NASA - National Aeronautics and Space Administration

An inlet collects ambient air from the free air stream and adds reagents, including O2 or N2 dilutents, and NO and SO2 reagent gases. This method, called "oxygen dilution modulation" leads to nearly 100% measurement of HO2 and RO2 in the O2 dilution/low reagent concentration mode, whereas RO2 is measured with less than 10% efficiency in the N2 dilution/higher reagent concentration mode. This is because the chemistry converts peroxy radicals to H2SO4 efficiently in the O2 mode, but RO2 radicals are converted to RONO in the N2 mode. The H2SO4 thus produced is ionized by reaction with NO3- ions. The reagent and product ions are detected by mass spectrometry using quadrupole mass filtering and counting by a channel electron multiplier operating in the negative ion mode.

The 1999 Leonid MAC campaign consisted of five consecutive nighttime flights including stops in the United States, England, Israel, and the Azores. The Space Dynamics Laboratory of Utah State University operated several instruments in the visible and infrared spectral bands. One system obtained high-resolution (4 cm-1) measurements of the night sky emission spectra in the 1 to 1.65-micrometer band. Measurements were obtained above the clouds providing exceptional viewing conditions. The OH airglow emission layer originates at an altitude of ~87 km and has a half-width of typically 8–10 km. Its behavior during the storm night of 17/18 November 1999 was of particular interest because the OH airglow emission may be affected by the Leonid meteor ablation products that can penetrate to altitudes as low as 80 to 90 km altitudes. Typical Leonid meteor end-heights are much higher above ~100 km. Variability of the OH emission was measured to investigate any changes that may result from meteor interactions with the atmosphere that could cause changes in the natural airglow emission via excitation caused by the meteor ablation products. It is also possible that organic materials in the meteors could be broken down into simpler products that include the OH hydroxyl radical.

To search for these effects, airglow data were collected by a Bomem Michelson M-150 interferometer. This interferometer operates at 4 cm-1 resolution (apodized) with a scan rate of about 1 scan every 3 seconds. The interferometer field of view is 1.5° and it is sensitive from 1 to 1.65 micrometers. An intensified Xibion camera recorded the instrument field of view during the flight, providing information on the pointing elevation and azimuth. This sensor operated almost continuously during the entire 1999 Leonid MAC campaign and collected an extensive set of night airglow spectra.

Titration of OH in H2SO4 and measurement of H2SO4 and MSA via proton exchange with NO3-. DMSO and DMSO2 are reacted with NH4+ ions. In all cases concentrations are determined by product/reactant ion ratios. Ion ratios are measured with quadrupole mass spectrometers.

OH measurements used to understand fast photooxidation chemistry; H2SO4 used to investigate particle nucleation; H2SO4 and MSA used to understand particle growth; DMSO and DMSO2 to investigate DMS oxidation process and its relation to particle production and growth.

OH is detected by direct laser induced fluorescence in the (0-1) band of the 2?-2? electronic transition. A pulsed dye-laser system produces frequency tunable laser light at 282 nm. An on-board frequency reference cell is used by a computer to lock the laser to the appropriate wavelength. Measurement of the signal is then made by tuning the laser on and off resonance with the OH transition.

Stratospheric air is channeled into the instrument using a double-ducted system that both maintains laminar flow through the detection region and slows the flow from free stream velocity (200 m/s) to 40 m/s. The laser light is beam-split and directed to two detection axes where it passes through the stratospheric air in multipass White cells.

Fluorescence from OH (centered at 309 nm) is detected orthogonal to both the flow and the laser propagation using a filtered PMT assembly. Optical stability is checked periodically by exchanging the 309 nm interference filter with a filter centered at 302 nm, where Raman scattering of N2 is observed.

HO2 is measured as OH after chemical titration with nitric oxide: HO2 + NO → OH + NO2. Variation of added NO density and flow velocity as well as the use of two detection axes aid in diagnosis of the kinetics of this titration. Measurements of ozone (by uv absorption) and water vapor (by photofragment fluorescence) are made as diagnostics of potential photochemical interference from the mechanism: O3 + hv (282 nm) → O(1D) + O2, followed by: O(1D) + H2O → OH + OH

ATHOS uses laser-induced fluorescence (LIF) to measure OH and HO2 simultaneously. OH is both excited and detected with the A2Σ+ (v’=0) → X2π (v”=0) transition near 308 nm. HO2 is reacted with reagent NO to form OH and is then detected with LIF. The laser is tuned on and off the OH wavelength to determine the fluorescence and background signals. ATHOS can detect OH and HO2 in clear air and light clouds from Earth's surface to the lower stratosphere. The ambient air is slowed from the aircraft speed of 240 m/s to 8-40 m/s in an aerodynamic nacelle. It is then pulled by a vacuum pump through a small inlet, up a sampling tube, and into two low-pressure detection cells - the first for OH and the second for HO2. Detection occurs in each cell at the intersection of the airflow, the laser beam, and the detector field-of-view.