bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of stratospheric input to the Arctic troposphere: 7Be and 10Be in aerosols at Alert, Canada
Authors: Dibb Jack E. , Meeker L. David , Finkel Robert C. , Southon John R. , Caffee Marc W. , Barrie Leonard A.
Published: February 2004

Concentrations of 7Be and 210Pb in 2 years of weekly high‐volume aerosol samples collected at Alert, Northwest Territories, Canada, showed pronounced seasonal variations. We observed a broad winter peak in 210Pb concentration and a spring peak in 7Be. These peaks were similar in magnitude and duration to previously reported results for a number of stations in the Arctic Basin. Beryllium 10 concentrations (determined only during the first year of this study) were well correlated with those of 7Be; the atom ratio 10Be/7Be was nearly constant at 2.2 throughout the year. This relatively high value of 10Be/7Be indicates that the stratosphere must constitute an important source of both Be isotopes in the Arctic troposphere throughout the year. A simple mixing model based on the small seasonal variations of 10Be/7Be indicates an approximately twofold increase of stratospheric influence in the free troposphere in late summer. The spring maxima in concentrations of both Be isotopes at the surface apparently reflect vertical mixing in rather than stratospheric injections into the troposphere. We have merged the results of the Be‐based mixing model with weekly O3 soundings to assess Arctic stratospheric impact on the surface O3 budget at Alert. The resulting estimates indicate that stratospheric inputs can account for a maximum of 10–15% of the O3 at the surface in spring and for less during the rest of the year. These estimates are most uncertain during the winter. The combination of Be isotopic measurements and O3 vertical profiles could allow quantification of the contributions of O3 from the Arctic stratosphere and lower latitude regions to the O3 budget in the Arctic troposphere. Although at present the lack of a quantitative understanding of the temporal variation of O3 lifetime in the Arctic troposphere precludes making definitive calculations, qualitative examples of the power of this approach are given.
Observations of the Earth's Radiation Budget in relation to atmospheric hydrology: 4. Atmospheric column radiative cooling over the world's oceans
Authors: Stephens Graeme L. , Slingo Anthony , Webb Mark J. , Minnett Peter J. , Daum Peter H. , Kleinman Lawrence , Wittmeyer Ian , Randall David A.
Published: February 2004

This paper introduces a simple method for deriving climatological values of the longwave flux emitted from the clear sky atmosphere to the ice‐free ocean surface. Simulations of the clear sky longwave fluxes to space and to the surface are employed in this study to assist in the development of this flux retrieval which requires monthly averaged column‐integrated water vapor w and the clear sky top‐of‐atmosphere (TOA) outgoing longwave flux (both available from satellite measurements). It is shown using both theory and data from simulations how the ratio of the surface to TOA flux is a simple function of w and a validation of the simple relationship is presented based on a limited set of surface flux measurements. The rms difference between the retrieved surface fluxes and the simulated surface fluxes is approximately 6 W m−2. The clear sky column cooling rate of the atmosphere is derived from the Earth Radiation Budget Experiment (ERBE) values of the clear sky TOA flux and the surface flux retrieved using Special Scanning Microwave Imager (SSM/I) measurements of w together with ERBE clear sky fluxes. The relationship between this column cooling rate, w, and the sea surface temperature (SST) is explored and it is shown how the cooling rate systematically increases as both w and SST increase. The uncertainty implied in these estimates of cooling are approximately ±0.2 K d−1. The effects of clouds on this longwave cooling are also explored in a limited way by placing bounds on the possible impact of clouds on the column cooling rate based on certain assumptions about the effect of clouds on the longwave flux to the surface. While a more global assessment of the cloud effect must await use of new satellite data that will allow us to estimate the contributions by clouds to these surface fluxes, it is shown in this paper how the longwave effects of clouds in a moist atmosphere where the column water vapor exceeds approximately 30 kg m−2 may be estimated from presently available satellite data with an uncertainty estimated to be approximately 0.2 K d−1. Based on an approach described in this paper, we show how clouds in these relatively moist regions decrease the column cooling by almost 50% of the clear sky values and the existence of significant longitudinal gradients in column radiative heating across the equatorial and subtropical Pacific Ocean.
Simultaneous observations of noctilucent clouds and mesopause temperatures by lidar
Authors: Hansen G. , von Zahn U.
Published: February 2004

During the night of August 5–6, 1989, geometric and optical properties of a noctilucent cloud (NLC) as well as the thermal structure of the atmosphere above the cloud were measured by ground‐based lidar. The data were collected at Andenes, Norway (69°N, 16°E). The cloud was observed from 2220 UT until 0010 UT, during which time its mean altitude was 82.6 km. Temperatures were measured from 2129 UT until 0017 UT in the altitude range of 86 to 94 km. A persistent temperature minimum was located in the 87‐ to 89‐km altitude region with temperatures dropping from close to 140 K at the beginning of the NLC observation to around 130 K after midnight. In a 2‐km‐wide layer below this minimum the temperature was persistently too high to provide supersaturation of water assuming realistic water concentrations. Comparison with other temperature measurements in the first half of August 1989 shows that the thermal conditions at the mesopause on August 5–6, 1989, were not especially favorable for the formation of NLCs, while on other occasions with lower mesopause temperatures no NLCs were observed. We conclude that the occurrence of NLCs only limitedly depends on the local thermal conditions at the display height and above. Instead, the conditions along the whole NLC particle trajectory have to be taken into account. This implies that horizontal and vertical advection are important factors controlling the occurrence of NLC displays in late summer at this latitude.
Further evidence for particle nucleation in clear air adjacent to marine cumulus clouds
Authors: Perry Kevin D. , Hobbs Peter V.
Published: February 2004

Observational evidence is presented for the nucleation of condensation nuclei (CN) in the clear air adjacent to an isolated, marine, cumulus cloud. Two separate regions of particle nucleation are identified: one located above the cloud top, and the second located downwind of the cloud near the level of the anvil outflow. The regions of high CN concentrations were located in extremely clean marine air, with unactivated aerosol surface area (excluding the nucleation mode) <2 μm2 cm−3, air temperature −31°C, and higher relative humidities than the undisturbed environment. Vertical profile measurements downwind of the cloud showed that CN concentrations at the level of the anvil outflow (4.9 km) were 8 times greater than at any other level between the surface and 5.3 km. A conceptual model is formulated in which aerosol particles, sulfur dioxide (SO2), sulfuric acid vapor (H2SO4), dimethyl sulfide (DMS), and ozone (O3) from the boundary layer are entrained into the cumulus cloud. Total aerosol number concentrations and unactivated aerosol surface area decrease with height in the cloud due to Brownian diffusion and diffusiophoresis of cloud interstitial aerosol to hydrometeors, coalescence scavenging by cloud droplets, collisional scavenging by ice particles, and subsequent removal by precipitation. The air that is detrained from the cloud raises the relative humidity and vents the clean air, SO2, H2SO4, DMS, and O3 to the near‐cloud environment. Hydroxyl radicals then oxidize the SO2 and DMS to H2SO4. Under the conditions of high relative humidity, low total aerosol surface area, low temperatures, and high SO2 concentrations near cloud top, significant concentrations of new particles can be produced by homogeneous‐bimolecular nucleation of sulfuric acid solution droplets from H2SO4 and H2O vapor molecules. The concentration of CN as a function of time is calculated for the case described in this paper using a bimodal integral nucleation model. The model results show that significant numbers of CN could have been produced within a few hours by the homogeneous‐bimolecular nucleation of sulfuric acid solution droplets under the observed conditions provided the concentration of SO2 near cloud top was enhanced by vertical transport.
Radio interferometric observations of cloud‐to‐ground lightning phenomena in Florida
Authors: Shao X. M. , Krehbiel P. R. , Thomas R. J. , Rison W.
Published: February 2004

Radio frequency observations of cloud‐to‐ground lightning in Florida have been analyzed to document a number of features of the lightning. The observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges. Observations are presented for about 50 radiating events during five multiple‐stroke, normal‐polarity flashes to ground. The results confirm and extend results of a similar study of New Mexico lightning by Rhodes et al. (1994). Dart leaders, in‐cloud “K” events, and attempted leaders are found to be the same phenomena, namely, a negative‐polarity streamer that propagates horizontally and/or vertically to ground at estimated speeds of about 107 ms−1 down to 106 ms−1. The step or impulsive fast electric field change produced by some K events is found to be caused by positive breakdown back at the starting point of the negative streamer, after the streamer has progressed over some or all of its distance. This breakdown appears to initiate a “forward” stroke along the streamer channel which can renew the breakdown at the front end of the channel. K breakdown which happens to occur during a continuing current discharge to ground produces an M event (channel brightening) when it connects with the conducting channel to ground. The step fast field change of the M events is found to be produced at the time of connection inside the cloud; no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke. M events can also be initiated by fast (107 ms−1) positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region. These streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground, as the M event. In all cases the current increase or channel brightening produced by K and M events is predicted to occur in a forward direction, that is, toward or down the channel to ground, in the same direction as the initiating negative streamer. Positive breakdown is detected only immediately following return strokes or during some K streamers. In both cases the electric stresses are expected to be very large, suggesting that positive breakdown which produces a conducting channel is difficult to initiate. Results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground; this forced the subsequent leaders to develop a new path to ground, first as series of attempted leaders and then as a dart‐stepped leader.
Evidence for interannual variability of the carbon cycle from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network
Authors: Conway Thomas J. , Tans Pieter P. , Waterman Lee S. , Thoning Kirk W. , Kitzis Duane R. , Masarie Kenneth A. , Zhang Ni
Published: February 2004

The distribution and variations of atmospheric CO2 from 1981 to 1992 were determined by measuring CO2 mixing ratios in samples collected weekly at a cooperative global air sampling network. The results constitute the most geographically extensive, carefully calibrated, internally consistent CO2 data set available. Analysis of the data reveals that the global CO2 growth rate has declined from a peak of ∼2.5 ppm yr−1 in 1987–1988 to ∼0.6 ppm yr−1 in 1992. In 1992 we find no increase in atmospheric CO2 from 30° to 90°N. Variations in fossil fuel CO2 emissions cannot explain this result. The north pole‐south pole CO2 difference increased from ∼3 ppm during 1981–1987 to ∼4 ppm during 1988–1991. In 1992 the difference was again ∼3 ppm. A two‐dimensional model analysis of the data indicates that the low CO2 growth rate in 1992 is mainly due to an increase in the northern hemisphere CO2 sink from 3.9 Gt C yr−1 in 1991 to 5.0 Gt C yr−1 in 1992. The increase in the north pole‐south pole CO2 difference appears to result from an increase in the southern hemisphere CO2 sink from ∼0.5 to ∼1.5 Gt C yr−1.
Total ozone and synoptic weather relationships over southern Africa and surrounding oceans
Authors: Barsby J. , Diab R. D.
Published: February 2004

Relationships between Total Ozone Mapping Spectrometer (TOMS) total ozone and synoptic weather systems for a region bounded by longitudes 10°W–50°E and latitudes 0°–50°S are presented. Least squares regression analysis revealed a generally poor but negative relationship between total ozone and the heights of the 500, 300, and 100 hPa geopotential surfaces at nine selected stations for the period 1987 to 1988. The relationship strengthens with increasing latitude and suggests a physical link with the midlatitude cyclone which facilitates the injection of stratospheric ozone rich air into the troposphere. In the tropics the link with middle to upper tropospheric meteorological parameters is weak. Analysis of daily relationships between gridded TOMS ozone and gridded European Centre for Medium‐Range Weather Forecasts (ECMWF) geopotential height data over the study area showed considerable variations, indicative of varying synoptic weather situations. Case studies of a cutoff low sequence and a subtropical anticyclone system were selected as examples of synoptic situations which accounted for the highest and lowest correlations, respectively. The contrasting dynamics operating in these systems are highlighted.
NCAR CCM2 simulation of the modern Antarctic climate
Authors: Tzeng Ren‐Yow , Bromwich David H. , Parish Thomas R. , Chen Biao
Published: February 2004

The National Center for Atmospheric Research community climate model version 2 (CCM2) simulation of the circumpolar trough, surface air temperature, the polar vortex, cloudiness, winds, and atmospheric moisture and energy budgets are examined to validate the model's representation of the present‐day Antarctic climate. The results show that the CCM2 can well simulate many important climate features over Antarctica, such as the location and intensity of the circumpolar trough, the coreless winter over the plateau, the intensity and horizontal distribution of the surface inversion, the speed and streamline pattern of the katabatic winds, the double jet stream feature over the southern Indian and Pacific oceans, and the arid climate over the continent. However, there are also some serious errors in the model. Some are due to old problems but some are caused by the new parameterizations in the model. The model errors over high southern latitudes can be summarized as follows: The circumpolar trough, the polar vortex, and the westerlies in midlatitudes are too strong; the semiannual cycle of the circumpolar trough is distorted compared to the observations; the low centers of the circumpolar trough and the troughs in the middle and upper troposphere are shifted eastward by 15°–40° longitude; the surface temperatures are too cold over the plateau in summer and over the coastline in winter; the polar tropopause continues to have a cold bias; and the cloudiness is too high over the continent. These biases are induced by two major factors: (1) the cloud optical properties in tropical and middle latitudes, which cause the eastward shift of troughs and surface low centers and the error in the semiannual cycle, and (2) the cold bias of the surface air temperature, which is attributed to the oversimulation of cloudiness over the continent, especially during summer, and the uniform 2‐m‐thick sea ice. The constant thickness of sea ice suppresses the energy flux from the ocean to the atmosphere and hence reduces the air temperature near the coast during winter. Finally, although the simulated Antarctic climate still suffers these biases, the overall performance of the CCM2 is much better than that of the CCM1 ‐ T42. Therefore the CCM2 is good enough to be used for climate change studies, especially over Antarctica.
Quasi 16‐day oscillation in the mesosphere and lower thermosphere
Authors: Forbes J. M. , Hagan M. E. , Miyahara S. , Vial F. , Manson A. H. , Meek C. E. , Portnyagin Y. I.
Published: February 2004

A quasi‐16‐day wave in the mesosphere and lower thermosphere is investigated through analyses of radar data during January/February 1979 and through numerical simulations for various background wind conditions. Previous workers have examined about 19 days of tropospheric and stratospheric data during January 10–28, 1979, and present conflicting evidence as to whether a large westward propagating wavenumber 1 oscillation observed during this period can be identified in terms of the second symmetric Rossby normal mode of zonal wavenumber 1, commonly referred to as the “16‐day wave.” In the present work we have applied spectral analysis techniques to meridional and zonal winds near 95 km altitude obtained from radar measurements over Obninsk, Russia (54°N, 38°E) and Saskatoon, Canada (52°N, 107°W). These data reveal oscillations of the order of ±10 m s−1 with a period near 16 days as well as waves with periods near 5 and 10 days. These periodicities all correspond to expected resonant frequencies of atmospheric disturbances associated with westward propagating free Rossby modes of zonal wavenumber 1. Numerical simulations are performed which demonstrate that the 95‐km measurements of the 16‐day wave are consistent with upward extension of the oscillation determined from the tropospheric and stratospheric data. Noteworthy features of the model in terms of its applicability in the mesosphere/lower thermosphere regime are explicit inclusion of eddy and molecular diffusion of heat and momentum and realistic distributions of mean winds, especially between 80 and 100 km. The latter include a westerly wind regime above the summer easterly mesospheric jet, thus providing a ducting channel enabling interhemispheric penetration of the winter planetary wave disturbance. This serves to explain the appearance of a quasi‐16‐day wave recently reported in the high‐latitude summer mesopause (Williams and Avery, 1992). However, the efficiency of this interhemispheric coupling may be reduced by gravity wave stress. No significant penetration of the 16‐day oscillation above about 100 km is predicted by the model. Reported signatures of a 16‐day periodicity in ionospheric data therefore require modulation of tidal or gravity wave accessibility to the thermosphere, or perhaps in situ excitation.
Electrodynamic characteristics of lightning and their influence on disturbances of high‐voltage lines
Authors: Kostenko M. V.
Published: February 2004

Nonlinear electrodynamic characteristics of the return‐stroke wave of cloud‐to‐round lightning equivalent Zeqimpedance and phase v velocity are estimated on the basis of the approximate description of the return stroke using quasi‐linear partial differential equations. These equations contain nonlinear parameters which depend on instantaneous values of the current in the front of the wave. Besides, a method for the calculation of overvoltages on the high‐voltage line is worked out for the case of direct lightning strokes, taking into account nonlinear impedance of lightning and wave processes in the line.
The variability of ClONO2 and HNO3 in the Arctic polar vortex: Comparison of Transall Michelson interferometer for passive atmospheric sounding measurements and three‐dimensional model results
Authors: Chipperfield M. P. , Pyle J. A. , Blom C. E. , Glatthor N. , Höpfner M. , Gulde T. , Piesch C. , Simon P.
Published: February 2004

A three‐dimensional radiative‐dynamical‐chemical model has been used to investigate measurements of column ClONO2 and HNO3 made by the airborne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument. MIPAS made measurements from the Transall aircraft in the northern hemisphere lower stratosphere from December 1992 to March 1993. The three‐dimensional model has a detailed stratospheric chemistry scheme including heterogeneous reactions on polar stratospheric clouds and sulfate aerosols. The circulation in the model is specified from the European Centre for Medium Range Weather Forecasts analyses. The MIPAS measurements reveal large variability in column ClONO2 at the edge of the polar vortex. For the measurements of January 27 and 31, 1993, the model experiments show that variability in ClONO2 observed over this period can be explained by polar stratospheric cloud processing and recovery. Measurements of ClONO2 on February 2, 1993, showed large variations depending on the orientation of the aircraft relative to the edge of the vortex. Results from the model show that this is qualitatively consistent with the aircraft flying near to the collar region with its associated strong horizontal gradients of ClONO2. The model's ability to simulate these strong gradients is limited by its relatively coarse resolution. In early March the vortex became very distorted. During this period MIPAS measured very large values of ClONO2 at high latitudes within the vortex but lower, although still large, values in the more southerly regions of the vortex. At this stage of the winter ClONO2 is the major chlorine species in the model at high latitudes. The model shows how the distortion of the vortex in March led to relatively high columns of ClONO2 in vortex air over southern Europe. The model also reproduces the observed gradient in ClONO2 within the vortex, and experiments show that these gradients are due, at least in part, to the availability of sunlight. This variability of ClONO2, and therefore active chlorine (ClOχ), implies that these tracers do not correlate well with potential vorticity. This places limitations on extrapolating localized measurements of anything but the longest lived chemical tracers to the whole of the polar vortex using potential vorticity, or indeed a long‐lived tracer, as part of a coordinate system.
Effects of electron and ion reactions on atmospheric lifetimes of fully fluorinated compounds
Authors: Morris Robert A. , Miller Thomas M. , Viggiano A. A. , Paulson John F. , Solomon Susan , Reid George
Published: February 2004

Atmospheric lifetimes are evaluated for the fully fluorinated compounds CF4, C2F6, c‐C4F8, C6F14, and SF6 using a two‐dimensional transport and chemistry model which includes removal by electrons and ions in the mesosphere and lower thermosphere. Laboratory measurements of the pertinent reaction rates were carried out at thermal energy for free electrons and for the atmospheric ions O+, O2+, O−, O2−, NO+, H3O+, NO3−, and CO3−. Atmospheric removal by electrons reduces the lifetimes of c‐C4F8 and SF6 from about 3200 years to 1400 and 800 years, respectively, only if the respective product anions C4F8− and SF6− do not subsequently regenerate the parent neutral compounds. Atmospheric removal by ion reactions is minor or negligible, with the largest effect (∼5%) being removal of C6F14 by O2+. Removal of CF4 and C2F6 by O+ is probably the most important single destruction process in the atmosphere for these two compounds, but their lifetimes are governed by removal at the Earth's surface in high‐temperature combustors. While we show that the lifetimes of c‐C4F8 and SF6 may be significantly shorter than previously estimated, these compounds remain extremely long‐lived with significant global warming potentials.
On the accuracy of TOVS temperature fields in an Arctic case study
Authors: Loechner F. , Buell R.
Published: February 2004

Temperature profiles from NOAA 10 data were retrieved with the International TOVS Processing Package software using different initial estimates and compared to radiosonde profiles and the European Center for Medium‐Range Weather Forecasts analysis. To assess the limits of their accuracy, the δ correction was updated for the microwave sounding unit with the analysis and the measurements of two days before and completed by its scan angle dependence. The HIRS cloud detection with the CO2 slicing method showed satisfactory results compared to a cloud mask from advanced very high resolution radiometer data. Error statistics for retrievals with 1 field of view (FOV) and 3×3 FOVs revealed similar values. With the forecast as a first guess, a rms <2 K and a bias <1 K for p < 850 hPa are found in the comparison with radiosondes but with a strong dependence on the errors of the δ correction. The error statistics of the forecast compared to analysis and radiosondes showed no advantages for TOVS retrievals in this case study.
Transport climatology of tropospheric ozone: Bermuda, 1988–1991
Authors: Moody Jennie L. , Oltmans Samuel J. , Levy Hiram , Merrill John T.
Published: February 2004

We determined the major transport patterns for Bermuda and quantified the degree to which they influenced variability in ozone concentrations by applying cluster analysis to isentropic trajectories from September 1988, through September 1991. Concentration distributions of ozone associated with these transport patterns were significantly different. The highest concentrations of ozone in each season were associated with transport off the North American continent; the lowest concentrations were during low‐level maritime transport around the Bermuda high. Using the vertical component of the isentropic trajectories, we also showed that the most extreme concentrations of ozone occurred with rapidly descending air from midtropospheric levels. This pattern was most pronounced in April and May when more than 50% of the O3 variability was related to transport differences. We conclude that this relatively remote marine site, which normally experienced low maritime ozone levels (∼30 parts per billion by volume (ppbv)), periodically entrained dry, ozone‐rich (∼55 ppbv) midtropospheric air in association with strong subsidence in high pressure behind spring low‐pressure systems. Although the ultimate source of these midtroposphere, midlatitude, elevated‐ozone concentrations is still being investigated, the synoptic meteorology associated with these transport patterns supports a significant contribution from the upper troposphere and lower stratosphere.
Use of backward trajectories to interpret the 5‐year record of PAN and O3 ambient air concentrations at Kejimkujik National Park, Nova Scotia
Authors: Sirois Alain , Bottenheim Jan W.
Published: February 2004

Air parcel trajectory data are used in two ways to elucidate the temporal trend analysis of 5 years of peroxyacetyl nitrate (PAN) and O3 observations at a rural site in eastern Canada. In the first method, “probability of residence” contours are constructed to determine the most probable origin of air parcels containing the highest and lowest 10% of PAN and O3 mixing ratios. High PAN is found to emanate always from areas of high anthropogenic activity, except when the transport path is to a large extent over the ocean, especially in the summer. High O3 originates from the same regions except in the winter when because of low photochemical activity, O3 is actually titrated and air from less populated areas is richer in O3. The trajectories indicate that transport at higher altitude leads to higher mixing ratios; this is especially the case for O3 in winter and spring. The complementary method of clustering the trajectories has allowed qualitative derivation of seasonal cycles for background and polluted air masses of different origin. Background air from the north shows a distinct PAN maximum in March; it is discussed that this could be either due to enhanced photochemistry or to the import of polluted air from the Arctic. Polluted air masses show the same March peak, but a second peak in late summer/early fall. Oceanic air from the south has a January maximum in PAN but otherwise is consistently low in PAN. O3 also has a spring maximum, but in polluted air it is broader stretching into the summer. It is postulated that this is due to additional O3 formation in the summer, while in the winter, actual O3 loss is indicated for polluted air. By inference it is deduced that the second PAN peak in polluted air is also due to additional formation in comparison with background air, while in the summer, extra PAN loss mechanisms operate that are less important for O3. Indications for both dry deposition and thermal decomposition have been found.
Application of the Langley plot method to the calibration of the solar backscattered ultraviolet instrument on the Nimbus 7 satellite
Authors: Bhartia P. K. , Taylor S. , McPeters R. D. , Wellemeyer C.
Published: February 2004

The concept of the well‐known Langley plot technique, used for the calibration of ground‐based instruments, has been generalized for application to satellite instruments. In polar regions, near summer solstice, the solar backscattered ultraviolet (SBUV) instrument on the Nimbus 7 satellite samples the same ozone field at widely different solar zenith angles. These measurements are compared to assess the long‐term drift in the instrument calibration. Although the technique provides only a relative wavelength‐to‐wavelength calibration, it can be combined with existing techniques to determine the drift of the instrument at any wavelength. Using this technique, we have generated a 12‐year data set of ozone vertical profiles from SBUV with an estimated accuracy of ±5% at 1 mbar and ±2% at 10 mbar (95% confidence) over 12 years. Since the method is insensitive to true changes in the atmospheric ozone profile, it can also be used to compare the calibrations of similar SBUV instruments launched without temporal overlap.
Origin of condensation nuclei in the springtime polar stratosphere
Authors: Zhao Jingxia , Toon Owen B. , Turco Richard P.
Published: February 2004

An enhanced sulfate aerosol layer has been observed near 25 km accompanying springtime ozone depletion in the Antarctic stratosphere. We use a one‐dimensional aerosol model that includes photochemistry, particle nucleation, condensational growth, coagulation, and sedimentation to study the origin of the layer. Annual cycles of sunlight, temperature, and ozone are incorporated into the model. Our results indicate that binary homogeneous nucleation leads to the formation of very small droplets of sulfuric acid and water under conditions of low temperature and production of H2SO4 following polar sunrise. Photodissociation of carbonyl sulfide (OCS) alone, however, cannot provide sufficient SO2 to create the observed condensation nuclei (CN) layer. When subsidence of SO2 from very high altitudes in the polar night vortex is incorporated into the model, the CN layer is reasonably reproduced. The model predictions, based on the subsidence in polar vortex, agree with in situ measurements of particle concentration, vertical distribution, and persistence during polar spring.
Reduction of Fe(III) with sulfite in natural waters
Authors: Millero F. J. , Gonzalez‐Davila M. , Santana‐Casiano J. M.
Published: February 2004

The Fe(III) in marine aerosols and rainwaters can be reduced to Fe(II) by photochemical processes and by reactions with sulfite. In this paper, measurements of the rates of reduction of nanomolar levels of Fe(III) with sulfite (without O2) have been determined in NaCl and seawater solutions as a function of temperature (0° to 40°C), pH (2 to 6.8), ionic strength (I = 0.1 to 6 M), and composition (Na+, Mg2+, Ca2+, F−, Cl−, Br−, HCO3−, SO42−). The overall rate constant (k, M−1 min−1) for the reaction,
products, is given by d[Fe(III)]/dt = −k[Fe(III)] [S(IV)]. The reaction was found to be first order with respect to Fe(III) and S(IV). The rate constants as a function of pH increased from a pH = 2 to 4 and decreased at higher pH. The effect of temperature and ionic strength on the rates could be represented by log k = log k0 + AI0.5/(1 + I0.5), where A = −1.1 in NaCl and −2.2 in seawater and log k0 = 25.39 − 6,323/T. The energy of activation was found by 121±6 kJ mol−1. The measured rates in seawater as a function of salinity were lower than the rates in NaCl at the same ionic strength. Measurements in NaCl solutions with added sea‐salt ions (Mg2+, Ca2+, F−, Br−, and SO42−) at pH = 3.5 indicate that the formation of inert FeF2+ may be responsible for the lower rates. The effect of changes in the composition on the rates was interpreted by examining the speciation of Fe(III) and S(IV). This analysis indicates that the rate‐determining steps from a pH of 2.5 to 4.0 are FeOH2+ + HSO3− ↔ HOFeSO3H + and
· and at pH of 4 to 6, the reactions Fe(OH)2+ + HSO3− ↔ (HO)2FeSO3H and
· become important. The changes in the concentration of FeOH2+ and HSO3− as a function of pH and composition can account for most of the changes in the rates. These kinetic studies indicate that the rates of reduction of Fe(III) with S(IV) in acidic water droplets at natural levels of S(IV) may be an important source of Fe(II).
Estimation of grain sizes and mixing ratios of fine powder mixtures of common geologic minerals
Authors: Hiroi Takahiro , Pieters Carlé M.
Published: February 2004

Two different approaches for modeling reflectance spectra of intimate mixtures, Hapke's model and the isograin model, are used to estimate grain sizes and mixing ratios of powder mixtures of three geologic minerals: olivine, orthopyroxene, and plagioclase. In Hapke's model, scattering and extinction efficiencies are mixed separately, and both models employ semiempirical refractive index spectra for component minerals. Mixing ratios of mixtures of grain size 45–75 μm are well estimated by both models assuming a common grain size of 60 μm and optimizing the constants for the single‐particle scattering. For each model, effective grain size ratios for mineral constituents in mixtures of grain size <25 μm, are derived successfully that allows mineral abundances to be accurately predicted within ∼4 wt%. On the other hand, neither model can accurately predict mineral reflectance spectra for its smaller grain sizes (<25 and 25–45 μm) using an absorption coefficient spectrum derived from a larger grain size (45–75 μm). The errors in both models are significantly reduced if surface roughness effects of the smaller grain‐size fractions are modeled.
Rate of observation of electrophonic meteor fireballs
Authors: Keay C. S. L. , Ceplecha Z.
Published: February 2004

The chances of witnessing a very large meteor fireball and hearing sounds coincident with its passage are calculated from two independent sets of observational data and from the known rate of incidence upon the Earth of meteoroids large enough to produce the phenomenon.
Bell Regio, Venus: Integration of remote sensing data and terrestrial analogs for geologic analysis
Authors: Campbell Bruce A. , Rogers Patricia G.
Published: February 2004

The geology and surface morphology of Bell Regio (18–42°N, 32–58°E) are investigated using a combination of Magellan, Venera, and analogous terrestrial data. The properties of surface units are compared to either direct terrestrial analog measurements or to the behaviors predicted by theoretical models. Five major volcanic sources are identified from geologic mapping (Tepev Mons, Nefertiti corona, a large shield volcano east of Tepev, and two small edifices southeast of Tepev). The volcano Api Mons lies northeast of the main Bell uplift. The oldest volcanic units are associated with an extensive low shield volcano east of Tepev Mons and a small edifice southeast of Tepev. The annular flow apron of Tepev Mons formed next, with volcanism at a second small edifice on the southeast flank of Tepev Mons producing the youngest flow units. Comparisons between Magellan data, terrrestrial radar images, and field topography profiles suggest that only three units resemble terrestrial a'a flows; the remainder are consistent with smoother pahoehoe‐type surfaces. This suggests that most of the flow units were erupted at relatively low volume effusion rates (<100 m3/s) over long periods of time or had very low viscosities. One flow unit has a knobby texture which resembles large tumuli or ridge structures. Much of the tectonic deformation in the area is in the form of fractures circumferential to the edifices and isolated blocks of tessera terrain; there are no rift zones such as those which occur at Beta Regio, Atla Regio, or Western Eistla Regio. Tepev Mons is characterized by very steep slopes (up to 40° along the east flank), a relatively flat summit, and two large (11 and 31 km) calderas. Though covered in places by high‐dielectric material, the Tepev Mons summit area is relatively smooth with the exception of annular rough deposits which ring the two calderas. These calderas are quite shallow (on the order of a few hundred meters at most), with no discernible difference in radius between individual altimeter footprints inside and outside the larger eastern feature. Magellan and Venera data suggest that the eastern caldera is covered by a layer of unconsolidated material. Ejecta from Miriam crater has produced a halo with a dielectric constant of 6–7 surrounding Nefertiti corona, and Potanina crater ejecta is the likely source material for radar‐dark wind streaks south of Tepev Mons. A low‐dielectric (ε = 2–3) triangular region on the south flank of Tepev Mons may be a pyroclastic or crater ejecta deposit which has been spread westward by the wind. This region has undergone a wide variety of volcanic processes, with a major shift in eruptive style from the older low‐relief eastern volcanic center to the steep slopes and large summit calderas characteristic of Tepev Mons and the two smaller edifices.
Reflectivity and other physicochemical properties of Mn‐substituted goethites and hematites
Authors: Vempati R. K. , Morris R. V. , Lauer H. V. , Helmke P. A.
Published: February 2004

Goethite‐bearing samples with values of Mns (Mn/(Mn+Fe) mole fraction) up to 0.206 were synthesized by precipitation from alkaline solution. Samples with Mns ≤ 0.061 were single‐phase Mn‐goethites; samples with higher Mns values contained another Mn‐bearing phase (probably jacobsite). Mn‐hematites were prepared by dehydroxylation of corresponding Mn‐goethites at 500°C. Orthorhombic a and b unit cell dimensions of Mn‐goethites changed in a linear manner with Mns, but not at rates predicted by the Vegard law. Hexagonal unit cell dimensions of Mn‐hematites did not vary with Mns. Mössbauer parameters isomer shift (IS), quadrupole splitting (QS), and hyperfine field (Bhf) were measured at 293 and 15 K. For all single‐phase Mn‐goethites and Mn‐hematites (Mns ≤ 0.061), magnetic splitting was observed at both temperatures. At 293 K, small but systematic decreases in Bhf were observed with increasing Mn substitution; IS and QS were not dependent on Mns. Mn substitution strongly lowered the Morin transition temperature of hematite. At 15 K, the Morin transition was not present for Mns > 0.020(4). The saturation magnetization of Mn‐goethites and Mn‐hematites (Mns ≤ 0.061) was the expected zero (within error) for antiferromagnetic goethite and for hematites obtained from dehydroxylation of goethites. Mn‐goethites with Mns > 0.061 were magnetic because of the presence of strongly magnetic jacobsite. For reflectivity spectra, bands resulting from Mn3+ were centered near 454 and 596 nm for Mn‐goethites and near 545 and 700 nm for Mn‐hematites. There is evidence for a ∼700 nm band in spectral data for Martian bright regions, but association of it with Mn3+ is not a unique interpretation. Comparison of laboratory and Martian spectral data implies that Mns < 0.032 for the Mn3+ content of Martian hematites.
Hotspot evolution and Venusian tectonic style
Authors: McGill George E.
Published: February 2004

Because hotspots represent an important manifestation of heat loss on Venus, their geological evolution is of fundamental importance for any attempt to understand Venusian tectonics. Eistla Regio is a ∼7500‐km‐long, moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots. It also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle. Central Eistla Regio includes two large volcanoes, Sappho and Anala, and several coronae in close proximity. Detailed mapping in this region results in two conclusions of tectonic significance: (1) Sappho and Anala occur near the intersection of two major extensional deformation zones, and (2) the coronae are older than the large volcanoes. Several of the coronae occur as a chain along Guor Linea, one of the major extensional deformation zones. Stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials. Thus Central Eistla Regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes. The expected result of such a swarm would be thermal thinning of the elastic lithosphere with time. However, model results, geological observations, and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time. This thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near‐global resurfacing. The global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes.
Igneous inclusions from ordinary chondrites: High temperature cumulates and a shock melt
Authors: Sack Richard O. , Ghiorso Mark S. , Wang Ming‐Sheng , Lipschutz Michael E.
Published: February 2004

We report microprobe, instrumental neutron activation analysis, and radiochemical neutron activation analysis data for three large igneous inclusions in the Yamato (Y‐)75097, Y‐793241, and Y‐794046 ordinary chondrites. The inclusions in the first two chondrites are troctolitic cumulates that have undergone appreciable reactions with their hosts either during emplacement and/or cooling. Olivine‐spinel Fe‐Mg exchange pairs in these two inclusions record equilibration temperatures of about 710°C, and these temperatures are similar to those exhibited by mineral pairs in the Y‐75097 and Y‐793241 hosts. The inclusion in Y‐794046 is texturally unique, consisting of fine‐grained, randomly distributed olivines, coarse (∼2 mm) fascicular pyroxene laths, and angular pockets of maskelynite/plagioclase feldspar. The phase compositions are readily interpreted as having resulted from extremely rapid, essentially isochemical cooling to temperatures <1000°C of a melt with an initial temperature >1670°C. We suggest that this igneous inclusion formed in situ by shock.
Chaotic obliquity and the nature of the Martian climate
Authors: Jakosky Bruce M. , Henderson Bradley G. , Mellon Michael T.
Published: February 2004

Recent calculations of the Martian obliquity suggest that it varies chaotically on timescales longer than about 107 years and varies between about 0 and 60°. We examine the seasonal water behavior at obliquities between 40 and 60°. Up to several tens of centimeters of water may sublime from the polar caps each year, and possibly move to the equator, where it is more stable. CO2 frost and CO2‐H2O clathrate hydrate are stable in the polar deposits below a few tens of meters depth, so that the polar cap could contain a significant CO2 reservoir. If CO2 is present, it could be left over from the early history of Mars; also, it could be released into the atmosphere during periods of high obliquity, causing occasional periods of more‐clement climate.
Reply [to “Comment on ‘The bromine content of micrometeorites: Arguments for stratospheric contamination’ by F. J. M. Rietmeijer”]
Authors: Rietmeijer Frans J. M.
Published: February 2004

Previous spectral analyses have given evidence of collisionally excited Jovian and (at times) Saturnian H2 Werner bands being absorbed by hydrocarbons at the shortest wavelengths along the auroral ovals, and of a longitudinal dependence of this absorption in the Jovian aurorae. This “color ratio” has been used to estimate the energy of the primary particles. In such estimates, particles are generally assumed to penetrate vertically into the atmosphere. However, the precipitating particle angular distribution is unknown, and a model developed for a diffuse aurora by Prangé and Elkhamsi (1991), for instance, predicts quite different possible distributions. We consider here the influence of the angular distribution used in the model, and show that distributions peaking far from vertical may increase the energy derived from a given color ratio by as much as a factor of 3. We discuss previous interpretations of the color ratio longitudinal modulation (variation of the auroral atmosphere structure, or of the incident particle energy) in view of the subsequent increase in energy input. We argue that an interpretation in terms of energy variations only is not consistent with the energy available in the magnetosphere if the aurorae are diffuse, and we discuss this finding in the context of recent Hubble Space Telescope images.
Floor‐fractured impact craters on Venus: Implications for igneous crater modification and local magmatism
Authors: Wichman R. W. , Schultz P. H.
Published: February 2004

Regional tectonism and volcanism affect crater modification and crater loss on Venus, but a comparison of Venusian craters to lunar floor‐fractured craters suggests that a third style of more localized, crater‐controlled magmatism also may occur on Venus. Based on lunar models for such magmatism, Venusian crustal conditions should generally favor crater‐filling volcanism over crater‐centered floor fracturing. Nevertheless, three craters on Venus strongly resemble extensively modified craters on the Moon where deformation can be attributed to failure over large crater‐centered intrusions. Models for crater modification over igneous intrusions indicate typical magmatic pressures beneath these three craters of ∼200–300 bars and intrusion depths of the order of 1–6 km. All three craters also share common settings and low elevations, whereas craters embayed by regional volcanism preferentially occur at much higher elevations on Venus. We suggest that the style of igneous crater modification on Venus thus may be elevation dependent, with crater‐centered intrusions primarily occurring at low elevations on Venus. This interpretation is consistent with theoretically predicted variations in magmatic neutral buoyancy depth as a function of atmospheric pressure suggested by other authors.