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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Photochemical ozone production in tropical squall line convection during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A

The role of convection was examined in trace gas transport and ozone production in a tropical dry season squall line sampled on August 3, 1985, during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A (NASA GTE/ABLE 2A) in Amazonia, Brazil. Two types of analyses were performed. Transient effects within the cloud are examined with a combination of two-dimensional cloud and one-dimensional photochemical modeling. Tracer analyses using the cloud model wind fields yield a series of cross sections of NO(x), CO, and O3 distribution during the lifetime of the cloud; these fields are used in the photochemical model to compute the net rate of O3 production. At noon, when the cloud was mature, the instantaneous ozone production potential in the cloud is between 50 and 60 percent less than in no-cloud conditions due to reduced photolysis and cloud scavenging of radicals. Analysis of cloud inflows and outflows is used to differentiate between air that is undisturbed and air that has been modified by the storm. These profiles are used in the photochemical model to examine the aftereffects of convective redistribution in the 24-hour period following the storm. Total tropospheric column O3 production changed little due to convection because so little NO(x) was available in the lower troposphere. However, the integrated O3 production potential in the 5- to 13-km layer changed from net destruction to net production as a result of the convection. The conditions of the August 3, 1985, event may be typical of the early part of the dry season in Amazonia, when only minimal amounts of pollution from biomass burning have been transported into the region.

Pickering, Kenneth E.↗
NASA NTRS1990

The Tropospheric Lifetimes of Halocarbons and Their Reactions with OH Radicals: an Assessment Based on the Concentration of CO-14

Chemical reaction with hydroxyl radicals formed in the troposphere from ozone photolysis in the presence of methane, carbon monoxide and nitrogen oxides provides an important removal mechanism for halocarbons containing C-H and C = C double bonds. The isotropic distribution in atmospheric carbon monoxide was used to quantify the tropospheric hydroxyl radical distribution. Here, this methodology is reevaluated in the light of recent chemical kinetic data evaluations and new understandings gained in the life cycles of methane and carbon monoxide. None of these changes has forced a significant revision in the CO-14 approach. However, it is somewhat more clearly apparent how important basic chemical kinetic data are to the accurate establishment of the tropospheric hydroxyl radical distribution.

Derwent, Richard G.↗
NASA NTRS1990

Tropospheric reactions of the haloalkyl radicals formed from hydroxyl radical reaction with a series of alternative fluorocarbons

In the present assessment, the hydrogen containing halocarbons being considered as alternatives to the the presently used chlorofluorocarbons are the hydrochlorofluorocarbons (HCFCs) 123 (CF3CHCl2), 141b (CFCl2CH3), 142b (CF2ClCH3), 22 (CHF2Cl) and 124 (CF3CHFCl) and the hydrofluorocarbons (HFCs) 134a (CF3CH2F), 152a (CHF2CH3) and 125 (CF3CHF2). All of these HCFCs and HFCs will react with the hydroxyl (OH) radical in the troposphere, giving rise to haloalkyl radicals which then undergo a complex series of reactions in the troposphere. These reactions of the haloalkyl radicals formed from the initial OH radical reactions with the HCFCs and HFCs under tropospheric conditions are the focus here.

Atkinson, Roger↗
NASA NTRS1992

Upper tropospheric equatorial waves in ECMWF analyses

An observational study is presented of travelling waves in the near-equatorial upper tropospheric wind field as revealed in operational analyses produced by the European Center for Medium-range Weather Forecasts (ECMWF) over the years 1980-1987. Space-time spectra are presented that reveal the seasonal and interannual variability of traveling waves in the tropical wind fields. The zonal wind spectra exhibit a majority of power in the lowest resolved frequencies, and there is a lack of notable spectral peaks. The upper tropospheric meridional wind spectra show maxima for westward-propagating zonal waves 4-7 with 6- to 10-day periods. The events are also longitudinally localized, with a preference for westward-propagating waves occurring over the eastern Pacific Ocean during August-September. The waves exhibit maximum amplitudes in the upper troposphere and weak coherence with the near-surface layers of the atmosphere.

Randel, William J.↗
NASA NTRS1992

Atmospheric nuclei in the remote free-troposphere

An extensive flight series to survey aerosol present in the upper-troposphere was undertaken aboard the NASA DC-8 as part of the Global Backscatter Experiment. Aerosol with diameters greater than 3 nm were counted and sized with a combination of condensation nuclei counters and optical particle counters. Aerosol number and mass concentrations were separately identified with regard to both refractory and volatile components. Regions of the free-troposphere with the lowest mass concentrations were generally found to have the highest number concentrations and appeared to be effective regions for new particle production. These new particle concentrations appear inversely related to available aerosol surface area, and their volatility suggests a sulfuric acid composition. The long lifetime of these new particles aloft can result in their growth to sizes effective as CN and CCN that can be mixed throughout the troposphere.

Clarke, Antony D.↗
NASA NTRS1992

Transport and mixing processes in the lower troposphere over the ocean

Aircraft observations in the lower troposphere over the eastern Pacific Ocean in summer are examined. The free troposphere was found to have a distinctly nonuniform, layered structure. In maritime air the layers consist of partially mixed air from several sources: moist ozone-depleted boundary layer air; very dry air with high ozone content that probably originated in the middle or upper troposphere; and air with relatively low ozone and moderate, varying moisture content that probably represents residues of convective clouds that had formed over the ocean upstream of the research area. The subtropical high results in stably stratified, subsiding air, where discrete horizontal layers with differing flow velocities transport air over long distances with little vertical mixing.

Paluch, I. R.↗
NASA NTRS1992

Distribution of tropospheric ozone in the tropics from satellite and ozonesonde measurements

Measurements from two independent satellite data sets have been used to derive the climatology of the integrated amount of ozone in the troposphere. These data have led to the finding that large amounts of ozone pollution are generated by anthropogenic activity originating from both the industrialized regions of the Northern Hemisphere and from the southern tropical regions of Africa. To verify the existence of this ozone anomaly over this region of the world, an ozonesonde capability has been established at Ascension Island, located downwind of the primary source region of this ozone pollution, which likely results from the photochemical oxidation of emissions emanating from the widespread burning of savanna. These first ozonesonde profiles suggest that much of the ozone generated over Africa during the 'burning season' (primarily July-October) reaches Ascension Island. These high levels of ozone in the lower troposphere become much lower by December. Elevated ozone concentrations in the middle troposphere are once again evident in February, which may be the result of biomass burning emissions being transported from western and northern Africa.

Fishman, J.↗
NASA NTRS1992

Distribution of tropospheric ozone at Brazzaville, Congo, determined from ozonesonde measurements

An analysis of 33 ozonesonde launches in Brazzaville, Congo (4 deg S, 15 deg E), between June 1990 and May 1991 is presented. The data indicate highest tropospheric amounts between June and early October, coincident with the dry season and with the presence of enhanced widespread biomass burning. The seasonal cycle of ozone derived from the ozonesonde measurements is in good agreement with the climatological seasonal cycle inferred from the use of satellite data amd both seasonal cycles peak in September. Averaged throughout the year, the integrated amount of ozone derived from the ozonesondes is 44 Dobson units (DU) and is 39 DU using the satellite data. Within the troposphere the highest partial pressures are generally found at pressure levels near 700 mbar (about 3 km). Using simultaneous ozonesonde data from Ascension Island (8 deg S, 15 deg W), examples are presented illustrating that differences in the troposphere are primarily responsible for the observed spatial gradients of total ozone observed by TOMS.

Cros, Bernard↗
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 NTRS1992

Tropospheric ozone and aerosol observations - The Alaskan Arctic

This paper discusses in situ measurements of O3, aerosol number density, and aerosol size obtained during NASA Arctic Boundary Layer Expedition aircraft flights over the Alaskan Arctic region. The major source of summer O3 for the troposphere in the intrusion of stratospheric air and subsequent transport to lower altitudes. Photochemistry of mixed layer emissions and O3 transported from high northern latitude urban/industrialized areas do not appear to play major roles as sources of O3 for the Alaska region. O3 gradients reflect the loss at the surface and supply from the stratosphere. Free tropospheric O3 averaged 74 ppbv compared to 32 ppbv for the mixed layer. O3 loss mechanisms are a combination of the destruction via photochemistry, chemical reaction with surface emissions, and direct loss through deposition to the surface. The boreal forest in the most efficient of the O3 sinks and has the largest increase in aerosol number density relative to the free troposphere.

Gregory, Gerald L.↗
NASA NTRS1992

Summertime tropospheric observations related to N(x)O(y) distributions and partitioning over Alaska - Arctic Boundary Layer Expedition 3A

Measurements of NO, NO2, PAN and NO(y) are presented for the summertime middle/lower troposphere over northern high latitudes. Chemical signatures from concurrent measurements of O3, CO, C2H2, C2H6, C3H8, C2Cl4, and H2O are used to characterize factors affecting the budget and distribution of N(x)O(y) in the Arctic and Sub-Arctic tropospheric air masses sampled over Alaska during the NASA Arctic Boundary Layer Expedition (ABLE 3A). The results implicate biomass burning in Siberia as the probable source of about one-third of the NO(y) abundance within the middle lower troposphere over Alaska and the downward transport of air from altitude in the vicinity of the tropopause as a major contributor to the abundance of NO(y) within the lower 6 km column over Alaska.

Sandholm, S. T.↗
NASA NTRS1991

Tropospheric Emission Spectrometer for the Earth Observing System

A Tropospheric Emission Spectrometer (TES) for the Earth Observing System (EOS) series of polar-orbiting platforms is described. TES is aimed at studying tropospheric chemistry, in particular, the exchange of gases between the surface and the atmosphere, urban and regional pollution, acid rain precursors, sources and sinks of greenhouse gases, and the interchange of gases between the troposphere and the stratosphere. TES is a high-resolution (0.025/cm) infrared Fourier transform spectrometer operating in the passive thermal-emission mode in a very wide spectral range (600 to 4350/cm; 2.3 to 16.7 microns). TES has 32 spatial pixels in each of four optically conjugated linear detector arrays, each optimized for a different spectral region.

Glavich, Thomas A.↗
NASA NTRS1993

Upper tropospheric ozone production following mesoscale convection during STEP/EMEX

The chemistry and dynamics of a convective system observed on February 2, 1987 in the EMEX and STEP campaigns are analyzed. Chemical and thermodynamic profiles in undisturbed air near the EMEX 9 system indicate that the troposphere was well mixed by previous convection. As a consequence there was little direct transport from the boundary layer to the upper troposphere and air transported upward was detrained throughout the middle and upper troposphere. There was minimal effect on O3 production. Other convective complexes located 800-900 km upstream produced greater perturbations on trace gas profile immediately below the tropopause. Ozone production was reduced by about 0.25 ppbv/d at these altitudes, representing a reduction in P(O3) of 15-20 percent over the column from 14.5 to 17 km. P(O3) from 12 to 17 km in a region distant from active convection and subject to lightning was 2-3 times higher than it would have been without lightning.

Pickering, Kenneth E.↗
NASA NTRS1987

Tropospheric ozone from satellite total ozone measurements

A method for determining the amount of ozone in the tropical troposphere from concurrent sets of satellite data is presented. This procedure is applied only in the Tropics and the results indicate that a significant longitudinal gradient is present at low latitudes and that the highest amounts of tropospheric ozone are located west (i.e., downwind) of Africa and South America. Such a distribution suggests that biomass burning, or another stationary source of continental origin, is the largest source of ozone in the Tropics. The integrated amount of ozone in the Tropics that is derived from this analysis is comparable to or only slightly less than the amount of ozone that is present in northern mid latitudes. If such an interpretation is valid, then it suggests that the magnitude of the amount of tropospheric ozone resulting from in situ photochemical production in this region is comparable to the in situ photochemical source from industrialized emissions.

Fishman, Jack↗
NASA NTRS1993

Radio metric errors due to mismatch and offset between a DSN antenna beam and the beam of a troposphere calibration instrument

Two components of the error of a troposphere calibration measurement were quantified by theoretical calculations. The first component is a beam mismatch error, which occurs when the calibration instrument senses a conical volume different from the cylindrical volume sampled by a Deep Space Network (DSN) antenna. The second component is a beam offset error, which occurs if the calibration instrument is not mounted on the axis of the DSN antenna. These two error sources were calculated for both delay (e.g., VLBI) and delay rate (e.g., Doppler) measurements. The beam mismatch error for both delay and delay rate drops rapidly as the beamwidth of the troposphere calibration instrument (e.g., a water vapor radiometer or an infrared Fourier transform spectrometer) is reduced. At a 10-deg elevation angle, the instantaneous beam mismatch error is 1.0 mm for a 6-deg beamwidth and 0.09 mm for a 0.5-deg beam (these are the full angular widths of a circular beam with uniform gain out to a sharp cutoff). Time averaging for 60-100 sec will reduce these errors by factors of 1.2-2.2. At a 20-deg elevation angle, the lower limit for current Doppler observations, the beam-mismatch delay rate error is an Allan standard deviation over 100 sec of 1.1 x 10(exp -14) with a 4-deg beam and 1.3 x 10(exp -l5) for a 0.5-deg beam. A 50-m beam offset would result in a fairly modest (compared to other expected error sources) delay error (less than or equal to 0.3 mm for 60-sec integrations at any elevation angle is greater than or equal to 6 deg). However, the same offset would cause a large error in delay rate measurements (e.g., an Allan standard deviation of 1.2 x 10(exp -14) over 100 sec at a 20-deg elevation angle), which would dominate over other known error sources if the beamwidth is 2 deg or smaller. An on-axis location is essential for accurate troposphere calibration of delay rate measurements. A half-power beamwidth (for a beam with a tapered gain profile) of 1.2 deg or smaller is desired for calibration of all types of radio metrics. A water-vapor radiometer calibration beam of this size with very low sidelobes would require a clear aperture antenna with a diameter of at least 1.5 m if the primary water vapor sensing channel were in the 20-22 GHz range.

R P Linfield↗
NASA NTRS1994

Absolute determination of local tropospheric OH concentrations

Long path absorption (LPA) according to Lambert Beer's law is a method to determine absolute concentrations of trace gases such as tropospheric OH. We have developed a LPA instrument which is based on a rapid tuning of the light source which is a frequency doubled dye laser. The laser is tuned across two or three OH absorption features around 308 nm with a scanning speed of 0.07 cm(exp -1)/microsecond and a repetition rate of 1.3 kHz. This high scanning speed greatly reduces the fluctuation of the light intensity caused by the atmosphere. To obtain the required high sensitivity the laser output power is additionally made constant and stabilized by an electro-optical modulator. The present sensitivity is of the order of a few times 10(exp 5) OH per cm(exp 3) for an acquisition time of a minute and an absorption path length of only 1200 meters so that a folding of the optical path in a multireflection cell was possible leading to a lateral dimension of the cell of a few meters. This allows local measurements to be made. Tropospheric measurements have been carried out in 1991 resulting in the determination of OH diurnal variation at specific days in late summer. Comparison with model calculations have been made. Interferences are mainly due to SO2 absorption. The problem of OH self generation in the multireflection cell is of minor extent. This could be shown by using different experimental methods. The minimum-maximum signal to noise ratio is about 8 x 10(exp -4) for a single scan. Due to the small size of the absorption cell the realization of an open air laboratory is possible in which by use of an additional UV light source or by additional fluxes of trace gases the chemistry can be changed under controlled conditions allowing kinetic studies of tropospheric photochemistry to be made in open air.

Armerding, Wolfgang↗
NASA NTRS1993

MOPITT - A gas correlation spectrometer for tropospheric Earth observations

MOPITT (Measurement of Pollution in the Troposphere) is an instrument that will fly on the EOS AM-1 Polar Orbiting Platform, to be launched in mid-1998. The instrument accurately measures the concentration of carbon monoxide (CO) and methane (CH4) gas in the troposphere, using novel and conventional gas correlation spectroscopy techniques. These techniques are used to measure both the column amount and the vertical profile of the two gases in the troposphere. The instrument is nadir viewing and has cross-track scanning ability. It measures upwelling IR radiation through the atmosphere in the 4.6-micron and 2.3-micron bands for CO detection, and 2.2-micron for CH4. The instrument performance is enhanced using Stirling cycle mechanical coolers to maintain the IR detectors at less than 100 K. MOPITT has a total of eight detector channels to measure the total column of atmospheric CH4 gas to a precision of 1 percent, and to measure CO profiles to a precision of 10 percent. The instrument is self-calibrating to ensure data validity using internal targets and an external cold space view.

Drummond, James R.↗
NASA NTRS1994

Tropospheric ozone in the western Pacific Rim: Analysis of satellite and surface-based observations along with comprehensive 3-D model simulations

Tropospheric ozone production and transport in mid-latitude eastern Asia is studied. Data analysis of surface-based ozone measurements in Japan and satellite-based tropospheric column measurements of the entire western Pacific Rim are combined with results from three-dimensional model simulations to investigate the diurnal, seasonal and long-term variations of ozone in this region. Surface ozone measurements from Japan show distinct seasonal variation with a spring peak and summer minimum. Satellite studies of the entire tropospheric column of ozone show high concentrations in both the spring and summer seasons. Finally, preliminary model simulation studies show good agreement with observed values.

Young, Sun-Woo↗