bluuebunny/crossref_metadata_2025 huggingface dataset:
A UARS study of lower stratospheric polar processing in the early stages of northern and southern winters
Authors: Yudin V. A. , Geller M. A. , Khattatov B. V. , Douglass A. R. , Cerniglia M. C. , Waters J. W. , Elson L. S. , Roche A. E. , Russell J. M.
Published: February 2004

UARS data and results from a three‐dimensional transport model have been used to compare and contrast the extent of the early stages of chemical processing by polar stratospheric clouds (PSCs) during the 1991–1993 northern and southern hemisphere winters of the Upper Atmosphere Research Satellite (UARS) mission. The modeled location and timing of regions of polar processing agree quite well with the UARS microwave limb sounder (MLS), cryogenic limb array etalon spectrometer (CLAES) and Halogen Occultation Experiment (HALOE) observations in that enhanced ClO concentrations are generally found in sunlit regions of polar‐processed air and decreases of ClONO2 and HNO3 accompany low temperatures and high values of the aerosol extinctions inside the vortex. For these winters it is found that there is a period of about 2 weeks during the northern hemisphere early winter where sporadic polar processing takes place. After this a persistent processing region is seen inside the polar vortex, and the vortex fills with polar‐processed air in a bit more than 2 weeks from the first persistent PSC occurrence. In the southern hemisphere, polar processing also fills the vortex in a bit over 2 weeks. Estimates of the spectral aerosol measure of the aerosol spectrum from CLAES observations illustrate that PSC particles are seen where cold temperatures occur inside the polar vortex and heterogeneous conversion of chlorine species on PSCs are expected.
Accurate determination of total ozone using SBUV continuous spectral scan measurements
Authors: Joiner J. , Bhartia P. K.
Published: February 2004

Over the last 2 decades, satellite data have been used to monitor long‐term global changes in stratospheric ozone. In order to measure relatively small ozone trends on timescales of the order of a decade, degradation of instrument components must be accounted for, and accurate interinstrument calibration must be maintained. In this paper, we have used a self‐calibrating method to accurately retrieve total ozone from backscatter ultraviolet spectra in the wavelength range 310 to 340 nm. Using the information contained in this spectral region, we correct for time dependent and time‐independent wavelength errors and calibration errors as well as radiative transfer modeling errors. We use continuous spectral scan data from the Nimbus 7 solar backscatter ultraviolet (SBUV) instrument to retrieve total ozone at latitudes between 40°S and 40°N over the time period 1979 to 1986. These total column ozone retrievals are used to independently validate retrievals from the both Nimbus 7 total ozone mapping spectrometer (TOMS) and SBUV in discrete mode. TOMS and SBUV in discrete mode have better temporal and spatial coverage than SBUV in continuous scan mode and are two of the primary instruments used to derive long‐term global ozone trends. The time dependence of the difference between total ozone retrieved from SBUV continuous scan and that derived from TOMS and SBUV discrete mode is 0.6±0.3%/decade or less depending on latitude. The SBUV continuous scan radiances are modeled to an accuracy of ±0.3% (1σ). Unfortunately, an instrument problem terminated continuous scan mode observations after 1986 so that the later years of TOMS and SBUV discrete mode observations cannot be validated. The methods developed here may also be applied to other continuous spectral backscatter ultraviolet instruments in order to intercalibrate total ozone retrievals. This will be particularly important when temporal overlap of ozone monitoring instruments does not occur as in the case of the Nimbus 7 and Earth Probe TOMS instruments. In addition, the methods used here are applicable to ground‐based spectral ultraviolet measurements.
Two‐dimensional speed and optical risetime estimates for natural and triggered dart leaders
Authors: Mach Douglas M. , Rust W. David
Published: February 2004

We report velocities, risetimes, and other optical measurements of a set of 35 natural and 26 triggered dart leaders. All of the dart leaders are from negative strokes. The data were taken with our return stroke velocity device mounted on the National Severe Storms Laboratory mobile laboratory. The average two‐dimensional (2‐D) speed for the natural leaders is 1.9±0.2×107 m s−1, while the triggered dart leader average 2‐D speed is 1.3±0.1×107 m s−1. These two averages are significantly different. We find no significant change in the dart leader 2‐D speed with height. The mean 10–90% optical risetime for the natural dart leaders is 2.6±0.4 μs. The optical risetime is evaluated from short channel segments (length of 3.2±1.7 m) located within 100 m of the ground. The corresponding risetime for triggered leaders is 1.4±0.4 μs. These averages are significantly different. We calculated an average dart leader head length of 35±5.4 m. We find no significant difference between natural and triggered dart leader head lengths. The channel traversed by the dart leader and before the return stroke is often bright enough to be detected by our instrument. We find that the ratio of the channel brightness after the dart leader head to the dart leader head is closer to unity for triggered dart leaders. We see no clear relationship between dart leader optical risetimes and subsequent return stroke peak transmission line model (TLM) current. We find that natural dart leader speeds are correlated with the return stroke peak TLM currents: the peak TLM current increases with increasing natural dart leader 2‐D speed. We see no correlation between triggered dart leader speeds and subsequent return stroke peak TLM currents. At a given dart leader speed, natural return strokes will have smaller peak TLM currents. Overall, these results indicate that there are significant differences between natural and triggered dart leaders.
Localization of methane consumption and nitrification activities in some boreal forest soils and the stability of methane consumption on storage and disturbance
Authors: Amaral John A. , Knowles Roger
Published: February 2004

Methane consumption was studied in intact cores and bulk samples of soils from aspen, pine, and spruce sites at the northern study area in Manitoba of the Boreal Ecosystem‐Atmosphere Study (BOREAS) program. Activity of aspen and spruce soil samples showed negligible change with time of refrigeration storage, but pine soil showed increased activity. Laboratory‐determined core fluxes were in agreement with field chamber flux measurements. Most methane consumption was restricted to the subsurface (lower organic or mineral) layers. Activity was slightly increased or not affected by physical disruption of intact core sections. Nitrification was restricted to horizons in which maximal methane consumption occurred and was low or negligible at pH<5. Potential for methane production was shown in pine organic horizons and sporadically in intact organic layers of aspen soil.
On the estimation of leaf size and crown geometry for tree canopies from hotspot observations
Authors: Goel Narendra S. , Qin Wenhan , Wang Bingquan
Published: February 2004

A computer‐graphics based model for radiation interception in vegetation canopies is used to investigate the feasibility of estimating leaf size and crown size, for deciduous and coniferous trees, from canopy reflectance in the hotspot region. For deciduous trees, as represented by aspen trees, it appears that under certain conditions one can estimate leaf size; we specify optimal Sun‐view geometry, wavelength, and index, which can minimize the impacts of other structural parameters (e.g., leaf area index, leaf angle distribution, and interplant spacing) for the most accurate estimation of leaf size. However, for coniferous trees an accurate estimation of crown size seems unlikely, except possibly for sparsely spaced canopies.
Some considerations of the origin of nighttime peroxy radicals observed in MLOPEX 2
Authors: Cantrell Christopher A. , Shetter Richard E. , Calvert Jack G. , Eisele Fred L. , Tanner David J.
Published: February 2004

The “chemical amplifier” (CA) was used to measure the concentration of the peroxy radicals present in the nighttime atmosphere at the Mauna Loa Observatory (MLO) during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) from April 15 to May 15, 1992. Simultaneous measurements were also made of peroxy radicals using a modified OH instrument during one period of 56‐hour duration. Both instruments observe experimentally significant nighttime signals, but the magnitude of the larger CA signal appears to result from organic peroxy radicals to which the modified OH instrument is insensitive. The data from about one half of the nights showed a decay of the radical signal from 2000 to 0400 HST the next morning. The decay rates followed roughly those expected for primary or secondary alkyl peroxy radicals. However, data from the other half of nights showed an increase in radical concentration with time, which reflected significant radical generation rates. Several alternatives are considered in explanation of the results. (1) The possible origin of the signals from the interference of HO2NO2, presumably transported from reservoirs at higher elevations, was considered. The data are inconclusive, and this hypothesis is tentatively rejected. (2) The nighttime chemistry of the O3 and NO3 was also considered. The observed trace gases provide in theory only a small fraction of the peroxy radical signal seen on many nights. However, the presence of 1–30 parts per trillion by volume (pptv) of CH3SCH3, a compound not measured but expected to be present at MLO, can react with NO3 to produce peroxy radical signals of the magnitude observed. It is concluded that this ubiquitous compound over the oceans offers the best current hypothesis to rationalize the nighttime generation of peroxy radicals at MLO.
Radiative forcing of climate change by CFC‐11 and possible CFC replacements
Authors: Christidis N. , Hurley M. D. , Pinnock S. , Shine K. P. , Wallington T. J.
Published: February 2004

The infrared absorption cross sections of CFC‐11 and 16 other halogenated compounds have been measured. These spectra were used in detailed line‐by‐line calculations to derive radiative forcing values. The radiative forcing values for 14 of these gases have not, to our knowledge, been previously reported in the literature. The accuracy of a computationally inexpensive narrowband scheme, which included the effect of clouds and stratospheric adjustment, was investigated. Global warming potentials are presented where atmospheric lifetimes are available. In light of the substantial disagreement in values for the forcing due to CFC‐11 reported in the literature and its use as a standard to which other halogenated gases are often compared, we examined the sensitivity of CFC‐11 forcing to a number of assumptions. We find that the uncertainties in the calculated value of the radiative forcing caused by neglect of the temperature and pressure dependence of the IR absorption spectrum are much smaller than those resulting from uncertainties in the absolute absorption cross sections or the vertical profile of CFC‐11. Our best estimate is 0.285 W m−2 ppbv−1, which is 30% higher than the value adopted by the Intergovernmental Panel on Climate Change and is believed to be accurate to within about 10%. For the other gases represented here the lack of detailed knowledge of the likely vertical and horizontal distribution probably represents the most significant uncertainty in evaluating their radiative forcing.
Cloud liquid water path comparisons from passive microwave and solar reflectance satellite measurements: Assessment of sub‐field‐of‐view cloud effects in microwave retrievals
Authors: Greenwald Thomas J. , Christopher Sundar A. , Chou Joyce
Published: February 2004

Satellite observations of the cloud liquid water path (LWP) are compared from special sensor microwave imager (SSM/I) measurements and GOES 8 imager solar reflectance (SR) measurements to ascertain the impact of sub‐field‐of‐view (FOV) cloud effects on SSM/I 37 GHz retrievals. The SR retrievals also incorporate estimates of the cloud droplet effective radius derived from the GOES 8 3.9‐μm channel. The comparisons consist of simultaneous collocated and full‐resolution measurements and are limited to nonprecipitating marine stratocumulus in the eastern Pacific for two days in October 1995. The retrievals from these independent methods are consistent for overcast SSM/I FOVs, with RMS differences as low as 0.030 kg m−2, although biases exist for clouds with more open spatial structure, where the RMS differences increase to 0.039 kg m−2. For broken cloudiness within the SSM/I FOV the average beam‐filling error (BFE) in the microwave retrievals is found to be about 22% (average cloud amount of 73%). This systematic error is comparable with the average random errors in the microwave retrievals. However, even larger BFEs can be expected for individual FOVs and for regions with less cloudiness. By scaling the microwave retrievals by the cloud amount within the FOV, the systematic BFE can be significantly reduced but with increased RMS differences of 0.046–0.058 kg m−2 when compared to the SR retrievals. The beam‐filling effects reported here are significant and are expected to impact directly upon studies that use instantaneous SSM/I measurements of cloud LWP, such as cloud classification studies and validation studies involving surface‐based or in situ data.
Global‐scale wave model estimates of nonmigrating tidal effects
Authors: Hagan M. E. , Chang J. L. , Avery S. K.
Published: February 2004

The global‐scale wave model (GSWM) is used to investigate the effects of mean winds and realistic dissipation on upward propagating nonmigrating diurnal tidal components. We explore the signatures of two plausible tropospheric sources of these waves, namely, latent heat release associated with deep convective activity and the absorption of solar infrared radiation which varies zonally with longitudinal variations in tropospheric water vapor. Our calculations suggest that while nonmigrating components are up to a factor of 3 times smaller than the migrating diurnal tide, they modulate the latter and introduce measurable (∼10 m/s) longitudinal variability in the mesosphere and lower thermosphere. These effects are most pronounced in the GSWM for the latent heat source and during northern hemisphere winter.
Spatial and seasonal characteristics of recent decadal trends in the northern hemispheric troposphere and stratosphere
Authors: Kodera Kunihiko , Koide Hiroshi
Published: February 2004

Linear trends in the northern hemispheric 500 hPa geopotential height, lower tropospheric temperature, and zonal‐mean zonal winds are investigated for the period 1965–1993. There is a general opinion that decadal trends in the atmospheric circulation are produced by changes in the equatorial sea surface temperatures (SSTs). In order to determine other factors, a trend analysis is conducted in the present study to remove the variabilities related to the equatorial SSTs as “known factors” by means of a linear regression. The results of the analysis suggest that during the winter, decadal trends originate from two different sources of variability. Trends over Pacific Ocean are related to changes in the equatorial SST. The decadal trends over Eurasian‐Atlantic sector, however, are related to the stratospheric polar‐night jet and the meridional propagation of planetary waves, which can be considered internal modes of variability in the winter circulation. During the summer, trends in the subtropical lower‐tropospheric temperature are explained as a direct response to changes in the equatorial SST. Trends over eastern Siberia, however, can not be explained in a similar manner. Instead, a relation to trends in the extent of spring snow cover is suggested. Accordingly, the possibility of producing long‐term variations through interactions among the atmosphere and land surface processes is also discussed.
Effect of fire on soil‐atmosphere exchange of methane and carbon dioxide in Canadian boreal forest sites
Authors: Burke Roger A. , Zepp Richard G. , Tarr Matthew A. , Miller William L. , Stocks Brian J.
Published: February 2004

During the spring and summer of 1994 we monitored soil‐atmosphere exchanges of methane and carbon dioxide at upland sites in the Canadian boreal forest near the northern study area (NSA) of the Boreal Ecosystem‐Atmosphere Study (BOREAS). The effects of fire on methane and carbon dioxide exchange in black spruce stands developed on clay soils were evaluated by measuring fluxes with dark chambers in unburned stands and stands burned in 1994, 1992, and 1987. Similar measurements were made in jack pine stands developed on sandy soils, one unburned and the other burned in 1989. All of the sites were net sinks of atmospheric methane with median fluxes ranging from −0.3 to −1.4 mg CH4‐C m−2 d−1. Median fluxes of carbon dioxide from the forest floor to the atmosphere ranged between 1 and 2 g C m−2 d−1. Both ecosystem characteristics (e.g., soil and vegetation type) and burning history (time since burn and fire intensity) appear to have some effect on atmospheric methane consumption and carbon dioxide emission by these forest soils. In general, the jack pine sites were stronger methane sinks and had lower carbon dioxide emissions than the black spruce sites. After a few years of recovery, the burned sites tended to be slightly stronger methane sinks than unburned controls. Our results suggest that soil CO2 effluxes from upland black spruce stands may not be immediately impacted by fire, possibly maintained at preburn levels by microbial decomposition of labile compounds released as a result of the fire. By 2 years postfire there appears to be a significant reduction in soil CO2 flux, due to the loss of tree root and moss respiration and possibly to the depletion of fire‐related labile compounds. The observed recovery of soil respiration rates to preburn levels by 7 years postburn is probably due to the respiration of regrowing vegetation and the combined effects of elevated soil temperatures (about 4° to 5°C warmer than unburned sites) and improved litter quality on soil microbial activities. We estimate that soil CO2 emissions from recently burned boreal forest soils in the northern hemisphere could be of the order of 0.35 Pg C yr−1, which is in good agreement with a previous estimate that was derived in a different manner.
Aircraft measurements made during the spring maximum of ozone over Hawaii: Peroxides, CO, O3, NOy, condensation nuclei, selected hydrocarbons, halocarbons, and alkyl nitrates between 0.5 and 9 km altitude
Authors: Ridley B. A. , Atlas E. L. , Walega J. G. , Kok G. L. , Staffelbach T. A. , Greenberg J. P. , Grahek F. E. , Hess P. G. , Montzka D. D.
Published: February 2004

Between April 22 and May 11, 1992, ten flights of the University of Wyoming King Air were made during the maximum in tropospheric ozone over the central North Pacific Ocean in conjunction with the spring intensive of the second Mauna Loa Observatory Photochemistry Experiment. During the first week of flights, an episode of remarkably large total reactive nitrogen, NOy (∼2 ppbv) persisted in the 5–9 km altitude region for 3–4 days. Backtrajectory calculations combined with the trace gas and aerosol measurements confirm that its source was due primarily to export from northern latitude continental surface regions. The total amount of odd nitrogen transported over Hawaii during this event was estimated to be 1–2% of the annual emissions from subsonic aircraft or from stratospheric input. Throughout the measurement program layers of elevated O3, NOy, condensation nuclei (CN), and other species were frequently found between the onset of the marine boundary layer temperature inversion and 4–5 km altitude. Structure and strong gradients within these layers contribute to the daily variations seen at the 3.4 km elevation of the Mauna Loa Observatory during the nighttime downslope flow. The dryness of these low‐altitude layers and the calculated air mass trajectories indicate that export from northern latitudes occurred mainly with subsidence to the Hawaii region rather than from transit just above the boundary layer inversion. There was no evidence of recent stratospheric input to the altitude region sampled below 9 km. However, the observations cannot distinguish whether O3 input from the stratosphere occurred earlier in the air mass histories at higher latitudes. Fine vertical scale anticorrelations between CN and O3 or NOy were also often observed particularly in the last week of the program when NOy mixing ratios were more typical of the remote troposphere. These features are attributed to new particle formation near the tops of cloud convection episodes and they illustrate the importance of such processes in contributing to the detailed layering and dilution of some chemical species in the free troposphere during this time of year. Mean and median profiles for many of the title species are given for high and low‐to‐moderate NOy categories.
Use of the isoprene algorithm for predicting the monoterpene emission from the Mediterranean holm oak Quercus ilex L.: Performance and limits of this approach
Authors: Ciccioli Paolo , Fabozzi Concetta , Brancaleoni Enzo , Cecinato Angelo , Frattoni Massimiliano , Loreto Francesco , Kesselmeier Jürgen , Schäfer Luise , Bode Kirsten , Torres Liberto , Fugit Jean‐Luc
Published: February 2004

The algorithm developed by Guenther et al. [1991] to describe the organic emission of isoprene‐emitting plants has been used for predicting monoterpene emission from Quercus ilex L., an evergreen oak typical of the Mediterranean basin. The dependence of monoterpene emission on photosynthetically active radiation and temperature has been verified through laboratory experiments carried out on single leaves as well as through field measurements at branch level. While the algorithm describes well monoterpene emission under stationary state conditions, it is less accurate when rapid fluctuations of light and temperature take place. Because of this, the isoprene algorithm is capable of predicting the response of Quercus ilex L. with an accuracy better than ±25% only in 65% of the environmental situations experienced by the plant. Field and laboratory observations consistently indicate that temperature oscillations can be an important source for the discrepancies between predicted and observed values as they can generate bursts of emission with values twice as high as those predicted by the algorithm. Possible causes generating these effects are analyzed and critically discussed. In spite of the observed limitations, the isoprene algorithm can successfully describe the biogenic emission from Quercus ilex L., and its use is advantageous as it greatly simplifies regional and global emission models, especially if the light dependence of monoterpene emission is proven to be a widespread phenomenon.
Interactions between sulfur and soot emissions from aircraft and their role in contrail formation: 1. Nucleation
Authors: Andronache C. , Chameides W. L.
Published: February 2004

A numerical model of the time evolution of subsonic aircraft exhaust is used to evaluate the possible activation of soot particles by collisions with SO2 and H2SO4 gas molecules and Brownian coagulation with H2SO4/H2O aerosol formed by homogeneous nucleation. The soluble mass fraction accumulated on soot by the three processes is estimated for emission indices of sulfur from 0.001 to 3 g kg−1. The calculations indicate that the soluble mass fraction of sulfate added to soot particles (assumed to be totally hydrophobic at the point of exhaust) can be large enough to form activated particles within the exhaust plumes of aircraft operating on fuels with typical sulfur contents. However, for emissions from aircraft operating on extremely low sulfur fuels, the soluble material added to soot particles is not sufficient to activate them within the time frame observed for contrail formation. This result, coupled with the Busen and Schumann [1995] observations of contrail formation from an aircraft using 0.004 g S kg−1 fuel, suggests that heterogeneous interactions between soot and sulfur within the exhaust plume are not sufficient to explain the presence of activated particles and contrails in the wakes of high altitude aircraft if the emitted sulfur is in the form of SO2 only. It is probable that soot particles already have enough soluble material when emitted from the engine exhaust, or/and a higher conversion of sulfur into H2SO4 enable them to act as cloud condensation nuclei (CCN) for contrails.
Use of a new database of lead 210 for global aerosol model validation
Authors: Preiss N. , Genthon C.
Published: February 2004

The new comprehensive global database of 210Pb surface concentration and deposition flux assembled by Preiss et al. [1996a] is synthesized by reduction and aggregation on a 5°×5° grid to facilitate and improve the analysis of a general circulation model‐based aerosol and climate model. The model is found to generally underestimate surface concentrations, and to overestimate (underestimate) deposition fluxes over source (non source) regions (respectively). We think that the simulated precipitation scavenging of aerosols is too efficient in the lower part of the troposphere, and too weak in the higher levels. This would be consistent with the spatial distribution of concentration and deposition errors in the model, and is backed up by the few available observed vertical profiles of 210Pb concentration. This paper demonstrates the usefulness, appropriateness and convenience for aerosol model validation of the synthesized 210Pb database, which is listed in detail for use by other modelers.
UV measurements at Mauna Loa: July 1995 to July 1996
Authors: Bodhaine Barry A. , Dutton Ellsworth G. , Hofmann David J. , McKenzie Richard L. , Johnston Paul V.
Published: February 2004

A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument, based on a commercially available double monochromator, uses a diffuser mounted as a horizontal receptor inside a quartz dome and views the whole sky. The instrument scans over the 290–450 nm spectral range with a band pass of about 1 nm for each 5° of solar zenith angle (SZA). The UV irradiances measured at MLO are much more intense than at low‐altitude midlatitude locations. For observations at SZA 45° the erythemally weighted UV irradiances can exceed 21 μW cm−2, which is approximately 15–20% greater than that seen at Lauder, New Zealand, for similar ozone amounts. The difference is primarily due to the higher altitude at MLO (3.4 km). For overhead Sun conditions at MLO the largest value of erythemal UV was 51.3±3.1 μW cm−2, which to our knowledge is the highest recorded any‐where at the Earth's surface. UV irradiance is strongly correlated (inversely) with Dobson spectrophotometer total ozone measurements at MLO, with higher correlations at shorter wavelengths. The radiative amplification factor (RAF) for erythema at MLO is about 1.33±0.2 at SZA 45°.
Subgrid‐scale rainfall variability and its effects on atmospheric and surface variable predictions
Authors: Zhang Shuxia , Foufoula‐Georgiou Efi
Published: February 2004

A new approach, which combines the Penn State/National Center for Atmospheric Research mesoscale model MM5 with a recently developed statistical downscaling scheme, has been investigated for the prediction of rainfall over scales (grid sizes) ranging from the atmospheric model scale (>10 km) to subgrid scale (around 1 km). The innovation of the proposed dynamical/statistical hybrid approach lies on having unraveled a link between larger‐scale dynamics of the atmosphere and smaller‐scale statistics of the rainfall fields [Perica and Foufoula‐Georgiou, 1996a], which then permits the coupling of a mesoscale dynamical model with a small‐scale statistical parameterization of rainfall. This coupling is two‐way interactive and offers the capability of investigating the feedback effects of subgrid‐scale rainfall spatial variability on the further development of a rainfall system and on the surface energy balance and water partitioning over the MM5 model grids. The results of simulating rainfall in a strong convection system observed during the Oklahoma‐Kansas Preliminary Regional Experiment for STORM‐Central (PRESTORM) on June 10–11, 1985 show that (1) the dynamical/statistical hybrid approach is a useful and cost‐effective scheme to predict rainfall at subgrid scales (around 1 km) based on larger‐scale atmospheric model predictions, and (2) the inclusion of the subgrid‐scale rainfall spatial variability can significantly affect the surface temperature distribution and the short‐term (<24 hour) prediction of rainfall intensity.
Angström turbidity parameters and aerosol optical thickness: A study over 500 solar beam spectra
Authors: Bokoye A. I. , de La Casinière A. , Cabot T.
Published: February 2004

There exists in the literature many methods of determining the Angström parameters for the measurement of the optical thickness of aerosols. This thickness is required in cases such as when the spectral beam solar irradiance at ground level has to be assessed. The present work studies the differences between some of these methods and establishes the relationships between their results. A systematic study investigating 509 direct solar irradiance spectra was undertaken. As a result, a new wavelength pair for filter measurements is proposed to calculate the Angström parameters with the Voltz technie, namely, the pair (0.400 μm, 0.750 μm). This new pair is the one which best describes the solar beam attenuation in the atmospheric transmittance models. For the considered atmospheric conditions (continental urban aerosols), the mean value of the Angstrom parameter α is found to be 1.0±0.3, which is relatively close to the conventional α=1.3 if taking into account the variance. The wavelength range explored is from 0.290 μm to 0.900 μm, which concerns many applications in bioscience.
Annually resolved southern hemisphere volcanic history from two Antarctic ice cores
Authors: Cole‐Dai Jihong , Mosley‐Thompson Ellen , Thompson Lonnie G.
Published: February 2004

The continuous sulfate analysis of two Antarctic ice cores, one from the Antarctic Peninsula region and one from West Antarctica, provides an annually resolved proxy history of southern semisphere volcanism since early in the 15th century. The dating is accurate within ±3 years due to the high rate of snow accumulation at both core sites and the small sample sizes used for analysis. The two sulfate records are consistent with each other. A systematic and objective method of separating outstanding sulfate events from the background sulfate flux is proposed and used to identify all volcanic signals. The resulting volcanic chronology covering 1417–1989 A.D. resolves temporal ambiguities about several recently discovered events. A number of previously unknown, moderate eruptions during late 1600s are uncovered in this chronology. The eruption of Tambora (1815) and the recently discovered eruption of Kuwae (1453) in the tropical South Pacific injected the greatest amount of sulfur dioxide into the southern hemisphere stratosphere during the last half millennium. A technique for comparing the magnitude of volcanic events preserved within different ice cores is developed using normalized sulfate flux. For the same eruptions the variability of the volcanic sulfate flux between the cores is within ±20% of the sulfate flux from the Tambora eruption.
A comparison of six methods for measuring soil‐surface carbon dioxide fluxes
Authors: Norman J. M. , Kucharik C. J. , Gower S. T. , Baldocchi D. D. , Crill P. M. , Rayment M. , Savage K. , Striegl R. G.
Published: February 2004

Measurements of soil‐surface CO2 fluxes are important for characterizing the carbon budget of boreal forests because these fluxes can be the second largest component of the budget. Several methods for measuring soil‐surface CO2 fluxes are available: (1) closed‐dynamic‐chamber systems, (2) closed‐static‐chamber systems, (3) open‐chamber systems, and (4) eddy covariance systems. This paper presents a field comparison of six individual systems for measuring soil‐surface CO2 fluxes with each of the four basic system types represented. A single system is used as a reference and compared to each of the other systems individually in black spruce (Picea mariana), jack pine (Pinus banksiana), or aspen (Populus tremuloides) forests. Fluxes vary from 1 to 10 μmol CO2 m−2 s−1. Adjustment factors to bring all of the systems into agreement vary from 0.93 to 1.45 with an uncertainty of about 10–15%.
Photochemistry of O3 and related compounds over southern Nova Scotia
Authors: Kleinman Lawrence I. , Daum Peter H. , Lee Jai H. , Lee Yin‐Nan , Weinstein‐Lloyd Judith , Springston Stephen R. , Buhr Martin , Jobson B. Thomas
Published: February 2004

Photochemical model calculations have been performed for air masses encountered by the National Research Council's Twin Otter aircraft during the 1993 summer North Atlantic Regional Experiment (NARE) intensive. These calculations use observed values of O3, NOy, CO, and hydrocarbons as constraints. NO is determined using the ratio NO/NOy measured from the National Center for Atmospheric Research King Air under comparable circumstances. Measurements over coastal locations indicate photochemically aged air masses with relatively low concentrations of NO and an OH reactivity that is dominated by CO and CH4. Samples over land have higher NO and an OH reactivity that is dominated by isoprene. Ozone production rates and H2O2 concentrations are analyzed using radical budget arguments that are applicable to low NOx conditions. The ozone production rate, P(O3), is predicted to be proportional to Q1/2[NO], where Q is the production rate for free radicals. This relation explains 99% of the variance in P(O3). Over 90% of the variance is explained by [NO] alone. P(O3) in the coastal samples is about a factor of 4 lower than previous estimates for the eastern United States. This is a consequence of low [NO] in the air masses that are advected to Nova Scotia.
Recent increase in H2O2 concentration at Summit, Greenland
Authors: Anklin Martin , Bales Roger C.
Published: February 2004

Prior measurements of hydrogen peroxide (H2O2) in Greenland ice suggested a 50% increase of the H2O2 concentration during the last 200 years, where most of the increase occurred between 1960 and 1988 [Sigg and Neftel, 1991]. In this work we present data from two shallow cores drilled at Summit, Greenland in 1995 that confirm the H2O2 increase found earlier and that show a further increase of the H2O2 concentration since 1988, leading to an overall increase of 60±12% during the last 150 years. The new shallow cores were drilled 6 years after the Eurocore, which allowed us to identify the influence of the firnification process on the mean annual H2O2 concentration recorded in the firn. We found that the H2O2 concentration in the upper snow/firn decreased until the layer was buried with at least 1 m of snow and that the mean annual H2O2 concentrations in deeper layers stayed essentially unchanged. Besides the increase in the mean annual concentration, the annual amplitude between winter minima and summer maxima has tripled since 1970. Since there has been no significant change in temperature during either the last 150 years or last 25 years, it is unlikely that the increasing H2O2 concentrations are temperature related. We cannot rule out the possibility that seasonal accumulation patterns at Summit have changed, which could make a small contribution toward the increase. A small part of the increase of both the mean annual concentration and the annual amplitude of H2O2 in recent years could be due to increasing UV‐B radiation caused by the depletion of stratospheric ozone, but a combination of changes in tropospheric chemistry apparently is involved.
Tropospheric aerosol backscatter at a midlatitude site in the northern and southern hemispheres
Authors: Rosen James M. , Kjome Norman T. , Liley J. B.
Published: February 2004

Monthly balloon borne aerosol backscatter measurements at ∼173° and wavelengths of 940 and 490 nm have been made at Lauder, New Zealand (45°S), and Laramie, Wyoming (41°N), since 1992. The presentation and analysis here focus on the tropospheric results which suggest that a similar quasi steady state background aerosol appears over both sites with superimposed seasonal disturbances that are probably related to arid region dust storms and biomass burning. Volcanic influences on the free troposphere are found to be minimal. In contrast to the background aerosol, the aerosol disturbances show a strong hemispheric difference with fewer perturbations over Lauder as well as significantly differing vertical profile structure, as might be expected at a remote site. Our interpretation of the higher resolution backscatter measurements substantially supports the conclusions drawn from the lower resolution stratospheric aerosol and gas experiment (SAGE) and stratospheric aerosol measurement (SAM) satellite observations.
Probability of occurrence of blowing snow
Authors: Li Long , Pomeroy John W.
Published: February 2004

Blowing snow (snow transport) affects snow cover distribution and snowmelt runoff patterns in cold, wind‐swept regions. This paper uses a statistical method to examine the occurrence of blowing snow and the meteorological conditions recorded for 16 stations on the prairies of western Canada over six winters. The results show that the occurrence probability is highly related to wind speed, air temperature and snow age. For the same air temperature and snow age, the occurrence probability increases with increasing wind speed. The probability distribution of occurrence with respect to wind speed approximates a cumulative normal probability distribution, depending on the mean and variance of wind speed: the location and scale parameters of the normal distribution. It was found that these two statistical parameters essentially indicate snow resistance and sensitivity to wind transport. Analysis of the probability distributions for the occurrence of blowing snow for different classes of air temperature and snow age reveals that the mean wind speed of the normal distribution generally increases with increasing air temperature and snow age, and the variance of wind speed increases with increasing air temperature. This leads to the development of a model which first estimates the two parameters of the normal distribution using air temperature and snow age, and then estimates the probability of the occurrence of blowing snow using wind speed and the two parameters with the cumulative normal probability function. Comparison of hours of occurrence of blowing snow and of fluxes of snow transport and sublimation estimated using the model to those determined using observations of occurrence of blowing snow shows good agreement. The results of this study can be used to estimate the frequency of blowing snow events using standard meteorological data, to determine the snow transport and snow sublimation fluxes, and to examine the effect of meteorological conditions on blowing snow processes.
Heterogeneous chlorine chemistry in the tropopause region
Authors: Solomon S. , Borrmann S. , Garcia R. R. , Portmann R. , Thomason L. , Poole L. R. , Winker D. , McCormick M. P.
Published: February 2004

Satellite observations of cloud optical depths and occurrence frequencies are used as input to a two‐dimensional numerical model of the chemistry and dynamics of the atmosphere to study the effects of heterogeneous reactions on cloud surfaces upon chemical composition and ozone depletion in the tropopause region. Efficient reactions of ClONO2 with HCl and H2O, and of HOCl with HCl, are likely to take place on the surfaces of cirrus clouds [Borrmann et al., 1996] and perturb chlorine chemistry, much as they do on polar stratospheric clouds present at higher altitudes and colder temperatures. Because of the very low predicted background abundances of ClO near the tropopause, such reactions could enhance the local ClO mixing ratios by up to 30‐fold at midlatitudes. Substantial perturbations are also predicted for related chemical species (e.g., HCl, HOCl, ClONO2, NO2, HO2) in the midlatitude and tropical tropopause regions due to these heterogeneous reactions. If cirrus clouds occur with sufficient frequency and spatial extent, they could influence not only the chemical composition but also the ozone depletion in the region near the tropopause. Because of variations in observed cloud occurrence frequency and in photochemical and dynamical timescales, the presence of cirrus clouds likely has its largest effect on ozone near the midlatitude tropopause of the northern hemisphere in summer.
Use of stable lead isotopes and trace metals to characterize air mass sources into the eastern North Atlantic
Authors: Véron Alain J. , Church Thomas M.
Published: February 2004

Stable lead isotopes (204Pb, 206Pb, 207Pb, 208Pb) and trace metals (Mn, Al, Fe, Ni, Cu, Cd, Zn, Pb) have been analyzed in aerosol collected during the Atlantic Stratocumulus Transition Experiment‐Marine Aerosol and Gas Exchange (ASTEX‐MAGE) cruise that transited between Miami and the Azores from May to July 1992. Our goal was to define the continental signatures of the air masses encountered between the Azores and the subtropical regions. The combination of air mass trajectories, trace metal concentrations and stable lead isotopes allowed us to characterize the anthropogenic character of encountered air masses. The average 206Pb/207Pb ratio was 1.148±0.021 and corresponded to a mixing between well defined European (such as Great Britain with 1.115<206Pb/207Pb<1.125 and France with 206Pb/207Pb=1.141±0.000) and North American sources (with 206Pb/207Pb=1.184±0.000). On the basis of air mass trajectories and trace metal concentrations, the background isotopic signature associated with the trade winds (206Pb/207Pb=1.161±0.004) is consistent with previous reports by Church et al. [1990] such as 206Pb/207Pb=1.154±0.004 in 1988, (Véron et al., 1993), 206Pb/207Pb=1.155±0.004 in 1989, and Hamelin et al. [1996] (206Pb/207Pb=1.158±0.006) in 1991. Short‐term variations of continental air mass sources was particularly investigated by considering the anthropogenic character of aerosols collected during two Lagrangian experiments conducted as part of the ASTEX‐MAGE cruise. We demonstrated the utility of stable lead isotopes to assign a “continental source signature” (or mixture thereof) to air masses beyond that normally possible by conventional air mass trajectory analysis in remote oceanic regions.
Accelerated monochromatic radiative transfer for scattering atmospheres: Application of a new model to spectral radiance observations
Authors: Moncet J. L. , Clough S. A.
Published: February 2004

A fast and numerically accurate model for monochromatic transfer in scattering atmospheres has been developed to extend the capabilities of the existing LBLRTM line‐by‐line model to the treatment of clouds and aerosols. The algorithm is based on the adding‐doubling method and is specifically designed to perform radiance calculations in both the thermal and the solar regimes using any specified number of computational streams. The efficient implementation of the adding‐doubling scheme makes it possible to use the multiple‐scattering algorithm in retrieval applications, an essential requirement for the intended use of the algorithm in atmospheric validation studies. The algorithm is applied to observations of water clouds from the ground‐based high spectral resolution atmospheric emitted radiance interferometer (AERI) made during the daytime. Retrieval of cloud mode radius, cloud liquid water, and effective cloud fraction is required to model the AERI radiance measurements in the 520–1500 cm−1 band and in the 1800–3020 cm−1 band which contains significant scattered solar energy. An initial assessment is made of the spectral information content of the AERI measurements for water cloud properties and of the quality of the spectral fits obtainable with those three parameters in the two spectral bands.
An investigation of measured temperature profiles and VHF mesosphere summer echoes at midlatitudes
Authors: Chilson Phillip B. , Czechowsky Peter , Klostermeyer Jürgen , Rüster Rüdiger , Schmidt Gerhard
Published: February 2004

During the summers of 1992 and 1994, experiments were conducted in Germany (52°N) to investigate the region of the summer mesopause in connection with mesospheric summer echoes (MSE). MSE form in layers and are associated with dramatically enhanced radar scattering cross sections. Furthermore, since the summer mesopause is characterized by very low temperatures, it has been proposed that MSE layers are related to the presence of subvisible ice particles. Parallel measurements were made using the Sounding System (SOUSY) VHF radar and Rayleigh lidar. Radar observations showing MSE signatures are presented together with accompanying temperature profiles obtained from the lidar data. Generally, the ambient temperatures in the summer mesosphere at midlatitudes are above that required to create ice crystals. However, the presence of waves can reduce the temperature. Our observations show this to be the case. In particular, during one of the MSE events, two long‐period inertia gravity waves were observed. These waves had estimated local periods of 6 and 8 hours with accompanying vertical wavelengths of 10 and 15 km, respectively. We find a good correlation between the locations of the calculated temperature minima and the observed MSE layers. Furthermore, the temperature minima were generally below the saturation temperature for water vapor, lending support to the supposition that the backscatter is connected with the presence of ice particles.
Spatial and temporal distribution of lightning activity around Java
Authors: Hidayat Syarif , Ishii Masaru
Published: February 2004

Cloud‐to‐ground lightning flashes on and around the island of Java, Indonesia, in the tropical maritime continent region, were observed continuously by a magnetic direction‐finder network for lightning location from December 1994 to January 1996. The annual ground flash density, estimated by correcting the raw data by using the detection efficiency of the network, ranged from less than 2 to about 16 flashes/y/km2 on the island. The average annual ground flash density over the entire island was 3.2 flashes/y/km2, as opposed to 0.24 flashes/y/km2 on the Indian Ocean about 100 km south of the island. The majority of the flashes on the land occurred during November to April in the rainy season. In the dry season, lightning was active only in the western part of the island. The amount of monthly precipitation over the island was related to the monthly number of ground flashes, but the ratio of the precipitation to the ground flash density differed from season to season, from 4×108 kg/flash in the rainy season to 1.3×109 kg/flash for other months. The ratio observed in the rainy season is equal to that for the break period in the rainy season observed at Darwin, Australia, in the same maritime continent region. The annual ratio for the entire island was 6×108 kg/flash. The diurnal variation of the lightning activity averaged over the island in the set season showed a single peak in the afternoon with the peak time of about 1530 LT, similar to those reported for land stations in the tropics. The diurnal variation over the ocean had a peak in the early morning, showing the same characteristic observed in the western Pacific. The diurnal variation of the lightning activity on the sea of offshore regions around Java showed an almost out‐of‐phase pattern to the variation on the land, indicating the influence of the land‐sea effect.