition of NO or NO2 calibration gas. The baseline of each measurement is determined in part by the addition of synthetic air that contains no reactive nitrogen. A continuous flow of water vapor is added directly to the sample flow in order to reduce the background signal in the detectors.

The sampling inlet for NOy is located outside the fuselage of the aircraft in a separate football-shaped housing. The shape of the housing allows for the inertial separation of large aerosols (> 5 µm diameter) from the NOy inlet at the downstream end of the housing.

NO is measured using a chemiluminescence detector. One of the four NO detectors is used for the NO measurements. NOy is measured simultaneously by catalytically converting it to NO on the surface of gold tubes heated to ±° C, with carbon monoxide (CO) acting as a reducing agent. The converter system is contained in a pod mounted outside the cabin to minimize the length of the inlet tubes. Gas phase-NOy measurements are made by sampling air through the rearward facing inlet which discriminates against particles of diameter larger than 1 mm. The mixing ratios of total NOy (gas phase-NOy + amplified particulate-NOy) are measured by sampling air through the forward facing inlet which is heated to 100° C. The mixing ratios of gas phase and total NOy are measured independently. A humidifier maintains the H2O mixing ratio in sample flows at a few % in order to stabilize the instrument background against humidity variations in the ambient air. The absolute sensitivities of the NO and NOy channels are measured every 80 minutes by adding NO or NO2 standard gases. The pressure in the gold catalytic converter for gas-phase NOy is maintained at a constant value of about 50 hPa, independent of the ambient pressure. The pressure is held constant by controlling the sample flow using a servo-controlled Teflon valve mounted upstream of the converter tube. All parts of the inlet system upstream of the gold catalyst are made of Perfluoroalkoxy (PFA) Teflon which is temperature controlled at 40˚C. The NO2 conversion efficiency is 99.0611%. The HCN conversion efficiency is lower than 5% for dry air with O3 mixing ratios lower than 100 ppbv. It decreases to 2% for humid air with an H2O mixing ratio of 0.1% and O3 mixing ratios lower than 100 ppbv. This instrument is also equipped with an NO2 photolytic converter combined with an NO detector in our first attempt to access the accuracy of the NO2 measurements.