bluuebunny/crossref_metadata_2025 huggingface dataset:
A Ewing Symposium on the Contemporary Carbon Cycle
Authors: Broecker Wallace S.
Published: February 2004

On October 27 and 28, 2000, a Ewing Symposium dealing with the contemporary carbon cycle was held at Lamont‐Doherty Earth Observatory of Columbia University. The central theme was understanding the now well‐documented large net uptake of CO2 by the terrestrial biosphere with focus on “why and where.” As emphasized by Martin Heimann, any deconvolution of the E‐W zonal components of terrestrial uptake from atmospheric CO2 distributions is subject to very large uncertainties. Hence additional information is required from other sources.
A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge
Authors: Gee Jeffrey S. , Webb Spahr C. , Ridgway Jeffrey , Staudigel Hubert , Zumberge Mark A.
Published: February 2004

We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high‐resolution map of the magnetic field anomaly within a 10 × 12 km region surrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near‐bottom magnetic anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11 SI) corresponds to ∼3.5% magnetite + pyrrhotite by volume, consistent with the abundance of these phases observed in drill core samples. However, this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization (∼10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near‐bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero, as had been inferred from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available.
Predicting plate velocities with mantle circulation models
Authors: Becker Thorsten W. , O'Connell Richard J.
Published: February 2004

We predict plate motions from a comprehensive inversion of theoretical estimates of tectonic forces in order to evaluate the relative importance of these and the uncertainties of such models. Plate‐driving forces from the mantle are calculated using global flow models that are driven by tomography and subduction‐derived density fields. Observed and predicted plate velocities agree well for a variety of models, leading to varied conclusions about the relative importance of forces. The dominance of the subduction related density pattern in the mantle is confirmed; it appears that P wave models do not satisfactorily image all of the slab‐associated anomalies in the upper mantle. Furthermore, lower mantle structure always improves the plate motion fit with respect to models that are based on upper mantle anomalies and lithospheric thickening only. We show that the average torques from the lower mantle scale with the radial flow through the 660‐km phase transition; the amplitude of the lower mantle torques will be significant for a range of models if there is mass flux through 660 km. We also evaluate parameterized edge forces and find that the additional inclusion of such torques does not significantly improve the model fit. The main reason for the nonuniqueness of the inversions is plate boundary geometry since all plate motions are dominated by the trench‐ridge system, and plates move from ridges to trenches.
Rhenium‐osmium isotope systematics and platinum group element concentrations: Loess and the upper continental crust
Authors: Peucker‐Ehrenbrink Bernhard , Jahn Bor‐ming
Published: February 2004

We investigate the use of loess as a proxy for the concentration and isotopic composition of highly siderophile elements, specifically Os, in the upper continental crust. The 187Os/188Os, platinum group element, and Re concentrations of 16 loess samples from China, Europe, and South America, previously analyzed for major, trace element, and Sr and Nd isotope composition, reveal subtle differences between loess provinces. Despite those differences, the 187Os/188Os of 1.05 ± 0.23 is surprisingly homogenous. Average 187Os/188Os as well as average Os (31 pg/g) and Ir (22 pg/g) concentrations are similar to the lower limit of previous estimates for average upper continental crust, whereas Ru, Pt, and Pd concentrations are intermediate between previous estimates. We argue that hydrogenous enrichment of Os in riverine sediments led Esser and Turekian [1993] to overestimate the Os concentration of upper continental crust (50 pg/g). On the basis of this argument and correlations with major and trace elements we propose that average platinum group element concentrations of loess (i.e., 31 pg Os/g, 22 pg Ir/g, 210 pg Ru/g, 510 pg Pt/g, 520 pg Pd/g) are a proxy for the upper continental crust. We further suggest that the nonchondritic average Os/Ir of 1.4 reflects the combined effects of radiogenic ingrowth of Os from Re decay over the mean lifetime of the upper continental crust and preferential return of Os to the crust during subduction. Rhenium concentrations scatter significantly, with highest values in loess derived from organic‐rich sedimentary rocks. Low median Re concentrations most likely reflect depletion of loess in organic matter, an important sink for Re in the upper continental crust. An average 187Re/188Os of 34.5 was calculated on the basis of the measured 187Os/188Os and Nd model ages. This value corresponds to a Re concentration of 198 pg/g. Correcting measured 187Os/188Os = 1.05 and inferred 186Os/188Os = 0.119871 (from 190Pt/188Os = 0.0176) for the older mean age (2.2 Gyr) of upper continental crust compared to loess (1.6 Gyr) yields average upper crustal 187Os/188Os of 1.40 and 186Os/188Os of 0.119885.
What caused the atmosphere's CO2 content to rise during the last 8000 years?
Authors: Broecker Wallace S. , Lynch‐Stieglitz Jean , Clark Elizabeth , Hajdas Irena , Bonani Georges
Published: February 2004

We make a case that the 20 ppm rise in atmospheric CO2 content over the last 8000 years was at least in part a consequence of the 500 Gt C increase in terrestrial biomass early in the present interglacial rather than of a 200 Gt C decrease in terrestrial biomass during the latter part of the Holocene as proposed by Indermühle et al. [1999]. In support of this claim, we present new 13C measurements from an Ontong Java Plateau box core, which do not reproduce the trend deduced from measurements on CO2 from the Taylor Dome ice core. In attempt to distinguish between scenarios put forth to accounting for the late Holocene rise in atmospheric CO2 content, we also made foraminifera shell weight measurements on three box cores from the Ontong Java Plateau. We were surprised to find that the early Holocene CaCO3 preservation event we sought was strongly depth dependent. The largest magnitude was at 4 km where CO3= ion concentrations appear to have been 30 μmol/kg higher than today's and hence nearly as high as those in today's North Atlantic Deep Water.
Coercivity analysis of magnetic phases in sapropel S1 related to variations in redox conditions, including an investigation of the S ratio
Authors: Kruiver Pauline P. , Passier Hilde F.
Published: February 2004

The most recent sapropel (S1) in the eastern Mediterranean has been extensively investigated with geochemical and rock‐magnetic techniques. Different redox conditions prevailed in different zones of the sediment through time. The oxidized sapropel zone is particularly interesting, because earlier studies indicated that new magnetic material was formed, including possible magnetosomes. Here we utilize component analyses of isothermal remanent magnetization (IRM) acquisition curves and the analysis of first‐order reversal curves (FORC) to further investigate the magnetic mineralogy. In the entire box core ABC26, the original input of eolian dust consisted of both magnetite and hematite. In the oxidized sapropel and in the active oxidation zone, an additional magnetite component is present. This magnetite component has a higher coercivity than the eolian magnetite and a very small coercivity dispersion, suggesting a narrow grain size distribution. This is a strong indication that magnetosomes, which are formed in the active oxidation zone, are the magnetic carriers of this coercivity fraction. FORC diagrams support these findings. The S ratio is forwardly modeled for mixed magnetic mineralogies with varying coercivity distributions to explain the down core S ratio behavior: close to 1 in the top sediment and in the oxidized sapropel, a drop in values in the active oxidation zone, and again close to 1 in the (syn)sapropel. The observed S ratio pattern in the oxidized sapropel and in the active oxidation zone can be explained by the recovered IRM components. Whereas both oxidized sapropel and active oxidation zone contain the extra magnetite component, their S ratios are different because of the differences in coercivity characteristics (coercivity and dispersion) of the two magnetites in these zones. Thus, in these zones the S ratio does not reflect variations in the relative contributions of hematite to magnetite but variations in the characteristics of the individual magnetite assemblages. Also in the rest of the core, the S ratio depends on the coercivity characteristics of the magnetite component rather than on the relative contributions of high‐ versus low‐coercivity minerals. The S ratio thus appears to be an unsuitable parameter to describe variations in the magnetic mineralogy, especially when more than two components are present. Therefore the classical interpretation of the S ratio should be treated with caution.
An interactive model study of the influence of the Mount Pinatubo aerosol on stratospheric methane and water trends
Authors: Considine D. B. , Rosenfield J. E. , Fleming E. L.
Published: February 2004

We have used the Goddard Space Flight Center interactive two‐dimensional model to evaluate the effects of the Mount Pinatubo volcanic aerosol cloud on subsequent trends in upper stratospheric CH4 and H2O. The effects of the aerosol cloud were included in the model photolysis, heating rate, and heterogeneous chemistry parameterizations, resulting in thermal, chemical, and dynamical responses to the aerosol forcing. The simulation used observed extinctions and surface area densities from the Stratospheric Gas and Aerosol Experiment II instrument between 1990 and 1997. Model upper stratospheric CH2 concentrations respond to a volcanically enhanced tropical vertical upwelling of up to 24%, which increases CH4 by ∼10% in early 1992. The decay of this CH4 pulse between 1992 and 1997 produces negative upper stratospheric trends which are in better agreement with observations than a nonvolcanic simulation but are still substantially weaker than the ∼30–40% CH4 decreases observed in the upper stratosphere by the Halogen Occultation Experiment between 1992 and 1997. Model temperature perturbations following the eruption of ∼2–3 K between 20 and 50 hPa agree well with observations in both magnitude, timing, and duration. The temperature of the model tropopause increases by ∼0.5 K and results in a substantial transfer of H2O into the stratosphere across the model tropopause. The H2O signal takes several years to propagate into the upper stratosphere, resulting in peak increases of ∼6% at the start of 1995. Middle to upper stratospheric model H2O trends between 1992 and 1997 agree better with observations than a nonvolcanic simulation, but atmospheric observations lack the strong signature of a pulse propagating from the tropical lower stratosphere as seen in the model simulation. These comparisons suggest that the aerosol from the Mount Pinatubo eruption could have contributed to the observed changes in CH4 and H2O following the eruption but was probably not the sole driver of the observed changes.
Modeling the formation of secondary organic aerosol within a comprehensive air quality model system
Authors: Schell Benedikt , Ackermann Ingmar J. , Hass Heinz , Binkowski Francis S. , Ebel Adolf
Published: February 2004

The Secondary Organic Aerosol Model (SORGAM) has been developed for use in comprehensive air quality model systems. Coupled to a chemistry‐transport model, SORGAM is capable of simulating secondary organic aerosol (SOA) formation including the production of low‐volatility products and their subsequent gas/particle partitioning. The current model formulation assumes that all SOA compounds interact and form a quasi‐ideal solution. This has significant impact on the gas/particle partitioning, since in this case the saturation concentrations of the SOA compounds depend on the composition of the SOA and the amount of absorbing material present. Box model simulations have been performed to investigate the sensitivity of the model against several parameters. Results clearly show the importance of the temperature dependence of saturation concentrations on the partitioning process. Furthermore, SORGAM has been coupled to the comprehensive European Air Pollution and Dispersion/Modal Aerosol Dynamics Model for Europe air quality model system, and results of a three‐dimensional model application are presented. The model results indicate that assuming interacting SOA compounds, biogenic and anthropogenic contributions significantly influence each other and cannot be treated independently.
Satellite retrievals of erythemal UV dose compared with ground‐based measurements at northern and southern midlatitudes
Authors: McKenzie R. L. , Seckmeyer G. , Bais A. F. , Kerr J. B. , Madronich S.
Published: February 2004

In recent years, estimates of UV radiation at the Earth's surface have been obtained from satellite retrievals. These retrievals have been compared with ground‐based measurements at a few sites, but so far there have been few comparisons from unpolluted sites. This study compares differences between satellite‐derived estimates of UV and ground‐based measurements at a clean air site in the Southern Hemisphere with those from more polluted sites in the Northern Hemisphere, to investigate the extent to which boundary layer extinctions are taken into account by frequently used satellite retrievals. It is found that the hemispheric differences inferred from the ground‐based measurements are much larger than those derived from the satellite retrievals. Since international intercomparison campaigns have shown the ground‐based spectroradiometers to be in agreement, it is concluded that differences in tropospheric extinctions (e.g., by ozone and aerosols) are not adequately taken into account in the satellite retrievals of UV radiation examined so far.
Coincident radio frequency and optical emissions from lightning, observed with the FORTE satellite
Authors: Light T. E. , Suszcynsky D. M. , Jacobson A. R.
Published: February 2004

We present long optical and radio frequency (RF) time series of lightning events observed with the FORTE satellite in January 2000. Each record contains multiple RF and optical impulses. We use the RF signatures to identify the general type of discharge for each impulse according to the discrimination techniques described by Suszcynsky et al. (2000) and reviewed herein. We see a large number of paired, impulsive events in the RF which allow us to study the heights within clouds of several events. We also see that the rate of RF/optical coincidence depends on the type of discharge: nearly 100% of VHF signals from first negative return strokes have an associated optical signal, whereas a mere 50% of impulsive intracloud events appear to have an optical counterpart. While the RF signals from ground strokes clearly coincide with simple optical signals in almost all cases, the intracloud lightning often shows nearly continuous, complicated RF and optical emissions which do not cleanly correlate with one another. The RF and optical pulses do not show a well‐defined relationship of intensities, for any lightning type. The observed delay between the RF and optical pulses we interpret as mainly an effect of the scattering experienced by the light as it traverses the cloud. For intracloud lightning, we find no evidence of an intrinsic delay at the source between the onset of the RF and optical signals. Impulsive in‐cloud RF events are seen to occur on average every 0.9 ms during a flash.
Atmospheric deposition of 7Be at a coastal Mediterranean station
Authors: Dueñas C. , Fernández M. C. , Carretero J. , Liger E. , Cañete S.
Published: February 2004

The bulk 7Be depositional flux was measured at Málaga (4°28′8″W; 36°43′40″N), a coastal Mediterranean station in the south of Spain, from 1992 to 1999. The annual 7Be depositional flux varied between 188 and 585 Bq m−2 yr−1 with a mean of 412 Bq m−2 yr−1, reflecting a Mediterranean climate where two well‐recognized wet and dry periods are dominant. Data on the 7Be depositional flux in Málaga show that the seasonal variation is not uniform from year to year and the amount of precipitation primarily controls the depositional fluxes. This study shows that the 7Be depositional fluxes during fall and winter were minimized and that the fallout maxima were confined to either spring and/or summer, possibly reflecting stratosphere/troposphere exchange. A comparison of our measurements and other data obtained at different locations in the world was also made.
Regional air pollution and its radiative forcing: Studies with a single‐column chemical and radiation transport model
Authors: Park R. J. , Stenchikov G. L. , Pickering K. E. , Dickerson R. R. , Allen D. J. , Kondragunta S.
Published: February 2004

This study focuses on the effects of subgrid (on general circulation model (GCM) scales) convective venting of the planetary boundary layer to the free troposphere and on the interactive effects of aerosols, ozone, UV actinic flux, and radiative forcing of climate. We developed a single‐column chemical transport model (SCCTM) consistent with the global Goddard Earth Observing System (GEOS) GCM and Chemical Transport Model (CTM). The SCCTM includes vertical transport by convection, turbulent mixing, a flexible photochemical scheme, and interactive calculations of radiative fluxes and photolysis rates. The model is designed as a chemical and physical core to be used in a completely interactive GCM. At this time it is driven by data from the GEOS Data Assimilation System archived by the Data Assimilation Office at NASA Goddard. We simulated an ozone/aerosol pollution episode in the Baltimore‐Washington region and a convective event in the central United States. These physically distinct case studies provide a thorough test for the chemical scheme and physical parameterizations employed in the SCCTM. The ozone episode simulation showed strong sensitivity to aerosol optical depth, with increased average PBL ozone due to the effects of aerosols on photolysis rates. Observed aerosols produced a surface cooling of up to 100 W m−2and stabilized the atmosphere suppressing convection. In the convective event over Oklahoma a squall line carried pollutants into the free troposphere increasing O3by up to 35 ppbv and peroxyacetylnitrate by up to 400 pptv. The maximum instantaneous radiative forcing due to this ozone reached 0.75 W m−2at the tropopause level. Accurate representation of the interaction among particles, trace gases, radiation, and convection is essential for global climate models.
An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET
Authors: Holben B. N. , Tanré D. , Smirnov A. , Eck T. F. , Slutsker I. , Abuhassan N. , Newcomb W. W. , Schafer J. S. , Chatenet B. , Lavenu F. , Kaufman Y. J. , Castle J. Vande , Setzer A. , Markham B. , Clark D. , Frouin R. , Halthore R. , Karneli A. , O'Neill N. T. , Pietras C. , Pinker R. T. , Voss K. , Zibordi G.
Published: February 2004

Long‐term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.
Charge transfer between colliding hydrometeors: Role of surface tension gradients
Authors: Graciaa A. , Creux P. , Lachaise J. , Schechter R. S.
Published: February 2004

A mechanism is postulated to account for the exchange of electrical charge that has been observed to take place when hydrometeors in a “cloud” created in the laboratory collide. A hydrodynamic flow induced by variation of the surface tension at an air/water interface is regarded as the main driving force. This flow created by forces acting within an interfacial region will transport the excess negative charge that normally resides at an air/water interface from one interfacial region to another, thereby yielding a separation of charge. An estimate of the quantity of charge exchanged by this process, using reasonable parametric values, is in accord with laboratory observations. In some interactions our proposed mechanism is only valid if a stable thick water film coats the surface of one of the colliding hydrometeors. Measurements are presented to show that the ice/water interface is negative, and this is a condition needed to confer stability of thick water films on ice.
Vertical and horizontal distributions of the aerosol number concentration and size distribution over the northern Indian Ocean
Authors: de Reus Marian , Krejci Radovan , Williams Jonathan , Fischer Horst , Scheele Rinus , Ström Johan
Published: February 2004

Airborne measurements of the aerosol number concentration and size distribution were conducted over the northern Indian Ocean during the Indian Ocean Experiment (INDOEX) in February–March 1999. Vertical profiles of the aerosol number concentration demonstrate elevated concentrations of nucleation mode particles in a layer between 8 and 12.5 km altitude. By using a novel combination of back trajectory information and cloud top temperatures retrieved from satellite images, it is shown that these particles most likely originated from the outflow of large convective clouds. Accumulation mode particles observed at these altitudes show indications for cloud processing. The aerosol size distributions observed in the layer between 4 and 8 km altitude show characteristics of an aged aerosol. In this layer the accumulation mode particle number concentration presents a minimum, a low variability, and very few particles larger than 0.7 μm diameter. The aerosol in the marine boundary layer can be characterized by high number concentrations of submicron and accumulation mode particles, which gradually decrease with distance from the Indian subcontinent. The particle loss rate is equivalent to 10–16 cm−3 per hour. This decrease takes place over the whole size distribution and can therefore not be explained by coagulation, but is likely due to precipitation and entrainment of free tropospheric air.
An aerosol model and aerosol‐induced changes in the clear‐sky albedo off the east coast of the United States
Authors: Hartley W. Seth , Hobbs Peter V.
Published: February 2004

Changes in the cloud‐free albedo of the Earth, and therefore direct aerosol radiative forcing, due to aerosol can be parameterized as a function of one extensive quantity that depends on the amount of aerosol present (e.g., the aerosol optical depth, τ) and two intensive quantities that depend on the internal characteristics of the aerosol (the aerosol single‐scattering albedo, ωo, and the aerosol asymmetry parameter, g). Airborne measurements of the dry aerosol scattering and absorption coefficients and dry particle size distributions made aboard the University of Washington's research aircraft during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) field campaign off the east coast of the United States in July 1996 are used in conjunction with measurements of the effects of humidity on aerosol light scattering to determine an optically equivalent model for the physical and optical properties of the ambient aerosol. This model is validated and used to derive values of ωo and g for the ambient (wet) aerosol. The average and standard deviation of the column‐mean, ambient values of ωo and g at a wavelength of 550 nm, determined from 12 vertical profiles, are 0.958±0.050 and 0.703±0.092, respectively. A pronounced increase in g with increasing relative humidity is observed. Using the aerosol model, the mean instantaneous change in the local albedo induced by the aerosol is found to be 0.027±0.018 at a wavelength of 550 nm (compared to an albedo of about 0.05 for the ocean).
Activation barrier for multicomponent droplet formation on partially soluble nuclei
Authors: Djikaev Y. S. , Donaldson D. J.
Published: February 2004

We present a thermodynamic treatment for obtaining the free energy surface as a function of droplet composition as well as the activation barrier for droplet formation in a multicomponent vapor mixture on a nucleating center which consists of both an insoluble core and some soluble species. An extension of Kuni's method [Kuni et al., 1993a] of studying the free energy of droplet formation to heterogeneous multicomponent condensation makes it possible to find all the main features of the free energy surface without explicitly knowing the free energy itself. Using that method, we obtain the inequalities that serve as criteria for whether heterogeneous condensation is barrierless or occurs in a fluctuational way or does not occur at all. An explicit expression for the height of the activation barrier for heterogeneous multicomponent condensation on mixed nuclei is proposed. The theoretical results are illustrated by numerical calculations for condensation on mixed nuclei in the water‐methanol and water‐sulfuric acid systems.
Transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM
Authors: Koch Dorothy
Published: February 2004

We simulate the major anthropogenic aerosols, sulfate, organic carbon and black carbon, in the Goddard Institute for Space Studies General Circulation Model (GISS GCM), and examine their transport, relative abundances, and direct radiative forcing. Both present‐day and projected future emissions are used, as provided by the IPCC SRES (A2) scenarios for 2030 and 2100. We consider the sensitivity of the black carbon distribution to the treatment of its solubility and allow solubility to depend upon exposure to gas phase production of sulfuric acid (case S), or time (case A), or a fixed rate (case C). We show that all three approaches can be tuned to give reasonable agreement with present‐day observations. However, case S has higher black carbon in the arctic winter, owing to reduced SO2 oxidation and black carbon solubility. This improves upon the arctic deficiency in previous models, though may be somewhat excessive in this model. We also show that with a different ratio of sulfur/carbonaceous emissions, the case S mechanism can give significantly different results compared to the other mechanisms. Thus, in the 2100 simulation, with reduced sulfur and increased black carbon emissions, the black carbon burden is ∼13% higher and the direct radiative forcing is ∼40% higher in case S compared to the other cases. We consider the relative abundances of carbonaceous and sulfate aerosols in different regions. In the current simulations, carbonaceous aerosols exceed sulfate at the surface in Asia and much of Europe and throughout the column in biomass burning regions. We show that the model ratio of carbonaceous to sulfate aerosols increases with altitude over many oceanic regions, especially in summertime, as was observed during the Tropospheric Aerosol Radiative Forcing Observational Experiment campaign; however, over land and during other seasons the ratio generally decreases with altitude. The (present day) direct radiative forcings for externally mixed (case A) black carbon, organic carbon, and sulfate are calculated to be 0.35, −0.30, and −0.65 W/m2, respectively. In the 2100 simulation these forcings are 0.89, −0.64, and −0.54 W/m2, respectively. The net anthropogenic aerosol global average forcing seasonality inverts between the current and future simulations: the forcing is most negative in (Northern Hemisphere) summertime in 2000 but is least negative or even positive during (NH) summer in 2100; this inversion is more extreme in the Northern Hemisphere.
Carbon and energy exchange by a black spruce‐moss ecosystem under changing climate: Testing the mathematical model ecosys with data from the BOREAS experiment
Authors: Grant R. F. , Goulden M. L. , Wofsy S. C. , Berry J. A.
Published: February 2004

There is some uncertainty whether net ecosystem productivity (NEP) of boreal black spruce forests is positive or negative under current climates and how NEP would change under hypothesized changes in future climates. The mathematical model ecosys was used to examine NEP of a boreal black spruce forest by testing rates of C transfer simulated from an integrated set of scientific hypotheses against rates measured with chamber, micrometeorological, isotopic and allometric techniques at the northern old black spruce site of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Daily aggregations of modeled and measured hourly C fluxes indicated that NEP rose with solar radiation if air temperatures remained below 15°C, but declined if air temperatures rose above 20°C. Daily NEP was thus largest under the higher radiation and lower temperatures of June, but declined under the lower radiation and higher temperatures of July and August. Yearly aggregations of modeled hourly C fluxes indicated that under current (1994–1996) climate NEP of the 150 year old northern old black spruce site was about 40 (wood) + 15 (soil) = 55 g C m−2 yr−1. Moss was estimated to contribute about 0.25 of net primary productivity (NPP) at this site and was thus an important component of ecosystem C exchange. The model was then used to predict changes in NEP at the old northern old black spruce site under the IS92a climate change scenario. After 150 years NEP was predicted to be 120 (wood) minus 10 (soil) = 110 g C m−2 yr−1. Cumulative gains of C under this scenario were predicted to be 5700 (wood) minus 700 (soil) = 5000 g C m−2 after 150 years. Gains of wood C would be vulnerable to loss by fire and insects, both of which could also increase under warmer climates.
Estimating the cloudy‐sky albedo of sea ice and snow from space
Authors: Key Jeffrey R. , Wang Xuanji , Stoeve Julienne C. , Fowler Charles
Published: February 2004

While satellites provide the means to monitor the temporal and spatial variability of surface albedo, their use has been limited to clear‐sky areas because clouds obscure the surface at wavelengths in the solar spectrum. However, the effect of clouds on the surface albedo, especially that of snow and ice, is significant and should be considered in satellite retrievals. In this paper theoretical and observational evidence is given that shows the snow/ice albedo to be on the average 4–6% (absolute) higher under cloud cover than for clear skies, with a range of slightly less than 0 to approximately 15%. A method for retrieving the clear‐sky broadband albedo of snow/ice from the advanced very high resolution radiometer is presented, and an adjustment for cloud optical depth is proposed. The cloudy‐sky adjustment is independent of sensor type and could also be used with nonsatellite data sets. An application of the algorithm to data from the Surface Heat Budget of the Arctic Ocean experiment demonstrates that clear‐ and cloudy‐sky snow surface albedo can be obtained from space with an uncertainty of approximately 7% absolute. While it may be sufficient to adjust a monthly clear‐sky surface albedo climatology for clouds by incorporating the mean cloud effect of approximately 5%, adjustments for cloud optical depth should be performed with instantaneous retrievals.
Intercomparison of BOREAS northern and southern study area surface fluxes in 1994
Authors: Barr Alan G. , Betts Alan K. , Black T. A. , McCaughey J. H. , Smith C. D.
Published: February 2004

Sensible and latent heat fluxes from the Boreal Ecosystem and Atmosphere Study (BOREAS) tower flux sites in 1994 are analyzed over both diurnal and seasonal cycles. We compare and contrast the southern and northern study areas and the behavior of five different land covers. For each land cover the evaporative fractions and surface conductances to water vapor are higher in the south than in the north, with the ranking from largest to smallest: aspen, fen, black spruce and jack pine. The conifer and, particularly, the jack pine sites show the greatest stomatal control of transpiration, as the vapor pressure deficit increases from morning to afternoon and as the soil dries during periods with low precipitation. The relation between surface conductance and the Priestley‐Taylor coefficient α is consistent between southern and northern study areas but varies among land covers. The aspen and fen sites have higher α values than the landscape mean, and the mature conifer sites have lower α values than the landscape mean. We attribute the differences to the impact of spatial heterogeneity at the landscape scale.
Fluxes of NH3 and CO2 over upland moorland in the vicinity of agricultural land
Authors: Milford Celia , Hargreaves Ken J. , Sutton Mark A. , Loubet Benjamin , Cellier Pierre
Published: February 2004

Intensive field measurements of NH3 and CO2 exchange were made over a wet heathland in the vicinity (<500 m) of sheep pastures in the Cairngorm mountains of Scotland for a two‐week period in the summer. Fluxes of NH3 were determined using the aerodynamic gradient method with a 3‐height continuous denuder system; fluxes of CO2 were determined using eddy correlation, while sensible and latent heat fluxes were determined by both methods. Few studies have measured NH3 and CO2 fluxes simultaneously, making these measurements relevant to compare exchange dynamics. Both NH3 and CO2 exchanged bidirectionally, in response to a combination of biological (foliar, soil) and physico‐chemical controls (solubility). NH3 was deposited rapidly to leaf surfaces, although during warm, dry daytime conditions periods of emission occurred, explained by the existence of a compensation point concentration for NH3. By contrast, CO2 followed a characteristic pattern of absorption during the day associated with net photosynthesis and emission at night. Both gases showed net uptake from the atmosphere, at 30 μmol NH3 m−2 d−1 and 74 mmol CO2 m−2 d−1. In southeast winds, NH3 emissions from the sheep pasture caused a significant advection error to the measured fluxes (>10%). Corrections were applied using a local‐scale dispersion‐exchange model. The analysis highlights how advection modifies the classical one‐dimensional inferential resistance approach. It is concluded that ecosystems in the vicinity of agricultural land receive more dry deposition than would be estimated using NH3 concentration monitoring and standard inferential models. In the present study, this effect represented an overall increase in total NH3 deposition of 32%.
Atmospheric methyl iodide: High correlation with surface seawater temperature and its implications on the sea‐to‐air flux
Authors: Yokouchi Yoko , Nojiri Yukihiro , Barrie Leonard A. , Toom‐Sauntry Desiree , Fujinuma Yasumi
Published: February 2004

Intensive measurements of atmospheric methyl iodide taken at high, middle, and low latitudes over a period of 3 years have provided evidence for its photochemical production in seawater and given new information that sea‐to‐air transport of CH3I is mainly controlled by surface seawater temperature (SST). These findings suggest a highly localized production and distribution of CH3I in the surface microlayer. As a result, the oceanic emission of CH3I is likely to be larger than previous estimates based on the classical two‐layer model. Owing to the SST dependence of atmospheric CH3I concentration, its impact on tropospheric or stratospheric ozone depletion would be increased by El Niño or future global warming.
Factors controlling tropospheric O3, OH, NOx and SO2 over the tropical Pacific during PEM‐Tropics B
Authors: Wang Yuhang , Liu Shaw C. , Wine Paul H. , Davis Douglas D. , Sandholm Scott T. , Atlas Eillot L. , Avery Melody A. , Blake Donald R. , Blake Nicola J. , Brune William H. , Heikes Brian G. , Sachse Glen W. , Shetter Richard E. , Singh Hanwant B. , Talbot Robert W. , Tan David
Published: February 2004

Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)‐Tropics B experiment (March‐April 1999) are analyzed. Concentrations of CO and long‐lived nonmethane hydrocarbons in the region are significantly enhanced due to transport of pollutants from northern industrial continents. This pollutant import also enhances moderately O3 concentrations but not NOx concentrations. It therefore tends to depress OH concentrations over the tropical Pacific. These effects contrast to the large enhancements of O3 and NOx concentrations and the moderate increase of OH concentrations due to biomass burning outflow during the PEM‐Tropics A experiment (September‐October 1996). Observed CH3I concentrations, as in PEM‐Tropics A, indicate that convective mass outflux in the middle and upper troposphere is largely independent of altitude over the tropical Pacific. Constraining a one‐dimensional model with CH3I observations yields a 10‐day timescale for convective turnover of the free troposphere, a factor of 2 faster than during PEM‐Tropics A. Model simulated HO2, CH2O, H2O2, and CH3OOH concentrations are generally in agreement with observations. However, simulated OH concentrations are lower (∼25%) than observations above 6 km. Whereas models tend to overestimate previous field measurements, simulated HNO3 concentrations during PEM‐Tropics B are too low (a factor of 2–4 below 6 km) compared to observations. Budget analyses indicate that chemical production of O3 accounts for only 50% of chemical loss; significant transport of O3 into the region appears to take place within the tropics. Convective transport Of CH3OOH enhances the production of HOx and O3 in the upper troposphere, but this effect is offset by HOx loss due to the scavenging of H2O2. Convective transport and scavenging of reactive nitrogen species imply a necessary source of 0.4–1 Tg yr−1 of NOx in the free troposphere (above 4 km) over the tropics. A large fraction of the source could be from marine lightning. Oxidation of DMS transported by convection from the boundary layer could explain the observed free tropospheric SO2 concentrations over the tropical Pacific. This source of DMS due to convection, however, would imply in the model free tropospheric concentrations much higher than observed. The model overestimate cannot be reconciled using recent kinetics measurements of the DMS‐OH adduct reaction at low pressures and temperatures and may reflect enhanced OH oxidation of DMS during convection.
Estimation of motion from sequences of images: Daily variability of Total Ozone Mapping Spectrometer ozone data
Authors: Gelpke V. , Künsch H. R.
Published: February 2004

The determination of smooth velocity fields from sequences of satellite images has received considerable attention in many domains in natural sciences such as atmospheric physics and physical oceanography. Two categories of methods have been discussed in the literature. The so‐called maximum cross correlation approach is a feature tracking method, whereas the gradient based approach relies on the fundamental assumption that the brightness of a moving point is invariant between time t and t + dt. The method we propose is related to the second approach, but we model displacement rather than velocity directly. We use a discretization of the continuity equation and assume absence of divergence. For the estimation of the displacement field, we use a penalized least squares method. The penalty term takes observation errors into account. It includes an estimate of the noise variance based on second‐order differences. We investigate how much of the day‐to‐day variability of total ozone of Total Ozone Mapping Spectrometer can be explained by advective displacement.
Fast‐turnaround alkyl nitrate measurements during the PROPHET 1998 summer intensive
Authors: Ostling Kristi , Kelly Brendan , Bird Stephen , Bertman Steven , Pippin Margaret , Thornberry Troy , Carroll Mary Anne
Published: February 2004

An automated gas chromatographic system for making fast‐turnaround measurements of alkyl nitrates was developed and tested. Every 30 min, samples were automatically acquired on a 1/16‐inch OD Tenax trap made from fused silica‐lined stainless steel tubing and injected by thermal desorption into the gas chromatograph. The system exhibited good chromatographic resolution, minimal breakthrough, and on‐trap stability for C3‐C5 nitrates. The detection limit of the instrument was <0.1 parts per trillion by volume for C3‐C5 nitrates with an overall uncertainty of 30% for isopropyl nitrate and 50% for all other quantified compounds. Overall uncertainty of the method was limited by calibration precision rather than by sampling method or chromatography. The system was deployed in the field as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport intensive held at the University of Michigan Biological Station in northern Michigan during the summer of 1998 from July 15 to August 22. Ambient results show a significant dynamic range with absolute levels affected by air mass history and pollution sources. The sum of measured C3‐C5 alkyl nitrates obtained during this intensive ranged from 3.45 to 65.8 pptv, with a mean of 18.6 pptv and contributed 1% on average to total NOy, with a range of 0.5% to 3%. Differences in the contribution to NOy are found when the data are filtered according to air mass origin. A positive relationship between n‐propyl nitrate and peroxypropionyl nitrate was observed over the course of the intensive.
Cloud cover analysis with METEOSAT‐5 during INDOEX
Authors: Sèze Geneviève , Pawlowska Hanna
Published: February 2004

During the Indian Ocean Experiment (INDOEX), METEOSAT‐5 positioned at 63°E provided observation of the visible and infrared radiance field over the Indian Ocean. A cloud classification process using the dynamic cluster method is applied to these data. For the 3 months of the experiment (January–March 1999), daily maps of the cloud cover type are built for 0730 and 0900 UTC. The occurrence frequency of clear sky, low‐ and high‐level cloud cover is examined. These frequencies are compared to the International Satellite Cloud Climatology Program (ISCCP) D1 data set for the period 1984 to 1994. The Indian Ocean region can be classified in three zones. In the north subtropics, clear sky and small cumulus occur at least 90% of the time. Near the coast of India, clear sky is as frequent as 80 to 100%. The Intertropical Convergence Zone, characterized by the occurrence frequency of high‐level clouds greater than 30%, spreads from Indonesia to North Madagascar. Near Indonesia, high‐level cloud cover occurs more than 55% of the time. In the south subtropics, low cloud cover is the most frequent. In the eastern part the occurrence frequency reaches 80%. This percentage decreases along the western side of the ocean where low clouds break up. Between the African coast and Madagascar, high‐level clouds are frequent. The mean spatial features found are in agreement with the ISCCP climatology, except for the eastern part of the south subtropics. A regional comparison shows the difficulty of making the analysis of interannual variations of cloud cover obtained from various cloud cover retrievals applied to different satellite data sets. This difficulty arises from the nonneglectable percentage of satellite pixels which can contain some very small low clouds.
Continuous measurements of size‐resolved particle chemistry during INDOEX‐Intensive Field Phase 99
Authors: Guazzotti Sergio A. , Coffee Keith R. , Prather Kimberly A.
Published: February 2004

During the Indian Ocean Experiment Intensive Field Phase 99, aerosol time‐of‐flight mass spectrometry (ATOFMS) was used for on‐line characterization of the size and chemical composition of individual particles. High temporal resolution single‐particle measurements were made on the Island of Kaashidhoo and on board the Research Vessel Ronald H. Brown during February and March, respectively. Sea‐salt and dust particles are the major contributors to the aerosol between 1.0 and 2.5 μm. For particles with aerodynamic diameters between 0.2 and 1.0 μm, carbon‐containing particles dominated. Differences in the chemical composition of the sampled aerosol were observed for different days and locations, and can be correlated with back trajectory information. For locations north of the Intertropical Convergence Zone (ITCZ), anthropogenic influences on the aerosol chemical composition were indicated by increased contributions from sea‐salt particles with non‐sea‐salt (nss) sulfates, dust with sulfates, and carbon‐containing particles with potassium and sulfates. In contrast, in regions close to and south of the ITCZ, the aerosol showed characteristics consistent with a more pristine marine environment. The relative contributions from sulfate‐associated particles were evaluated from the single‐particle data, as well. The observed sulfate aerosols over the size range analyzed in this study (i.e., aerodynamic diameter between 0.2 and 2.5 μm) consist exclusively of sulfates associated with sea salt, dust, elemental carbon, and/or organic matter. Quantitative contributions from the different types of sulfate‐associated particles are evaluated at different locations during the IFP: southern latitude, midlatitude, northern latitude at the Arabian Sea, and mid/northern latitude at the Bay of Bengal.
Generalized model for NOx and N2O emissions from soils
Authors: Parton W. J. , Holland E. A. , Del Grosso S. J. , Hartman M. D. , Martin R. E. , Mosier A. R. , Ojima D. S. , Schimel D. S.
Published: February 2004

We describe a submodel to simulate NOx and N2O emissions from soils and present comparisons of simulated NOx and N2O fluxes from the DAYCENT ecosystem model with observations from different soils. The N gas flux submodel assumes that nitrification and denitrification both contribute to N2O and NOx emissions but that NOx emissions are due mainly to nitrification. N2O emissions from nitrification are calculated as a function of modeled soil NH4+ concentration, water‐filled pore space (WFPS), temperature, pH, and texture. N2O emissions from denitrification are a function of soil NO3− concentration, WFPS, heterotrophic respiration, and texture. NOx emissions are calculated by multiplying total N2O emissions by a NOx:N2O equation which is calculated as a function of soil parameters (bulk density, field capacity, and WFPS) that influence gas diffusivity. The NOx submodel also simulates NOx emission pulses initiated by rain events onto dry soils. The DAYCENT model was tested by comparing observed and simulated parameters in grassland soils across a range of soil textures and fertility levels. Simulated values of soil temperature, WFPS (during the non‐winter months), and NOx gas flux agreed reasonably well with measured values (r2 = 0.79, 0.64, and 0.43, respectively). Winter season WFPS was poorly simulated (r2 = 0.27). Although the correlation between simulated and observed N2O flux was poor on a daily basis (r2 = 0.02), DAYCENT was able to reproduce soil textural and treatment differences and the observed seasonal patterns of gas flux emissions with r2 values of 0.26 and 0.27, for monthly and NOr flux rates, respectively.