bluuebunny/crossref_metadata_2025 huggingface dataset:
Composition and sources of aerosol in a central African rain forest during the dry season
Authors: Roberts Gregory C. , Andreae Meinrat O. , Maenhaut Willy , Fernández‐Jiménez Maria‐Teresa
Published: February 2004

During the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO‐96), size‐fractionated aerosol samples were collected in November and December 1996 at a ground site in the tropical rain forest at the N'doki National Park (NNP) in the Republic of Congo. The samples were analyzed for up to 26 elements using particle‐induced X‐ray emission. Elements related to mineral dust and pyrogenic aerosol exhibited greater concentrations during the daytime, while aerosol produced by the rain forest exhibited higher concentrations at night. Samples were also collected at two levels on the tower, above and below the canopy, to characterize vegetation sources. Absolute principal component analysis (APCA) identified three major aerosol source types in each size fraction, which explained more than 90% of the data variance. The fine‐size fraction contained mineral dust (Al, Si, Ca, Ti, and Fe), pyrogenic (black carbon, K, and Zn), and marine/anthropogenic sulfur components. The coarse‐size fraction included a mineral dust (Al, Si, Ca, Ti, Mn, and Fe) and two primary biogenic components consisting of K, P, Zn, and S. Absolute principal component scores were calculated for the components of APCA, and temporal trends were compared to 7 day isopycnic backward trajectories. Consistent relationships between the temporal trends of the fine fraction aerosol components and meteorological patterns were observed. Trade wind air masses transported biomass burning and mineral dust aerosol to NNP during the first half of the experiment. The fine fraction sulfur component correlated well with the pyrogenic activity before the change in meteorological patterns halfway through the experiment. The fine and coarse sulfur concentrations nearly doubled in the latter part of the experiment as a monsoon circulation brought sulfur‐enriched aerosol from the Atlantic Ocean. Various industrial activities on the coast of Cameroon and Gabon probably contributed to the high sulfur concentrations as well.
Parameterizations of INDOEX microphysical measurements and calculations of cloud susceptibility: Applications for climate studies
Authors: McFarquhar Greg M. , Heymsfield Andrew J.
Published: February 2004

This study develops parameterizations of cloud microphysical properties in terms of bulk variables, provides new calculations of cloud susceptibility, and explores applications for climate studies. On the basis of cloud microphysical properties acquired during the Indian Ocean Experiment (INDOEX), the cloud effective radius re is observed to be proportional to the cube root of the ratio of cloud liquid water content (LWC) to total cloud droplet concentration Nt. A shape parameter k, depending on the skewness and dispersion of the cloud droplet size distribution, determines the proportionality constant and is highly dependent on the number of aerosols Na observed; k values range from 0.83±0.07 in pristine clouds (Na < 500 cm−3) to 0.73±0.08 in polluted clouds (Na > 1500 cm−3). Further, Nt is parameterized as a function of Na and cloud vertical velocity or LWC. Although the parameterization coefficients vary between pristine and polluted clouds, the parameterization equations are similar to those developed for other geographic locations and other cloud types, suggesting that they might be applied in climate models with varying coefficients. Using an empirically derived relation between Nt and the projected area of the cloud droplet distributions ac, the cloud susceptibilities are 2.16 times larger than those calculated using the original definition of susceptibility which does not take into account changes in cloud spectral width. Because susceptibility is shown to not vary significantly with wavelength in the solar regime, the average visible susceptibilities of 1.4×10−3 cm3 and 0.4×10−3 cm3 for pristine and polluted clouds are representative of the albedo effect. Observed susceptibilities and re agree with those calculated from the parameterization mostly within 30%, suggesting the parameterization can be applied in models to predict indirect radiative forcing.
Heterogeneous processes involving nitrogenous compounds and Saharan dust inferred from measurements and model calculations
Authors: Galy‐Lacaux C. , Carmichael G. R. , Song C. H. , Lacaux J. P. , Al Ourabi H. , Modi A. I.
Published: February 2004

Experimental data on aerosol chemical composition and gaseous concentrations in various African ecosystems have been obtained under the IGAC DEBITS AFRICA (IDAF) program. In this paper, data covering a complete wet and dry season (1996 and 1998) in the semiarid savanna of the Sahelian region of Niger are presented. The analysis of the aerosol chemical composition and the gas phase concentrations at the Banizoumbou station indicates two strong signatures: a nitrogenous component composed of nitric acid, ammonia, particulate ammonium, and nitrates; and a terrigenous component originating from semiarid and desert soils (calcium, carbonates, magnesium, potassium, sulfate). To further investigate the interactions between gas and particles and to help interpret the IDAF experimental data, these data are analyzed using a gas aerosol equilibrium model (Simulating Composition of Atmospheric Particles at Equilibrium (SCAPE)). The model is found to accurately represent the mean aerosol composition for the dry and the wet season of the studied region. It is found that heterogeneous processes involving terrigenous compounds are important and play a major role in partitioning semivolatile species, such as nitric acid, between the gas and aerosol phases. The important role of these heterogeneous processes in the atmospheric chemistry in the Sahelian region is discussed. To compare results obtained in the semiarid savanna of Niger and other African ecosystems, SCAPE model is also applied to humid savanna and forest using IDAF and Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) measurements.
Comparison of cross‐tropopause transport and ozone in the upper troposphere and lower stratosphere region
Authors: Morgenstern Olaf , Carver Glenn D.
Published: February 2004

Wintertime tracer transport in the atmospheric “middle world” is analyzed for five winters using three different advection methods, namely Eulerian advection, reverse domain‐filling trajectory calculations, and contour advection with a diagnosis of filaments. In all approaches, isentropic motion is assumed. The results are compared to synopses of ozone derived from the Measurement of Ozone and Water Vapor by Airbus In‐Service Aircraft (MOZAIC) airborne ozone measurement program. Tracer and ozone data are cast into the “reduced” space spanned by equivalent latitude, potential temperature, and time. In this representation some anomalies of MOZAIC ozone are mirrored in corresponding features of stratospheric tracer mass mixing ratios. Eulerian and reverse domain‐filling trajectory studies give similar results, suggesting that numerical diffusion only plays a minor role. Tracer transport out of the stratosphere minimizes in the height range of 340 to 350 K. The trajectory studies reveal preferred regions of isentropic stratosphere‐troposphere exchange associated with the positions of storm tracks. These asymmetries are similar to asymmetries in the correlation of potential vorticity (PV) and ozone found in the MOZAIC data. Zonal asymmetries are also found in the occurrence of filamentary structure in the contour advection simulations with filamentation maximizing downstream of the North Atlantic storm track. Hemispheric‐ and seasonal‐mean filamentation activity is strongly correlated with stratosphere‐troposphere exchange discerned from the Lagrangian transport experiments. In both signals the winter of 1997/1998 shows the lowest activity of the five winters analyzed here. A drop in activity in the winter of 1995/1996 may, however, be related to the introduction of a more advanced data assimilation scheme to the European Centre for Medium‐Range Weather Forecasts (ECMWF) analysis cycle. This change may also be partly responsible for a somewhat improved correlation of PV and MOZAIC ozone at high equivalent latitudes after the winter of 1994/1995.
A new retrieval for cloud liquid water path using a ground‐based microwave radiometer and measurements of cloud temperature
Authors: Liljegren James C. , Clothiaux Eugene E. , Mace Gerald G. , Kato Seiji , Dong Xiquan
Published: February 2004

A new method to retrieve cloud liquid water path using 23.8 and 31.4 GHz microwave radiometer brightness temperature measurements is developed. This method does not depend on climatological estimates of either the mean radiating temperature of the atmosphere Tmr or the mean cloud liquid water temperature Tcloud. Rather, Tmr is estimated from surface temperature and relative humidity measurements, while Tcloud is estimated using millimeter‐wave cloud radar data, together with atmospheric temperature profiles obtained from either radiosonde or rapid update cycle (RUC) model output. Simulations demonstrate that the new retrieval method significantly reduces the biases in the liquid water path estimates that are apparent in a site‐specific retrieval based on monthly stratified, local climatology. An analysis of the liquid water path estimates produced by the two retrievals over four case study days illustrates trends and retrieval performances consistent with the model simulations.
Saharan dust in Brazil and Suriname during the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA) ‐ Cooperative LBA Regional Experiment (CLAIRE) in March 1998
Authors: Formenti P. , Andreae M. O. , Lange L. , Roberts G. , Cafmeyer J. , Rajta I. , Maenhaut W. , Holben B. N. , Artaxo P. , Lelieveld J.
Published: February 2004

Advection of Saharan dust was observed via chemical and optical measurements during March 1998 in Brazil and Suriname during the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA)‐Cooperative LBA Airborne Regional Experiment (CLAIRE)‐98 experiment. In Brazil the dust outbreak produced an increase of a factor of 3 in the daily mean mass concentration (up to 26±7 μg m−3) of particles smaller than 10 μm equivalent aerodynamic diameter (EAD), and in the daily mean aerosol particle scattering coefficient σs (up to 26±8 Mm−1 STP, ambient humidity). Background levels of aerosol scattering (ambient) were σs ∼ 10 Mm−1. The effect of dust advection was evident for all major crustal elements (Al, Si, Ca, Ti, Mn, and Fe), as well as the sea‐salt elements (Na, Cl, and S), as the dust layer was transported at low altitude (below 800 hPa). Coarse P and organic carbon (OC) concentrations were not influenced by the occurrence of dust, and were mainly emitted by the rain forest. The dry scattering mass efficiency of dust (particles smaller than 10 μm EAD) was estimated to be between 0.65 (±0.06) and 0.89 (±0.08) m2 g−1. Airborne profiles of aerosol scattering showed two distinct types of vertical structure in the dust layer over Suriname, either vertically uniform (15, 26 March), or plume‐like (25 March). Dust layers extended generally up to 700 hPa, while scattering layers occasionally encountered at higher altitudes resulted from smoke emitted by biomass burning in Venezuela and Colombia. Observations in South America were supported by measurements in Israel and Tenerife (Canary Islands), where the dust outbreaks were also detected.
Measurements of OH aboard the NASA P‐3 during PEM‐Tropics B
Authors: Mauldin R. L. , Eisele F. L. , Cantrell C. A. , Kosciuch E. , Ridley B. A. , Lefer B. , Tanner D. J. , Nowak J. B. , Chen G. , Wang L. , Davis D.
Published: February 2004

Airborne measurements of the hydroxyl radical were performed aboard the NASA P‐3B using the Selected Ion Chemical Ionization technique during the Pacific Exploratory Mission (PEM)‐Tropics B study. Typical midday boundary layer OH concentrations of 6–7 × 106 molecule cm−3 were observed in the vicinity of Christmas Island (0°‐3°N)) and 7–8 × 106 molecule cm−3 in the vicinity of Tahiti (17°‐22°S). Photochemical box model calculations of the concentration of OH ([OH]) throughout the entire mission yield generally good agreement (slope = 1.26) with a tendency for the model to slightly overestimate the measured OH. Boundary layer measurements of OH in an air mass containing low ozone concentrations (<5 ppbv) yielded a midday [OH] of ∼5×106 molecule cm−3, some 45% lower than 7×106 molecule cm−3 observed in air masses at the same latitude with more typical ozone concentrations (30–40 ppbv). Measurements performed under clouds yielded [OH] of ∼2.5×106 molecule cm−3, a value which is a factor of 2 lower when compared to data taken on either side of the cloud. This decrease corresponds to the decrease in j(O3) to produce O(1D).
Ozone in the Pacific tropical troposphere from ozonesonde observations
Authors: Oltmans Samuel J. , Johnson Bryan J. , Harris Joyce M. , Vömel Holger , Thompson Anne M. , Koshy Kanatathu , Simon Patrick , Bendura Richard J. , Logan Jennifer A. , Hasebe Fumio , Shiotani Masato , Kirchhoff Volker W. J. H. , Maata Matakite , Sami Gopal , Samad Ansari , Tabuadravu Jioji , Enriquez Humberto , Agama Mario , Cornejo Jaime , Paredes Francisco
Published: February 2004

Ozone vertical profile measurements obtained from ozonesondes flown at Fiji, Samoa, Tahiti, and the Galapagos are used to characterize ozone in the troposphere over the tropical Pacific. There is a significant seasonal variation at each of these sites. At sites in both the eastern and western Pacific, ozone mixing ratios are greatest at almost all levels in the troposphere during the September‐November season and smallest during March‐May. The vertical profile has a relative maximum at all of the sites in the midtroposphere throughout the year (the largest amounts are usually found near the tropopause). This maximum is particularly pronounced during the September‐November season. On average, throughout the troposphere, the Galapagos has larger ozone amounts than the western Pacific sites. A trajectory climatology is used to identify the major flow regimes that are associated with the characteristic ozone behavior at various altitudes and seasons. The enhanced ozone seen in the midtroposphere during September‐November is associated with flow from the continents. In the western Pacific this flow is usually from southern Africa (although 10‐day trajectories do not always reach the continent) but also may come from Australia and Indonesia. In the Galapagos the ozone peak in the midtroposphere is seen in flow from the South American continent and particularly from northern Brazil. High ozone concentrations within potential source regions and flow characteristics associated with the ozone mixing ratio peaks seen in both the western and eastern Pacific suggest that these enhanced ozone mixing ratios result from biomass burning. In the upper troposphere, low ozone amounts are seen with flow that originates in the convective western Pacific.
Climatological characteristics of the tropical tropopause as revealed by radiosondes
Authors: Seidel Dian J. , Ross Rebecca J. , Angell James K. , Reid George C.
Published: February 2004

A temporally and spatially comprehensive depiction of the tropical tropopause is presented, based on radiosonde data from 83 stations. Climatological statistics for 1961–1990 are computed for three levels: the conventional lapse‐rate tropopause (LRT), the cold‐point tropopause (CPT), and the 100 hPa level. Mean values and seasonal and interannual variations of temperature, pressure, height, potential temperature, and water vapor saturation mixing ratio at these levels are compared. The tropopause is higher, colder, and at lower pressure in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH) in NH winter. This pattern reverses in NH summer, except that the tropopause remains colder in the NH than in the SH. The climatological locations of minimum tropopause temperature differ from those of maximum height and minimum pressure: In NH winter the tropopause is coldest over the western tropical Pacific warm pool region, but it is highest and at lowest pressure over the western Atlantic. Correlations of interannual anomalies in zonal‐mean characteristics reveal that the height of the tropopause reflects the temperature of the underlying troposphere. Tropopause temperature, on the other hand, shows little association with tropospheric characteristics but is significantly correlated with the temperature and pressure of the lower stratosphere. The 100 hPa level is a poor surrogate for the tropical tropopause. Changes in radiosonde instrumentation limit the potential for detecting tropopause trends. However, the following (nonmonotonic) trends in the tropopause in the deep tropics during 1978–1997 seem robust: an increase in height of about 20 m decade−1, a decrease in pressure of about 0.5 hPa decade−1, a cooling of about 0.5 K decade−1, little change in potential temperature, and a decrease in saturation volume mixing ratio of about 0.3 ppmv decade−1.
Effect of tidal variability on the mean diurnal variation of noctilucent clouds
Authors: Klostermeyer J.
Published: February 2004

Lidar observations of noctilucent clouds (NLCs) have shown that the mean diurnal variations of the NLC altitude and backscatter ratio are dominated by semidiurnal components that are at odds with previous model simulations using mean tidal temperature variations. Measured tides, however, exhibit a strong day‐to‐day variability so that, as a consequence of the highly nonlinear response of NLC models to temperature changes, the mean diurnal NLC variation cannot be obtained from the mean diurnal temperature variation but has to be simulated by averaging over NLC samples computed from individual diurnal temperature variations. Therefore a simple model of tidal variability based on means and standard deviations of the diurnal and semidiurnal temperature amplitudes and phases is set up so that samples of the diurnal temperature variation and corresponding NLC samples can be computed. The resulting mean diurnal variations of the NLC altitude and backscatter ratio show a good overall agreement with the observed variations and clearly reflect the strong semidiurnal components. Discrepancies between observed and simulated results can be attributed at least partly to deficiencies of the tidal variability model which, in particular, does not take into account any coherence between tidal and planetary wave‐associated temperature variations.
Subdaily variations of atmospheric dimethylsulfide, dimethylsulfoxide, methanesulfonate, and non‐sea‐salt sulfate aerosols in the atmospheric boundary layer at Dumont d'Urville (coastal Antarctica) during summer
Authors: Legrand Michel , Sciare Jean , Jourdain Bruno , Genthon Christophe
Published: February 2004

A study of atmospheric dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) was conducted on a subdaily basis during austral summer months (450 samples from mid‐December 1998 to late‐February 1999) at Dumont d'Urville, a coastal Antarctic site (66° 40′S, 140° 01′E). In addition, subdaily aerosol samplings were analyzed for particulate methanesulfonate (MSA) and non‐sea‐salt sulfate (nssSO42−). During these summer months, DMS and DMSO levels fluctuated from 34 to 2923 pptv (mean of 290±305 pptv) and from 0.4 to 57 pptv (mean of 3.4±4.4 pptv), respectively. Mean MSA and non‐sea‐salt sulfate (nssSO42−) mixing ratios were close to 12.5±8.2 pptv and 68.1±35.0 pptv, respectively. In two occasions characterized by stable wind conditions and intense insolation, it was possible to examine the local photochemistry of DMS. During these events, DMSO levels tracked quite closely the solar flux and particulate MSA levels were enhanced during the afternoons. Photochemical calculations reproduce quite well observed diurnal variations of DMSO when we assume an 0.8 yield of DMSO from the DMS/OH addition channel and an heterogeneous loss rate of DMSO proportional to the OH radical concentration: 0.5×10−10 [OH] + 5.5x10−5 (in s−1). If correct, on a 24 hour average the heterogeneous loss of DMSO is estimated to be 2 times faster than the DMSO/OH gas phase oxidation in these regions. Very low levels of DMSO were found in the aerosol phase (less than 0.01 pptv), suggesting that an efficient oxidation of DMSO subsequently takes place onto the aerosol surface. The observed increase of MSA levels which takes place quasi‐immediately after the noon DMSO maximum suggests that an heterogeneous oxidation of DMSO onto aerosols represents a more efficient pathway producing MSA compared to the gas phase DMSO/OH pathway. Since only a third of the total amount of DMSO lost can be explained by the observed enhancement of MSA levels, further studies investigating other species including methanesulfinic acid and dimethylsulfone (DMSO2) formed during the oxidation of DMS are here needed. When katabatic winds took place, bringing continental Antarctic air at the site, enrichments of DMSO relative to DMS and MSA relative to non‐sea‐salt sulfate levels were observed. That is in agreement with the hypothesis of an accumulation of DMSO and probably of gaseous MSA in the free Antarctic troposphere in relation to a less efficient heterogeneous loss rate of DMSO.
Seasonal variations of atmospheric dimethylsulfide, dimethylsulfoxide, sulfur dioxide, methanesulfonate, and non‐sea‐salt sulfate aerosols at Dumont d'Urville (coastal Antarctica) (December 1998 to July 1999)
Authors: Jourdain Bruno , Legrand Michel
Published: February 2004

A study of oxidation products of dimethylsulfide (DMS) was conducted at Dumont d'Urville (DDU), coastal Antarctica, from summer 1998/1999 to midwinter 1999. The study involved multiple daily measurements of DMS and dimethylsulfoxide (DMSO) using a gas chromatograph at the site. Methanesulfonate (MSA), and non‐sea‐salt sulfate (nssSO42−) aerosols were studied on a daily basis. A few studies of size‐segregated aerosol chemistry indicate that both MSA and nssSO42− are present in a dominant submicronic mode at 0.3 μm. MSA and nssSO42− levels exhibit a well‐marked seasonal cycle characterized by summer maxima (0.6±0.3 and 3.8±1.4 nmol m−3 of MSA and nssSO42− in January 1999, respectively) and winter minima (0.01±0.004 and 0.21±0.22 nmol m−3 of MSA and nssSO42− in July, respectively). In contrast, weak seasonal cycles of DMS (from 13.1±6.1 nmol m−3 in January to 3.9±1.3 nmol m−3 in July) and DMSO (from 0.15±0.1 nmol m−3 in January to 0.06±0.01 nmol m−3 in July) are observed there. A few SO2 samplings indicate a seasonal cycle with 2.8±0.9 nmol m−3 in January and levels close to the detection limit (0.25 nmol m−3) in winter. The major finding of this study is the presence of large amounts of DMS and DMSO in winter, whereas MSA levels are strongly decreased. These winter DMS levels may be due to small DMS emissions from open water present in sea ice located offshore DDU and/or advection from further north in conjunction with a long lifetime of DMS. The hypothesis of a heterogeneous uptake of DMSO onto aerosols followed by a rapid oxidation into MSA could explain the seasonal DMSO and MSA changes. With respect to the summer situation, in winter, DMSO levels of a tenths of nmol m−3 would result from transport of air masses located further north associated with a lifetime of DMSO of 2 days (instead of a few hours in summer) and a local production of DMSO from DMS oxidation. Such a winter DMSO production in spite of decreased DMS/OH addition pathway (50 times slower than in summer) results from decreased heterogeneous uptake (30 times slower) partly driven by reduction of available aerosol surface by a factor of 15. Finally, when katabatic regime took place bringing air from inland Antarctica, it is shown that the free troposphere above the Antarctic plateau in summer is enriched in DMSO and MSA with respect to DMS and nssSO42−, respectively. That supports the assumption of a different chemistry of DMS taking place in the free troposphere over Antarctica due to dry air conditions and absence of high aerosol levels.
Sulfur dioxide uptake and oxidation in sea‐salt aerosol
Authors: Hoppel William , Pasternack Louise , Caffrey Peter , Frick Glendon , Fitzgerald James , Hegg Dean , Gao Song , Ambrusko John , Albrechcinski Thomas
Published: February 2004

Measurements of SO2 and O3 uptake by sea‐salt and NaCl aerosol were made in a 600 m3 environmental chamber by measuring the rate of SO2 and O3 depletion during nebulization of seawater and NaCl solutions. The experiments were carried out with starting relative humidity between 80% and 92%, with SO2 concentrations between 35 and 60 ppb, and ozone concentrations between O and 110 ppb. For NaCl, no SO2 or O3 uptake was observed. For sea‐salt aerosol, uptake in the range of 0.21 and 1.2 millimoles of S per liter of (nebulized) seawater was observed. Surprisingly, no O3 uptake was observed even though the residence time of the aerosol in the chamber was long compared to the time required for the predicted S(IV)‐O3 reaction to occur. Several S(IV) oxidation schemes are considered to explain these observations. The Cl‐catalyzed aerobic mechanism as formulated by Zhang and Millero [1991] from empirical data best explains our observations. The Cl‐catalyzed S(IV) reaction decreases rapidly with decreasing pH, making it important only at pH>∼5.5. This rapid decrease with pH explains why SO2 uptake was not observed in the NaCl aerosol and observed at a level approaching the sea‐salt alkalinity in the case of sea‐salt aerosol.
Investigations on long‐term temperature changes in the upper stratosphere using lidar data and NCEP analyses
Authors: Keckhut Philippe , Wild Jeannette D. , Gelman Melvyn , Miller Alvin J. , Hauchecorne Alain
Published: February 2004

OHP lidar data and National Centers for Environmental Prediction (NCEP) stratospheric temperature analyses provide long and continuous databases for the middle and upper stratosphere that are highly valuable for long‐term studies. However, each data set has limitations. Comparisons between lidar data from 1979 to 1993 and NCEP data interpolated from the global analyses to the lidar location reveal significant mean temperature differences. Insight into the origin of the differences offers an opportunity to improve the overall quality of temperature monitoring in the stratosphere. Some of the differences can be explained by instrumental effects in the lidar system. In the stratosphere most of the limitations in lidar temperatures appear below 35–40 km, due to events of lidar misalignment (as large as 10 K) or to the effects on lidar data of volcanic aerosols (as large as 15 K). Changing biases between lidar and NCEP temperatures above 5 hPa coincide with replacement of satellites used in the NCEP analyses. However, some bias differences in upper stratospheric temperatures remain even after NCEP adjustments are made, based on rocketsonde comparisons. While these biases have been already suspected, they had never been explained. Here we suggest that the remaining bias (2–4 K) is caused by tidal influences, heretofore not accounted for by the NCEP adjustment procedure. Lidar profiles have been filtered in their lower part for misalignment and aerosol contamination. Long‐term changes have been compared, and a factor of 2 in trend differences have been reported. No significant trends (at 95% confidence) have been detected except with lidar around the stratopause and with NCEP analyses at 5 and 10 hPa. According to instrumental limitations of both data sets the temperature trend may vary from 1 to 3 K with altitude (10–0.4 hPa). Because only satellite data can provide global trend estimates and because lidar data have been chosen for ground‐based stratospheric monitoring programs, we suggest some plans to overcome these difficulties for past and future measurements. This should allow a more confident use for future trend estimates from both data sets.
A numerical model for an alternative origin of Lake Vostok and its exobiological implications for Mars
Authors: Duxbury N. S. , Zotikov I. A. , Nealson K. H. , Romanovsky V. E. , Carsey F. D.
Published: February 2004

In connection with recent Galileo images of the Jovian satellite Europa, there has been a significantly increased interest in the subglacial Lake Vostok in central East Antarctica. Since the theoretical prediction by Zotikov [1961] of the existence of lakes (one of which later was named Vostok) under ∼4 km of ice in Antarctica and its confirmation by radar measurements from aircraft in 1974 [Robin et al., 1977] and from the ERS‐1 satellite in 1993, it was tacitly assumed that the lake originated from the basal melting of ice [e.g., Zotikov, 1961, 1977, 1986; Kapitsa et al., 1996]. Recently, Zotikov and Duxbury [2000] proposed an alternative hypothesis that if Lake Vostok had existed on the surface before the Antarctic perennial glaciation began 5–30 Myr ago, then it was buried under the thermally protecting ice and never froze all the way to the bottom. In this study we support our hypothesis by comprehensive numerical modeling and evaluate its interesting implications for Mars and exobiology. Our numerical experiments on the sensitivity of the freezing depth to different upper boundary temperatures and different internal heat flows and the recent Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA) data support our model of the origin of possible subpolar water on Mars versus the old basal melting hypothesis. We also propose here experimental tests for our hypothesis. As a result of our computations, we conclude that Lake Vostok has survived the Antarctic glaciation without freezing completely, if it were initially an open lake with a surprisingly small water depth (greater than ∼53 m). The current water depth at a single point is as much as 500 m [Verkulich et al., 1996]. This maximum freezing depth of ∼53 m was reached in only 3300 years when the total upper ice cover was only ∼600 m. The computed critical water depth is applicable to other Antarctic lakes under a thick ice cover. For the Martian internal heat flow we have computed the critical water depth to be ∼63 m. The existence of the lake before the accumulation of ice in Antarctica implies that possible microorganisms and their remnants in the lake can be older than the Antarctic ice sheet, i.e., older than 5–30 Myr. If future observations find water under the Martian polar caps, this subpolar water, according to our model, has originated from lakes that were on the Martian surface before the formation of Mars' polar caps and have not completely frozen. Such environments would be ideal candidates for extant or extinct life on the red planet. This argues for a drilling mission at the Martian polar caps.
Sputtering of carbon monoxide ice by hydrogen ions
Authors: Schou Jørgen , Pedrys Roman
Published: February 2004

Carbon monoxide is an important constituent of comet comae, of icy surfaces of planetary bodies, and of interstellar grains. We present new laboratory studies of sputtering of frozen CO by hydrogen ions for energies below 10 keV. The sputtering yield turned out to depend critically on the energy and the electronic stopping power, (dE/dx)e, for the ions. The yield for a 9 keV H+ incident on solid CO was ∼34 CO/H+. For proton bombardment the yield is proportional to (dE/dx)e1.3, similar to the behavior of the sputtering yield for water ice. It means that the particle ejection occasionally requires species from two ionization/excitation events. For molecular ions the yield for CO ice increases with the square of the stopping power (dE/dx)e. The distribution of the sputtered CO molecules exhibits a maximum at 13 meV and falls off strongly with ejection energy E1. A complicating feature is the formation of a residue, possibly CO2 ice, during bombardment of solid CO. The sputtering yield depends slightly on the initial residue, similar to the case of sputtering by keV electrons. The sputtering yield and the chemical efficiency are high compared to similar properties of solid N2, which means that mixtures of these solids are preferentially depleted of CO.
Introduction to special section: New Views of the Moon II, a series of papers related to the lunar science initiative “New views of the moon enabled by combined remotely sensed and lunar sample data sets”
Authors: Jolliff Bradley L.
Published: February 2004

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.
Short‐time exosphere evolution following an impulsive vapor release on the Moon
Authors: Goldstein David B. , Austin J. Victor , Barker Edwin S. , Nerem R. Steven
Published: February 2004

The impact of the Lunar Prospector spacecraft in a lunar polar crater was used in an (unsuccessful) attempt to detect the release of vapor from volatilized water ice. The short‐time evolution of such an impulsively produced atmosphere is discussed. The effects of the initial vapor temperature (and thus its initial molecular thermal velocity) and the nature of the gas‐surface interaction are examined parametrically. It is shown that the exosphere quickly diffuses to the nightside of the Moon. The results have implications for observations of impulsive gas releases on airless bodies in general.
Latent outflow activity for western Tharsis, Mars: Significant flood record exposed
Authors: Dohm J. M. , Anderson R. C. , Baker V. R. , Ferris J. C. , Rudd L. P. , Hare T. M. , Rice J. W. , Casavant R. R. , Strom R. G. , Zimbelman J. R. , Scott D. H.
Published: February 2004

Observations permitted by the newly acquired Mars Observer Laser Altimeter data have revealed a system of gigantic valleys northwest of the huge Martian shield volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys (NSVs)). These features, which generally correspond spatially to gravity lows, are obscured by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials. Geologic investigations of the Tharsis region suggest that the system of gigantic valleys predates the construction of Arsia Mons and its extensive associated lava flows of mainly late Hesperian and Amazonian age and coincides stratigraphically with the early development of the outflow channels that debouch into Chryse Planitia. Similar to the previously identified outflow channels, which issued tremendous volumes of water into topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of immense magnitude and, as such, a source of water for a northern plains ocean.
A survey of Io's volcanism by adaptive optics observations in the 3.8‐μm thermal band (1996–1999)
Authors: Marchis Franck , Prangé Renée , Fusco Thierry
Published: February 2004

In this study, we analyze a series of images of Io obtained with the European Southern Observatory adaptive optics system (Adaptive Optics Near Infrared System, ADONIS) at 3.8 μm from 1996 to 1999, with particular emphasis on the observations carried out in late 1999 in support of the Galileo flybys of Io. Use of a new myopic deconvolution method, Myopic Iterative Step Preserving Algorithm (MISTRAL), especially designed for planetary objects, significantly improves the quality and the reliability of the reconstructed images. Using a simulation of artificial images of Io, we estimate the extent to which this algorithm is able to prevent noise amplification, to better restore sharp edges, and to preserve the initial photometry. Once this algorithm has been applied to our data and the solar‐reflected background has been subtracted, we have 89 images of Io available for search and temporal survey of the hot spots over 4 years. In most cases, the data, acquired during two consecutive nights, provide an almost global view of the surface of Io. We identify 20 hot spots for which we determine the coordinates and the brightness at 3.8 μm (as a function of time). More than half of the hot spots were detected on all the images and were considered as persistent. Particular emphasis has been put on the brightest two hot spots, Loki and Pele, for which we obtain a center‐to‐limb variation curve (a cosine curve for Loki and a curve decreasing slightly faster than a cosine law for Pele with possible physical interpretations). The temporal variations of their activity have been derived over these 4 years and, for Loki, compared with the results of other ground‐based surveys. The variability of the other, fainter, sources has also been derived but with less accuracy. We have also identified a few new hot spots, some of which are transient, such as a diffuse emission seen in September and November 1999 at the location of the upcoming Tvashtar outburst. The data are based on observations (58.F‐1003, 63.S‐0260, and 64.S‐0661) collected at the European Southern Observatory in La Silla, Chile.
Ocean thermohaline circulation and sedimentary 231Pa/230Th ratio
Authors: Marchal Olivier , François Roger , Stocker Thomas F. , Joos Fortunat
Published: February 2004

Holocene sediments from the Atlantic are characterized by 231Pa/230Th ratios below the production ratio of the two radionuclides in the water column (0.093), whereas Holocene sediments from the Southern Ocean have 231Pa/230Th > 0.093. This pattern of 231Pa deficit and excess was ascribed to southward 231Pa export from the Atlantic by the Atlantic thermohaline circulation (THC) as Pa is scavenged less efficiently by marine particles and more effectively transported by the THC than Th. The same pattern is observed in deposits of the Last Glacial Maximum (LGM), which led to a previous contention that the THC strength did not vary markedly through the last glacial termination. Here we embed a description of trace metal scavenging into a zonally averaged, circulation‐biogeochemistry ocean model to explore the sensitivity of 231Pa/230Th in Atlantic and Southern Ocean sediments to THC changes. Our results show that the production of biogenic opal (which, unlike other marine particles, poorly fractionates Th and Pa) in the Southern Ocean water column determines the spatial pattern of the sensitivity. Also, 231Pa/230Th increases in the North Atlantic but changes little in the South Atlantic and decreases in the Southern Ocean as THC is reduced. The mean 231Pa/230Th of the whole Atlantic is therefore less sensitive to THC changes than the mean 231Pa/230Th of the North Atlantic. The current uncertainties in Atlantic mean 231Pa/230Th are too large to rule out a twofold reduction of the THC at the LGM. However, the increase in North Atlantic mean 231Pa/230Th simulated in response to a twofold THC reduction is larger than the observed change in the North Atlantic mean 231Pa/230Th from the LGM to Holocene. Comparing this change with the modeled sensitivity of North Atlantic 231Pa/230Th to THC variations indicates that the THC at the LGM could not have been reduced by >30% of its present strength. Experiments of transient THC changes indicate that high‐resolution 231Pa/230Th records from North Atlantic sediments could also document thermohaline oscillations on century‐to‐millennial timescales.
Holocene sediment records from the continental shelf of Mac. Robertson Land, East Antarctica
Authors: Sedwick Peter N. , Harris Peter T. , Robertson Lisette G. , McMurtry Gary M. , Cremer Maximilian D. , Robinson Philip
Published: February 2004

Geochemical records are presented for five sediment cores from basins on the continental shelf of Mac. Robertson Land, East Antarctica. The cores contain 2–4 m thick sequences of hemipelagic, siliceous mud and ooze (SMO) deposited under seasonally open marine conditions. The inner and middle shelf SMO sequences are massive dark olive green material, whereas the outer shelf SMO sequences are dark olive material interspersed with light olive green layers ∼1–10 cm thick. The biogenic material is dominated by marine diatoms including Fragilariopsis curta, Fragilariopsis cylindrus, and Chaetoceros spp. in the dark‐colored SMO and Corethron criophilum in the light‐colored layers. Radiocarbon dates suggest that the cores provide continuous accumulation records extending from < 1 kyr before present (B.P.) back as far as 4–15 kyr B.P., with estimated accumulation rates of 0.07–5 mm yr−1. The three core records from the middle and outer shelf suggest six episodes of increased accumulation of biogenic material at ∼5.5 kyr B.P. (all three cores), 1, 2, and 6.2 kyr B.P. (two of the three cores), and 3.8 and 10.8 kyr B.P. (one core), most of which coincide with Corethron layers. We interpret these features as the result of enhanced diatom production over the outer shelf, possibly related to climatic warm periods. The absence of such features in the inner shelf core records is thought to reflect a relatively constant level of seasonal diatom production in adjacent waters maintained by a coastal polynya.
Similar glacial and Holocene Southern Ocean hydrography
Authors: Matsumoto Katsumi , Lynch‐Stieglitz Jean , Anderson Robert F.
Published: February 2004

We present new Holocene and glacial δ18O data measured on planktonic foraminifera Neogloboquadrina pachyderma and benthic foraminifera Cibicidoides species from the Atlantic and Pacific sectors of the Southern Ocean in order to better understand its glacial hydrography. Combined with previously published data, the latitudinal δ18O distributions of these foraminifera show no appreciable difference during the Holocene and the Last Glacial Maximum. This suggests that the basic Southern Ocean hydrography, including stratification and the mean position of the Antarctic Circumpolar Current frontal system, remained largely unchanged, although the small number of Pacific data render any conclusion in that sector preliminary.
Multidecadal variations in the Early Holocene outflow of Red Sea water into the Arabian Sea
Authors: Jung S. J. A. , Ganssen G. M. , Davies G. R.
Published: February 2004

We present Holocene stable oxygen isotope data from the deep Arabian Sea off Somalia at a decadal time resolution as a proxy for the history of intermediate/upper deep water. These data show an overall δ18O reduction by 0.5‰ between 10 and ∼6.5 kyr B.P. superimposed upon short‐term δ18O variations at a decadal‐centennial timescale. The amplitude of the decadal variations is 0.3‰ prior, and up to 0.6‰ subsequent, to ∼8.1 kyr B.P. We conclude from modeling experiments that the short‐term δ18O variations between 10 and ∼6.5 kyr B.P. most likely document changes in the evaporation‐precipitation balance in the central Red Sea. Changes in water temperature and salinity cause the outflowing Red Sea Water to settle roughly 800 m deeper than today.
The effect of snow accumulation on imaging riometer performance
Authors: Rose Mike C. , Jarvis Martin J. , Clilverd Mark A. , Maxfield David J. , Rosenberg Theodore J.
Published: February 2004

In January 1998 an imaging riometer system was deployed at Halley, Antarctica (76°S, 27°W), involving the construction of an array of 64 crossed‐dipole antennas and a ground plane. Weather conditions at Halley mean that such an array will rapidly bury beneath the snow, so the system was tuned to operate efficiently when buried. Theoretical calculations indicate that because the distance between the ground plane and the array was scaled to be 1/4λ in the snow, as snow fills the gap the signal will increase by 0.6–2.5 dB. Similarly, the short antennas are resonant when operated in snow, not in air. Theoretical calculations show that the largest effect of this is the mismatch of their feed point impedance to the receiver network. As the signal for each riometer beam is composed of a contribution from all 64 antennas, for each antenna that buries the signal level will increase by 1/64 of ∼9 dB. The measured response of the system to burial showed significant changes as snow accumulated in and over the array during 1998. The changes are consistent with the magnitude of the effects predicted by the theoretical calculations. The Halley imaging riometer system, having now been buried completely, is operating more efficiently than if a standard air‐tuned configuration had been deployed. The results are of considerable relevance to the ever‐increasing community of imaging riometer users regarding both deployment and the subsequent interpretation of scientific data. Some systems will experience similar permanent burial, while others will be subject to significant annual variability as a result of becoming snow‐covered during winter and clear during summer.
Volume‐integral‐equation‐based analysis of transient electromagnetic scattering from three‐dimensional inhomogeneous dielectric objects
Authors: Gres Noel T. , Ergin Arif A. , Michielssen Eric , Shanker Balasubramaniam
Published: February 2004

A novel technique for analyzing transient electromagnetic scattering from three‐dimensional inhomogeneous dielectric targets is proposed. An integral equation for the electric flux density throughout the scatterer is constructed by invoking the electromagnetic volume equivalence principle. This equation is solved using a marching‐on‐in‐time scheme in which the electric flux density is expanded in space by volumetric rooftop basis functions defined on a tetrahedral mesh and in time by piecewise polynomials. The proposed method is validated for representative dielectric structures by comparison via Fourier transformation of scattering data obtained with this method and various frequency domain techniques.
Design and calculation of the directional leaky coaxial cables
Authors: Wang Jun Hong , Mei Kenneth K.
Published: February 2004

Directional leaky coaxial cables (DLCX) with periodic slots are proposed. The difference between this kind of leaky cable and the conventional LCX is that there are two rows of slots located oppositely on the outer shield of the coaxial cable. Different radiation patterns in the r‐ф plane can be obtained by adjusting the relative distance between the two arrays. The radiation patterns and coupling loss are calculated by finite difference time domain method and dyadic Green's function.