Argus is a new, lightweight spectrometer designed for measuring the stratospheric nitrous oxide (N2O) and methane (CH4) tracer fields in situ from balloons and aircraft. It is a second-generation instrument drawing on the experience gained with the very successful Atmospheric Laboratory for Applications and Science (ATLAS) instrument measuring N2O and flying on the NASA ER-2 since 1986.
During the past several years Argus was part of the Observations from the Middle Stratosphere (OMS) balloon payload probing the tropical and mid-latitude stratosphere in studies of transport issues related to dispersion of aircraft exhaust. A major concern motivating these studies was the potential for nitrogen oxides (NOx) generated in the exhaust plume of a supersonic high flyer to reach the tropical ozone production region by rapid transport from a midlatitude flight corridor. NOx is effective in destroying ozone (O3) in a well-known catalytic cycle that can proceed rapidly under stratospheric photochemical conditions.
Several results from research carried out with the Argus instrument are described below.
Tracer Laminae in the Tropics - Transport of air from the midlatitudes into the tropics in the stratosphere is known to be inhibited by a "transport barrier." The existence of such a barrier was already well known from satellite studies of dispersion of the Mt. Pinatubo aerosol cloud, following Pinatubo's eruption in June, 1991. On the other hand, some transport of stratospheric air into the tropics is known to occur from the detailed study of tracer mean vertical profiles in the tropics. Interest in this issue centers on the fate of aircraft exhaust products injected into the midlatitude stratosphere, and their potential impact on the tropical ozone production region.
During an OMS balloon launch from 7° S in Brazil in November, 1997, thin laminar regions of low N2O on the normal tropical profile of this tracer were observed. (N2O is lower in the midlatitude stratosphere than in the tropics at the same altitude.) The data indicated clearly midlatitude values of N2O in the tropics. This appears to be a direct observation of the process of rapid transport into the tropics of midlatitude stratospheric air masses. The observations suggest that this transport is episodic, possibly the result of breaking of midlatitude stratospheric waves (Rossby waves).
Long-Lived Arctic Winter Vortex Remnant - During an OMS balloon launch (in conjunction with ER-2 flights) from Fairbanks, Alaska, in late June, 1997, several layers of unusually low values in the N2O and CH4 tracer fields were encountered. These were clearly "remains" of the winter polar vortex with the observed filaments preserving very low tracer values characteristic of the interior of the vortex which had subsided from higher altitudes during the previous winter. The unusual, possibly unique, quality of the discovery was that these layers had maintained their integrity for about two months, from the time of vortex breakup sometime in late April until their observation in late June.
Intercomparison Flights with ATLAS and Other Tracer Instruments - Plans to design a new ER-2 instrument or to fly Argus on the ER-2, in both cases for the purpose of replacing the older and heavier ATLAS instrument, has motivated two separate flight intercomparison campaigns. In the fall of 1998, Argus and ATLAS flew together on the ER-2 in an otherwise unrelated atmospheric radiation campaign called CiRex (Cirrus Radiation Experiment). In the fall of 1999, a comparison of several N2O instruments was carried out in preparation for the SOLVE (SAGE II Ozone Loss and Validation Experiment) Arctic Ozone mission to take place in January through March of 2000.
Both of these intercomparison campaigns provided an opportunity to pass the pedigree of N2O tracer field measurements made by ATLAS, and widely accepted as a standard in the ER-2 airborne measurement community, on to the new Argus instrument. The importance of this transfer is the significant weight reduction realized by the dual channel Argus instrument, thus allowing space for additional payload in the wide-ranging ER-2 atmospheric chemistry payload suite.
Point of Contact: M. Loewenstein
(650) 604-5504
mloewenstein@mail.arc.nasa.gov
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