The dynamics of the stratospheric polar vortex and its relation to springtime ozone depletions

Dramatic springtime depletions of ozone in polar regions require that polar stratospheric air has a high degree of dynamical isolation and extremely cold temperatures necessary for the formation of polar stratospheric clouds. Both of these conditions are produced within the stratospheric winter polar vortex. Recent aircraft missions have provided new information about the structure of polar vortices during winter and their relation to polar ozone depletions. The aircraft data show that gradients of potential vorticity and the concentration of conservative trace species are large at the transition from mid-latitude to polar air. The presence of such sharp gradients at the boundary of polar air implies that the inward mixing of heat and constituents is strongly inhibited and that the perturbed polar stratospheric chemistry associated with the ozone hole is isolated from the rest of the stratosphere until the vortex breaks up in late spring. The overall size of the polar vortex thus limits the maximum areal coverage of the annual polar ozone depletions. Because it appears that this limit has not been reached for the Antarctic depletions, the possibility of future increases in the size of the Antarctic ozone hole is left open. In the Northern Hemisphere, the smaller vortex and the more restricted region of cold temperatures suggest that this region has a smaller theoretical maximum for column ozone depletion, about 40 percent of the currently observed change in the Antarctic ozone column in spring.

Variable pressure infrared diode laser spectroscopy: a new method for trace-gas monitoring

A new method for measuring trace concentrations of atmospheric pollutants by infrared diode laser spectroscopy has been devised. This method relies on the increase of the signal as the pressure inside the cell increases, while the frequency of the diode is stabilized on the line, even if it is unresolved. Performances of this method were tested with N2O and with 1,3-butadiene. As an example of application, we measured the butadiene emitted by car exhausts. Sensitivity and rapidity of this method are equivalent to the usual scanning method in which the whole line is described, but this new method benefits from its simplicity and robustness.

Stable isotope laser spectrometer for exploration of Mars

On Earth, measurements of the ratios of stable carbon isotopes have provided much information about geological and biological processes. For example, fractionation of carbon occurs in biotic processes and the retention of a distinctive 2-4% contrast in 13C/12C between organic carbon and carbonates in rocks as old as 3.8 billion years constitutes some of the firmest evidence for the antiquity of life on the Earth. We have developed a prototype tunable diode Laser spectrometer which demonstrates the feasibility of making accurate in situ isotopic ratio measurements on Mars. This miniaturized instrument, with an optical path length of 10 cm, should be capable of making accurate 13C/12C and 15N/14N measurements. Gas samples for measurement are to be produced by pyrolysis using soil samples as small as 50 mg. Measurements of 13C/12C, 18O/16O and 15N/14N have been made to a precision of better than 0.1% and various other isotopes are feasible. This laser technique, which relies on the extremely narrow emission linewidth of tunable diode lasers (<0.001 cm(-1)) has favorable features in comparison to mass spectrometry, the standard method of accurate isotopic ratio measurement. The miniature instrument could be ready to deploy on the 2003 or other Mars lander missions.

Interhemispheric Differences in Polar Stratospheric HNO3, H2O, CIO, and O3

Simultaneous global measurements of nitric acid (HNO(3)), water (H(2)O), chlorine monoxide (CIO), and ozone (O(3)) in the stratosphere have been obtained over complete annual cycles in both hemispheres by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. A sizeable decrease in gas-phase HNO(3) was evident in the lower stratospheric vortex over Antarctica by early June 1992, followed by a significant reduction in gas-phase H(2)O after mid-July. By mid-August, near the time of peak CIO, abundances of gas-phase HNO(3) and H(2)O were extremely low. The concentrations of HNO(3) and H(2)O over Antarctica remained depressed into November, well after temperatures in the lower stratosphere had risen above the evaporation threshold for polar stratospheric clouds, implying that denitrification and dehydration had occurred. No large decreases in either gas-phase HNO(3) or H(2)O were observed in the 1992-1993 Arctic winter vortex. Although CIO was enhanced over the Arctic as it was over the Antarctic, Arctic O(3) depletion was substantially smaller than that over Antarctica. A major factor currently limiting the formation of an Arctic ozone "hole" is the lack of denitrification in the northern polar vortex, but future cooling of the lower stratosphere could lead to more intense denitrification and consequently larger losses of Arctic ozone.

Aircraft (ER-2) laser infrared absorption spectrometer (ALIAS) for in-situ stratospheric measurements of HCI, N(2)O, CH(4), NO(2), and HNO(3)

The Aircraft Laser Infrared Absorption Spectrometer (ALIAS) instrument is a high-resolution (0.0003 cm-(1)) scanning tunable-diode-laser spectrometer designed, tested, and flown more than 30 times on the National Aeronautics and Space Administration's high-altitude ER-2 aircraft in the Airborne Arctic Stratospheric Expedition of 1991-1992. Using long-path infrared laser absorption spectroscopy to detect optical absorptions as small as 10-(5), ALIAS provides fast, continuous in-situ measurements of key atmospheric gases, with gas detection sensitivities of tens of parts in 10(12). With four lasers and detectors in a single liquid-nitrogen Dewar, simultaneous measurements of HCI, NO(2), HNO(3), CH(4), and N(2)O are made using laser sources at 3.4-8 µm, injected into a 1-m-long, 80-pass Herriott cell.

Airborne tunable diode laser spectrometer for trace-gas measurement in the lower stratosphere

This paper describes the airborne tunable laser absorption spectrometer, a tunable diode laser instrument designed for in situ trace-gas measurement in the lower stratosphere from an ER-2 high-altitude research aircraft. Laser-wavelength modulation and second-harmonic detection are employed to achieve the required constituent detection sensitivity. The airborne tunable laser absorption spectrometer was used in two polar ozone campaigns, the Airborne Antarctic Ozone Experiment and the Airborne Arctic Stratospheric Expedition, and measured nitrous oxide with a response time of Is and an accuracy ≤ 10%.

Ozone Loss Inside the Northern Polar Vortex During the 1991-1992 Winter

Measurements made in the outer ring of the northern polar vortex from October 1991 through March 1992 reveal an altitude-dependent change in ozone, with a decrease at the bottom of the vortex and a substantial increase at the highest altitudes accessible to measurement. The increase is the result of ozone-rich air entering the vortex, and the decrease reflects ozone loss accumulated after the descent of the air through high concentrations of reactive chlorine. The depleted air that is released out of the bottom of the vortex is sufficient to significantly reduce column ozone at mid-latitudes.

Stable isotope analysis using tunable diode laser spectroscopy

Measurements of ratios of stable isotopes are used in such diverse fields as petroleum prospecting, medical diagnostics, and planetary exploration. The narrow emission linewidth available from tunable diode lasers permits high-resolution infrared absorption measurements of closely spaced isotopic rovibrational lines. Our dual beam spectrometer uses the sweep integration technique in a spectral region where adjacent spectral lines are of approximately equal absorbance at the expected isotopic abundances. The experimental results reported here indicate that isotopic ratios of carbon in carbon dioxide can be measured to an accuracy of better than 0.4%. This laser spectroscopic technique offers an alternative to the mass spectrometric technique for in situ isotopic analysis in field studies, as well as flight and space applications.

Versatile integrated tunable diode laser system for high precision: application for ambient measurements of OCS

A versatile and integrated tunable diode laser system for high precision measurements of the important sulfur gas carbonyl sulfide is described. We explicitly address some of the-major factors affecting tunable diode laser measurement precision as well as accuracy and have implemented a number of new features for increased system control and versatility. The system described herein provides the capability for measuring ambient concentrations of this gas with a precision in the range from +/-0.3 to +/-1% over time periods of many hours. Such a precision provides us with an important new capability for measuring true spatial and temporal variations of carbonyl sulfide in the atmosphere.