bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial and temporal variability of nonmethane hydrocarbon mixing ratios and their relation to photochemical lifetime
Authors: Jobson B. T. , Parrish D. D. , Goldan P. , Kuster W. , Fehsenfeld F. C. , Blake D. R. , Blake N. J. , Niki H.
Published: February 2004

The relationship between temporal and spatial variability of C2‐C8 nonmethane hydrocarbon mixing ratios and their HO lifetimes (τ) is presented for samples collected during the 1993 North Atlantic Regional Experiment (NARE) and from other urban and remote sites. The C2‐C4 alkanes, acetylene and benzene typically define a trend of the form slnx = Aτ−b where slnx is the standard deviation of the ln of the mixing ratio. The relationship extended over a wider range of hydrocarbons in winter. The exponent b ranged in value from 0.28±0.023 for winter urban data where C2‐C8 hydrocarbons defined a strongly correlated trend, to 0.56±0.15 for C2‐C4 hydrocarbons at a coastal site in Nova Scotia during NARE. The trends are significantly different from that given by the Junge relationship [Junge, 1974]. Data from the Azores do not display such a trend and were likely influenced by local emissions. Variance trends are a useful analytical tool for examining the validity of hydrocarbon measurements.
Some aspects of the sensitivity of stratospheric climate simulation to model lid height
Authors: Lawrence Bryan N.
Published: February 2004

Two ensembles of simulations of the middle atmosphere are compared for early northern hemisphere winter. The two ensembles differ only in the position of the model lid (80 km or 96 km). Two sets of comparisons are carried out using a primitive equation mechanistic model; the first uses Rayleigh. friction in the mesosphere to mimic the effect of gravity waves, and the second uses the Hines gravity wave parameterization. Simulations which use Rayleigh friction are found to show little climatic sensitivity to the position of the lid. Simulations which use the Hines parameterization are shown to be very sensitive to the position of the lid, and this sensitivity is seen directly up to six scale heights below the lid. Indirect effects may extend throughout the middle atmosphere. This sensitivity is believed to be due to real circulation changes and not any numerical artifacts resulting from reflections from the model lid.
Observation of the transport of polluted air masses from the northeastern United States to Cape Sable Island, Nova Scotia, Canada, during the 1993 NARE summer intensive
Authors: Knapp K. G. , Balsley B. B. , Jensen M. L. , Hanson H. P. , Birks J. W.
Published: February 2004

Vertical profiles of ozone, temperature, pressure, and water vapor mass mixing ratio obtained using a parafoil kite platform during the North Atlantic Regional Experiment (NARE) 1993 summer intensive at Cape Sable Island, Nova Scotia, Canada, demonstrate the of use of kite platforms for the collection of vertically and temporally resolved data over a fixed location. During the period August 8–28, 1993, 39 profiles of the lower atmosphere were collected. Data collected as part of this field campaign illustrate the complex vertical stratification and temporal variability of pollutants transported into the Maritime Provinces of Canada. Transport phenomena resulted in pollution events in which ozone at the ground level remained in the 20–40 parts per billion by volume (ppbv) range, while mixing ratios of 90–130 ppbv were observed above ∼300 m. Back trajectories indicate that these highly elevated levels of ozone are attributable to source regions in the heavily industrialized northeastern United States. Vertical stratification of the lower atmosphere was also present during transport of Canadian air to the sampling site, with layers of both elevated and diminished ozone observed, while marine air did not exhibit layering characteristic of air masses originating from continental source regions.
Carbon monoxide column abundances and tropospheric concentrations retrieved from high resolution ground‐based infrared solar spectra at 43.5°N over Japan
Authors: Zhao Y. , Kondo Y. , Murcray F. J. , Liu X. , Koike M. , Kita K. , Nakajima H. , Murata I. , Suzuki K.
Published: February 2004

High spectral resolution (0.0024 cm−1), ground‐based infrared (IR) solar spectra were recorded at Rikubetsu, Japan (43.5°N, 143.8°E), by a Fourier transform infrared (FTIR) spectrometer covering the 4.8‐μm fundamental absorption band of carbon monoxide (CO). Total vertical CO columns and tropospheric concentrations, from May 1995 to May 1996, were retrieved from these spectra using an iterative inversion algorithm. Microwindows at 2111.00–2112.00 cm−1 and 2157.40–2159.20 cm−1 were used for the retrievals. Tropospheric CO concentrations above Rikubetsu retrieved from the IR solar spectra are in good agreement with aircraft measurements performed in the nearby area in recent years. The data exhibit seasonal variations in tropospheric CO concentrations. The averaged mixing ratios between 0 and 3 km are 219±26 parts per billion by volume (ppbv) in April and 115±7 ppbv in September. The data also show CO concentrations and seasonal variations decreasing with altitude. The vertical change in CO concentration is the greatest in spring and the smallest in fall. A maximum CO column of 3.31±0.34×1018 molecules cm−2 in April and a minimum of 2.04±0.10×1018 molecules cm−2 in September were observed. The CO column above Rikubetsu is consistent with observations at Kitt Peak, the International Scientific Station of the Jungfraujoch (ISSJ), and Zvenigorod, considering differences in altitude and latitude between the observatories.
Expansion of the luminous region of the lightning return stroke channel
Authors: Takagi N. , Wang D. , Watanabe T. , Arima I. , Takeuchi T. , Simizu M. , Katuragi Y. , Yokoya M. , Kawashima Y.
Published: February 2004

We developed a high‐speed line‐scanning camera (HSLSC) for measuring the radial variation of optical intensity of lightning channels. With this system we succeeded in obtaining a few examples of data for natural lightning. From these data we found that the luminous region of return stroke channels may expand with a velocity of more than 105 m/s during the initial stage and then reach the maximum diameter of several tens of meters after about 100 μs from the initiation of the return stroke and finally decrease with time. This suggests the occurrence of lateral discharges along the lightning channel during the propagation of return strokes.
Correction of Rayleigh scattering effects in cloud optical thickness retrievals
Authors: Wang Menghua , King Michael D.
Published: February 2004

We present results that demonstrate the effects of Rayleigh scattering on the retrieval of cloud optical thickness at a visible wavelength (0.66 μm). The sensor‐measured radiance at a visible wavelength (0.66 μm) is usually used to infer remotely the cloud optical thickness from aircraft or satellite instruments. For example, we find that without removing Rayleigh scattering effects, errors in the retrieved cloud optical thickness for a thin water cloud layer (τc=2.0) range from 15 to 60%, depending on solar zenith angle and viewing geometry. For an optically thick cloud (τc≳10), on the other hand, errors can range from 10 to 60% for large solar zenith angles (θ0≳60°) because of enhanced Rayleigh scattering. It is therefore particularly important to correct for Rayleigh scattering contributions to the reflected signal from a cloud layer both (1) for the case of thin clouds and (2) for large solar zenith angles and all clouds. On the basis of the single scattering approximation, we propose an iterative method for effectively removing Rayleigh scattering contributions from the measured radiance signal in cloud optical thickness retrievals. The proposed correction algorithm works very well and can easily be incorporated into any cloud retrieval algorithm. The Rayleigh correction method is applicable to cloud at any pressure, providing that the cloud top pressure is known to within ±100 hPa. With the Rayleigh correction the errors in retrieved cloud optical thickness are usually reduced to within 3%. In cases of both thin cloud layers and thick clouds with large solar zenith angles, the errors are usually reduced by a factor of about 2 to over 10. The Rayleigh correction algorithm has been tested with simulations for realistic cloud optical and microphysical properties with different solar and viewing geometries. We apply the Rayleigh correction algorithm to the cloud optical thickness retrievals from experimental data obtained during the Atlantic Stratocumulus Transition Experiment (ASTEX) conducted near the Azores in June 1992 and compare these results to corresponding retrievals obtained using 0.88 μm. These results provide an example of the Rayleigh scattering effects on thin clouds and further test the Rayleigh correction scheme. Using a nonabsorbing near‐infrared wavelength (0.88 μm) in retrieving cloud optical thickness is only applicable over oceans, however, since most land surfaces are highly reflective at 0.88 μm. Hence successful global retrievals of cloud optical thickness should remove Rayleigh scattering effects when using reflectance measurements at 0.66 μm.
Dust emission from Chinese desert sources linked to variations in atmospheric circulation
Authors: Zhang Xiao Y. , Arimoto Richard , An Zhi S.
Published: February 2004

Estimates of atmospheric dust deposition to five Asian/Pacific regions indicate that ∼800 Tg of Chinese desert dust is injected into the atmosphere annually; about 30% of this is redeposited onto the deserts, 20% is transported over regional scales, primarily within continental China. The remaining 50% of the dust is subject to long‐range transport to the Pacific Ocean and beyond. Elemental tracers based on several dustderived elements (Al, Fe, Mg, and Sc) reveal high‐frequency variability in the contributions of the western desert sources versus northern high‐dust and low‐dust desert sources to eolian deposits from the center of the Loess Plateau. Comparisons of the patterns uncovered with climate signals from the remote North Atlantic region for the last glaciation show that shifts in source areas of Asian dust were synchronous with large‐scale variations in atmospheric circulation.
A wind profiler climatology of boundary layer structure above the boreal forest
Authors: Wilczak James M. , Cancillo M. Luisa , King Clark W.
Published: February 2004

During the Boreal Ecosystem‐Atmosphere Study (BOREAS) summer 1994 campaign, a 915 MHz wind profiling radar and surface meteorological station were operated continuously from May 21 through September 20, 1994, near the old jack pine site in the southern study area. The data provided by the wind profiler are vertical profiles of wind speed and direction and virtual temperature, as well as boundary layer depth and the presence of precipitation. From this data set, we have documented the diurnal evolution of the atmospheric boundary layer and lower troposphere, especially changes in the diurnal pattern during the course of the summer growing season. Boundary layer depths are found to be surprisingly large, with summer‐averaged midafternoon values nearly 1.75 km and individual daily maximum depths often greater than 3 km. Monthly averaged profiler winds show a diurnal variation in scalar wind speed in the lowest kilometer, with a minimum reached near the time of greatest surface heating and a maximum reached shortly after midnight, with the difference as large as 2.4 m s−1. We interpret this to be the result of greater coupling between the atmosphere and Earth's surface during the day and a subsequent inertially generated nocturnal low‐level jet. Also evident in the summer‐averaged wind profiler statistics is the semidiurnal atmospheric tide. The amplitude of the tide, approximately 0.5 m s−1, is in agreement with theory and with previous measurements, as are the times of the maxima in north and west wind components. Frequency of precipitation as measured by the radar profiler shows a clear late afternoon and early evening maxima for the first half of the summer, and little diurnal variation in the second half. Finally, mean statistics from each Intensive Field Campaign (IFC) have been compared to the longer term averages to check for the meteorological representativeness of the IFCs. Similarly, mean statistics for the Golden Days within each IFC have been compared to the IFC for representativeness.
Interhemispheric asymmetry in the 1 mbar O3 trend: An analysis using an interactive zonal mean model and UARS data
Authors: Considine D. B. , Dessler A. E. , Jackman C. H. , Rosenfield J. E. , Meade P. E. , Schoeberl M. R. , Roche A. E. , Waters J. W.
Published: February 2004

Trends in O3 calculated from solar backscattered ultraviolet (SBUV) observations near 1 mbar are more negative at high latitudes in the southern hemisphere than in the northern hemisphere [Hood et al., 1993]. A mechanism is presented that produces an interhemispheric O3 trend asymmetry similar to the observed asymmetry in the Goddard Space Flight Center dynamically interactive zonal mean model. Upper Atmosphere Research Satellite (UARS) data are then examined for evidence that the atmospheric trend asymmetry is produced by a similar mechanism. The model O3 trend asymmetry is mainly due to interhemispheric differences in odd chlorine (Cly) partitioning. The asymmetry in Cly partitioning is caused primarily by lower amounts of CH4 and NO in the southern hemisphere than the northern hemisphere high latitudes, due to differences in the dynamical behavior of the model hemispheres. Symmetric increases in Cly are accompanied by a southern hemisphere increase in ClO that is larger than in the northern hemisphere. Concentrations of CH4 and N2O retrieved by the cryogenic limb array étalon spectrometer aboard UARS during 1992 are lower in the southern hemisphere fall and winter seasons at high latitudes in the upper stratosphere than in the northern hemisphere, favoring higher southern hemisphere ClO values. However, observations of ClO by the microwave limb sounder on UARS do not show consistently higher values in the southern hemisphere compared with the northern hemisphere in 1992, 1993, or 1994. The UARS data therefore do not confirm that a mechanism similar to the model mechanism occurs in the real atmosphere and is the cause of the SBUV O3 trend asymmetry.
Effects of fluid‐dynamical stirring and mixing on the deactivation of stratospheric chlorine
Authors: Tan D. G. H. , Haynes P. H. , MacKenzie A. R. , Pyle J. A.
Published: February 2004

We quantify the sensitivity to mixing of chlorine deactivation and ozone depletion in simplified models of the northern hemisphere, middle‐latitude lower stratosphere. A photochemical box model augmented by a volume exchange model of mixing was used to identify situations for which effects of ClOx deactivation through mixing with NOx‐rich air are at least as important as photochemical effects alone. In one simulation representative of a filament of high‐latitude, ClOx‐activated air mixing with low‐latitude air, the O3 depletion rate was found to be 0.287%, 0.127%, or 0.08% day−1, for a volume exchange rate of 0, 0.06, or 0.48 day−1, respectively. If we take the first rate as representative of Lagrangian models, the second rate as typical of the lower stratosphere, and the third rate as typical of those grid‐based models that do not resolve the real mixing length scales, then our results suggest that Lagrangian models that do not represent mixing processes can, in certain circumstances, be in error by as much or more than grid‐based models. The box model results allowed the formulation of a simplified, yet reasonably accurate reaction scheme for mixing‐induced chlorine deactivation which was then implemented in a two‐dimensional model of quasi‐horizontal transport along isentropic surfaces. The two‐dimensional model represents mixing by an effective horizontal diffusivity that accounts for vertical diffusivity and horizontal strain, and for advective transport it uses winds from lower stratospheric observational analyses. Mixing‐induced chlorine deactivation was found to exhibit substantial sensitivity to the effective diffusivity of the two‐dimensional model. The associated O2 depletion from the Molina and Molina [1987] ClO‐dimer cycle was found to exhibit sensitivity in a number of different regimes. The sensitivity increases with time and also depends on other details of the wind fields and the NOx concentration field. We suggest that O3 loss in low‐resolution models is sensitive to perturbations in the NO2 field, whereas, by contrast, O3 loss in the lower stratosphere is probably far less sensitive to such perturbations. To obtain estimates of O3 loss over 11 days that are not sensitive to the diffusivity employed, it was necessary to use effective horizontal diffusivities DH < 105 m2 s−1, corresponding to horizontal features of about 200 km and hence requiring spatial resolution of about 40 km, which is much higher resolution than routinely employed. The results from the box model and the two‐dimensional model are combined to assess the importance of mixing‐induced chlorine deactivation relative to photolysis‐induced deactivation.
Relationship between chemistry of air, fresh snow and firn cores for aerosol species in coastal Antarctica
Authors: Wolff E. W. , Hall J. S. , Mulvaney R. , Pasteur E. C. , Wagenbach D. , Legrand M.
Published: February 2004

Aerosol and fresh snow concentrations have been determined at three coastal Antarctic stations, Dumont d'Urville, Halley, and Neumayer. Model estimates suggest that dry deposition, including that caused by wind pumping, is only a minor contributor (of order 1%) to chemical fluxes at these sites with relatively high snow accumulation. Larger dry deposition fluxes are possible for very large aerosol particles, including sea‐salt aerosol. Measurements of surface snow on successive days provide experimental data that constrain the contribution of dry deposition to probably less than 10% of annual fluxes for all ions, although very high episodic fluxes of giant sea‐salt aerosol cannot be ruled out. Spatial variability, and frequent snow, fog and drift events, make it difficult to improve this quantification. Both theory and measurement suggest that fog deposition is also a minor contributor to the annual flux (probably <1%). Sublimation of surface snow and of blowing snow may increase snow concentrations by a few percent, with a larger role in summer, but should not affect fluxes. Wet deposition in falling snow appears to be by far the major contributor. However, the relationship between concentrations in snow and in simultaneously sampled aerosol at ground level was poor for most species. Scavenging ratios derived from these data are higher than those from the limited data previously available, but have huge uncertainties associated with them. Particularly at sites with frequent drifting snow, groundlevel aerosol measurements may be inappropriate for deriving scavenging ratios. Despite this, there is a general seasonal coincidence of high aerosol concentrations and high snow concentrations. We are also able to trace the chemistry of fresh snowfall to an ice core collected up to 2 years later. Although some major snowfall events may be missing, it seems that, as expected, there is no significant postdepositional modification of chemistry for aerosol species in the top meter of firn.
Comparison of model estimated and measured direct‐normal solar irradiance
Authors: Halthore Rangasayi N. , Schwartz Stephen E. , Michalsky Joseph J. , Anderson Gail P. , Ferrare Richard A. , Holben Brent N. , Ten Brink Harry M.
Published: February 2004

Direct‐normal solar irradiance (DNSI), the energy in the solar spectrum incident in unit time at the Earth's surface on a unit area perpendicular to the direction to the Sun, depends only on atmospheric extinction of solar energy without regard to the details of the extinction, whether absorption or scattering. Here we report a set of closure experiments performed in north central Oklahoma in April 1996 under cloud‐free conditions, wherein measured atmospheric composition and aerosol optical thickness are input to a radiative transfer model, MODTRAN 3, to estimate DNSI, which is then compared with measured values obtained with normal incidence pyrhe‐liometers and absolute cavity radiometers. Uncertainty in aerosol optical thickness (AOT) dominates the uncertainty in DNSI calculation. AOT measured by an independently calibrated Sun photometer and a rotating shadow‐band radiometer agree to within the uncertainties of each measurement. For 36 independent comparisons the agreement between measured and model‐estimated values of DNSI falls within the combined uncertainties in the measurement (0.3–0.7%) and model calculation (1.8%), albeit with a slight average model underestimate (−0.18±0.94%) for a DNSI of 839 W m−2 this corresponds to −1.5±7.9 W m−2. The agreement is nearly independent of air mass and water‐vapor path abundance. These results thus establish the accuracy of the current knowledge of the solar spectrum, its integrated power, and the atmospheric extinction as a function of wavelength as represented in MODTRAN 3. An important consequence is that atmospheric absorption of short‐wave energy is accurately parametrized in the model to within the above uncertainties.
Parameterization of urban subgrid scale processes in global atmospheric chemistry models
Authors: Calbó Josep , Pan Wenwei , Webster Mort , Prinn Ronald G. , McRae Gregory J.
Published: February 2004

We have derived a parameterization consisting of a set of analytical expressions that approximate the predictions by the California Institute of Technology ‐ Carnegie‐Mellon University (CIT) Urban Airshed Model for the net export to the environment (i.e., effective emissions) of several chemical species, as functions of 14 input parameters. For each species, effective emissions are a function of actual urban emissions of this and other species and of other urban domain properties such as meteorology. Effective emissions may be “aged” emissions of primary pollutants or actual production of secondary pollutants. To develop the parameterization we have applied the probabilistic collocation method, which uses the probability density functions of the inputs to generate a set of orthogonal polynomials. These polynomials are then used as the basis for a polynomial chaos expansion that approximates the actual response of the CIT model to its inputs. We assume that seasonal variations can be represented by sinusoidal functions. The parameterization provides a computationally very efficient simulation of the actual model behavior. We have compared the outputs of the parameterization with the outputs of the CIT model, and we conclude that it gives a quite good approximation for effective emissions, at least in the regions of highest probability of the input parameters. This parameterization is applicable to detailed uncertainty and sensitivity analyses and enables computationally efficient inclusion of urban‐scale processes as subgrid scale phenomena in global‐scale models.
Retrieval of Pinatubo aerosol optical depth and surface bidirectional reflectance from six years of AVHRR global vegetation index data over boreal forests
Authors: Molineaux Benoît , Royer Alain , O'Neill Norm
Published: February 2004

Six years (1989–1994) of weekly composite satellite data are analyzed with the objective of retrieving the aerosol optical depth over boreal forests. The average surface bidirectional reflectance distribution (BRD) was estimated from a sample of the original data, yielding similar results in different regions around the world. Four analytical models are compared in their ability to reproduce the surface BRD. In channel 1 (0.58–0.68 μm) the surface reflectance averages 0.05 and exceeds 0.10 in the backscattering region. Thus the non‐Lambertian ground contribution could not be neglected in the aerosol retrievals. A relevant radiative transfer code was run in iterative mode to retrieve the aerosol optical depth which best accounts for the difference between modeled surface BRD and measured satellite reflectance in all conditions. From comparisons with Sun photometer data we conclude that a large‐scale, persistent aerosol enhancement such as that resulting from Mount Pinatubo's eruption in 1991 is clearly discernible from weekly composite AVHRR data over boreal forests.
Air mass characteristics, aerosol particle number concentrations, and number size distributions at Macquarie Island during the First Aerosol Characterization Experiment (ACE 1)
Authors: Brechtel Fred J. , Kreidenweis Sonia M. , Swan Hilton B.
Published: February 2004

During the First Aerosol Characterization Experiment (ACE 1), continuous measurements were made of the particle number size distribution (between 18 and 540 nm diameter (Dp)) and total particle number concentration (Dp > 3 nm and Dp > 12 nm) on Macquarie Island, Tasmania (54°30′S, 158°57′E, 7 m above sea level). Periodic real‐time measurements of dimethyl sulfide were also made. Sampled air masses were separated into clean marine and those influenced by Tasmania or Antarctica. Observations were compared to those from a southern hemisphere midlatitude site (Cape Grim) and to sites on the Antarctic continent. It was found that the average total number concentration observed during clean marine conditions, 675 cm−3, was about 21% higher than values observed at Cape Grim during ACE 1 and was similar to the high end of the historical range of number concentrations reported by Gras [1995] for Cape Grim during the same time of year. During both clean marine and influenced conditions, the Aitken and accumulation modes dominate the number size distribution, with a Young Aitken mode observed less often. The number size distribution between 18 and 540 nm exhibited two and three modes 75% and 25% of the time, respectively, during clean marine conditions, more consistent with previous observations at Cape Grim than with those from coastal Antarctica. The typical bimodal number distribution at Macquarie Island exhibited average modal diameters of 33 and 113 nm during clean marine conditions, corresponding to the smaller Aitken mode and larger accumulation mode, respectively. The 50 to 70 nm diameter range corresponds to the minimum in the bimodal size distribution at Macquarie Island, except for continentally influenced periods when the size distribution exhibits an Aitken mode near 50 nm and an accumulation mode near 128 nm. The Young Aitken mode appeared most often during or immediately after periods of precipitation associated with both warm and cold fronts, when the Aitken and accumulation mode number concentrations were depleted. Evidence for possible cloud processing of aerosol was found during two Antarctic influenced periods. Variability in observed aerosol characteristics was found to coincide with changes in air mass source region as indicated by back trajectories and frontal passages.
Mesoscale spectral analysis of in situ horizontal and vertical wind measurements at 6 km
Authors: Gao X. , Meriwether J. W.
Published: February 2004

As part of the ALOHA/ ANLC‐93 campaign to investigate atmospheric dynamics, in situ wind measurements collected at 6 km with the gust probe sensor attached to the National Center for Atmospheric Research Electra aircraft were analyzed for 16 flight segments between 61°N and 14°S and between 92°W and 170°W over land and water. Horizontal wavenumber spectra for the vertical wind and both zonal and meridional horizontal‐wind components were calculated. The zonal and meridional wavenumber spectra exhibited a decrease in the spectral slope from ∼−1.98±0.02 for wavelengths between 2 km and 100 km to ∼−3.18±0.06 for the longer wavelengths between ∼100 km and ∼330 km. The wavenumber spectrum for the vertical wind component showed a −2 power law dependence for wavelengths in excess of 5 km and a generally flat shape at shorter wavelengths. An increase in power at the wavelength of ∼10 km was detected for the meridional wavenumber spectrum. These results were compared with predictions from two spectral models and with spectral results from other observations.
Lightning flash rate and type in convective storms
Authors: Solomon Robert , Baker Marcia
Published: February 2004

We use observations and results of a numerical thunderstorm model [Solomon and Baker, 1994] with the addition of a lightning parameterization [Solomon and Baker, 1996] to examine thunderstorm development and lightning flash rates in a tropical maritime, subtropical continental, and midlatitude continental storms, We examine the dependence of lightning occurrence and flash rate on cloud condensation nucleus concentrations, primary and secondary glaciation mechanisms, liquid water flux, and updraft velocities.
Interpretations of observed climatological patterns in stratospheric gravity wave variance
Authors: Alexander M. J.
Published: February 2004

Observational analyses of gravity waves in the stratosphere have revealed various climatological patterns in gravity wave activity. Seasonal, geographical, and vertical variations have all been observed. In this work, a linear model of gravity wave propagation is applied to investigate the underlying causes of some of the observed patterns. A collection of monochromatic gravity waves that represent a broad spectrum of wavenumbers and frequencies is input at 6‐km altitude in the model. Propagation of the waves through realistic background atmospheric wind and stability fields is treated with linear ray theory and a simple saturation condition to limit amplitudes to stable values. The wave spectrum at the 6‐km source height is specified to be constant at all latitudes, longitudes, and times, so the variability that appears at higher altitudes is due entirely to background atmosphere variations. Before the model results are compared to the observations, the spectrum of waves is filtered in a way that mimics the limitations of each of the observation techniques. The filtering is described in terms of vertical wavelength and is referred to as the “observational filter.” In a vertically varying background wind, gravity waves are Doppler‐shifted in intrinsic frequency and refracted to different vertical wavelengths as they propagate vertically through the atmosphere. The observational filter and the wave refraction effects can thus couple in interesting ways that have not been explicitly considered in previous work. The model shows that this coupling can give rise to geographical, seasonal, and vertical variations in gravity wave observations without any variations in the spectrum or amplitude of gravity wave sources in the troposphere. Thus careful consideration of both the background wind profile and observational filter can greatly affect the interpretation of the observed climatological patterns in gravity wave activity.
Theoretical models for gravity wave horizontal wave number spectra: Effects of wave field anisotropies
Authors: Gardner Chester S.
Published: February 2004

Theoretical models are derived for the gravity wave one‐dimensional (1‐D) and 2‐D horizontal wave number spectra which include the effects of wave field anisotropies. The new models are based on the diffusive filtering [Gardner, 1994] and linear instability [Dewan and Good, 1986] wave dissipation paradigms and are generalizations of our earlier models for isotropic spectra. They are derived by making two crucial assumptions, namely, that waves propagating at different azimuth angles are statistically independent because they originate at different locations from different sources, and all waves, including those in the so‐called saturation regime, obey the polarization and dispersion relations. The unambiguous 2‐D spectra can be expressed as functions of (h, ϕ) or (k, l), where k = h sin ϕ is the zonal wave number, l = h cos ϕ is the meridional wave number, h = (k2 + l2)1/2, and ϕ = tan−1k/l is the azimuth angle of propagation. For a given azimuth angle, the unambiguous (h, ϕ) spectrum is simply the 1‐D horizontal wave number spectrum associated with just the waves propagating in the ϕ direction scaled by 2π/h. The diffusive filtering theory spectra are separable in h and ϕ, while the linear instability theory spectra are not. These results can be used to characterize the wave fields generated by realistic source distributions in the lower atmosphere and are particularly useful for developing accurate parameterizations of gravity wave effects in global circulation models.
Photochemical alkene formation in seawater from dissolved organic carbon: Results from laboratory experiments
Authors: Ratte M. , Bujok O. , Spitzy A. , Rudolph J.
Published: February 2004

The production mechanism of light alkenes, alkanes, and isoprene was investigated in laboratory experiments by measuring their concentrations in natural seawater as a function of spectral range, exposure time and origin, and concentration of dissolved organic carbon (DOC). The production mechanism of alkanes and of isoprene could not be clarified. Ethene and propene are produced photochemically from DOC. The relevant spectral range is UV and short‐wavelength visible light. Initial production rates (up to day 10 of exposure) were in the range of several pmol L−1 h−1 (mg DOC)−1; the corresponding mean quantum yields for the spectral range of 300–420 nm were about 10−8. Generally, the production rates and the quantum yields for ethene were about 2 times that of propene. The key factors in the total column integrated oceanic alkene production are the solar photon flux at sea surface, the penetration depth of the light into the ocean (especially the relation between different light absorbers, i.e., the extinction due to absorption of DOC), and the wavelength‐ and DOC‐dependent quantum yields. As a result of the high variability of these parameters, actual local alkene production rates for a specific oceanic region may differ considerably from the globally averaged oceanic alkene production rates. The latter were estimated to be at most 5 Mt yr−1.
Surface air mass origins during the First Aerosol Characterization Experiment (ACE 1)
Authors: Whittlestone S. , Gras J. L. , Siems S. T.
Published: February 2004

During the First Aerosol Characterization Experiment (ACE 1) a number of meteorological and atmospheric composition measurements were made to identify anthropogenically impacted conditions and to infer information about air parcels during transport to the sampling point. Radon was used as an indicator of land contact supported by back trajectory calculations; additional information on anthropogenic sources was obtained from the concentrations of condensation nuclei and elemental carbon. A limited extension of the air mass categorization utilizes altitude profiles of potential temperature and water content derived from balloon soundings at the fixed sites. The four surface platforms were found to sample a very similar range of conditions, from very clean marine air masses to those strongly influenced by anthropogenic sources. Although this was expected because the study area is essentially encompassed by the one meteorological system, the precise origins of air masses were usually different for each platform, even, for example, when the Discoverer was within a hundred kilometres of Cape Grim. There was good agreement between the different air mass indicators under conditions when they could be compared. Radon was present when the back trajectory indicated passage over land at low altitude. Vertical profiles of potential temperature and water concentration were generally consistent with the origin of back trajectories. Excursions in the concentrations of condensation nuclei and elemental carbon occurred during periods of elevated radon concentrations when the path from land was short. However, these coincident periods of elevated concentration were rare when there was a long enough time from land for the different processing of each species to take effect.
Compositional mapping with the NEAR X ray/gamma ray spectrometer
Authors: Trombka J. I. , Floyd S. R. , Boynton W. V. , Bailey S. , Brückner J. , Squyres S. W. , Evans L. G. , Clark P. E. , Starr R. , Fiore E. , Gold R. , Goldsten J. , McNutt R.
Published: February 2004

The X ray/gamma ray spectrometer (XGRS) instrument on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft will map asteroid 433 Eros in the 0.7 keV to 10 MeV energy region. Measurements of the discrete line X ray and gamma ray emissions in this energy domain can be used to obtain both qualitative and quantitative elemental compositions with sufficient accuracy to enable comparison to the major meteorite typies. It is believed that Eros is an S‐type asteroid, the most common of the near‐Earth asteroids. The determination of whether Eros consists of either differentiated or undifferentiated materials is an important objective of this mission. Observations of Eros during the NEAR mission will contribute significantly to our understanding of the structure and composition of this asteroid. The NEAR spacecraft was successfully launched on February 17, 1996. The NEAR XGRS was turned on during the week of April 7, 1996, and all detector systems operated nominally. Background spectra have been obtained.
Chemical schemes for surface modification of icy satellites: A road map
Authors: Delitsky Mona L. , Lane Arthur L.
Published: February 2004

The icy Galilean satellites of Jupiter are subject to magnetospheric plasma ion bombardment, which induces chemical changes within the ice. The possible detection of CO2 on the surface of Ganymede by the Galileo spacecraft makes for a more complicated chemistry and increases the number of chemical compounds that may then be present. We outline chemical schemes for the irradiation of pure and mixed ices H2O/CO2 and suggest species which observers may detect on Europa, Ganymede, and Callisto, such as C3O2, H2CO3, H2O2, CO3, HO2, CO, H2CO, CH2CO, as well as K2O, KOH, and SO3, from plasma implantation. Column abundances of compounds in the ice are calculated using a specified energy input and G values (yield per 100 eV).
Constraints on the thermal and mixing history of lunar surface materials and comparisons with basaltic meteorites
Authors: Batchelor J. D. , Symes S. J. K. , Benoit P. H. , Sears D. W. G.
Published: February 2004

We have measured the induced thermoluminescence (TL) properties of mare basalts, highland rocks, glasses, regolith breccias, soils and core samples. We also performed a series of heating experiments and made cathodoluminescence (CL) observations of Apollo 16 soils. The data are readily interpreted in terms of feldspar being the dominant source of TL and CL, the known luminescence properties of feldspar and history of the samples. For example, the TL sensitivity of the mare basalts is lower than that of the highland basalts by about an order of magnitude, probably due to the differing FeO content of their feldspars. Similarly, the TL properties of regolith breccias can readily be explained in terms of thermal processes similar to those experienced by the soils and by mixing of highland and mare components. Our major new observations and interpretations include (1) that there are maturity‐dependent variations in the TL and CL properties of the core samples which reflect thermal annealing and melting during regolith working, (2) that the most “primitive” material in lunar samples in terms of their thermal histories is located in the immature lunar soils, and (3) that there is no TL evidence for widespread long‐term thermal metamorphism of lunar samples, TL being particularly sensitive to low‐level metamorphism in extraterrestrial materials. In this latter respect, lunar samples differ from basaltic meteorites which otherwise have very similar properties and histories. We argue that this reflects a greater tendency toward thick regoliths on asteroid‐sized bodies.
Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact
Authors: Pope Kevin O. , Baines Kevin H. , Ocampo Adriana C. , Ivanov Boris A.
Published: February 2004

A comprehensive analysis of volatiles in the Chicxulub impact strongly supports the hypothesis that impact‐generated sulfate aerosols caused over a decade of global cooling, acid rain, and disruption of ocean circulation, which contributed to the mass extinction at the Cretaceous/Tertiary (K/T) boundary. The crater size, meteoritic content of the K/T boundary clay, and impact models indicate that the Chicxulub crater was formed by a short period comet or an asteroid impact that released 0.7–3.4×1031 ergs of energy. Impact models and experiments combined with estimates of volatiles in the projectile and target rocks predict that over 200 gigatons (Gt) each of SO2 and water vapor, and over 500 Gt Of CO2, were globally distributed in the stratosphere by the impact. Additional volatiles may have been produced on a global or regional scale that formed sulfate aerosols rapidly in cooler parts of the vapor plume, causing an early, intense pulse of sulfuric acid rain. Estimates of the conversion rate of stratospheric SO2 and water vapor to sulfate aerosol, based on volcanic production of sulfate aerosols, coupled with calculations of diffusion, coagulation, and sedimentation, demonstrate that the 200 Gt stratospheric SO2 and water vapor reservoir would produce sulfate aerosols for 12 years. These sulfate aerosols caused a second pulse of acid rain that was global. Radiative transfer modeling of the aerosol clouds demonstrates (1) that if the initial rapid pulse of sulfate aerosols was global, photosynthesis may have been shut down for 6 months and (2) that for the second prolonged aerosol cloud, solar transmission dropped 80% by the end of first year and remained 50% below normal for 9 years. As a result, global average surface temperatures probably dropped between 5° and 31°K, suggesting that global near‐freezing conditions may have been reached. Impact‐generated CO2 caused less than 1°K greenhouse warming and therefore was insignificant compared to the sulfate cooling. The magnitude of sulfate cooling depends largely upon the rate of ocean mixing as surface waters cool, sink, and are replaced by upwelling of deep ocean water. This upwelling apparently drastically altered ocean stratification and circulation, which may explain the global collapse of the delta 13C gradient between surface and deep ocean waters at the K/T boundary.
Dust ablation during the Shoemaker‐Levy 9 impacts
Authors: Moses Julianne I.
Published: February 2004

Because of the complexity of the physical and chemical processes that occurred during the collision of Comet Shoemaker‐Levy 9 with Jupiter, our understanding of the observed phenomena is not complete. In this paper, models of the ablation of small particles from both the incoming cometary comae and the refalling impact plumes are used to better define the physics and chemistry of the impacts. The incomplete ablation of small refractory plume grains can explain both the timing and relative strengths of the metal emission lines (e.g., Mg and Na) that were observed many minutes after the impacts. Model‐data comparisons show that silicate dust was present in the fastest moving portions of the plume and that some of the plume material (at least during the L impact) was ejected at velocities in excess of 22 km s−1 (for a 45° ejection angle). The models also show that the plume dust is not completely ablated at typical plume reentry velocities; therefore, a good estimate for the maximum size of the plume grains is the final observed radius of the high‐altitude dust (e.g., <0.15 μm), and abundances inferred from the metallic emission features observed after the impacts do not provide a good measure of the amount of silicate material in the plume. Other implications for the physical and chemical properties of the comet/impact plume and for the timing and other features of the observations are discussed in detail.
Micropaleontology and palynology of core PAR87A‐10: A 23,000 year record of paleoenvironmental changes in the Gulf of Alaska, northeast North Pacific
Authors: de Vernal Anne , Pedersen Thomas F.
Published: February 2004

Micropaleontological data of core PAR87A‐10 reveal that the last glacial interval, prior to 13 ka, was marked by low biogenic fluxes and poor CaCO3 preservation. Quantitative estimates of sea‐surface conditions based on dinocyst assemblages suggest that cold temperatures and freezing winter conditions existed during this period. The glacial to interglacial transition, i.e., the 13–8 ka interval, was characterized by an increase in fluxes of microfossils indicating enhanced productivity in surface waters. A higher biogenic carbonate production probably resulted in better preservation of CaCO3. This interval was marked by relatively low salinity and by sea‐surface temperatures increasing toward modern values. Relatively high pollen flux during the transition suggests nutrient inputs through atmospheric and/or fluvial transport from the adjacent North American continent. After 8 ka, diminished fluxes of plankton, concomitant with a decline in pollen input, are associated with decreasing nutrient supply as predominantly eastward winds became established over the North Pacific.
Paleocene to Early Eocene paleoceanography of the Middle East: The δ13C and δ18O isotopes from foraminiferal calcite
Authors: Charisi Stella D. , Schmitz Birger
Published: February 2004

Paleocene to early Eocene benthic foraminiferal δ13C and δ18O records from southern Tethyan sections at Ben Gurion, Israel (paleodepth 500–700 m), and Gebel Aweina, Egypt (paleodepth 150–200 m), show generally similar trends but 1–3‰ more negative values than coeval deep‐sea isotopic records. In both Tethyan sections a negative δ13C excursion of 2.5–3‰ marks the benthic extinction event in the latest Paleocene. For at least 1 m.y. after this event, δ13C values were 1.5–2‰ more negative on the shelf than at upper bathyal depths, reflecting a deepening of the oxygen minimum zone, possibly related to an increase or spatial shift in upwelling. Benthic δ18O records indicate a 2–4°C temperature gradient between the shelf and upper bathyal depths. Temperature‐salinity reconstructions suggest that upwelling was a dominant mechanism for surface water formation in this part of the southern Tethys during the late Paleocene.
The Holocene preservation history of equatorial Pacific sediments
Authors: Oxburgh Rachel
Published: February 2004

A model of the sediment mixed layer is used to quantify the glacial‐Holocene changes in calcite preservation (productivity minus dissolution) at water depths that span the transition between lysocline and calcite compensation depth in the equatorial Pacific. The mean preservation fluxes required by the model to account for the observed changes in sediment composition indicate, in most cases, that calcite dissolution exceeds supply and that there has been net removal of calcite from surface sediments over the Holocene. The sediment mixed layer is therefore not at steady state with respect to CaCO3 fluxes. The model is also used to show that an observed increase in the radiocarbon age of the core top sediments with increasing extent of dissolution cannot be explained by chemical erosion of underlying glacial material. Dissolution taking place at or just below the sediment‐water interface is the probable cause of this phenomenon.