bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of nitrogen oxides by electrical discharges and implications for atmospheric lightning
Authors: Stark M. S. , Harrison J. T. H. , Anastasi C.
Published: February 2004

The mechanism of production of nitrogen oxides by electrical discharges has been examined. The velocity of shock fronts generated by laboratory scale discharges have been measured and are found to be too slow to raise the air temperature to the ∼3000 K necessary for nitrogen fixation by the Zel'dovich mechanism. The freeze‐out mixing ratio of NOx in air has been measured directly for low‐pressure discharges and is found to be of the order expected from the Zel'dovich mechanism for gas cooling over a timescale far longer than the duration of the shock front. Therefore it is concluded that NOx is formed in the gas in the slowly cooling hot channel region and not in the rapidly cooling shock front. Also, it is argued that NOx formation occurs by a freeze out mechanism due to a rapid drop in temperature, not density as has been suggested. NO2 production is found to be significant, with the [NO2]/[NO] ratio being strongly dependent on the water content of the air. Discrepancies between previous experimental studies of the [NO2]/[NO] ratio and the quantity of NOx formed per unit energy (P) are discussed. P is also found to vary with spark gap and ambient pressure. It is thought that these effects may be due to a significant loss of heat from the spark gap to the electrodes. The inclusion of atmospheric levels of N2O, CH4, and CO2 are found to have no measurable effect on the yields of NO or NO2.
A modeling and observational study of East Antarctic surface mass balance
Authors: Connolley W. M. , King J. C.
Published: February 2004

We examine simulations of the surface mass balance of the sector of the East Antarctic Ice Sheet between 2.4°W and 110.5°E as produced by the United Kingdom Meteorological Office Unified Climate Model. Estimates of the actual mass balance of this sector can be obtained from glaciological observations of snow accumulation and from studies of the atmospheric water vapor budget using radiosonde observations. The former technique gives an average sector accumulation of 104 mm yr−1, and the latter yields 157 mm yr−1. The modeled accumulation in this sector, 122 mm yr−1, lies between these two estimates, suggesting that the model can accurately represent the processes controlling surface mass balance. However, examination of the atmospheric water vapor budget in the model shows that only 30% of the water vapor precipitated in this sector is carried by resolved‐scale transport. Although the model produces the “correct” accumulation in present‐day climate simulations, it is not clear that this would change appropriately if the model were used to simulate future climates. By producing synthetic estimates of water vapor transport at radiosonde station locations we have used the model data to investigate the uncertainties in estimating sector accumulation from radiosonde data. The results of this study indicate that the radiosonde technique will tend to overestimate sector accumulation, thus reconciling the two observational estimates.
Three‐dimensional model studies of the effect of NOx emissions from aircraft on ozone in the upper troposphere over Europe and the North Atlantic
Authors: Flatøy Frode , Hov Øystein
Published: February 2004

A mesoscale chemistry transport model is coupled to a numerical weather prediction model (NWP) for Europe and the North Atlantic. It is applied to show that air traffic emissions significantly increase the concentrations of NOx as well as the net chemical formation rate of ozone over both the Atlantic Ocean and over central Europe. The time period studied was July 1–10, 1991, a period characterized by a high‐pressure ridge over North Europe. In convection the vertical gradient of NOx is strongly reduced because the midtropospheric concentrations increase. In the updraft region of convective plumes, chemical ozone formation up to 1.8 ppbv/h is calculated in the upper troposphere, and this is about a factor 50 higher than the average for 10.5 km over continental Europe during the 10‐day period. The high production rates are not sustained for more than a few hours, and it would be difficult to identify the effect of the process in a local measurement of the ozone concentration. The calculations indicate that 10–25 molecules of O3 (0.03–0.05 ppb/h) are generated on the average per NOx molecule emitted from aircraft (2–3 ppt/h). On convective days over regions with significant surface sources of NOx, ozone formation in the upper troposphere due to NOx brought there by convection, outweighs the role of aircraft NOx emissions. The net chemical formation rate of ozone across the model domain depends on the boundary concentrations of NOx and O3, but the change in the upper troposphere in the net chemical formation rate of ozone induced by aircraft NOx emissions is quite independent of the concentrations of NOx or O3.
Rocket vapor trail releases revisited: Turbulence and the scale of gravity waves: Implications for the imaging Doppler interferometry/incoherent scatter radar controversy
Authors: Roper R. G.
Published: February 2004

The rate of diffusion of a trimethyl aluminum trail released by a Skylark rocket over Woomera, Australia (31°S, 137°E), in 1969 is interpreted in terms of molecular and turbulent diffusion, and is related to the scale of internal atmospheric gravity wave winds. Implications for disagreement between imaging Doppler interferometry and incoherent scatter radar winds are explored.
Seasonal variation of isentropic transport out of the tropical stratosphere
Authors: Waugh Darryn W.
Published: February 2004

The seasonal variation of the isentropic transport out of the tropical stratosphere is examined using contour advection calculations with winds from meteorological analyses for the period July 1991 to June 1994. Calculations using winds from three different meteorological analyses are compared. There is good qualitative agreement between the different calculations for extratropical structures but not for structures in the tropics. There is also reasonable agreement between area diagnostics (for transport out of the tropics) derived from these calculations. Transport out of the tropics occurs in Rossby wave breaking events in which filaments of tropical air are drawn into middle latitudes. In all years these wave breaking events occur during the fall to spring period when there are westerlies throughout middle latitudes (late September to early May in the northern hemisphere). This includes periods before the formation of, and after the breakup of, the polar vortex. In the northern hemisphere the rate of transport out of the tropics during fall to spring fluctuates around a constant value, whereas in the southern hemisphere there is a relatively quiet midwinter period, with maxima in the transport in early and late winter.
Tropospheric formaldehyde concentration at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2
Authors: Zhou Xianliang , Lee Yin‐Nan , Newman Leonard , Chen Xiaohui , Mopper Kenneth
Published: February 2004

The concentration of formaldehyde at Mauna Loa Observatory, Hawaii, was determined during four Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) measurement intensives between September 1991 and August 1992. The observed diurnal variations, 200–900 parts per trillion by volume (pptv) during daytime and 60–200 pptv during nighttime, resulted mainly from the local air circulation pattern whereby island modified marine boundary layer air prevailed during the day and free tropospheric air dominated during the night. A seasonal variation was also observed; the median/mean values of all data points are: 149/196, 129/149, 143/178, and 181/211 pptv for autumn, winter, spring, and summer intensives, respectively. During nighttime downslope flow periods which brought in free tropospheric air to the measurement site, the formaldehyde concentrations (median/mean) were 122/123, 110/112, 120/125, and 140/137 pptv for autumn, winter, spring, and summer, respectively. This seasonal dependence may be attributable to changes in solar insolation and NO concentrations. A simple box model calculation constrained by the experimentally determined concentrations of CH3OOH yielded a formaldehyde concentration (without/with heterogeneous removal) for free tropospheric air, at 7°C, of 155/140, 125/115, 210/195, and 220/205 pptv for autumn, winter, spring and summer, respectively. The calculated values are in good agreement with the measured concentrations for winter (within 27/15%, without/with heterogeneous removal) and fall (within 14/5%), but are significantly higher for spring (75/63%) and summer (57/46%).
Lidar, nephelometer, and in situ aerosol experiments in southern Ontario
Authors: Hoff Raymond M. , Guise‐Bagley L. , Staebler R. M. , Wiebe H. A. , Brook J. , Georgi B. , Düsterdiek T.
Published: February 2004

During August 1992, a complex aerosol and optical experiment was performed at Egbert, Ontario, Canada. In situ data from Berner impactors, three active scattering aerosol spectrometer probes, a differential mobility analyzer, filter, and denuder samples were intercompared with optical measurements from a nephelometer and a lidar. A haze event during the study period has been modeled using the measured in situ particle size data and a hygroscopic growth model. Agreement between the modeled and the measured data is good, except in periods of relative humidities in excess of 90%. Explosive particle growth during the midmorning hours is explained by gas‐particle conversion processes of sulphur‐bearing species. The light scattering measurements lead to a specific scattering coefficient of the dry aerosol of 3.2 m2 g−1, but the result is clearly dependent on air mass origin and relative humidity. The latter result is important in global models of radiative response to anthropogenic aerosol inputs (sulphate aerosol, lidar, nephelometer, light scattering, aerosol radiative forcing).
Particulate emissions in the exhaust plume from commercial jet aircraft under cruise conditions
Authors: Hagen D. E. , Whitefield P. D. , Schlager H.
Published: February 2004

In situ measurements of total concentration, size distribution, and hydration properties of jet engine exhaust from a range of commercial transports is reported. Significant concentration enhancements (above ambient background) for aircraft exhaust particulates is reported permitting the detection of not only newly formed but also aged plumes, even in the presence of considerable ambient pollution. Two types of particle size distributions are found in the near‐field (∼8 km behind source) exhaust plume from jet aircraft operating under cruise conditions. One type exhibits the form of the Junge distribution with exponential coefficient −2.4. The second exhibits the Junge distribution form in the small‐particle region, below about 50 nm, followed by a larger‐particle mode between 0.1 and 0.2 μm. Neither of these observed types of distributions exhibit the sharp drop‐off in particle concentrations at the small‐particle end of the spectrum that was found in ground‐based engine tests. Binary nucleation of sulfuric acid aerosols and heterogeneous nucleation on ion clusters are postulated for particles in this size range. This is supported by the finding of significant numbers of particles having high soluble mass fractions. These data are compared with those taken in ground test cells and those reported by other investigators.
Light carboxylic acids in Greenland ice: A record of past forest fires and vegetation emissions from the boreal zone
Authors: Legrand Michel , De Angelis Martine
Published: February 2004

Measurements of light carboxylic acids (formate, HCOO−; acetate, CH3COO−; glycolate, C2H3O3−; and oxalate, C2O4−−) were performed along Summit (central Greenland) ice cores covering the last two centuries and a complete glacial‐interglacial cycle. High‐latitude biomass burning contributes between 20% and 30% to the formate, oxalate, glycolate, and ammonium deposited on central Greenland over the last 200 years. This biomass burning contribution is very weak for nitrate and acetate. A decreasing trend in formate, parallel to the increasing strong acidity (H2SO4 and HNO3) is observed over the last decades. This lack of record of the growing anthropogenic emissions of formic acid and of their precursors (nonmethane hydrocarbons and CH4) likely results from a less efficient scavenging of this weak acid from the gas phase into precipitation due to the recent acidification of the atmosphere. In contrast, the acetate profile shows a well‐marked increase over the last three decades. This difference in the formate and acetate trends remains difficult to explain in the light of our present knowledge of anthropogenic emissions. The role of peroxyacetyl nitrate (PAN) acting as a reservoir for acetic acid in polar regions is a possibility which needs to be investigated. Aside from high‐latitude biomass burning events which frequently occurred in the past except during the ice age maximum (15,000 to 40,000 years B.P.), the background levels of HCOO− and CH3COO− are lower during the ice age with respect to values observed during warm stages. Changes in vegetation emissions from North America are suggested to be responsible for the variations of formate background level observed in Greenland ice over the last 100,000 years. Greenland ice therefore contains information on the history of the Laurentide ice sheet, suggesting a well‐established Laurentide ice sheet between 15,000 and 78,000 years B.P. and large fluctuations of its extent before 70,000 years B.P. Although the interpretation of the acetate profile remains more speculative, our data suggest that marine emissions of hydrocarbons may be an additional significant source of acetate.
First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) in 1995
Authors: Hall F. G. , Sellers P. J.
Published: February 2004

The history and goals of FIFE are briefly reviewed. The papers in this special issue are summarized under the headings of “Remote Sensing Science” and “Modeling.” The significant contributions of FIFE to Earth System Science are itemized and discussed.
Schumann resonance spectra in a two‐scale‐height Earth‐ionosphere cavity
Authors: Sentman D. D.
Published: February 2004

New expressions are developed for the amplitude and power spectra of the electric and magnetic components of lightning‐excited Schumann resonances. A classical normal mode approach is used, yielding expressions directly in terms of the complex eigenfrequencies of the Earth‐ionosphere cavity. Using previous simplified formulations of the eigenvalue problem in terms of a two‐scale‐height model of a dissipative Earth‐ionospheric cavity, these results permit calculation of spectra in terms of geometric properties of the system. The expressions are in a convenient form for application in correlation studies involving use of the resonance intensities as a proxy measure of other Earth system parameters, such as average tropical continental temperature [Williams, 1992].
ClONO2 total vertical column abundances above the Jungfraujoch Station, 1986–1994: Long‐term trend and winter‐spring enhancements
Authors: Rinsland C. P. , Zander R. , Demoulin P. , Mahieu E.
Published: February 2004

Total vertical column abundances of chlorine nitrate (ClONO2) have been retrieved from 0.006 cm−1 resolution solar absorption spectra recorded at the International Scientific Station of the Jungfraujoch (ISSJ) in the Swiss Alps (altitude 3.58 km, latitude 46.5°N, longitude 8.0°E;) on 105 days between June 1986 and November 1994. The analysis is based on spectral fittings of the ClONO2 ν4 band Q branch at 780.21 cm−1 and the interferences occurring in the same spectral region. The ISSJ measurements show a regular long‐term increase in the ClONO2 column with an occasional factor of 2 to 3 enhancements during the midwinter to early spring. Excluding data from this time of the year, the ISSJ database reflects a linear rate of increase and 1σ uncertainty equal to (4.0 ± 0.7)% yr−1 referenced to 1990.0. The corresponding ClONO2 total vertical columns for mid‐1986 and mid‐1994 are equal to 0.92 and 1.26 × 1015 molecules cm−2, respectively. The high ClONO2 columns and high HF/HCl column ratios sometimes measured during winter indicate the occasional presence of chemically processed air above the station. This is corroborated by trajectories calculated for the stratospheric air masses sounded on these occasions.
In situ observations of particles in jet aircraft exhausts and contrails for different sulfur‐containing fuels
Authors: Schumann U. , Ström J. , Busen R. , Baumann R. , Gierens K. , Krautstrunk M. , Schröder F. P. , Stingl J.
Published: February 2004

The impact of sulfur oxides on particle formation and contrails is investigated in the exhaust plumes of a twin‐engine jet aircraft. Different fuels were used with sulfur mass fractions of 170 and 5500 ppm in the fuel, one lower than average, the other above the specification limit of standard Jet‐Al fuel. During various phases of the same flight, the two engines burnt either high‐ or low‐sulfur fuel or different fuels in the two engines. Besides visual, photographic, and video observations from close distance, in situ measurements were made within the plumes at plume ages of 20 to 30 s, at altitudes between 9 and 9.5 km, and temperatures between −49 and −55°C, when the visible contrail was about 2 km long. The data include particle number densities for particles larger than 7 nm, 18 nm, 120 nm, and 1 μm in diameter, together with wind, temperature and humidity measurements. The observations show visible and measurable differences between contrails caused by the different sulfur levels. At ambient temperatures 5 K below the threshold temperature for contrail onset, the plume became visible about 10 m after the engine exit for high sulfur content, but 15 m after the engine exit for low sulfur content. The higher sulfur emission caused a larger optical thickness of the contrail shortly after onset, with slightly brown‐colored contrail when the Sun was behind the observer, and more contrast when viewed against the Sun. The high‐sulfur contrail grew more quickly but also evaporated earlier than the low‐sulfur contrail. At plume ages of about 20 s, each engine plume was diluted to an effective diameter of 20 m. The plumes contained many subvisible particles. Peak number densities were 30,000 cm−3 for particles of diameter above 7 nm and 15,000 cm−3 above 18 nm. The latter is a little larger than the estimated number of soot particles emitted. The high‐sulfur plume shows more particles than the low‐sulfur plume. The differences are about 25% for particles above 7 nm and about 50% above 18 nm. The results indicate that part of the fuel sulfur is converted to sulfuric acid which nucleates with water vapor heterogeneously on soot or nucleates acid droplets homogeneously which then coagulate partly with soot. During descent through the level of contrail onset, the high‐sulfur contrail remained visible at slightly lower altitude (25 to 50 m) or higher temperature (0.2 to 0.4 K). At least for average to high sulfur contents, aircraft generate an invisible aerosol trail which enhances the background level of condensation nuclei, in particular in regions with dense air traffic at northern latitudes and near the tropopause.
Multiwavelength comparison of modeled and measured remote tropospheric aerosol backscatter over Pacific Ocean
Authors: Cutten D. R. , Pueschel R. F. , Bowdle D. A. , Srivastava V. , Clarke A. D. , Rothermel J. , Spinhirne J. D. , Menzies R. T.
Published: February 2004

Aerosol concentrations and size distributions in the middle and upper troposphere over the remote Pacific Ocean were measured with a forward scattering spectrometer probe (FSSP) on the NASA DC‐8 aircraft during NASA's Global Backscatter Experiment (GLOBE) in May–June 1990. The FSSP size channels were recalibrated based on refractive index estimates from flight‐level aerosol volatility measurements with a collocated laser optical particle counter (LOPC). The recalibrated FSSP size distributions were averaged over 100‐s intervals, fitted with lognormal distributions and used to calculate aerosol backscatter coefficients at selected wavelengths. The FSSP‐derived backscatter estimates were averaged over 300‐s intervals to reduce large random fluctuations. The smoothed FSSP aerosol backscatter coefficients were then compared with LOPC‐derived backscatter values and with backscatter measured at or near flight level from four lidar systems operating at 0.53, 1.06, 9.11, 9.25, and 10.59 μm. Agreement between FSSP‐derived and lidar‐measured backscatter was generally best at flight level in homogeneous aerosol fields and at high backscatter values. FSSP data often underestimated low backscatter values especially at the longer wavelengths due to poor counting statistics for larger particles (>0.8 μm diameter) that usually dominate aerosol backscatter at these wavelengths. FSSP data also underestimated backscatter at shorter wavelengths when particles smaller than the FSSP lower cutoff diameter (0.35 μm) made significant contributions to the total backscatter.
Concentrations of carbonyl compounds and the carbon isotopy of formaldehyde at a coastal site in Nova Scotia during the NARE summer intensive
Authors: Tanner Roger L. , Zielinska Barbara , Uberna Ewa , Harshfield Gregory , McNichol Ann P.
Published: February 2004

The concentrations of gaseous atmospheric carbonyl compounds have been measured at a coastal site on the southern tip of Nova Scotia during August 1993 as part of the National Oceanic and Atmospheric Administration's North Atlantic Regional Experiment (NARE) summer intensive. The DNPH‐cartridge technique was used for sampling, with analysis by HPLC with UV absorbance detection. The carbon isotopic content of formaldehyde was measured using a novel collection technique on bisulfite‐coated filters followed by accelerator mass spectrometric determination of 14C content and 13C/12C isotopic ratios. Results from this study indicate that quantifiable levels above about 100 parts per trillion by volume were found at the Chebogue Point, Nova Scotia, site for several carbonyls, formaldehyde, acetaldehyde, acetone, propionaldehyde, methyl ethyl ketone, and benzaldehyde, for most 6‐hour sampling periods. Six‐hour averaged formaldehyde levels of <0.5 to about 3.5 parts per billion were found during periods of local influence and during influence of continental air masses crossing eastern Canada and northeastern US. In contrast, background levels of formaldehyde of 200–400 pptv were found under conditions of predominantly marine back trajectories. Large fractions of contemporary carbon (of the order of 80%) were found in collected formaldehyde samples for all transport regimes and formaldehyde concentrations, suggesting that biogenic sources of hydrocarbon precursors to formaldehyde predominated during the summer season.
On the formation and persistence of subvisible cirrus clouds near the tropical tropopause
Authors: Jensen Eric J. , Toon Owen B. , Selkirk Henry B. , Spinhirne James D. , Schoeberl Mark R.
Published: February 2004

We have used a detailed cirrus cloud model to evaluate the physical processes responsible for the formation and persistence of subvisible cirrus near the tropical tropopause and the apparent absence of these clouds at midlatitudes. We find that two distinct formation mechanisms are viable. Energetic tropical cumulonimbus clouds transport large amounts of ice water to the upper troposphere and generate extensive cirrus outflow anvils. Ice crystals with radii larger than 10 – 20 μm should precipitate out of these anvils within a few hours, leaving behind an optically thin layer of small ice crystals (τvis ≃ 0.01 – 0.2, depending upon the initial ice crystal size distribution). Given the long lifetimes of the clouds, wind shear is probably responsible for the observed cloud thickness ≤1 km. Ice crystals can also be generated in situ by slow, synoptic scale uplift of a humid layer. Given the very low temperatures at the tropical tropopause (≃−85°C), synoptic‐scale uplift can generate the moderate ice supersaturations (less than 10%) required for homogeneous freezing of sulfuric acid aerosols. In addition, simulations suggest that relatively large ice crystal number densities should be generated (more than 0.5 cm−3). The numerous crystals cannot grow larger than about 10–20 μm given the available vapor, and their low fall velocities will allow them to remain in the narrow supersaturated region for at least a day. The absorption of infrared radiation in the thin cirrus results in heating rates on the order of a few K per day. If this energy drives local parcel temperature change, the cirrus will dissipate within several hours. However, if the absorbed radiative energy drives lifting of the cloud layer, the vertical wind speed will be about 0.2 cm‐s−1, and the cloud may persist for days with very little change in optical or microphysical properties. The fact that these clouds form most frequently over the tropical western Pacific is probably related (through the nucleation physics) to the very low tropopause temperatures in this region. Simulations using midlatitude tropopause temperatures near −65°C suggest that at the higher temperatures, fewer ice crystals nucleate, resulting in more rapid crystal growth and cloud dissipation by precipitation. Hence, the lifetime of thin cirrus formed near the midlatitude tropopause should be limited to a few hours after the synoptic‐scale system that initiated cloud formation has passed.
Role of vegetation in sustaining large‐scale atmospheric circulations in the tropics
Authors: Eltahir Elfatih A. B.
Published: February 2004

The focus of this paper is the role of rain forests in large‐scale atmospheric circulations. The significance of this role is investigated by studying the response of the tropical atmosphere to a perturbation in the state of vegetation (deforestation) over three regions: the Amazon, Congo, and Indonesia. A theory is developed to relate tropical deforestation and the resulting changes in the large‐scale atmospheric circulation. Field observations and numerical simulations support the argument that tropical deforestation reduces the total net surface radiation, including terrestrial and solar forms. However, the energy balance at the land‐atmosphere boundary dictates that for equilibrium conditions, any reduction in net surface radiation has to be balanced by a similar reduction in the total flux of heat, including sensible and latent forms. Since these fluxes supply heat as well as entropy from the forest into the atmospheric boundary layer, a reduction in the total flux of heat reduces the boundary layer entropy. In a moist atmosphere, that satisfies a quasi equilibrium between moist convection and radiative forcing, the equilibrium temperature profile is uniquely related to the boundary layer entropy. Under such conditions, large‐scale deforestation reduces boundary layer entropy relative to the surroundings, cools the upper troposphere, and results in subsidence, divergent flow in the boundary layer, and weakening of the large‐scale circulation. These changes are simulated using a simple linear model of atmospheric flow. The comparison of the model predictions with observations of atmospheric circulations over the Amazon, Congo, and Indonesia suggests a significant role for vegetation in maintaining large‐scale atmospheric circulations in the tropics.
Observations of the impact of volcanic activity on stratospheric chemistry
Authors: Coffey M. T.
Published: February 2004

The basic stratospheric chemical and radiative processes which could be modified by volcanic injections to the stratosphere are reviewed. Observed effects after two major volcanic eruptions (El Chichón and Mount Pinatubo) are reported. Measurements of SO2, NO, NO2, HNO3, HCl, and O3 clearly show the impact of volcanic injections to the stratosphere. Large amounts of SO2 (up to 20 Mt) are observed to be injected by energetic volcanoes. Gaseous SO2 is converted into sulfate aerosols within about 30 days. Reactive nitrogen (NO and NO2) are reduced by up to 50% of their column amounts in midlatitudes. Some observations have shown HNO3 amounts to be increased where NO2 is decreased; other observations have not shown an HNO3 increase. Heterogeneous reactions on the surfaces of sulfate aerosol particles are implicated in the conversion of NO and NO2 into HNO3. The direct injection of HCl by volcanic eruptions may increase the local column by up to 40%. Satellite observations have revealed local ozone decreases in the range of 5 to 10% of the column following El Chichón and Mount Pinatubo eruptions.
Model study indicating halogen activation and ozone destruction in polluted air masses transported to the sea
Authors: Sander Rolf , Crutzen Paul J.
Published: February 2004

A chemical box model of the marine boundary layer has been developed. It treats reactions in the gas phase and in deliquesced sea‐salt aerosol particles. A quasi‐size‐dependent approach was used to obtain mean values of those aerosol properties that depend on the droplet radii. A residence time of 2 days was assumed for the return of the aerosol particles to the sea surface. Emission and deposition fluxes simulate exchange processes with seawater and air masses surrounding the air parcel. Apart from the well‐known reactions, the chemical reaction mechanism includes a large set of reactions of halogen compounds that are of potential importance for the ozone budget. Photochemical reactions are switched on during the day, assuming a semisinusoidal diurnal cycle of the photolysis rates. In our model runs, a heavily polluted urban air mass (in which O3 has been formed by photochemical smog reactions over land) is advected over the ocean. We found two processes that convert aqueous phase bromide into reactive bromine compounds. First, when concentrations of nitrogen oxides are still high, NO3 is scavenged during nighttime by the aerosol particles and oxidizes bromide: Br− + NO3 → Br + NO−3. The second process is a cycle in which the aqueous phase reaction H+ + HOBr + Br− → Br2 + H2O plays a central role. Br2 is only slightly soluble, volatilizes, and is dissociated into Br atoms. Subsequently, rapid destruction of ozone takes place via the gas phase reactions Br + O3 → BrO + O2, BrO + HO2 → HOBr + O2, and HOBr + hv → Br + OH. Sensitivity analyses have been performed to investigate how our results are influenced by input parameters that are not well known.
Stratospheric horizontal wavenumber spectra of winds, potential temperature, and atmospheric tracers observed by high‐altitude aircraft
Authors: Bacmeister Julio T. , Eckermann Stephen D. , Newman Paul A. , Lait Leslie , Chan K. Roland , Loewenstein Max , Proffitt Michael H. , Gary Bruce L.
Published: February 2004

Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N2O mixing ratios, as measured in the mid‐stratosphere during 73 ER‐2 flights (altitude ≈ 20km) are presented. The velocity and potential temperature spectra in the 100 to 1‐km wavelength range deviate significantly from the uniform −5/3 power law expected for the inverse energy‐cascade regime of two‐dimensional turbulence and also for inertial‐range, three‐dimensional turbulence. Instead, steeper spectra approximately consistent with a −3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between −1.5 and −2.0. For vertical velocity the spectrum at longer scales becomes flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley‐Shur‐Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N2O mixing ratio in the 100 to 1‐km wavelength range do conform to an approximately uniform −5/3 power law. It is argued that this may reflect interactions between gravity wave air‐parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstrophy‐cascading two‐dimensional turbulence.
Direct observations of growing cracks in ice
Authors: Arakawa Masahiko , Maeno Norikazu , Higa Michiya
Published: February 2004

The crack growth process in the high‐velocity impact of water ice was directly observed with an image‐converter camera which stored 24 successive images at speeds of 2×105 and 5×105 frames/s. At lower‐velocity impacts, the maximum growth velocity of a crack was found to be 1050±130 m/s. At higher‐velocity impact experiments, 140–650 m/s, a shear fracture region was observed to grow from the impact point hemispherically with a velocity between 2.0 and 3.5 km/s depending on the impact velocity, which is larger than the shear wave velocity of water ice, 2.05 km/s. Then the fracture velocity gradually decreases, and the growth of the shear fracture region stops when the growth velocity is below 1 km/s, whereas radial cracks continue to grow outside the region. The subsequent radial growth of cracks was similar to that found at lower‐velocity impacts.
Neutron‐capture 36Cl, 41Ca, 36Ar, and 150Sm in large chondrites: Evidence for high fluences of thermalized neutrons
Authors: Bogard D. D. , Nyquist L. E. , Bansal B. M. , Garrison D. H. , Wiesmann H. , Herzog G. F. , Albrecht A. A. , Vogt S. , Klein J.
Published: February 2004

We have measured significant concentrations of 36Cl, 41Ca, 36Ar from decay of 36Cl, and 150Sm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 36Cl and 41Ca, corrected for a high‐energy spallogenic component and a terrestrial age of ∼50 ka, give average neutron‐capture production rates of 208 atoms/min/g‐Cl and 1525 atoms/min/kg‐Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm2/s and 4.3 n/cm2/s, respectively. If sustained for the ∼65 Ma single‐stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ∼1.3×1016 and 0.8 × 1016 n/cm2 for 36Cl and 41Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 36Ar/38Ar ratios as large as ∼9. The correlation of high 36Ar/38Ar with high Cl/Ca phases in neutron‐irradiated Chico indicates that the excess 36Ar above that expected from spallation is due to decay of neutron‐produced 36Cl. Excess 36Ar in Chico requires a thermal neutron fluence of 0.9–1.7×1016 n/cm2. Decreases in 149Sm/152Sm due to neutron‐capture by 149Sm correlate with increases in 150Sm/152Sm for three samples of Chico, and one of the Torino H‐chondrite. The 0.08% decrease in 149Sm/152Sm shown by Chico corresponds to a neutron fluence of 1.23×1016 n/cm2. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 36Ar identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of ∼0.2–0.6×1016 n/cm2. Depletion of 149Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8×1016 n/cm2 and 0.25×1016 n/cm2, respectively. Neutron fluences of ∼1016 n/cm2 in Chico are almost as large as those previously observed for some lunar soils. Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron‐capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron‐capture has the potential for identifying large meteorites with complex exposure histories.
Factors influencing the capability of lava to erode its substrate: Application to Venus
Authors: Bussey D. B. J. , Sørensen S.‐A. , Guest J. E.
Published: February 2004

There is evidence that a uniquely Venusian type of channel may be at least partly erosional in origin. A number of published models discuss the erosion process on various planetary bodies. However, these relate to lavas of specifically different compositions. Here a numerical model is derived to investigate the sensitivity of the erosion process to various physical parameters of the lava and substrate, placing constraints on the physical characteristics required to obtain a suitably rapid erosion rate. The model allows for the effects of latent heat and also considers the presence of a liquid substrate boundary layer between the lava and solid substrate. The results indicate that the thermal erosion process is particularly sensitive to the melting temperature, when the melting temperature is doubled from 700 to 1400 K, the erosion rate decreases by approximately an order of magnitude. Also, the erosion rate associated with a venusian initial substrate temperature is approximately 1.5 times greater than the equivalent terrestrial one. Mechanical erosion is considered by altering the percentage of latent heat required to melt the substrate; as this percentage is varied from 0 to 100, the erosion rate decreases by about a factor of 2.
Remote sensing and geologic studies of the Schiller‐Schickard region of the Moon
Authors: Blewett David T. , Hawke B. Ray , Lucey Paul G. , Taylor G. Jeffrey , Jaumann Ralf , Spudis Paul D.
Published: February 2004

Near‐infrared reflectance spectra, multispectral images, and photogeologic data for the Schiller‐Schickard (SS) region were obtained and analyzed in order to determine the composition and origin of a variety of geologic units. These include light plains deposits, Orientale‐related deposits, mare units, and dark‐haloed impact craters (DHCs). Spectral data indicate that the pre‐Orientale highland surface was dominated by noritic anorthosite. Near‐IR spectra show that DHCs in the region have excavated ancient (>3.8 Ga) mare basalts from beneath highland‐bearing material emplaced by the Orientale impact. Ancient mare basalts were widespread in the SS region prior to the Orientale event, and their distribution appears to have been controlled by the presence of several old impact basins, including the Schiller‐Zucchius basin and a basin previously unrecognized. Both near‐IR spectra and multispectral images indicate that light plains and other Orientale‐related units in the SS region contain major amounts of local, pre‐Orientale mare basalt. The amounts of local material in these deposits approach, but seldom exceed, the maximum values predicted by the local mixing hypothesis of Oberbeck and co‐workers.
Collisional history of H chondrites
Authors: Graf Thomas , Marti Kurt
Published: February 2004

The cosmic ray exposure histories of ordinary chondrites were recently reviewed (Marti and Graf, 1992). Here, we discuss in detail currently available cosmic ray exposure records and the adopted exposure ages of individual H chondrites. We further present analyses of the distinct histograms of petrographic subgroups and the implications for orbital evolution and collisional records. In addition to the stochastic events 7 to 8 m.y. ago, a peak at 33 Ma represents a statistically significant deviation from an exponential age distribution, and signals a major collisional event at this time. The H6 subgroup also recorded a similar event 24 m.y. ago. These records present significant arguments against a continuous supply of H chondrites from the asteroid belt. We show that the records of fall times for H5s are distinct from other subgroups and that a different orbit for the H5 parent is indicated in collisional events 7 to 8 m.y. ago. The systematics of noble gas losses in the H5 subgroup are consistent with this interpretation. The evidence for stochastic fragmentation and subsequent delivery to Earth of H chondrites suggests that their present predominance among falls may represent a deviation from a representative population distribution.
Floor‐fractured craters in Mare Smythii and west of Oceanus Procellarum: Implications of crater modification by viscous relaxation and igneous intrusion models
Authors: Wichman R. W. , Schultz P. H.
Published: February 2004

Endogenic modification in lunar floor‐fractured craters can constrain spatial variations in early lunar conditions. The nature of these constraints, however, depends on the assumed mechanism of crater modification. For viscous relaxation, the extent of crater modification depends on the surrounding crustal viscosity and thus provides loose constraints on the history of crustal heating within a region. For igneous intrusion models, the extent of crater modification reflects magmatically driven deformation and can be inverted to estimate both local magma pressure and intrusion depth. Both models indicate clear differences between regional conditions at Mare Smythii and in the highlands west of Oceanus Procellarum. The uniformly shallow crater depths in Mare Smythii probably indicate a long‐lived period of extreme crater relaxation, whereas the wide range of modified crater depths in the western highlands suggest a much shorter period of partial crater relaxation. For comparable relaxation times, the average viscosity derived for Mare Smythii is over 10 times lower than the average viscosity inferred for the western highlands. Alternatively, if modification reflects deformation over crater‐centered, laccolithlike intrusions, the derived magma pressures indicate a broad, uniform magma source beneath Mare Smythii, whereas the spatial variation of estimated magma pressures in the western highlands suggests the presence of several, smaller magma sources. The derived intrusion depths are partly a function of crater size, but range from ∼1 to 10 km in depth for both regions and may be slightly greater on average in the western highlands. While both viscous relaxation and igneous intrusions can explain the modification of individual craters, the regional variations in these derived modification conditions also allow further testing of each modification model. In particular, the correlation of the lowest viscosities or the longest relaxation times with the smallest craters in both Mare Smythii and the western highlands seems inconsistent with crater modification by the relaxation mechanism. The onset of total relaxation at progressively smaller topographic wavelengths over time might produce such a trend in Mare Smythii, but total relaxation cannot be invoked for the western highlands, where many large craters still preserve significant fractions of their initial relief. In addition, the relaxation of the smallest craters in both regions suggests the presence of exceptionally high near‐surface thermal gradients (∼150–200 K/km). Since the observed regional variations in crater modification can be easily attributed to variations in magma pressure as a function of mantle topography, we conclude that crater floor fracturing in both regions is more consistent with the igneous intrusion mechanism than with viscous relaxation during a crustal heating event.
Low brightness temperatures of Martian polar caps: CO2 clouds or low surface emissivity?
Authors: Forget François , Hansen Gary B. , Pollack James B.
Published: February 2004

One of the major surprises from the spacecraft missions to Mars of the 1970s was the finding of anomalously low brightness temperatures in the winter polar regions, far below the expected CO2 frost point. Since then, many explanations have been advanced for this puzzling behavior, including the low emissivities of carbon dioxide frost and the presence of carbon dioxide clouds, but no conclusion has been reached. We have carefully analyzed the measurements obtained by the Mariner 9 infrared interferometer spectrometer (IRIS) and the Viking infrared thermal mapper (IRTM). Besides their low brightness temperatures, the anomalous areas are characterized by their high variability and their complex spectral signature. Also, there is evidence suggesting that their occurrence is related to the condensation of CO2 in the atmosphere. We have used a combination of data analysis and modeling to compare these observations with simulated spectra obtained with radiative models of CO2 ice clouds and CO2 ice deposits. We show that precipitating CO2 cloud with particle radius larger than 10 μm and CO2 snow deposits with millimeter‐sized grains are able to produce the observed features. In both cases, matching the IRIS spectra requires the CO2 ice particles to be mixed with small amounts of water or dust, as expected for the northern winter cap observed by the Mariner 9 mission. Nonprecipitating CO2 clouds, if they exist, should be transparent in the infrared. On the other hand, CO2 ice deposits composed of large grains or monolithic ice which have directly condensed on the ground could have an emissivity close to unity and in any case much higher than that of small CO2 ice particles originating from atmospheric condensation. We conclude that the low brightness temperatures are likely to be created by CO2 snow falls and that both falling snow particles and fresh snow deposits could contribute to create the observed features.
The degradation of impact craters in Maja Valles and Arabia, Mars
Authors: Barlow Nadine G.
Published: February 2004

Photoclinometry is being utilized to quantify the degree of degradation suffered by Martian impact craters (1‐ to 5‐km‐diameter range) in the Arabia and Maja Valles regions. Results indicate that present crater depths can vary substantially from crater depths expected for the original fresh craters at all sizes examined. Localized regions of high, moderate, and low degradation of the order of 102 km2 have been delineated using the percentage change in crater depth/diameter between current and assumed fresh crater morphology. Regions of low degradation occur randomly across these regions and probably represent recently produced impact craters which have undergone little degradation since their formation. Aeolian activity, including current mantling deposits, appears to dominate the processes degrading craters in the Arabia region, with fluvial and impact processes playing a secondary role. In the Maja Valles region, crater degradation is associated primarily with fluvial outflow channel activity, with aeolian and impact processes contributing secondary effects to the destruction of impact craters.
Prediction of aeolian features on planets: Application to Titan paleoclimatology
Authors: Lorenz R. D. , Lunine J. I. , Grier J. A. , Fisher M. A.
Published: February 2004

We examine the conditions necessary to form aeolian features on planets such as ripples and dunes. Specifically, we compute saltation and suspension speeds for the terrestrial planets, Triton, Titan, and two possible primordial Titan atmospheres, and compare these with theoretical estimates of friction speeds. We examine the availability of sediments and the mass transport rates needed to form aeolian features on Titan, and compare likely feature dimensions with the resolution of instruments on the Cassini mission to evaluate the detectability of such features. A novel element is the consideration of a Weibull wind speed distribution, adding a probabilistic element to the formation and destruction of dunes. We find dunes unlikely to form in any Titan atmospheres, suggesting that dunelike bedforms are only likely to be found in areas that were once submerged by liquids.