bluuebunny/crossref_metadata_2025 huggingface dataset:
Long‐term measurements of free‐tropospheric sulfate at Mauna Loa: Comparison with global model simulations
Authors: Huebert B. J. , Phillips C. A. , Zhuang L. , Kjellström E. , Rodhe H. , Feichter J. , Land C.
Published: February 2004

We report here on a comparison of three models, Model of the Global Universal Tracer Transport in the Atmosphere (MOGUNTIA, abbreviated here as MOG), European Center/Hamburg (GCM Model) 3 (ECHAM3, here E3), and European Center/Hamburg (GCM Model) 4 (ECHAM4, here E4), with a decade‐long time series of sulfate aerosol measurements (Obs) from the Mauna Loa Observatory (MLO), Hawaii. The observations were sorted with four criteria to eliminate any contamination from the local volcanoes. E3 was run for 8 years, and E4 was run for 5 years to assess interannual variability. In terms of the climatological average concentration, E3 (51 pmol mol−1) was very close to Obs (53 pmol mol−1), while MOG (33 pmol mol−1) and E4 (134 pmol mol−1) predicted different concentrations. The interannual variability of the annual average was somewhat larger in Obs than in E3 and E4. The large interannual variability is noteworthy; it demonstrates that multiyear time series are essential for comparison with climatological models. The seasonal cycle of Obs was reproduced surprisingly well by E3. E4 predicted much higher concentrations, although the seasonal amplitude was reasonable. MOG showed much less seasonal variability, which suggests that stochastic processes (which are not described by climatological models) may be responsible for much of the transport from continents to remote regions. Using tagged sources, E4 found that urban/industrial sources in SE Asia and volcanoes were responsible for most of the sulfate at MLO. The interannual variability of each month was similar for Obs, E3, and E4. When E4 was run in a nudged mode for 1 year (so that it tended to reproduce the actual meteorology) and compared day‐by‐day to Obs, the R2 was 0.73. Thus, in spite of significant magnitude differences, E4 clearly reproduces many of the features that cause free‐tropospheric sulfate concentrations to vary from day‐to‐day. When we moved the continual 5–8 km volcanic source in E4 to lower altitudes, the large overestimate was reduced but not eliminated, suggesting that the high‐altitude releases alone cannot explain the high concentrations predicted by E4. Missing processes and the parameterizations of vertical exchange and scavenging are among the possible reasons for the differences between the simulations. While the discrepancy between the observations and E4 suggests that we first look for shortcomings in E4's sulfur processing, we cannot exclude the possibility that compensating errors in E3 cause its better agreement with the observations.
Global implications of ozone loss in a space shuttle wake
Authors: Danilin Michael Y. , Ko Malcolm K. W. , Weisenstein Debra K.
Published: February 2004

Existing global model calculations of ozone depletion due to solid‐fueled rocket motor (SRM) launches [Prather et al., 1990; Jackman et al., 1998] take into account the effect of globally dispersed chlorine emissions and ignore the ozone loss in the rocket wake. This ozone depletion in the wake could be substantial (up to 100% in the lower stratosphere during the first hour after exhaust [Ross et al., 1997a, 2000]). In this paper, we provide an estimate of whether wake ozone loss could accumulate after each SRM launch, leading to a larger ozone depletion on the global scale. To address this issue, we estimate an upper bound of the ozone loss in a space shuttle wake and use the Atmospheric and Environmental Research, Inc. two‐dimensional model to simulate the global effect. For the scenarios considered, the global impact of the localized ozone loss in the wakes is at least an order of magnitude less than the effects from global dispersion of the SRM chlorine emissions alone (on the order of 10−3–10−4% versus 10−2% in the ozone column near 30°N). Additional sensitivity studies performed for different wake dilution rates, seasons, locations, and local times of the shuttle launches and accounting for chlorine activation via ClONO2 + HCl → Cl2 + HNO3 on alumina particles did not change this conclusion.
Climate response to basin‐specific changes in latitudinal temperature gradients and implications for sea ice variability
Authors: Rind D. , Chandler M. , Lerner J. , Martinson D. G. , Yuan X.
Published: February 2004

We use experiments with the GISS general circulation model to investigate how changes in latitudinal temperature gradients affect atmospheric circulation in different ocean basins, with particular attention paid to the implications for high‐latitude sea ice. The results are relevant to both estimated past climate changes, current climate gradient changes (e.g., El Niño‐Southern Oscillation events), and proposed future climate responses to greenhouse gases. Sea surface temperature gradients are increased/decreased in all ocean basins, and in the Pacific and Atlantic separately, without changing sea ice or global average temperature. Additional experiments prescribe sea ice growth/reduction with global cooling/warming. As expected, increased gradients strengthen the subtropical jet stream and deepen the subpolar lows in each hemisphere, but results in the Northern and Southern Hemispheres differ in fundamental ways. In the Northern Hemisphere, increased storm intensities occur in the ocean basin with the increased gradient; in the Southern Hemisphere the deeper storms occur in the ocean basin with the decreased gradient. Alterations of the gradient in one ocean basin change longitudinal temperature gradients; an increased gradient in one basin from tropical heating results in subsidence in the tropics in the other basin, mimicking the effect of a decreased gradient in that basin. The subtropical jet is therefore strengthened over the basin with the increased gradient and decreased over the other ocean basin. Hence in many respects, regional effects, such as the strength of subpolar lows in an individual basin, are amplified when the gradient changes are of opposite sign in the two ocean basins. The Southern Hemisphere response occurs because gradient increases in one ocean basin, by strengthening the subtropical jet, shift storm tracks equatorward and away from the potential energy source associated with cold air advection from Antarctica. At the same time, with a weaker subtropical jet in the other basin, storms move poleward and strengthen. This latter effect may explain observed sea ice variations that are out of phase in the Atlantic and Pacific Ocean basins in the Southern Hemisphere (referred to as the Antarctic dipole) as well as upper ocean variability in the Weddell gyre. Gradient changes produce little effect on sea level pressure in the Arctic, unless sea ice is changed. With Arctic sea ice reductions, the sea ice response acts as a positive feedback, inducing cyclonic circulation changes that would enhance its removal, as may be occurring due to the current high phase of the North Atlantic Oscillation.
Measurements of gravity wave activity in the lower stratosphere by Doppler lidar
Authors: Hertzog A. , Souprayen C. , Hauchecorne A.
Published: February 2004

This study presents results obtained with a Rayleigh‐Mie Doppler wind lidar on mesoscale fluctuations of horizontal winds in the lower stratosphere. The instrument, which is located at the Observatory of Haute‐Provence, France (44°N, 6°E), and its capabilities are briefly described. The technique enables the computation of the kinetic energy content in the [13 km; ≈20 km] altitude range, as well as the vertical wavenumber and apparent frequency spectra, respectively in the [≈300 m; 7 km] and [20 min; ≈10 hours] spectral domains. A large variability of gravity wave activity is found. The kinetic energy can be as low as 3 J kg−1 or as high as 50 J kg−1 with an annual mean of 10 J kg−1. This variability seems to be rather independent of the season, and no annual cycle of gravity wave activity has been detected. As an example of mesoscale fluctuations which may be responsible for the variability of the spectra, we analyzed a high‐amplitude (≈10 m s−1), inertio‐gravity wave detected in the lower stratosphere and propagating energy upward. According to this, the vertical wavenumber spectrum does not show any significant annual variation but exhibits important variability at shorter timescales. The mean spectrum has a spectral index of −2.8 in the high wavenumber regime and a characteristic vertical wavelength (1/m*) of 2 km. The apparent frequency spectrum exhibits a −1.7 slope in the whole frequency domain studied.
Lagrangian reconstruction of ozone column and profile at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) throughout the winter and spring of 1997–1998
Authors: Orsolini Yvan J. , Hansen Georg , Manney Gloria L. , Livesey Nathaniel , Hoppe Ulf‐Peter
Published: February 2004

We present a Lagrangian model‐based technique to reconstruct ozone profile and column in support of the interpretation of ozone lidar observations made at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR). High‐resolution ozone profiles as well as column ozone are reconstructed locally at ALOMAR, several times a day, regularly throughout the winter and spring of 1997/1998. The approach consists of calculating a large number of back trajectories to determine the origin of air parcels above ALOMAR and of using satellite observations to determine their ozone content. The blend of satellite observations with limited spatial and temporal coverage, and global gridded meteorological data is akin to a simplified form of data assimilation. The usefulness of the approach is demonstrated by a systematic comparison between reconstructed and observed ozone profiles and column and, in particular, their day‐to‐day variability. Abrupt changes in reconstructed and observed profile shape are caused by polar vortex displacements, deep intrusions of midlatitude air, or vortex edge filamentation. Prominent laminae are seen in the spring, as the vortex breaks down. Short‐lived, large‐amplitude total ozone peaks are seen in the model and observations, most prominently in February and March, when they are in excess of 100 Dobson units. They are shown to result from a combination of a lowering of isentropes in the lowermost stratosphere and of advection of ozone‐rich air from the base of the polar vortex.
Interactive soil dust aerosol model in the GISS GCM: 1. Sensitivity of the soil dust cycle to radiative properties of soil dust aerosols
Authors: Perlwitz Jan , Tegen Ina , Miller Ron L.
Published: February 2004

The sensitivity of the soil dust aerosol cycle to radiative forcing by the soil dust aerosol particles themselves is studied. Four experiments with the NASA GISS atmospheric general circulation model, which includes a soil dust aerosol model, are compared, all using a prescribed climatological sea surface temperature as a lower boundary condition. In one experiment, dust is included as a dynamic tracer with no radiative effect, whereas dust interacts with radiation in the other simulations. The single‐scattering albedo of dust particles is prescribed to be globally uniform in the experiments with radiatively active dust, although this albedo is varied from experiment to experiment. On a global scale the radiative forcing by dust generally causes a reduction in the atmospheric dust load, corresponding to a decreased dust source flux. The dust source flux and its changes are analyzed in more detail for the main source regions. This analysis shows that the reduction varies both with the season and with the single‐scattering albedo of the dust particles. The experiments show that dust radiative forcing can lead to significant changes both in the soil dust cycle and in the climate state. To estimate dust concentration and radiative forcing by dust more accurately, dust size distributions and dust single‐scattering albedo in the model should be a function of the source region, because dust concentration and the climate response to dust radiative forcing are sensitive to dust radiative parameters.
Atmospheric dust aerosol over the Tibetan Plateau
Authors: Zhang Xiao Y. , Arimoto Richard , Cao Jun J. , An Zhi S. , Wang D.
Published: February 2004

Cascade impactor and bulk samples of atmospheric aerosol particles were collected at three sites on the Tibetan Plateau: Udaoliang (September 1993 to May 1994) and Lhasa and Gongga (May 1998). The average dust concentration at these groundbased sites was 82 μg m−3; this is lower than the lowest average concentration observed over the nine major Chinese deserts (arithmetic mean value of 270 μg m−3), and it also is lower than that over the Loess Plateau (170 μg m−3). The elemental mass‐particle size distributions for several major dust‐derived elements (Al, Ca, Fe, K, Mn, Si, and Ti) were approximately lognormal, but those for Al, Fe, and Ti differed from Ca, K, and Si. Dry deposition velocities calculated from the observed size distributions and a two‐layer deposition model averaged 2.7 cm−1 for Al, Fe, and Ti. For Ca, K, and Si, which appear to be more strongly affected by local Tibetan sources, the average calculated dry deposition velocity was 4.3 cm s−1. Wet deposition fluxes estimated from scavenging ratios indicate that less than 10% of the total deposition on the Tibetan Plateau is attributable to wet removal during the 9 months for which data are available. The yearly mean dust deposition rate (local plus remote and dry plus wet) is estimated to be of the order of ∼100 g m−2 yr−1, which is lower than that onto the Chinese desert regions (320 g m−2 yr−1) or the Loess Plateau (250 g m−2 yr−1), suggesting that the sources upwind of China, and the Tibetan Plateau itself, are not major sources for Asian dust.
Oxygen isotopic composition of winter precipitation in central Japan
Authors: Suzuki Keisuke , Endo Yasoichi
Published: February 2004

The oxygen isotopic composition of the winter precipitation was observed in Tohkamachi, central Japan, in 1993/1994 and 1994/1995 winter. Synoptic meteorological conditions of winter precipitation in this area are classified as winter monsoons, cyclones positioned over the Sea of Japan, and cyclones located over the Pacific Ocean near Honshu (main Japanese Island). The oxygen isotopic composition of precipitation does not correlate with surface air temperature (r = 0.012). A close relationship is instead observed between temperature at the cloud top and the oxygen isotopic composition of precipitation (r = 0.662), and the mutual relationships are recognized for each weather condition formed under that winter precipitation (r = 0.772 for winter monsoon, 0.935 for cyclone positioned over the Pacific Ocean near Honshu). Winter precipitation formed during the winter monsoon has a higher value of oxygen isotopic composition than those formed when a cyclone is positioned over the Pacific Ocean near Honshu at the same temperature.
Nighttime observations of anomalously high levels of hydroxyl radicals above a deciduous forest canopy
Authors: Faloona Ian , Tan Dave , Brune William , Hurst Julia , Barket Dennis , Couch Tara L. , Shepson Paul , Apel Eric , Riemer Daniel , Thornberry Troy , Carroll Mary Anne , Sillman Sanford , Keeler Gerald J. , Sagady Jessica , Hooper Dianne , Paterson Kurt
Published: February 2004

Diurnal measurements of hydroxyl and hydroperoxy radicals (OH and HO2) made during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer intensive of 1998 indicate that these key components of gas phase atmospheric oxidation are sustained in significant amounts throughout the night in this northern forested region. Typical overnight levels of OH observed were 0.04 parts per trillion (pptv) (1.1 × 106 molecules/cm3), while HO2 concentrations ranged from 1 to 4 pptv. Results of diagnostic testing performed before, after, and during the deployment suggest little possibility of interferences in the measurements. Collocated measurements of the reactive biogenic hydrocarbon isoprene corroborate the observed levels of OH by exhibiting significant decays overnight above the forest canopy. The observed isoprene lifetimes ranged from 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidation by the measured OH abundances. Possible dark reactions that could generate such elevated levels of OH include the ozonolysis of extremely reactive biogenic terpenoids. However, in steady state models, which include this hypothetical production mechanism, HO2 radicals are generated in greater quantities than were measured. Nonetheless, if the measurements are representative of the nocturnal boundary layer in midlatitude temperate forests, this observed nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low‐NOx environments.
Western European N2O emissions: A top‐down approach based on atmospheric observations
Authors: Schmidt Martina , Glatzel‐Mattheier Holger , Sartorius Hartmut , Worthy Douglas E. , Levin Ingeborg
Published: February 2004

We present a 3 year record of continuous gas chromatographic nitrous oxide (N2O) observations performed at the urban station Heidelberg (Germany) together with weekly flask data from a remote continental site, Schauinsland (Black Forest, Germany), and two‐weekly integrated data from the maritime background station Izaña (Canary Islands). These data are supplemented by continuous atmospheric radon 222 observations. Mean rates of increase of N2O of 0.70–0.78 ppb yr−1 were observed over the continent and in maritime background air (Izaña). The well‐mixed continental mixing ratio was found to be higher by only 1.1 ppb (Schauinsland) and 2.4 ppb (Heidelberg) than for maritime background air. Specially tailored data selection of the Heidelberg record allowed the changing influence of a regional N2O point source (adipic acid production, BASF AG) to be clearly identified. The radon (222Rn) tracer method was applied to nighttime N2O observations at Heidelberg to estimate mean regional emissions, which changed from (161 ± 32) μg N2O‐N m−2 h−1 in 1996–1997 to (77 ± 10) μg N2O‐N m−2 h−1 in 1998 as a consequence of 90% emission reduction from BASF. An estimate of the continental N2O flux from southwestern Europe based on further selected Heidelberg data (only well‐mixed, late afternoon situations) and observations from the Schauinsland station yielded mean N2O fluxes of (43 ± 5) μg N2O‐N m−2 h−1 and (42 ± 4) μg N2O‐N m−2 h−1. These results compare well with statistical emissions inventories, when taking into account possible systematic errors of the radon tracer method of 30–35%.
Chemistry‐climate interactions in the Goddard Institute for Space Studies general circulation model: 1. Tropospheric chemistry model description and evaluation
Authors: Shindell Drew T. , Grenfell J. Lee , Rind David , Grewe Volker , Price Colin
Published: February 2004

A tropospheric chemistry model has been developed within the Goddard Institute for Space Studies general circulation model (GCM) to study interactions between chemistry and climate change. The model uses simplified chemistry based on CO‐NOx‐HOx‐Ox‐CH4 and also includes a parameterization for isoprene emissions, the most important non‐methane hydrocarbon. The model reproduces observed annual cycles and mean distributions of key trace gases fairly well. It simulates preindustrial to present‐day changes similar to those seen in other simulations. For example, the global tropospheric ozone burden increases 45%, within the 25%–57% range of other studies. Annual average zonal mean surface ozone increases more than 125% at northern midlatitudes. Comparison between runs that allow calculated ozone to interact with the GCM and those that do not shows only minor ozone differences. The common usage of non‐interactive ozone seems adequate to simulate ozone distributions. However, use of coupled chemistry does alter the tropospheric oxidation capacity, enlarging the preindustrial to present‐day OH decrease by about 10% (−5.3% global annual average uncoupled, −5.9% coupled). Thus simulation of changes in oxidation capacity may be systematically biased (though a 10% difference is within the uncertainty). Global annual average radiative forcing from preindustrial to present‐day ozone change is 0.32 W m−2. The forcing seems to increase by ∼10% with coupled chemistry. Forcing greater than 0.8 W m−2 is seen over the United States, the Mediterranean area, central Asia, and the Arctic, with values greater than 1.5 W m−2 over parts of these areas during summer. Though there are local differences, the radiative forcing is overall in good agreement with other modeling studies in both magnitude and spatial distribution, demonstrating that the simplified chemistry is adequate for climate studies.
Characteristics of Chinese aerosols determined by individual‐particle analysis
Authors: Gao Yuan , Anderson James R.
Published: February 2004

Tropospheric aerosols that originate in China and are transported over the North Pacific Ocean have potentially significant impacts on regional and global climate. These aerosols are complex mixtures of soil dust and anthropogenic particles from a variety of sources, including fossil fuel combustion, biomass burning, mining, smelting, and other industrial processes, plus reaction products of heterogeneous processes that affect these particles during transport. In the coastal marine atmosphere, these particles could be further mixed with marine aerosols. To provide examples of the diversity of chemical and physical properties of east Asian aerosols in the spring, individual aerosol particle samples were collected in April and May 1999 in three different environments in China: Qingdao on the coast of the East China Sea, Beijing in the northeast interior, and Mount Waliguan in remote northwestern China. Results reveal that aerosols in this region are complex and heterogeneous. In addition to significant differences in aerosol composition and size distributions among the samples, each sample contains a large number of polyphase aggregates. Many of the particles also have irregular shapes; for a number of the particle types, the irregular shapes should persist even at high ambient RH. Because composition, degree and nature of polyphase aggregation, and shape all effect aerosol radiative properties, the complex state of east Asian aerosols presents a challenge for the modeling of aerosol radiative forcing in the region.
Sampling error in eddy correlation flux measurements
Authors: Finkelstein Peter L. , Sims Pamela F.
Published: February 2004

Sampling errors in eddy correlation flux measurements arise from the small number of large eddies that dominate the flux during typical sampling periods. Several methods to estimate sampling, or random error in flux measurements, have been published. These methods are compared to a more statistically rigorous method which calculates the variance of a covariance when the two variables in the covariance are auto‐ and cross‐correlated. Comparisons are offered between the various methods. Compared to previously published methods, error estimates from this technique were 20 to 25% higher because of the incorporation of additional terms in the estimate of the variance. This new approach is then applied to define the random error component of representative eddy correlation flux measurements of momentum, sensible and latent heat, carbon dioxide, and ozone from five field studies, three over agricultural crops (corn, soybean, and pasture), and two from towers over forests (deciduous and mixed). The mean normalized error for each type of flux measurement over the five studies ranged from 12% for sensible heat flux to 31% for ozone flux. There were not large or significant differences between random errors for fluxes measured over crops versus those measured over forests. The effects of stability, flux magnitude, and wind speed on measurement error are discussed.
Saharan dust profiles measured by lidar at Lampedusa
Authors: di Sarra A. , Di Iorio T. , Cacciani M. , Fiocco G. , Fuà D.
Published: February 2004

Lidar observations of the tropospheric aerosols were carried out at the island of Lampedusa (35.5°N, 12.6°E) in the Mediterranean during May‐June 1999, within the Photochemical Activity and Ultraviolet Radiation II experiment. The measurements indicate that the troposphere is often loaded with large amounts of aerosol particles, producing relatively large values of the depolarization ratio. The aerosol content below and above 2 km shows a somewhat different behavior. In the upper region, large aerosol concentrations last for a few days; during these events aerosol is often detected up to 7 or 8 km. Large amounts were detected in mid‐May and were very often observed in June. By using meteorological analyses and isentropic backward trajectories, the aerosol behavior above Lampedusa has been related to the large‐scale transport patterns and to the source regions. Large aerosol loads are clearly due to dust transport from Africa, occurring through two main paths: from central Sahara, when a high‐pressure system was centered over northern Libya, and following the northwestern African coast, often along the Atlas Mountains, when the anticyclone is over Algeria or Libya, at latitudes lower than 30°N. Large aerosol loads are observed even when the air mass trajectories marginally overpass Africa, often up to 5–6 km. According to the isentropic trajectories, large vertical motions occur when the air masses travel over Africa. Significant differences in the aerosol profiles are found, depending on the origin of the air masses, and on the strength of the vertical motion. All the air masses that have traveled over Africa show an enhanced aerosol concentration, indicating that in this period the conditions were always favorable to dust mobilization and vertical propagation. The identified transport patterns appear to mainly affect the southern Mediterranean; in rare cases, forward trajectories reached southern Italy, Greece, Turkey, and the eastern Mediterranean.
Analysis of polar stratospheric cloud measurements from AVHRR
Authors: Hervig Mark E. , Pagan Kathy L. , Foschi Patricia G.
Published: February 2004

This work investigates thermal emission from polar stratospheric clouds (PSCs) measured at 10.9 and 11.9 μm wavelengths (channels 4 and 5) by the advanced very high resolution radiometer (AVHRR). PSCs can be broadly categorized by particle composition as either ice or nitric acid mixtures. This work shows that nitric acid PSCs are invisible to AVHRR, while some ice PSCs can be detected. Methods were developed to discriminate ice PSCs from other cloud types in AVHRR imagery based on the brightness temperature difference between channels 4 and 5. When PSCs are identified in AVHRR imagery, it is possible to estimate the PSC optical depth, effective radius, and ice water path using relationships reported here, however, these estimates may have large uncertainties.
Atmospheric chemistry of small organic peroxy radicals
Authors: Tyndall G. S. , Cox R. A. , Granier C. , Lesclaux R. , Moortgat G. K. , Pilling M. J. , Ravishankara A. R. , Wallington T. J.
Published: February 2004

Global atmospheric models play a key role in international assessments of the human impact on global climate and air pollution. To increase the accuracy and facilitate comparison of results from such models, it is essential they contain up‐to‐date chemical mechanisms. To this end, we present an evaluation of the atmospheric chemistry of the four most abundant organic peroxy radicals: CH3O2, C2H5O2, CH3C(O)O2, and CH3C(O)CH2O2. The literature data for the atmospheric reactions of these radicals are evaluated. In addition, the ultraviolet absorption cross sections for the above radicals and for HO2 have been evaluated. The absorption spectra were fitted to an analytical formula, which enabled published spectra to be screened objectively. Published kinetic and product data were reinterpreted, or in some case reanalyzed, using the new cross sections, leading to a self‐consistent set of kinetic, mechanistic, and spectroscopic data. Product studies were also evaluated. A set of peroxy radical reaction rate coefficients and products are recommended for use in atmospheric modeling. A three‐dimensional global chemical transport model (the Intermediate Model for the Global Evolution of Species, IMAGES) was run using both previously recommended rate coefficients and the current set to highlight the sensitivity of key atmospheric trace species to the peroxy radical chemistry used in the model.
Turbulent aerosol fluxes over the Arctic Ocean: 2. Wind‐driven sources from the sea
Authors: Nilsson E. D. , Rannik Ü. , Swietlicki E. , Leck C. , Aalto P. P. , Zhou J. , Norman M.
Published: February 2004

An eddy‐covariance flux system was successfully applied over open sea, leads and ice floes during the Arctic Ocean Expedition in July‐August 1996. Wind‐driven upward aerosol number fluxes were observed over open sea and leads in the pack ice. These particles must originate from droplets ejected into the air at the bursting of small air bubbles at the water surface. The source flux F (in 106 m−2 s−1) had a strong dependency on wind speed,
and
, over the open sea and leads, respectively (where
is the local wind speed at about 10 m height). Over the open sea the wind‐driven aerosol source flux consisted of a film drop mode centered at ∼100 nm diameter and a jet drop mode centered at ∼1 μm diameter. Over the leads in the pack ice, a jet drop mode at ∼2 μm diameter dominated. The jet drop mode consisted of sea‐salt, but oxalate indicated an organic contribution, and bacterias and other biogenic particles were identified by single particle analysis. Particles with diameters less than −100 nm appear to have contributed to the flux, but their chemical composition is unknown. Whitecaps were probably the bubble source at open sea and on the leads at high wind speed, but a different bubble source is needed in the leads owing to their small fetch. Melting of ice in the leads is probably the best candidate. The flux over the open sea was of such a magnitude that it could give a significant contribution to the condensation nuclei (CCN) population. Although the flux from the leads were roughly an order of magnitude smaller and the leads cover only a small fraction of the pack ice, the local source may till be important for the CCN population in Arctic fogs. The primary marine aerosol source will increase both with increased wind speed and with decreased ice fraction and extent. The local CCN production may therefore increase and influence cloud or fog albedo and lifetime in response to greenhouse warming in the Arctic Ocean region.
Sudden changes in aerosol and gas concentrations in the central Arctic marine boundary layer: Causes and consequences
Authors: Bigg E. Keith , Leck Caroline , Nilsson E. Douglas
Published: February 2004

Measurements of aerosol number size distributions and concentrations of the precursor gases dimethyl sulfide, sulfur dioxide and ammonia were made within the pack ice region of the central Arctic Ocean during July and August 1996 from the icebreaker Oden. Changes in concentration, sometimes exceeding the entire seasonal variation, often occurred within an hour and attempts to find the reasons for them are described. Vertical profiles of aerosol concentration in Aitken and accumulation mode particles obtained on helicopter flights revealed intense concentration gradients in the lowest 1000 m. Those below 100 m were common. Concentrations of accumulation mode particles were usually greater near the surface than at 100 m. Four representative case studies for which vertical aerosol profiles were obtained are presented. Observations of rapid large changes in near‐surface concentration of aerosols in different size ranges are compared with the vertical profiles, meteorological information, and acoustic or optical remote sensing to infer processes causing the changes. Comparison of simultaneous variations in aerosols and precursor gas concentrations are used to define the vertical profiles of the gases. It was found that dimethyl sulfide and ammonia concentrations usually must have been strongly depleted near the surface relative to concentrations at about 100 m. Sulfur dioxide profiles appeared to be more complex. Turbulence or vertical air motions initiated by atmospheric wave motions trapped within the stable boundary layer appeared to be directly responsible for many of the sudden concentration changes, through interaction with concentration gradients close to the surface. The presence of low‐level jets also had direct or indirect influences on mixing in the lowest few hundred meters. The extent to which aerosols measured near the surface can determine the microphysics of central Arctic marine boundary layer clouds is examined.
An overview of microphysical properties of Arctic clouds observed in May and July 1998 during FIRE ACE
Authors: Lawson R. Paul , Baker Brad A. , Schmitt Carl G. , Jensen T. L.
Published: February 2004

Microphysical data were collected by the NCAR C‐130 research aircraft during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment Arctic Cloud Experiment (FIRE ACE). Boundary layer clouds 100 to 400 m thick were observed on 11 of the 16 missions. The all‐water clouds varied from being adiabatic and homogeneous with monomodal drop spectra to subadiabatic and inhomogeneous with bimodal drop spectra and drizzle. The subadiabatic clouds were observed to be actively mixing near cloud top. The adiabatic clouds provided a test of the performance of the liquid water content (LWC) probes but only in low LWC conditions. A mixed‐phase boundary layer cloud displayed striking variability in the hydrometeor fields on a horizontal scale of 10 km and a vertical scale of 100 m. Cloud Particle Imager (CPI) data showed separate regions with small supercooled cloud drops, supercooled drizzle (at −25°C) and graupel particles. A deep stratus cloud with its base at 2 km (+2°C) and top at 6 km (−25°C) contained drizzle near cloud top and (lower in the cloud) very high (2500 to 4000 L−1) concentrations of ice particles in conditions that did not meet all the Hallett‐Mossop criteria. CPI data showed that an Arctic cirrus cloud was composed of very high (∼100,000 L−1) concentrations of small ice particles interspersed with single, large (mostly bullet rosette) crystals. The data showed that the cirrus cloud was inhomogeneous on scales down to tens of meters. The average ice particle concentrations measured in the cirrus by the FSSP and CPI probes were several hundred to a few thousand per liter, much higher than commonly found in the literature.
Modeling the atmospheric life cycle and radiative impact of mineral dust in the Hadley Centre climate model
Authors: Woodward S.
Published: February 2004

A parameterization of mineral dust within the Hadley Centre atmospheric general circulation model (AGCM) is described, modeled dust distributions are compared with observations, and estimates of the radiative forcing due to the inclusion of dust in the model are obtained. The parameterization uses six particle size divisions in the range 0.3–30 μm radius, and all calculations are performed on each division independently, using the GCM's prognostic variables. The dust production scheme works within the GCM and includes dependencies on particle size distribution, soil moisture, vegetation, and friction velocity. Dust transport is carried out by the GCM's tracer advection scheme and includes vertical motion due to convection, gravitational settling, and turbulent mixing in the boundary layer. Wet and dry deposition processes are included within the GCM's precipitation schemes. Representative dust radiative parameters are incorporated into the GCM's two stream radiation code. Modeled monthly average near‐surface dust concentrations are compared with observations: good agreement is seen in most locations, though the dust scheme tends to produce too little dust from China and too much from Australia in the southern spring. Global annual mean direct forcing due to the inclusion of dust in the GCM is +0.07 W m−2 at the top of the atmosphere (TOA) and −0.82 W m−2 at the surface. The geographical distributions of annual mean forcings are very inhomogeneous, with peak values exceeding the global means by a factor of approximately 2 orders of magnitude.
Analysis and validation of GPS/MET radio occultation data
Authors: Gorbunov Mikhail E. , Kornblueh Luis
Published: February 2004

This paper describes the results of the statistical comparison of the inversions of GPS/MET radio occultation data with analysis data of the European Centre for Medium‐Range Weather Forecast (ECMWF). The Prime Time 4 period (February 2–16, 1997) GPS/MET data were analyzed. The data analysis algorithms include the derivation of refraction angles from the measured phase excess, the back‐propagation method for handling data in multipath regions, ionospheric correction and noise reduction, and the Abel inversion of the refraction angle profiles and the derivation of dry temperature. We use a forward model of radio occultation experiments in order to produce artificial occultation data for the ECMWF global fields of atmospheric variables. The forward model exists in two variants, which are geometric optics propagator and wave optics propagator. The wave optics propagator allows for the simulation of diffraction effects and multipath propagation, which made this operator the choice for the validation. The artificial occultation data are processed by exactly the same inversion algorithm as the GPS/MET data. This allows for the adequate comparison of the retrieved temperatures. For regions with dense observational networks (Europe, China, and United States), the comparison between the satellite data inversions and the model analysis fields is characterized by biases inside 0.5 K and mean square deviations of 1.5–2 K. In the Southern Hemisphere, where observational data are sparse, the bias can reach 2 K, and the mean square deviation is 2–3 K. Because the quality of GPS/MET data must not be different in different hemispheres, the difference in the bias is assumed to be based on the used ECMWF analysis data.
Forecast and simulation of stratospheric ozone filaments: A validation of a high‐resolution potential vorticity advection model by airborne ozone lidar measurements in winter 1998/1999
Authors: Heese Birgit , Godin Sophie , Hauchecorne Alain
Published: February 2004

In the framework of the Third European Stratospheric Experiment on Ozone project Meridional Transport of Ozone in the Lower Stratosphere (METRO) an airborne ozone lidar has been flown on the French Falcon (Mystere 20) during winter 1998/1999 to investigate polar and subtropical filaments at midlatitudes. The objective of the METRO project is to quantify the proportion of the transport of polar and subtropical air and their mixing into midlatitude air masses. The dynamical evolution of the northern winter hemisphere was simulated using a high‐resolution advection model for potential vorticity (PV): Modele Isentropique de transport Mesoechelle de l'Ozone Stratospherique par Advection (MIMOSA). To validate the model for further studies, it was first utilized to forecast the appearance of filaments inside the range of the airplane so that each flight of the airborne campaign could be planned precisely. The vertical ozone distribution measured along the flight tracks was then compared to the respective PV distribution simulated by the advection model. Correlation coefficients between 0.5 and 0.7 found over the altitude range where the filaments were observed show a good agreement between PV and ozone filaments. An improvement of the correlation up to 0.8 by horizontal shifting of the ozone profiles against the PV evolution showed that small displacements of less than 1° of the modeled PV filaments can occur. These displacements can be explained by the uncertainties of the input wind velocity data and of the lidar data. Thus we can conclude that the PV advection model MIMOSA reproduces the position, size, and structure of polar filaments and subtropical intrusions well in the range of the expected accuracy. The model is a suitable tool for further studies of the quantification of the global, long‐term transport and mixing of polar and subtropical air into mid‐latitudes.
Lightning activity for the July 10, 1996, storm during the Stratosphere‐Troposphere Experiment: Radiation, Aerosol, and Ozone‐A (STERAO‐A) experiment
Authors: Defer Eric , Blanchet Patrice , Théry Claire , Laroche Pierre , Dye James E. , Venticinque Martin , Cummins Kenneth L.
Published: February 2004

We have analyzed the lightning activity recorded during the Stratosphere‐Troposphere Experiment: Radiation, Aerosols, and Ozone (STERAO‐A) July 10, 1996, storm by the Office National d'Etudes et de Recherches Aérospatiales (ONERA) lightning VHF interferometer and the National Lightning Detection Network (NLDN) system. Both cloud‐to‐ground and total lightning activity were observed and studied for the entire 5‐hour life of the storm. The July 10 storm was a multicellular complex, which became unicellular during the last hour. It primarily exhibited high intracloud activity with only 1.5% cloud‐to‐ground flashes. The maximum value of the total flash rate was 58 flashes per minute. Cloud‐to‐ground (CG) flashes occurred after some intracloud flashes with a delay ranging from 3 to 26 min for the different cells of the storm. Our study revealed that measured flash duration ranged from 23 μs to 1.8 s. Flash duration, averaged over 5‐min periods, increased during the storm life. Short‐duration flashes (<1 ms) did not occur until 30 min after the initial flash in the storm when the 50 dBZ vertical profile reached 8 km mean sea level (msl). The short‐duration flashes were recorded in cells where high reflectivity reached high altitude. Detailed analysis showed that the ONERA and NLDN reports were temporally and spatially consistent in the measurement of the cloud‐to‐ground flashes. Finally, we developed a new technique to distinguish negative CG flashes from other flashes by identifying the VHF signature of the negative downward stepped leader‐return stroke process in the flash VHF signal.
Polymeric substances and biofilms as biomarkers in terrestrial materials: Implications for extraterrestrial samples
Authors: Westall Frances , Steele Andrew , Toporski Jan , Walsh Maud , Allen Carlton , Guidry Sean , McKay David , Gibson Everett , Chafetz Henry
Published: February 2004

Organic polymeric substances are a fundamental component of microbial biofilms. Microorganisms, especially bacteria, secrete extracellular polymeric substances (EPS) to form slime layers in which they reproduce. In the sedimentary environment, biofilms commonly contain the products of degraded bacteria as well as allochthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps. Biofilms are almost ubiquitous wherever there is an interface and moisture (liquid/liquid, liquid/solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly recognized as stromatolites. We also discuss the distinction between bacterial biofilms and prebiotic films. The EPS and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mineralization of the films. EPS, biofilms, and their related components thus have strong preservation potential in the rock record. Fossilized microbial polymeric substances (FPS) and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to the Early Archaean (3.5 b.y.). We describe FPS and biofilms from hot spring, deep‐sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 b.y. in age. FPS and biofilms are more commonly observed than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials.
Motion of a fragmented meteoroid through the planetary atmosphere
Authors: Artemieva N. A. , Shuvalov V. V.
Published: February 2004

Meteor observations and crater field investigations support the view that some meteoroids undergo fragmentation into a finite number of splinters which move a long distance without further disruption. The motion of these fragments cannot be described in the frame of liquid‐like models. The primary purpose of this study is to achieve some success in improving the alternative model of separate fragments, which is based on studying the motion of a finite number of fragments interacting with each other through the air pressure. The three‐dimensional (3‐D) numerical technique is elaborated for the direct modeling of the flow around several arbitrarily arranged fragments. The model allows us to calculate the ablation mass rate and aerodynamic loading for each fragment. The new approach (hybrid model) based on the simultaneous solution of the 3‐D hydrodynamic equations for the airflow and the differential equations for the motion of discrete particles is proposed to study the debris cloud evolution, taking into account successive fragmentation. The simplified version of this model was applied to the Sikhote‐Alin meteorite shower, Benešov bolide, and small meteoroids' impacts against the Martian surface.
Extraterrestrial iridium, sediment accumulation and the habitability of the early Earth's surface
Authors: Anbar Ariel D. , Zahnle Kevin J. , Arnold Gail L. , Mojzsis Stephen J.
Published: February 2004

We have measured the concentrations of iridium and platinum in ancient metasediments from Akilia Island, southern West Greenland, to obtain the first direct constraints on the extraterrestrial mass flux at the Earth's surface before 3.8 Gyr. The lunar impact record indicates this was a time of heavy bombardment; the average lunar cratering rate between 4.15 and 3.8 Gyr was elevated ∼150 times the modern value. Hence, heightened Ir and Pt abundances are possible in sediments from this time. However, the observed concentrations of Ir and Pt in the Akilia metasediments are extremely low. In most samples, Ir<3 ppt and Pt<20 ppt. In contrast, the average modern crustal abundances of Ir and Pt are ∼50 ppt and ∼2 ppb, respectively. The very low concentrations observed in the Akilia metasediments (cherts and banded iron formations) can be reconciled with the lunar record by quantifying the effects of sedimentation rate on the efficiency with which the extraterrestrial mass flux is sampled. The observations are explained if rapidly accumulating sediments sampled stochastic bombardment by an impactor population governed by a power law mass distribution, such that exogenous Ir and Pt are concentrated in rare horizons. The required sedimentation rates are geologically plausible. This explanation has two consequences. First, evidence of massive impact events may be rare in sediments older than 3.8 Gyr despite heavy bombardment. Second, conditions at the Earth's surface may have been conducive for extensive accumulation of sedimentary sequences and for microbial life during most of the period 4.15–3.8 Gyr. It is possible that the Earth's surface hosted microbial habitats during most of this time if microbes living in deep‐sea or deep‐crustal environments survived photic zone vaporizing impacts that would have occurred every ∼30 Myr. Hence, evidence of life is expected even in the earliest sedimentary sequences.