bluuebunny/crossref_metadata_2025 huggingface dataset:
Gravity‐destabilized nonwetting phase invasion in macroheterogeneous porous media: Near‐pore‐scale macro modified invasion percolation simulation of experiments
Authors: Glass Robert J. , Conrad Stephen H. , Yarrington Lane
Published: February 2004

We reconceptualize “macro” modified invasion percolation (MMIP) at the near pore (NP) scale and apply it to simulate the nonwetting phase invasion experiments of Glass et al. [2000] conducted in macroheterogeneous porous media. For experiments where viscous forces were nonnegligible, we redefine the total pore filling pressure to include viscous losses within the invading phase as well as the viscous influence to decrease randomness imposed by capillary forces at the front. NP MMIP exhibits the complex invasion order seen experimentally with characteristic alternations between periods of gravity‐stabilized and destabilized invasion growth controlled by capillary barriers. The breaching of these barriers and subsequent pore‐scale fingering of the nonwetting phase is represented extremely well, as is the saturation field evolution and total volume invaded.
Limitations and potential of commercially available rhodamine WT as a groundwater tracer
Authors: Sutton D. J. , Kabala Z. J. , Francisco A. , Vasudevan D.
Published: February 2004

We conducted chemical characterization, batch, column, and modeling studies to elucidate the sorption and transport of rhodamine WT (RWT) in the subsurface. The sand‐pack material from the Lizzie field site near Greenville, North Carolina, served as our porous media. Our study confirms earlier results that RWT consists of two isomers with different sorption properties. It also shows that the two isomers have distinct emission spectra and are equally distributed in the RWT solution. The presence of the two isomers with different sorption properties and distinct emission spectra introduces an error in measuring the RWT concentration with fluorometers during porous media tracer studies. The two isomers become chromatographically separated during transport and thus arrive in a different concentration ratio than that of the RWT solutions used for fluorometer calibration and test injection. We found that this groundwater tracer chromatographic error could be as high as 7.8%. We fit six different reactive‐solute transport models of varying complexity to our four column experiments. A two‐solute, two‐site sorption transport model that accounts for nonequilibrium sorption accurately describes the breakthrough curves of the shorter‐timescale column experiments. However, possibly due to the groundwater tracer chromatographic error we discovered, this model, or a similar one that accounts for a Freundlich isotherm for one of the solutes, fails to describe the RWT transport in the longer‐timescale column experiments. The presence of the two RWT isomers may complicate the interpretation of field tracer tests because a shoulder, or any two peaks in a breakthrough curve, could result from either aquifer heterogeneity or the different arrival times of the two isomers. In cases where isomer 2 sorbs to such an extent that its breakthrough is not recorded during a test, only isomer 1 is measured, and therefore only 50% of the injected mass is recorded. Isomer 1 of RWT can be accurately modeled with a one‐solute, two‐site, nonequilibrium sorption model. This conclusion and the results from our batch studies suggest that RWT isomer l is an effective groundwater tracer but that the presence of isomer 2 hampers its effectiveness.
Effect of spatial variation of textural layers on regional field water balance
Authors: Li Weidong , Li Baoguo , Shi Yuanchun , Jacques Diederik , Feyen Jan
Published: February 2004

The spatial variability of textural layers is a major factor influencing the field water and solute transport in alluvial soils. To quantify the water transport process at a regional scale accurately, one has to take the spatial variability of textural layers into account. In this paper, a recently presented Markov chain simulation model for soil textural profiles was coupled with a deterministic field water balance model to conduct a stochastic analysis of the field water balance in a 15 km2 alluvial soil region. The aim is to assess the effect of spatial variability of textural layers on the field water balance at a regional scale. By combining simulated soil textural profiles with the field water balance model, the mean values, extreme values, and probability distributions of field water balance variables were calculated. Results showed large differences in the magnitude of soil water balance variables between different profiles. The extreme difference in the water storage in 1 m depth soil during the winter wheat growth varied with time between 175 mm and 180 mm, which accounted for 86.5–135.0% of the mean of the soil water storage. This indicates that the soil water balance variables derived from only a few soil profiles are not representative for the situation of field water balance in the entire region. The simulated root water uptake showed different types of probability density functions when the soil water storage and the deficit of soil water in the field changed with time. The simulated water storage in 1 m depth soil showed obviously a lognormal distribution, but the measured data showed an approximate normal distribution. It may be heterogeneous irrigation and cropping and some other factors that induce this discrepancy.
An analysis of terrestrial water storage variations in Illinois with implications for the Gravity Recovery and Climate Experiment (GRACE)
Authors: Rodell M. , Famiglietti J. S.
Published: February 2004

Variations in terrestrial water storage affect weather, climate, geophysical phenomena, and life on land, yet observation and understanding of terrestrial water storage are deficient. However, estimates of terrestrial water storage changes soon may be derived from observations of Earth's time‐dependent gravity field made by NASA's Gravity Recovery and Climate Experiment (GRACE). Previous studies have evaluated that concept using modeled soil moisture and snow data. This investigation builds upon their results by relying on observations rather than modeled results, by analyzing groundwater and surface water variations as well as snow and soil water variations, and by using a longer time series. Expected uncertainty in GRACE‐derived water storage changes are compared to monthly, seasonal, and annual terrestrial water storage changes estimated from observations in Illinois (145,800 km2). Assuming those changes are representative of larger regions, detectability is possible given a 200,000 km2 or larger area. Changes in soil moisture are typically the largest component of terrestrial water storage variations, followed by changes in groundwater plus intermediate zone storage.
Testing a blowing snow model against distributed snow measurements at Upper Sheep Creek, Idaho, United States of America
Authors: Prasad Rajiv , Tarboton David G. , Liston Glen E. , Luce Charles H. , Seyfried Mark S.
Published: February 2004

In this paper a physically based snow transport model (SnowTran‐3D) was used to simulate snow drifting over a 30 m grid and was compared to detailed snow water equivalence (SWE) surveys on three dates within a small 0.25 km2 subwatershed, Upper Sheep Creek. Two precipitation scenarios and two vegetation scenarios were used to carry out four snow transport model runs in order to (1) evaluate the blowing snow model, (2) evaluate the sensitivity of the snow transport model to precipitation and vegetation inputs, and (3) evaluate the linearity of snow accumulation patterns and the utility of the drift factor concept in distributed snow modeling. Spatial comparison methods consisted of (1) pointwise comparisons of measured and modeled SWE, (2) visual comparisons of the spatial maps, (3) comparisons of the basin‐wide average SWE, (4) comparisons of zonal average SWE in accumulation and scour zones, and (5) comparisons of distribution functions. We found that the basin average modeled SWE was in reasonable agreement with observations and that visually the spatial pattern of snow accumulation was well represented except for a pattern shift. Pointwise comparisons between the modeled and observed SWE maps displayed significant errors. The distribution functions of SnowTran‐3D‐modeled drift factors from two precipitation scenarios on three dates were compared with the distribution function of observation‐based drift factors obtained previously by calibration to evaluate the assumption of linearity. We found only a 14% reduction in explained variance in the distribution function of drift factors for a 69% increase in precipitation, suggesting that the simplification provided by the use of drift factor distributions will result in errors that are tolerable in many cases.
One last visit to the capillarity correction for free surface flow
Authors: Fink James P. , Parlange J.‐Y. , El‐Kadi Aly I.
Published: February 2004

When a capillarity correction is incorporated into the diffusion equation to describe the sudden drawdown of a water table, the resulting differential equation preserves, instead of smoothes, irregularities in the initial data. We describe how this problem can be handled by a proper and careful interpretation of the initial conditions, explain why it works, and illustrate the resulting behavior with numerical experiments.
Isotopic evolution of a seasonal snowpack and its melt
Authors: Taylor Susan , Feng Xiahong , Kirchner James W. , Osterhuber Randall , Klaue Björn , Renshaw Carl E.
Published: February 2004

The study of isotopic variation in snowmelt from seasonal snowpacks is useful for understanding snowmelt processes and is important for accurate hydrograph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between the isotopic composition of the original snow and its melt. This work studies the isotopic composition of new snow and its modification by snow metamorphism and melting. To distinguish individual snowstorms, we applied solutions of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less variable than the new snow, reflecting isotopic redistribution during metamorphism and melting. The snowmelt had low δ18O values early in the season and became progressively enriched in 18O as the pack continued to melt. On a given day, meltwater δ18O was systematically lower whenever melt rates were low than when melt rates were high. The progressive enrichment in δ18O of snowmelt and the dependence of δ18O on melt rates can be explained by isotopic exchange between liquid water and ice. A one‐dimensional (1‐D) model of the melting process, including advection and water‐ice isotopic exchange kinetics, reproduces the observed progressive 18O enrichment of snowmelt.
Sensitivity of bed load transport in Harris Creek: Seasonal and spatial variation over a cobble‐gravel bar
Authors: Hassan Marwan A. , Church Michael
Published: February 2004

Bed load sediment was caught in pit traps at several locations on a bar in Harris Creek, a cobble‐gravel stream with a nivally dominated hydrograph, a structurally highly organized bed, and very low rates of bed material transport. Observations were made during two spring freshets. In order to obtain representative grain size distributions of the material in transport, the traps were left for periods of up to 24 hours, so that samples of up to 30 kg were recovered. We examine the sensitivity of bedload flux to flow variations via trap‐specific ratings for narrowly defined textural subranges. All the ratings are very sensitive, indicating that bed load flux remains in the regime of “partial transport.” The ratings also exhibited seasonal hysteresis and varied from trap to trap and from year to year. At one trap the ratings for large material are distinctly segmented, with no strong correlation for the highest flows. Significant transport for material >8 mm in size begins at ∼7 m3 s−1, considerably higher than for sand. At flows competent to move the local bed gravel the portion of the sand load between 0.25 and 0.50 mm goes into suspension (finer material being dominantly suspended at all flows). Most of the sand transported at <7 m3 s−1 is washed in from upstream while the local bed remains stable. Auxiliary tracer studies demonstrate that at no observed stage (up to mean annual flood level) was the local bed generally mobilized.
Nonlinear and scaling spatial properties of soil geochemical element contents
Authors: Olsson Jonas , Berndtsson Ronny , Bahri Akissa , Persson Magnus , Jinno Kenji
Published: February 2004

The present study aimed at investigating whether a nonlinear and scaling approach is suitable for statistically characterizing the spatial variability of soil geochemical element contents at field conditions. Spatial distributions of 20 soil geochemical elements' contents in an agricultural field soil were investigated. Two indicators of nonlinear scaling were employed, empirical probability distribution functions (pdfs) and structure functions. For the pdfs, a trend was discovered with major elements naturally occurring in the soil being hyperbolic and minor trace elements being close to Gaussian. Further, fertilizer management was found to modify the behavior of related elements. The structure functions were generally nonscaling for major elements and scaling for minor elements, however, without any noticeable effect of fertilizer application. The scaling was of both monoaffine and multiaffine type. This suggests that the random cascade models recently used within geophysics may be useful also for geochemical element variability in soil.
Network modeling of dispersion: Transition from Taylor Dispersion in homogeneous networks to mechanical dispersion in very heterogeneous ones
Authors: Bruderer Céline , Bernabé Yves
Published: February 2004

The purpose of this work is to model dispersion at the macroscale as the result of mechanisms acting at the microscale. We simulated advection and molecular diffusion of a passive tracer in networks of capillaries with different radius distributions. The solute transport process asymptotically reached a Fickian regime at the end of a transitory period of variable duration. One effect of heterogeneity was to increase the asymptotic dispersion coefficients and the transient time. Another, more important, result is the observation of a transition of the longitudinal dispersion from the Taylor‐Aris dispersion in homogeneous networks to the so‐called mechanical dispersion in highly heterogeneous ones. The existence of this transition is supported by previous numerical and experimental studies. In the case of transverse dispersion, the asymptotic transverse coefficient appeared very weakly related to the Peclet number at all heterogeneity levels. We propose an explanation to this apparent contradiction to experimental observations.
A comparison of methods to determine measurement support volumes
Authors: Beckie Roger
Published: February 2004

We examine methods that can be used to identify the spatial filter function associated with the measurement of quantities on two scales. We divide the methods into two general classes: deconvolution methods and sensitivity‐based methods. We briefly describe the two classes of methods, provide examples of each from the literature, and introduce a new sensitivity‐based method inspired by Backus‐Gilbert model resolution ideas. We compare the approaches by examining the filter function that relates smaller‐scale grid block parameters to parameters determined by a Theis model interpretation of drawdown data. In general, we find that sensitivity approaches are easier to implement numerically and are less subject to the problems of identifiability, stability, and uniqueness associated with deconvolution methods.
Dissolution of residual tetrachloroethylene in fractional wettability porous media: incorporation of interfacial area estimates
Authors: Bradford Scott A. , Abriola Linda M.
Published: February 2004

Available data from a recent study of entrapped tetrachloroethylene (PCE) dissolution [Bradford et al., 1999] reveal that soil wettability and grain size distribution characteristics can dramatically influence dissolution behavior. This paper explores the modeling of the long‐term dissolution of residual nonaqueous phase liquids (NAPLs) entrapped in fractional wettability porous media. Here dissolution is modeled with a linear driving force expression, and the lumped mass transfer coefficient is represented using independent estimates of the film mass transfer coefficient and the NAPL‐water interfacial area. In fractional wettability porous media the NAPL saturation is assumed to occur as both NAPL films and ganglia, and NAPL‐water interfacial area is thus modeled as the sum of contributions from films and ganglia. The interfacial area model is calibrated by fitting several parameters to the available experimental data. Trends in fitted parameters suggest that (1) residual NAPL ganglia are less accessible to a flowing aqueous phase in finer textured soils; (2) the dissolution rate depends on the interfacial area in a nonlinear manner; and (3) NAPL ganglia entrapment depends on system wettability and grain size distribution. Mass transfer coefficient correlations are established from these fitted parameters. The calibrated model predicts that for a given NAPL saturation, an increasing PCE‐wet sand fraction results in longer periods of high effluent concentrations, followed by increased rates of concentration reduction and more persistent subsequent low‐ concentration tailing. The dependence of dissolution behavior on mean grain size is also captured by this model. Simulations demonstrate a significant improvement in dissolution predictions using the proposed model in comparison with those obtained using a previously developed mass transfer correlation based upon a power law function of saturation.
Solute transport through a heterogeneous coupled vadose‐saturated zone system with temporally random rainfall
Authors: Foussereau X. , Graham W. D. , Akpoji G. A. , Destouni G. , Rao P. S. C.
Published: February 2004

The transport of nonreactive solutes through a coupled, two‐dimensional, randomly heterogeneous vadose‐saturated zone system subject to temporally random rainfall is predicted by Monte Carlo simulation and compared with previously published analytic results for three different rainfall patterns. The relative contributions of the uncertain inputs (i.e., rainfall and saturated conductivity) to the prediction uncertainty of solute transport are quantified in terms of the statistical moments of the pore water velocity, the plume spatial moments, and solute flux breakthrough curves at downstream control planes. Results show that the mean and variance of the saturated zone pore water velocity were approximately equivalent for the cases of uniform and random rainfall and were well predicted by the analytical relationships developed by Rubin and Bellin [1994]. As a result, the mean plume displacement, estimated by the trajectory of the mean plume center of mass, was found to be nearly identical for these cases. In the temporally random rainfall case, the saturated zone mean plume experienced more spread in the direction of mean flow at early times. However, the asymptotic rates of spatial spreading of the mean solute plumes were found to be approximately equivalent for the uniform and random rainfall cases and well predicted by the approximate expressions for longitudinal macrodispersivity in nonuniform flow proposed by Destouni and Graham [1995]. Random rainfall and random soil properties increased prediction uncertainty of the solute plume behavior in the vadose zone by an order of magnitude when compared with the uniform rain and random soil case. This effect was reduced considerably when the solute entered the saturated zone, where random rainfall produced only slightly larger prediction uncertainty than the uniform rainfall case. The analytic model developed by Destouni and Graham [1995] accurately predicted the temporal breakthrough of the mean solute plume at saturated zone control planes for all cases, if transport through the unsaturated zone accounted for the effects of temporally random rainfall using the methodology developed by Foussereau et al. [2000a, 2000b]. Results of this work indicate that for the humid climates studied here, uncertain rainfall patterns dominate transport prediction uncertainty in the shallow unsaturated zone, while uncertain solute breakthrough to the saturated zone and uncertain hydraulic conductivity dominate prediction uncertainty in the saturated zone.
Paleohydrology of flash floods in small desert watersheds in western Arizona
Authors: House P. Kyle , Baker Victor R.
Published: February 2004

In this study, geological, historical, and meteorological data were combined to produce a regional chronology of flood magnitude and frequency in nine small basins (7–70 km2). The chronology spans more than 1000 years and demonstrates that detailed records of flood magnitude and frequency can be compiled in arid regions with little to no conventional hydrologic information. The recent (i.e., post‐1950) flood history was evaluated by comparing a 50‐year series of aerial photographs with precipitation data, ages of flood‐transported beer cans, anthropogenic horizons in flood sediments, postbomb 14C dates on flotsam, and anecdotal accounts. Stratigraphic analysis of paleoflood deposits extended the regional flood record in time, and associated flood magnitudes were determined by incorporating relict high‐water evidence into a hydraulic model. The results reveal a general consistency among the magnitudes of the largest floods in the historical and the paleoflood records and indicate that the magnitudes and relative frequencies of actual large floods are at variance with “100‐year” flood magnitudes predicted by regional flood frequency models. This suggests that the predictive equations may not be appropriate for regulatory, management, or design purposes in the absence of additional, real data on flooding. Augmenting conventional approaches to regional flood magnitude and frequency analysis with real information derived from the alternative methods described here is a viable approach to improving assessments of regional flood characteristics in sparsely gaged desert areas.
How significantly does coagulational scavenging limit atmospheric particle production?
Authors: Kerminen Veli‐Matti , Pirjola Liisa , Kulmala Markku
Published: February 2004

A model study was performed to investigate the initial fate of nanometer‐size nuclei formed in the atmosphere by homogeneous nucleation. Two important results were obtained: (1) coagulational scavenging into preexisting aerosol particles is an extremely strong sink for nuclei formed recently in the atmosphere, and (2) the growth of fresh nuclei to detectable sizes and above requires in most cases the presence of nonvolatile condensable vapors other than H2SO4(g). These vapors are probably secondary organics, and their total required concentration level ranges from <107 up to 108 molecules cm−3 depending on the preexisting particle population. The fact that newly formed particles are scavenged significantly or even totally before reaching detectable sizes has important consequences when interpreting atmospheric measurements. First, atmospheric nucleation events are probably much more frequent than what has been believed based on earlier observations, and second, the nucleation rates derived from atmospheric observations are likely to be substantially lower than the rates at which new particles are really formed by nucleation. Since newly formed nuclei will be scavenged away unless they grow sufficiently fast, there must be some lower limit for the nuclei growth rate below which the nucleation event will not be recorded using the current measurement techniques. The mimimum growth rate was estimated to be below 1 nm h−1 in “clean” environments and of the order of several nm h−1 in “polluted” environments. This requirement may explain why observed atmospheric particle formation events are accompanied frequently by the growth of new particles to sizes typical of the Aitken mode.
Surface pressure estimates from satellite data in the oxygen A‐band: Applications to the MOS sensor over land
Authors: Dubuisson P. , Borde R. , Schmechtig C. , Santer R.
Published: February 2004

A fast method for the Apparent Pressure Retrieval (APR method) over land from satellite data, based on a two band ratio in the oxygen A‐band (759–770 nm), is described. This method is devoted to the cloud detection and atmospheric corrections. Parameterizations are performed from line‐by‐line calculations assuming a pure absorbing medium. Moreover, we defined a corrective factor to account for scattering effects of the atmosphere. We validated this method with measurements of the MOS sensor (Modular Optoelectronic Scanner), whose spectral characteristics are appropriate. Comparisons with ECMWF (European Centre for Medium‐Range Weather Forecasts) pressures showed the need to perform in‐flight calibrations over a reference scene to account for spectral shifts of filter responses. Therefore we selected bright surfaces for the calibration, such as deserts, because of their major contribution to the satellite signal. After calibration the accuracy of the method is about 10 hPa over bright surfaces. Comparisons for various meteorological and geographical conditions showed that deviations between ECMWF pressures and MOS apparent pressures are generally less than 30 hPa using scattering corrections. These deviations are multiplied by 2 without correction. The APR method has been included in the cloud detection and atmospheric correction algorithms for the MOS data processing over land. Theoretical studies showed that the APR method is suitable for the cloud discrimination and that an error of 30 hPa on the surface pressure retrieval has no noticeable effect on the geophysical products of these algorithms, such as aerosol optical thickness or surface reflectance. Consequently, this method is potentially applicable to the retrieval of the apparent pressure over land with the MOS algorithms, as well as other similar satellite sensors such as the Medium Resolution Imaging Spectrometer.
Airglow observations of dynamical (wind shear‐induced) instabilities over Adelaide, Australia, associated with atmospheric gravity waves
Authors: Hecht J. H. , Walterscheid R. L. , Vincent R. A.
Published: February 2004

While several observations have been made in recent years of instability features in airglow images of atmospheric gravity waves (AGWs), such measurements are still rare. To date, these features are characterized by appearing to be aligned perpendicular to the AGW wave fronts. Multi‐instrument observations confirm the theoretical prediction that such features are caused by convective instabilities where the AGW‐induced temperature variation causes the total lapse rate to exceed the adiabatic lapse rate. In February 2000, airglow observations were obtained at Buckland Park, Australia, which showed instability features with a different characteristic. These images showed small‐scale (less than 10 km horizontal wavelength) features aligned parallel to the larger scale AGW wave fronts. These features were only seen in OH images, not in O2A images, indicating that they originate below 90 km altitude. Simultaneous MF radar wind data reveal the presence of a mean wind shear which, during the period of the small‐scale features, was aligned nearly in the direction of AGW propagation. In addition, the larger scale AGW approached a critical level near 90 km altitude. While the wind shear itself is not large enough to cause an instability, an analysis of the data suggests that the small‐scale features are the result of a dynamic (wind shear‐induced) instability in the 87–90 km altitude region. The instability was due to a combination of the background wind shear and the large shear induced by the passage of the larger scale AGW as it approached the critical level.
Airborne vacuum ultraviolet resonance fluorescence instrument for in situ measurement of CO
Authors: Takegawa N. , Kita K. , Kondo Y. , Matsumi Y. , Parrish D. D. , Holloway J. S. , Koike M. , Miyazaki Y. , Toriyama N. , Kawakami S. , Ogawa T.
Published: February 2004

An airborne instrument for fast‐response, high‐precision measurement of tropospheric carbon monoxide (CO) was developed using a vacuum ultraviolet (VUV) resonance fluorescence technique. The excitation radiation is obtained by a DC discharge CO resonance lamp combined with an optical filter for the CO fourth positive band emission around 150 nm. The optical filter consists of a VUV monochromator and a crystalline quartz window (<147‐nm cutoff). The crystalline quartz window ensures a sharp discrimination against wavelengths below 135.7 nm that yield a positive interference from water vapor. Laboratory tests showed that the optical system achieved a precision of 1.1 parts per billion by volume (ppbv) at a CO concentration of 100 ppbv for a 1‐s integration period, and the flow system provided a response time (1/e time constant) of ∼2 s. The aircraft measurement campaign Biomass Burning and Lightning Experiment‐phase B (BIBLE‐B) was conducted between August and September 1999 over the western Pacific and Australia. The flight data obtained during this campaign were used to demonstrate the high precision and fast response of the instrument. An intercomparison of the VUV CO measurement and a gas chromatographic CO measurement was conducted during BIBLE‐B. Overall, these two independent measurements showed good agreement, within the experimental uncertainties.
Relation of size and displacement of the 300 mbar north circumpolar vortex to QBO, El Nino, and sunspot number, 1963–2000
Authors: Angell James K.
Published: February 2004

The size of the 300 mbar north circumpolar vortex, and its eastern, western, date line, and Greenwich hemisphere components, is estimated for the period 1963–2000 by planimetering the area poleward of 300 mbar contours in the main belt of westerlies on the mean‐monthly polar stereographic analyses of the Institute of Meteorology of the Free University of Berlin. On the basis of the superposed epoch method, there is little evidence of a relation between vortex size and phase of the QBO, but significant at the 90% level or better is the tendency for the vortex to be less displaced into the eastern hemisphere in the east‐wind phase of the QBO, for the vortex to be expanded near the time of Nino 3 sea surface temperature maximum (El Nino) but contracted 3–4 seasons after, and for the vortex to be displaced farther into the date line hemisphere when there is an El Nino. There is also an impressive tendency for the winter vortex to be less displaced into the eastern hemisphere at the time of El Nino. The tendency for the vortex to be contracted near sunspot maximum, and expanded near sunspot minimum, is significant at only about the 80% level because of the small sample size.
Summertime formaldehyde at a high‐elevation site in Quebec
Authors: Macdonald A. M. , Makar P. A. , Anlauf K. G. , Hayden K. L. , Bottenheim J. W. , Wang D. , Dann T.
Published: February 2004

Measurements of formaldehyde were made during the summer of 1996 at a high‐elevation site in Quebec as part of the North American Research Strategy on Tropospheric Ozone‐Canada East (NARSTO‐CE) measurement program. Gas phase mixing ratios were determined continuously by removing formaldehyde from the air in a glass coil scrubber, and producing a fluorescent dimer through the Hantzsch reaction. Average mixing ratios of formaldehyde were 1.3 and 0.8 ppbv for dry and wet periods, respectively. Highest values of HCHO were observed July 1–2 with a maximum mixing ratio of 4.6 ppbv. Fog water samples were also collected and analyzed for HCHO on five afternoon periods. Comparison of HCHO in the gas and aqueous phases shows reasonable agreement with Henry's law equilibrium. For dry periods July 1–12, relationships were examined between formaldehyde and other chemical species also measured at the site. Data were segregated based on the ratio of NOx to NOy and on the level of anthropogenic hydrocarbons present in the air mass. For the majority of the data, formaldehyde increased with both ozone and products of NOx oxidation (NOz) and was inversely related to the NOx/NOy, ratio. During the high HCHO episode July 1–2, HCHO was correlated with neither ozone nor NOz illustrating the different chemistry at the site on these days. A chemical box model was used to examine sources of HCHO July 1–4. The model suggests that biogenic hydrocarbons contribute on average 53% of the locally produced formaldehyde, the remainder resulting from the oxidation of methane (19%), anthropogenic VOCs (16%), acetaldehyde (7%), and organic peroxides (3%). The model cannot account for the July 1–2 formaldehyde mixing ratios from the chemistry measured at the site. This implies that an additional HCHO source not included in the model was responsible for the high levels on those days.
Measurement of aerosol sulfuric acid: 1. Experimental setup, characterization, and calibration of a novel mass spectrometric system
Authors: Curtius J. , Arnold F.
Published: February 2004

An instrument for the direct in situ measurement of aerosol sulfuric acid in the atmosphere has been developed, characterized and calibrated. The instrument termed Volatile Aerosol Component Analyzer (VACA) utilizes chemical ionization mass spectrometry technique for the detection of sulfuric acid. Prior to the mass spectrometric identification the sample flow is heated, and sulfuric acid is evaporated from the aerosol. The VACA system is characterized as a function of temperature, residence time in the flow reactor, aerosol size, and chemical composition. A unit consisting of an aerosol generator, a differential mobility analyzer, a hygrometer, and a condensation nuclei counter is used to calibrate the VACA instrument.
Ultraviolet radiation environment of Antarctica: 1. Effect of sea ice on top‐of‐atmosphere albedo and on satellite retrievals
Authors: Lubin Dan , Morrow Esther
Published: February 2004

The backscattered ultraviolet radiance measured by the Total Ozone Mapping Spectrometer (TOMS) over the Southern Ocean is influenced by both cloud cover and sea ice concentration. In TOMS data alone, these influences cannot be separated. To assess the relative importance of cloud opacity and sea ice concentration, TOMS level 2 data are colocated with AVHRR and SSM/I data. AVHRR provides independent cloud identification at a spatial resolution sufficient to estimate cloud fraction within a TOMS level 2 footprint, while the SSM/I provides useful estimates of sea ice concentration over clear and cloudy scenes. The sea ice cover is shown to have a stronger influence than cloud cover on the backscattered ultraviolet radiance at the top of the atmosphere. Over overcast scenes the mean TOMS reflectivity increases from 45 to 84% as the underlying sea ice concentration increases from 0 to 1. Over scenes containing sea ice concentrations greater than 0.5, the increase in TOMS‐measured radiance with increasing cloud amount (0–1) is generally less than 30% and is negligible over high sea ice concentrations. Over clear‐sky scenes the characteristic UV‐A albedos of the sea ice components of the scenes are retrieved from the TOMS data. These albedos range from 0.19±0.14 for sea ice concentration 0.1, increasing rapidly to 0.53±0.15 for sea ice concentration 0.3, and then approximately linearly to 0.80±0.11 for sea ice concentration 1.0. There is the potential to develop a climatology of surface ultraviolet and photosynthetically active radiation for southern high latitudes, which utilizes a combination of TOMS and SSM/I data. Such a climatology could cover the entire Southern Ocean throughout the duration of the modern springtime ozone depletion phenomenon. Analysis of uncertainties related to sea ice concentration retrieval from SSM/I, and related uncertainties in surface albedo identification and their influence on the estimated surface radiative flux, shows that such a climatology would have the most quantitative value for sea ice concentrations less than 0.5.
An extension of Measurement of Ozone and Water Vapour by Airbus In‐service Aircraft (MOZAIC) ozone climatologies using trajectory statistics
Authors: Stohl Andreas , James Paul , Forster Caroline , Spichtinger Nicole , Marenco Alain , Thouret Valérie , Smit Herman G. J.
Published: February 2004

This study investigates the distribution and the sources of ozone (O3) observed in the uppermost troposphere and lowermost stratosphere within the framework of the Measurement of Ozone and Water Vapor by Airbus InService Aircraft (MOZAIC) program. The O3 measurement data taken aboard five commercial aircraft during a 1‐year period are combined with 8‐day three‐dimensional back trajectories ending at the aircraft positions. Potential vorticity along the trajectories is used to form subsets of the data taken in the troposphere, in the stratosphere, and in stratosphere‐troposphere exchange flows. A measured O3 value is then attributed to the entire path of the corresponding trajectory. A statistical analysis of this data set extends previous global MOZAIC O3 climatologies which are restricted to the major flight corridors. Furthermore, it allows source region identification of the measured O3. It is found that the North American and Eurasian continental boundary layers are the major sources of upper tropospheric O3 in spring and summer, while the African boundary layer is the greatest Os source in fall and winter. The seasonal O3 variation is strongest in air masses exported from central Asia, followed by North America. The central Asian O3 maximum is higher and occurs earlier in the year than the North American maximum, which possibly indicates a great influence from biomass burning emissions in Asia. In the lowermost stratosphere the dominant structures in the O3 distribution are a strong north‐south gradient and a wave‐like pattern with O3 maxima over North America and eastern Asia. O3 concentrations in stratosphere‐troposphere exchange flows are higher than in tropospheric air throughout the year and thus provide an important O3 source for the troposphere. However, our analysis suggests that photochemical O3 formation is more important than stratosphere‐troposphere exchange during most of the year.
Three‐dimensional simulation of stratospheric background aerosol: First results of a multiannual general circulation model simulation
Authors: Timmreck Claudia
Published: February 2004

A sulfuric acid aerosol model has been implemented in the global general circulation model ECHAM4. This model treats the formation, the development, and the transport of stratospheric sulfuric acid aerosol. The aerosol size distribution and the sulfuric acid mass fraction are calculated as a function of the H2SO4/H2O concentration, temperature, and air pressure in a size range between 0.001 μ and 2.58 μ. Binary homogeneous nucleation of H2SO4/H2O, condensation and evaporation of H2SO4 and H2O, Brownian coagulation and gravitational sedimentation are included. The microphysical model for stratospheric sulfate aerosol and a stratospheric sulfur chemistry are combined with a representation of the tropospheric sulfur chemistry. This tropospheric scheme accounts for the natural and anthropogenic emissions, chemistry, and dry and wet deposition of DMS, SO2, and SO42−. Globally and seasonally different SO2− and SO42− sources for stratospheric aerosol can therefore be taken into account. Results of a multiannual simulation show that the simulated SO2 and H2SO4 concentrations are generally in good agreement with available observations. The formation of new particles through homogeneous nucleation takes place in the tropical lower stratosphere and upper troposphere and in polar spring. The aerosol surface area density and the aerosol mass concentration reproduce lower stratospheric background conditions quite well. Effective radius and aerosol mixing ratio agree also with satellite and in situ measurements at Northern Hemisphere midlatitudes.
Modal structure of chemical mass size distribution in the high Arctic aerosol
Authors: Hillamo Risto , Kerminen Veli‐Matti , Aurela Minna , Mäkelä Timo , Maenhaut Willy , Leek Caroline
Published: February 2004

Chemical mass size distributions of aerosol particles were measured in the remote marine boundary layer over the central Arctic Ocean as part of the Atmospheric Research Program on the Arctic Ocean Expedition 1996 (AOE‐96). An inertial impaction method was used to classify aerosol particles into different size classes for subsequent chemical analysis. The particle chemical composition was determined by ion chromatography and by the particle‐induced X‐ray emission technique. Continuous particle size spectra were extracted from the raw data using a data inversion method. Clear and varying modal structures for aerosols consisting of primary sea‐salt particles or of secondary particles related to dimethyl sulfide emissions were found. Concentration levels of all modes decreased rapidly when the distance from open sea increased. In the submicrometer size range the major ions found by ion chromatography were sulfate, methane sulfonate, and ammonium. They had most of the time a clear Aitken mode and one or two accumulation modes, with aerodynamic mass median diameters around 0.1 μm, 0.3 μm, and between 0.5–1.0 μm, respectively. The overall submicron size distributions of these three ions were quite similar, suggesting that they were internally mixed over most of this size range. The corresponding modal structure was consistent with the mass size distributions derived from the particle number size distributions measured with a differential mobility particle sizer. The Aitken to accumulation mode mass ratio for nss‐sulfate and MSA was substantially higher during clear skies than during cloudy periods. Primary sea‐salt particles formed a mode with an aerodynamic mass median diameter around 2 μm. In general, the resulting continuous mass size distributions displayed a clear modal structure consistent with our understanding of the two known major source mechanisms. One is the sea‐salt aerosol emerging from seawater by bubble bursting. The other is related to dimethylsulfide (DMS) emissions from biogenic processes in seawater, followed by gas‐to‐particle conversion, formation of particulate sulfate and methane sulfonate (MSA) and neutralization by ammonia.
Experimental evidence for the importance of convected methylhydroperoxide as a source of hydrogen oxide (HOx) radicals in the tropical upper troposphere
Authors: Ravetta F. , Jacob D. J. , Brune W. H. , Heikes B. G. , Anderson B. E. , Blake D. R. , Gregory G. L. , Sachse G. W. , Sandholm S. T. , Shetter R. E. , Singh H. B. , Talbot R. W.
Published: February 2004

Concurrent measurements of OH, HO2, H2O2, and CH3OOH concentrations were made during an aircraft flight over the tropical South Pacific that followed a back‐and‐forth pattern at constant 10 km altitude for 4 hours. One end of the pattern sampled an aged convective outflow, while the other end sampled the background atmosphere. Concentrations of HO2 and CH3OOH in the convective outflow were elevated by 50 and 350% relative to background, respectively, while concentrations of OH and H2O2 were not elevated. The high CH3OOH concentrations in the outflow were due to convective pumping from the marine boundary layer. In contrast to CH3OOH, H2O2 was not enhanced in the outflow because its high water solubility allows efficient scavenging in the convective updraft. A photochemical model calculation constrained with the ensemble of aircraft observations reproduces the HO2 enhancement in the convective outflow and attributes it to the enhanced CH3OOH; the calculation also reproduces the lack of OH enhancement in the outflow and attributes it to OH loss from reaction with CH3OOH. Further analysis of model results shows substantial evidence that the rate constant used in standard mechanisms for the CH3O2 + HO2 reaction is about a factor of 3 too low at the low temperatures of the upper troposphere. A sensitivity simulation using a value of 3.4×10−11 cm3 molecule−1 s−1 at 233 K for this rate constant yields better agreement with observed HO2 concentrations and better closure of the chemical budgets for both CH3OOH and H2O2. The CH3O2 + HO2 reaction then becomes the single most important loss pathway for HOx radicals (HOx = OH + peroxy radicals) in the upper troposphere.
Evaluation of the atmospheric lifetime and radiative forcing on climate for 1,2,2,2‐tetrafluoroethyl trifluoromethyl ether (CF3OCHFCF3)
Authors: Jain Atul K. , Li Zhuangjie , Naik Vaishali , Wuebbles Donald J. , Good David A. , Hansen Jaron C. , Francisco Joseph S.
Published: February 2004

The compound 1,2,2,2‐Tetrafluoroethyl Trifluoromethyl Ether, CF3OCHFCF3 (HFE‐227), is currently being considered as a potential replacement for certain halocarbons, particularly for perfluorocarbons (PFCs), as a dry etching gas in the semiconductor industry. For this reason, it is important to determine the potential environmental effects resulting from the use and emissions of this compound. In this paper, the atmospheric lifetime, radiative forcing, and Global Warming Potentials (GWPs), an important measure of the potential effects of a gas on climate, are evaluated for this compound using our zonally averaged chemical transport and radiative transfer models of the atmosphere. To our knowledge, this is the first time this compound has been evaluated with such atmospheric models. In order to calculate the lifetime and radiative forcing, the rate constants and infrared cross sections of this compound were measured in laboratories at Illinois and Purdue, and results are reported here. The model‐evaluated atmospheric lifetime is 11.3 years, mainly due to reaction with OH radicals. The model‐evaluated instantaneous‐clear‐sky radiative forcing is 0.38 W m−2 ppbv−1, about 45% lower than previously estimated [Imasu et al., 1995]. However, the model‐estimated cloudy‐sky adjusted forcing, needed to calculated GWPs, is about 25% lower than the model‐estimated instantaneous‐clear‐sky forcing. The GWPs are calculated to be 3400, 1200, and 370 for 20, 100, and 500 year time horizons, respectively.
Receptor modeling for smoke of 1998 biomass burning in Central America
Authors: Cheng Meng‐Dawn , Lin Che‐Jen
Published: February 2004

The potential source contribution function (PSCF) is a receptor modeling technique for identifying emission sources and transport pathways. This technique has been widely used in the determination of potential emission sources and preferred transport pathways of a variety of air pollutants for more than a decade. However, the model has not been objectively evaluated against well‐documented real‐world data and the performance of the model is virtually untested. In this study we verified the PSCF model and tested the model performance by taking the opportunity of the 1998 Central America smoke events that produced elevated levels of aerosols detected at the Southern Great Plains site operated by the U.S. Department of Energy's Atmospheric Radiation Measurement. Our results show that the PSCF model could correctly identify the emission source locations and transport pathways of the smoke particles. However, one cannot take the PSCF results for granted without further examination of back trajectories that are responsible for the transport. False positives could result from the tailing of the back trajectories that travel over the true source locations. Model performance analysis suggests that using the mean or median as the criterion value yielded satisfactory PSCF results. Use of a high criterion value (e.g., the 90th percentile) and a larger database could improve the resolution of PSCF source identification. Trajectory arrival height above the surface sampling station should be carefully chosen to be within the column mixing height to ensure an effective coupling of transport and surface measurement.