Engineering topics

Brune, William H.

Publications and source records attributed to Brune, William H..

27 records · Page 2

NASA NTRS1994

In situ detection of tropospheric OH and HO2 by laser-induced fluorescence in a detection chamber at low pressure

Just as in the method of Hard and O'Brien, ambient air is pulled through an approximately 1 mm diameter inlet into a detection chamber that is maintained at a pressure of 2.4 torr. The ambient air stream travels through the detection chamber with a velocity of greater than 100 m sec(exp -1) in a narrow stream, constrained by the addition of an inert gas flow (0.4 torr). The OH molecule is both excited and detected in the A(exp 2)Sigma (v' = 0) yields X(exp 2)II (v'' = 0) transition at 308 nm. Light from a copper vapor-pumped dye laser (rep. rate = 10kHz; pulse length = 20 ns; linewidth = .1 cm(exp -1), and average power = 15 mW), resonant with the Q(sub 1)(3) transition, is multipassed through a White cell and intercepts the air stream as 24 non-overlapping 2mm by 5mm beams. A fast microchannel plate detector is turned off during the laser pulse to prevent saturation of the detector due to Rayleigh and chamber scattering. It is turned on 30-100 nsec after the end of the laser pulse for 300 ns to collect resonance fluorescence from OH. HO2 is detected by chemical conversion to OH by reaction with reagent NO, followed by OH detection. Both the detection sensitivity and the inlet characteristics must be understood for any in situ instrument. For the calibration of the detection sensitivity, OH is produced quantitatively by the fast H + NO2 yields OH + NO reaction in a low pressure, flowing discharge tube connected to the detection chamber. The inlet transmission of OH inlet is calibrated separately.

Brune, William H.↗

NASA NTRS1993

In situ detection of tropospheric OH, HO2, NO2, and NO by laser-induced fluorescence in detection chambers at reduced pressures

This report is a brief summary of the status of work on the grant entitled 'In situ detection of tropospheric OH, HO2, NO2, and NO by laser induced fluorescence in detection chambers at low pressures.' The basic instrument characteristics have been established, and have been reported in a manuscript, included as an appendix to this report, that has been accepted by the Journal of Geophysical Research. Currently, two efforts are under way. First, instrument tests and calibrations are continuing. These efforts include field measurements and an informal inter comparison in Colorado last August and September. Second, new technologies in lasers and detectors are being implemented to make the instrument smaller, lighter, and more energy efficient. Such instrument modifications are essential for measurements from aircraft, high scaffolding in forests, and ships.

Brune, William H.↗

NASA NTRS1993

An aircraft instrument design for in situ tropospheric OH measurements by laser induced fluorescence at low pressures

The hydroxyl radical (OH) is important for many processes involved in tropospheric chemistry. For instance, it initiates the photochemical degradation of gases that cause global climate change, such as methane and the chlorofluorocarbon substitutes (HCFCs). Because of its reactivity, its abundances are less than 0.1 pptv. Thus, OH has been very difficult to measure accurately, despite its importance. Techniques have evolved, however, so that good measurements of tropospheric OH abundances are now possible. One of these techniques that is adaptable to aircraft measurements is the laser induced fluorescence detection of the OH radical in a detection chamber at low pressures. The current ground-based instrument, which can be readily adapted to aircraft, can detect OH abundances of 1.4 x 10 exp 5 OH molecules/cu cm with S/N = 2 in 30 sec, and 5 x 10 exp 4/cu cm in 5 min.

Brune, William H.↗

NASA NTRS1992

In situ detection of tropospheric OH, HO2, NO2, and NO by laser-induced fluorescence in detection chambers at reduced pressures

This report is a brief summary of the status of work on the grant entitled 'In situ detection of tropospheric OH, HO2, NO2, and NO by laser induced fluorescence in detection chambers at low pressures'. The first version of the instrument is essentially complete and operational for about six months, and we continue to make improvements on the instrument sensitivity and reliability. We are focusing our efforts on improving our understanding of the operating characteristics of the instrument - particularly the inlet transmission for OH and HO2, the exact character of the air flow around and within the instrument, and the efficiency of the chemical conversion of HO2 to OH. We are also in the process of converting this laboratory instrument into a field worthy instrument that we can take to remote sites for measurements.

Brune, William H.↗

NASA NTRS1992

In situ detection of tropospheric OH, HO2, NO2, and NO by laser-induced fluorescence in detection chambers at reduced pressures

For detection of OH and HO2, we have met or exceeded most of the goals of the proposal for this grant. We have succeeded in building a prototype instrument for the detection of OH and HO2 that has a detection sensitivity of about 2 x 10 exp 4 (OH molecules/cc)/(ct/s), about five times greater than proposed. The current minimum detectable OH of 1.4 x 10 exp 5 OH molecules/cc (S/N=2; 30 second integration) is less than half that proposed, and will be much lower once scattered light levels are reduced. The instrument displays other important properties. First, interfering signals from other gases and OH losses on the inlet appear to be small under laboratory and field conditions. Second, three different calibration methods, two external to the inlet, give similar results, to within 30 percent. Third, the chemical conversion efficiency of HO2 to OH by addition of reagent NO is better than 90 percent. All of these factors give us great confidence that this technique works. The instrument has gone through several variations that have not affected its potential performance but have affected its use. We were able to undergo the first field trials away from State College in June 1992, when we were able to participate informally in the ROSE experiment in rural Alabama, a year ahead of the proposed schedule for field studies. Because this field experiment was our first and the data analysis was complicated by instrument instabilities, we have not yet released the observations. None-the-less, we have gained insight into instrument design. We have not yet completed all of the proposed work. The calibration systems need to be improved to reduce the 50 percent to 100 percent uncertainties to less than 30 percent. Interference signals from ambient gases and possible losses on the inlet under field conditions need to be quantified. Finally, the detection of NO2 and NO with laser induced fluorescence is only now being seriously pursued.

Brune, William H.↗

NASA NTRS1991

Aerodynamic design of gas and aerosol samplers for aircraft

The aerodynamic design of airborne probes for the capture of air and aerosols is discussed. Emphasis is placed on the key parameters that affect proper sampling, such as inlet-lip design, internal duct components for low pressure drop, and exhaust geometry. Inlet designs that avoid sonic flow conditions on the lip and flow separation in the duct are shown. Cross-stream velocities of aerosols are expressed in terms of droplet density and diameter. Flow curvature, which can cause aerosols to cross streamlines and impact on probe walls, can be minimized by means of a proper inlet shape and proper probe orientation, and by avoiding bends upstream of the test section. A NASA panel code called PMARC was used successfully to compute streamlines around aircraft and probes, as well as to compute to local velocity and pressure distributions in inlets. A NACA 1-series inlet with modified lip radius was used for the airborne capture of stratospheric chlorine monoxide at high altitude and high flight speed. The device has a two-stage inlet that decelerates the inflow with little disturbance to the flow through the test section. Diffuser design, exhaust hood design, valve loss, and corner vane geometry are discussed.

Soderman, Paul T.↗

NASA NTRS1991

Stratospheric processes: Observations and interpretation

Explaining the observed ozone trends discussed in an earlier update and predicting future trends requires an understanding of the stratospheric processes that affect ozone. Stratospheric processes occur on both large and small spatial scales and over both long and short periods of time. Because these diverse processes interact with each other, only in rare cases can individual processes be studied by direct observation. Generally the cause and effect relationships for ozone changes were established by comparisons between observations and model simulations. Increasingly, these comparisons rely on the developing, observed relationships among trace gases and dynamical quantities to initialize and constrain the simulations. The goal of this discussion of stratospheric processes is to describe the causes for the observed ozone trends as they are currently understood. At present, we understand with considerable confidence the stratospheric processes responsible for the Antarctic ozone hole but are only beginning to understand the causes of the ozone trends at middle latitudes. Even though the causes of the ozone trends at middle latitudes were not clearly determined, it is likely that they, just as those over Antarctica, involved chlorine and bromine chemistry that was enhanced by heterogeneous processes. This discussion generally presents only an update of the observations that have occurred for stratospheric processes since the last assessment (World Meteorological Organization (WMO), 1990), and is not a complete review of all the new information about stratospheric processes. It begins with an update of the previous assessment of polar stratospheres (WMO, 1990), followed by a discussion on the possible causes for the ozone trends at middle latitudes and on the effects of bromine and of volcanoes.

Brune, William H.↗

NASA NTRS1991

Tropospheric processes: Observations and interpretation

Three aspects of tropospheric chemical processes imposed by manmade emission of source gases will be discussed. First, the implications for the OH distribution and thereby for the lifetime of source gases which are controlled by reactions with OH in the troposphere (e.g., CH4 and HCFC) are investigated. This is of importance for stratosphere ozone and climate. Second, we will study the impact of source gas emission on tropospheric ozone and discuss the possibility to estimate indirect climate effects from the changes in ozone and other climate gases. Finally, the degradation of HFC and HCFC's is discussed.

Isaksen, Ivar S. A.↗

NASA NTRS1986

In situ observations of midlatitude stratospheric ClO and BrO

A balloon-borne experiment to measure midlatitude stratospheric BrO and ClO concentrations by NO chemical conversion/atomic resonance fluorescence was flown from Palestine, Texas, on May 20 1986. In this first study of BrO, no signal attributable to BrO was detected, and upper limits (2 sigma uncertainty) between 35 and 24 km altitude give BrO mixing ratios less than 15 pptv. Current models predict mixing ratios that are 1.7 times larger. Measurements of ClO were obtained at less than 0.2-km altitude resolution from 41 to 22 km. The smoothly varying altitude profile lies within the range of two-dimensional model calculations.

Brune, William H.↗