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Stewart, R. W.

Publications and source records attributed to Stewart, R. W..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

An 11-year Solar-Cycle in Tropospheric Ozone from TOMS Measurements

Tropospheric column ozone derived from Nimbus 7 total ozone mapping spectrometer (TOMS) footprint measurements in the tropical western Pacific for 1979-1992 provide the first observational evidence of changes in tropospheric ozone which are out of phase with the stratospheric ozone changes on a time scale of a solar cycle. The estimated changes in tropospheric and stratospheric column ozone over a solar cycle are respectively -2.97 and 8.83 DU (Dobson units) or - 12.7 and 3.7% from the solar minimum to solar maximum. These observations are qualitatively consistent with a modulation effect on tropospheric ozone photochemistry by UV-induced changes in stratospheric ozone. However, in the low NO(x) regime of the marine atmosphere, the observed changes are significantly larger than estimated from a photochemical model. The solar cycle detected in tropospheric ozone is estimated to produce approximately 0.1-0.25 W m(exp -2) anomalous radiative forcing in the tropics. The combination of this with other radiative sources may have substantial influence on both regional and global climate.

Chandra, S.↗
NASA NTRS1983

A model study of the effects of intermittent loss on odd nitrogen concentrations in the lower troposphere

A time-dependent box model of the lower troposphere which includes a description of photochemical and physical processes has been developed. This model has been applied to the calculation of nitric acid and NO(x)(NO + NO2) concentrations over a diurnal cycle which includes precipitation. Nitric acid concentrations and the HNO3/NO(x) ratio are found to be highly variable under the assumptions regarding the frequency, duration, and intensity of precipitation employed in this model. The chemistry of odd nitrogen compounds during the night is potentially important in establishing the level of nitric acid in the lower troposphere. These calculations also indicate that relatively large errors may occur when the continuity equation describing nitric acid variations is averaged over a diurnal cycle which includes precipitation. Interpretation of simultaneous measurements of HNO3 and NO(x) will require some knowledge of the history of the observed air mass and may require an improved understanding of nighttime odd nitrogen chemistry.

Stewart, R. W.↗
NASA NTRS1983

Latitudinal variation of tropospheric ozone in a photochemical model

A zonally, vertically, and annually averaged numerical model of trace gases in the troposphere is used to examine the variation of ozone as a function of latitude. Processes included in the ozone budget are a source due to downward transport of ozone from the stratosphere, deposition at earth's surface, north-south diffusive transport, and photochemical reactions. Sensitivity of calculated ozone concentrations to the latitude variation and magnitude of the stratospheric source is presented. The model indicates a net photochemical source of ozone in the Northern Hemisphere and a net chemical sink in the tropical latitudes. Calculated variation of ozone concentration with latitude is in qualitative agreement with observations.

Hameed, S.↗
NASA NTRS1981

Implications of natural sources for the latitudinal gradients of NO/y/ in the unpolluted troposphere

The latitudinal distributions of tropospheric odd nitrogen (NO/y/) due to lightning discharges and stratospheric injection are studied by estimating latitude dependences of the sources and the rainout sink from data obtained in August-September, 1977, and May-June, 1978, during project GAMETAG. A one-dimensional (latitude dependent) model is developed for the annually, zonally, and vertically averaged concentration of trace species, NO(y) being treated as a single species, which decouples the NO(y) calculation from the rest of the model since NO(y) has no chemical sources or sinks in the troposphere. The equation governing the NO(y) distribution is solved using finite differences with 33 uniformly spaced grid points in the micron range of -1 to 1. The distribution due to the stratospheric source has a minimum in the tropics, and there are broad maxima centered in the 25-30 degree region in both hemispheres. The calculated NO(y) due to lightning has a maximum near 15 degrees S, and it decreases to nearly a fourth of the maximum value near the poles. The drop off from the maximum is sharper in the Southern Hemisphere in comparison with the Northern Hemisphere.

Hameed, S.↗
NASA NTRS1980

Development of a pulsed 9.5 micron lidar for regional scale O3 measurement

A pulsed infrared lidar system designed for application to the remote sensing of atmospheric trace gases from an airborne platform is described. The system is also capable of measuring the infrared backscatter characteristics of the ocean surface, terrain, cloud, and aerosol targets. The lidar employed is based on dual wavelength pulse energy measurements in the 9-11 micrometer wavelength region.

Stewart, R. W.↗
NASA NTRS1979

Latitudinal distribution of the sources of carbon monoxide in the troposphere

A vertically and zonally averaged model of the troposphere has been constructed which calculates photochemical interactions and diffusive North-South transport of trace species. The model can be used to calculate the latitudinal distribution of the source function of a species if its concentration distribution is known. This procedure has been applied to carbon monoxide and large sources have been found outside the industrialized belt in the Northern Hemisphere.

Hameed, S.↗
NASA NTRS1979

Remote measurement of tropospheric ozone

It is shown that a differential absorption lidar employing a pulsed CO2 laser and a direct detection receiver is capable of significantly improving the existing data base on the tropospheric ozone burden. As a ground-based system, the lidar could obtain urban to regional scale O3 measurements with a vertical or horizontal resolution of at least 1 km in the troposphere. As a space-based system, it could obtain global scale coverage of the O3 burden below the stratospheric maximum of O3.

Bufton, J. L.↗
NASA NTRS1979

Tropospheric UV flux calculations and photolysis rates for use with zonally and diurnally averaged models

A computationally fast and efficient method for calculating solar UV fluxes in the troposphere is developed. Calculated fluxes compare favorably with more rigorous multiple scattering results, and qualitatively correct behavior of the direct and diffuse flux components is maintained. In view of the relatively large uncertainties in other aspects of tropospheric photochemical models, the use of approximate UV flux calculations appears warranted. The technique is applied to the calculation of photolysis rates for twelve significant photolytic reactions of nine species.

Matloff, G. L.↗
NASA NTRS1979

Sensitivity of the predicted CO-OH-CH4 perturbation to tropospheric NOx concentrations

Measurements indicate that ambient NOx(NO + NO2) concentrations vary over at least an order of magnitude in the troposphere. Thus it is of interest to study atmospheric chemistry with NOx concentrations covering this range. In this paper we present steady state calculations which show that as NOx concentrations are increased from very low values, the hydroxyl radical concentration increases at first and then decreases. This change of behavior occurs at a concentration of NO2 of about 0.23 ppb in the model. Also, the response of the atmosphere to increased CO fluxes undergoes a qualitative change as a function of the concentration of NOx. At low levels of NOx concentration, an increase in CO fluxes into the atmosphere depletes the OH concentration and increases the concentrations of CH4 and H2, as reported in the literature. However, at high values of NOx concentration, increased CO fluxes give rise to an increase in the OH concentration and, consequently, a decrease in the concentrations of CH4 and H2.

Hameed, S.↗
NASA NTRS1979

Application of a microprocessor controlled lidar to tropospheric ozone measurements

A microprocessor controlled lidar system under construction at the NASA Goddard Space Flight Center is described and the problems in making space-based measurements of tropospheric ozone are considered. The differential absorption lidar using a dual wavelength, pulsed CO2 laser and direct detection receiver can significantly improve the existing global data base on tropospheric ozone burden. Sensitivity to tropospheric ozone can be obtained in the spaceborne version of lidar by selecting laser lines located in the wings of the target zone lines. Simulation studies using various laser line pairs in the P-branch of the CO2 9.4 micron band show that the ozone burden retrieval may be weighted to particular altitude regions. These simulation studies are based on numerical integration of differences in absorption coefficient at the two selected laser wavelengths using the AFGL absorption line parameter compilations, U.S. Standard Atmosphere ozone profile, and laser software. Simulation, data collection and reduction are performed by a microprocessor subsystem of the CO2 lidar.

Stewart, R. W.↗
NASA NTRS1978

The natural and perturbed troposphere

A quantitative assessment of the chemical and climatic effects of industrial emissions into the atmosphere requires an understanding of the complex interactions of species within the atmosphere and of the atmosphere with other physical systems such as the oceans, lithosphere, and biosphere. The concentration of a particular species is determined by competition between various production and loss processes. The abundances of tropospheric gases are examined. The reactions of the members of the oxygen group are considered along with the models which have been developed to describe the involved relationships. Attention is also given to the natural carbon cycle, perturbations to the carbon cycle, the natural nitrogen cycle, perturbations to the nitrogen cycle, the hydrogen group, the sulfur group, and the halogen group.

Stewart, R. W.↗
NASA NTRS1978

Tropospheric ozone

Photochemical equilibrium models have been used to study the effect of recent revisions in important chemical reaction rates on the calculated tropospheric ozone concentration. Reasonable agreement with observed values is obtained if the NO(x) background is approximately 0.02 to 0.03 ppb, about an order of magnitude less than used in many previous model studies, but consistent with present nitrogen budget considerations and with observations of NO2. Chemical production and loss of ozone may dominate transport terms depending on values of key chemical rate constants and of the NO(x) background. This is consistent with the structure observed in the mean annual latitudinal distribution of ozone since ozone is its own chemical precursor and regions of enhanced stratosphere-troposphere exchange are also regions of enhanced photochemical activity.

Stewart, R. W.↗
NASA NTRS1977

Photochemistry of tropospheric ozone

A photochemical equilibrium model of minor constituent chemistry in the troposphere was constructed to investigate the photochemistry of tropospheric ozone. We find that photochemical production must be supplemented by additional sources to account for observed ozone levels in clean air, while photochemical loss is more important than surface destruction as an ozone loss mechanism. Photochemically produced ozone is sensitive to the NO(x) background in the model, exhibiting a maximum of about 0.5 ppb of NO(x).

Stewart, R. W.↗
NASA NTRS1976

A model study of the impact of emission control strategies on Los Angeles air quality

A generalized cell model is developed for the calculation of city-wide averages of photochemical smog components in Los Angeles. This model takes into account the effects of variations with time and within the city of the source strengths, the wind field, and the mixing depth. The effect of the influx of background pollution from outside the modeled volume is also included. Several control strategies for reducing automobile emissions are then introduced into the model, and their impact on predicted pollutant levels, particularly those of O3, are investigated.

Hameed, S.↗
NASA NTRS1975

Stratospheric ozone - Fragile shield

Atmospheric models that have been used in major studies on the possible impact of SST exhausts and Freons on stratospheric ozone are discussed and compared. An overview is given of ozone-reduction estimates that they produce, together with an assessment of possible effects of atmospheric testing of thermonuclear bombs in an attempt to find direct observational evidence for ozone depletion resulting from human activities. It is concluded that clear validation of atmospheric-model predictions is lacking.

Hoffert, M. I.↗
NASA NTRS1975

A chemical model of the troposphere and stratosphere

A time-dependent, one-dimensional model of the coupled chemistry and vertical mixing of the atmosphere is used to compute the distribution for many atmospheric constituents of the troposphere and stratosphere. The model treats the photochemistry of the carbon-hydrogen-oxygen-nitrogen system and is (with the exception of H2O) self-consistent in the sense of requiring no assumptions regarding minor constituent distributions.

Stewart, R. W.↗