NO_y

Measurement Description: The NO_y instrument has three independent chemiluminescence detectors for simultaneous measurements of NO_y, NO₂, and NO (see Figure 1). 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. NO_y is the sum of the individual reactive nitrogen oxide species present in the lower stratosphere:

Figure 1. Three channel NO_y instrument layout. The internal components reside inside the ER-2 fuselage in the Q-bay. The external inlet is located approximately 48 cm below the Q-bay fuselage opening. The cooled detectors are Hamamatsu R1333 photomultiplier tubes.

Because NO_y species other than NO do not respond in the chemiluminescence detector, NO_y 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 NO_y is then detected by chemiluminescence in the detector module. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses (see Figure 2). NO₂ is photolytically converted to NO in a glass cell in the presence of intense UV light between 300 and 400 nm (see Figure 3). 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 NO_y is located outside the fuselage of the aircraft in a separate football-shaped housing (see Figure 2). The shape of the housing allows for the inertial separation of large aerosols (> 5 µm diameter) from the NO_y inlet at the downstream end of the housing.

Figure 2. Cross-sectional view of the external inlet housing (see Figure 1) on the ER-2 aircraft, showing the sampling points for the total water, ozone, and NO_y measurements and some components of the NO_yand total water sampling lines.

Figure 3. The NO₂ photolysis system using a Heraeus UV lamp and glass photolysis cell. Borosilicate glass and color glass filters remove unwanted UV and IR radiation from the lamp flux. Air from the fuselage is used to cool the cell in flight.

For smaller particles, the difference between the sample flow velocity in the inlet opening and the aircraft velocity cause ambient aerosol particles to be oversampled in the NO_y inlet. This feature assists in the identification of aerosol particles that contain NO_y.