bluuebunny/crossref_metadata_2025 huggingface dataset:
A critical review of marine sedimentaryδ13Corg‐pCO2 estimates: New palaeorecords from the South China Sea and a revisit of other low‐latitude δ13Corg‐pCO2 records
Authors: Kienast Markus , Calvert Stephen E. , Pelejero Carles , Grimalt Joan O.
Published: February 2004

In an attempt to understand better the local biogeochemistry of the South China Sea (SCS) and to unravel the contribution of this marginal low‐latitude basin to changes in atmospheric CO2 concentrations, we analyzed the carbon isotopic composition of organic matter (δ13Corg) in four sediment cores from throughout the SCS covering the last 220 kyr. Higher values (around −19.5 to −20.5‰) mark glacial stages, while lower values (around −21 to −22.5‰) are characteristic of interglacials. Following well established procedures, the δ13Corg records are converted to local pCO2 estimates. On the basis of these and other low‐latitude δ13Corg−pCO2 estimates from the literature, we present a critical evaluation of the use of δ13C of bulk sedimentary organic matter to hindcast past changes in local CO2(aq). Three crucial pitfalls are identified. (1) Given the present inability to quantify precisely the time‐varying amount of terrigenous Corg input to marine sediments, absolute values of pCO2 estimates based on bulk sedimentary Corg are questionable. (2) None of the low‐latitude sedimentary δ13Corg−pCO2 records shows the expected correlation between temporal changes in upwelling intensity and CO2 estimates, most likely due to the antagonistic influences of CO2(aq) and phytoplankton growth rate on δ13Corg. (3) A detailed comparison of marine δ13Corg−pCO2 records with the Vostok CO2 record reveals significant differences in phasing, specifically at the end of the last deglaciation and during the oxygen isotope stage 5/4 transition. However, in areas where equilibrium between oceanic and atmospheric CO2 occurs, for example the SCS and the Mediterranean, the timing of changes in δ13Corg should agree with the CO2 record from ice cores if δ13Corg is a reliable proxy for changes in CO2(aq). Taken together, the compilation of records presented here cautions the use of δ13Corg as an unambiguous tracer of dissolved molecular CO2 in the surface ocean and calls for a re‐evaluation of the role of the low‐latitude ocean on temporal changes in atmospheric CO2.
Biogenic nitric oxide emissions upscaling: An approach for Zimbabwe
Authors: Kirkman G. A. , Yang W. X. , Meixner F. X.
Published: February 2004

A mechanistic upscaling scheme was developed to estimate temporal nitric oxide [NO] emissions from soils for three distinct land use classes by spatial extrapolation of laboratory measurements for Zimbabwe (383,667 km2). Laboratory measurements on miombo woodland, grassland, and agricultural Zimbabwean soils were used to derive moisture‐ and temperature‐dependent NO emission algorithms. By combining monthly fields of modeled soil moisture and temperature (based on climate means, vegetation indices, soil, and terrain elevation data) a mean monthly soil NO flux, including rainfall‐induced elevated flux events (pulsing), was obtained. Countrywide emission rates ranged from 0.1–0.4 ng N m−2 s−1 for the dry season to wet season mean fluxes of 3.7–9.4 ng N m−2 s−1 for miombo, 4.4–7.0 ng N m−2 s−1 for grassland, and 4.6–10.9 ng N m−2 s−1 for agriculture. Annual net soil NO emissions (less canopy reduction) for Zimbabwe were 32.9 Gg N yr−1, with miombo woodlands (66% of Zimbabwe) contributing 63% to the regional NO budget. Rainfall‐induced NO pulsing contributed an additional 14% to countrywide annual bulk NO emissions. The majority of these pulses occurred during the first half of the rainy season (November‐December), with agricultural soils contributing the most and grassland soils contributing the least to pulsing. The upscaling approach provided a first countrywide estimate of spatiotemporal soil NO emissions for a hitherto largely unresolved source: the soil‐moisture‐limited African subtropical savannas.
Denitrification and N2 fixation in the Pacific Ocean
Authors: Deutsch Curtis , Gruber Nicolas , Key Robert M. , Sarmiento Jorge L. , Ganachaud Alexandre
Published: February 2004

We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large‐scale circulation. A water column denitrification rate of 48±5 Tg N yr −1 is calculated for the Eastern Tropical Pacific using N* [Gruber and Sarmiento, 1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove an additional 15±3 Tg N yr−1. We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of −4±12 Tg N yr−1. Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59±14 Tg N yr−1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron suppy.
TRAGNET analysis and synthesis of trace gas fluxes
Authors: Ojima D. , Mosier A. , Del Grosso S. , Parton W. J.
Published: February 2004

A critical synthesis of information on the biotic and abiotic controls of trace gas fluxes was needed in order to advance our ability to determine regional estimates of various trace gas compounds. In response to this need, the U.S. Trace Gas Network (TRAGNET), as part of an International Global Atmospheric Chemistry (IGAC) activity, developed an accessible database of multiyear trace gas flux (CH4, NOx, and N2O) and ancillary data from a range of ecosystems (tropical to arctic) across North America, Europe, and Central America. These data have been collected by a number of independent research programs. Through the support of National Science Foundation (NSF), United States Department of Agriculture/Agriculture Research Service (USDA/ARS), Natural Resource Ecology Laboratory at Colorado State University, and National Center for Ecological Analysis and Synthesis (NCEAS), TRAGNET has assembled the data sets and has begun to analyze and synthesize these trace gas data on regional to global scales to (1) determine a generalized understanding of environmental factors controlling trace gas fluxes in order to develop methods for spatial and temporal interpolation and regional extrapolation, (2) test and validate trace gas models across different spatial, temporal, and process scales, and (3) determine regional trace gas fluxes from the measured set of fluxes in the database.
Millennial‐scale oscillation of denitrification intensity in the Arabian Sea during the Late Quaternary and its potential influence on atmospheric N2O and global climate
Authors: Suthhof Andreas , Ittekkot Venugopalan , Gaye‐Haake Birgit
Published: February 2004

The intensity of denitrification in the Arabian Sea during the last 65 kyr is reconstructed using high‐resolution δ15N records of three sediment cores in conjunction with other geochemical tracers for water column oxygenation and productivity. The results reveal a close link to the Greenland ice core record with low or absent water column denitrification during the Last Glacial Maximum, the stadials, and at the time of the Heinrich Events including the Younger Dryas. In contrast, denitrification was high during the Holocene and the interstadials. The intensification of denitrification is related to stronger SW monsoonal upwelling, which enhances organic matter flux and degradation, resulting in a strengthening of the midwater oxygen deficiency. Such a combination of enhanced upwelling and denitrification has also been implied for the Eastern Tropical North Pacific (ETNP), where these events occur during the Holocene and to some extent during the interstadials, too. Today, the Arabian Sea and the ETNP together contribute substantially to the global marine water column denitrification, and a significant fraction of the ocean‐atmosphere N2O flux originates from these areas. Changes in N2O emissions from these areas could thus have effected the recently described stadial/interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores. Moreover, denitrification is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory, which in turn influences global primary productivity and CO2 sequestration by the biological pump. Short‐term switches between a nondenitrification mode and a denitrification mode in these marine regions therefore have an impact on global climate.
Characterization and transport of aerosols over equatorial eastern Africa
Authors: Gatebe C. K. , Tyson P. D. , Annegarn H. J. , Helas G. , Kinyua A. M. , Piketh S. J.
Published: February 2004

Measurements of the composition of aerosol partitioned into two size fractions, fine (particle aerodynamic equivalent diameter, dp ≤ 2.5 μm) and coarse (2.5 < dp ≤ 10 μm) were made at a high‐altitude site over equatorial eastern Africa on Mount Kenya to study long‐range transport of aerosol and to determine the extent of interhemispheric transport at the equator. The two size fractions allow long‐range transport aerosol to be distinguished from those more locally derived. Background midtropospheric elemental aerosol concentrations observed on Mount Kenya, for example, S = 0.0157 μg m−3, are lower than commensurate findings for remote areas in industrialized countries and elsewhere in Africa but are 3 factors higher than concentrations reported from the South Pole. Locally derived aerosols are frequently transported up the slopes of Mount Kenya by day in valley winds but are rarely transported at night in the stable conditions characterized by downslope katabatic flow. Nocturnal conditions favor the measurement of long‐range, midtropospheric transport products to Kenya from afar. Case studies reveal that anthropogenically derived S and Fe aerosols from both southernmost and central southern Africa may be observed on Mount Kenya after transport over distances >6000 km. So‐called pristine marine air from the central Indian Ocean is shown to contain aged and recirculated Si that could only have been derived from land areas after transport over long distances. The nature of the aerosols measured on Mount Kenya depends critically on regional patterns of aerosol transport. Interregional transfers seem to be a feature of the transport climatology. Likewise, interhemispheric transport across the equator in east Africa is observed.
Effect of climatic variability from 1980 to 1997 on simulated methane emission from a boreal mixed mire in northern Sweden
Authors: Granberg Gunnar , Ottosson‐Löfvenius Mikaell , Grip Harald , Sundh Ingvar , Nilsson Mats
Published: February 2004

The objective of this study was to evaluate the interannual variation during 1981–1997 in methane emission from an oligotrophic lawn plant community in a boreal mire. Daily mean air temperatures and daily accumulated precipitation figures were used as forcing variables for a mechanistic soil physical model, to generate estimates for the depths of snow, soil frost, and water table, together with soil temperature profiles. Methane emissions were then simulated with a mechanistic mixed mire methane model, described in this paper. The methane model simulated total methane emission rates separated into diffusion, ebullition, and plant‐mediated transport. The climate record for 1981–1997 represented almost the total range in growing season temperature sums in northern Sweden during the twentieth century. The average temperature sum for 1980–1997 was 735 (range 553–981) degree days. The average accumulated annual precipitation during the same period was 283 (range 140–397) mm. The integrated simulated annual methane emission during 1981–1997 varied almost threefold among years, with a mean of 17.4 ±1.1 (SE) and a range of 11 to 27 g m−2. The simulated annual emissions for the three calibration years 1995–1997 were 18, 14, and 22 g m−2, respectively, slightly higher than the integrated measured emissions over the growing season (May‐September), which were 16, 13, and 18 g m−2 y−1, respectively. Given the model formulation and parameterization, the single most important climatic predictor of simulated annual methane emission was mean water table position (r2 = 0.58). Adding annual soil temperature sum at 26 cm in a multiple regression solution significantly increased the explained variance (R2 = 0.85). The study emphasizes that interannual variability in methane emission may be large, and therefore extrapolations of annual methane emissions must be based on time series that adequately span the interannual variability in the local climate.
Controls on palagonitization versus pedogenic weathering of basaltic tephra: Evidence from the consolidation and geochemistry of the Keanakako'i Ash Member, Kilauea Volcano
Authors: Schiffman Peter , Spero Howard J. , Southard R. J. , Swanson D. A.
Published: February 2004

On the summit of Kilauea Volcano, incipient palagonitization and consolidation of vitric tephras from the <500‐year‐old Keanakako'i Ash Member of the Puna Basalt have occurred only adjacent to caldera‐bounding faults. Calcite intergrown with palagonitized glass has δ18O values indicative of a hydrothermal origin. Whereas the δ13C of the calcite suggests that it has sequestered carbon from atmospheric as well as mantle sources, radiocarbon dates from the calcites, in addition to calcites from modern steam vents at Kilauea, indicate that >90% of the carbon is mantle derived. We hypothesize that the palagonitized zones may denote the location of fossil steam vents, similar to active vents exposed along circumferential faults on both the north and the south sides of the Kilauea Caldera. Elsewhere, Keanakako'i tephras are weathering into pedogenic products that closely reflect modern environmental conditions. Under acidic soil conditions (pH <6.0), tephra is undergoing dissolution, with the development of opaline crusts on outcrop faces. Under more neutral soil pH (6.5–7.8), the pedogenic products reflect the degree of desilication of tephra mitigated by leaching: dominantly smectite where mean annual rainfall is <50 cm/yr and dominantly kaolinite, allophane, and/or imogolite where rainfall exceeds 250 cm/yr.
Behavior of the cotectic curve En‐Ol in the system leucite‐olivine‐quartz under dry conditions to 2.0 GPa
Authors: Conceição R. V. , Green D. H.
Published: February 2004

The system leucite‐olivine (Fo90)‐quartz has been experimentally studied from 0.5 to 2.0 GPa under dry conditions, and the position of the invariant point involving olivine, enstatite, K‐rich phase (leucite or sanidine), and liquid has been determined. We find that at 0.5 GPa the olivine (Ol) + liquid (Liq1) ↔ enstatite (En) + leucite (Lc) + liquid (Liq2) peritectic occurs at 1205°C and Lc31.7Qz66.5Ol1.8, while at 1.0 GPa, at 1265°C, it has moved to Lc38Qz58Ol4 as the leucite field contracts. A second peritectic reaction, Lc + Liq3 ↔ Sa + En + Liq4, occurs at 1.0 GPa and ∼1260°C (Lc37Qz60Ol3). With increasing pressure from 1.0 GPa the stability field of sanidine increases such that at 1.5 GPa the subsolidus reaction En + Lc = Ol + San intersects the solidus, defining an invariant point at 1290°C (Lc46Qz47.5Ol6.5). At 2.0 GPa the leucite field contracts farther toward the leucite corner, and the peritectic reaction En + Liq6 ↔ Ol + Sa + Liq7 takes place at 1350°C (Lc50Qz43Ol7). The Ol + En + Liq boundary moves from the silica‐oversaturated field at 0.5 and 1.0 GPa into the silica‐saturated field at 1.5 GPa and to the silica‐undersaturated field at 2.0 GPa. Our new data, together with earlier studies of anhydrous and hydrous Lc‐Ol‐Qz systems for 2.8 GPa, suggest that highly potassic (i.e., with very low Na2O contents), silica‐oversaturated magmas with minor contents of water could be direct partial melts of a potassic‐harzburgite or phlogopite‐bearing lherzolite mantle at pressures of nearly 1.5 GPa and temperatures of about 1200°–1300°C. Generation of strongly undersaturated K‐rich magmas requires higher pressures (>2.0 GPa) and is favored by the presence of dissolved carbon as CO3= and/or fluorine.
Transfer of atmospheric boron from the oceans to the continents: An investigation using precipitation waters and epiphytic lichens
Authors: Rose E. F. , Carignan J. , Chaussidon M.
Published: February 2004

The variation of B concentration in atmospheric deposition was studied from the analysis of 35 individual rain events, 17 snow packs, and 17 lichens sampled over NE North America (south from Hudson Bay) and Asia (from the coast of Bangladesh to the high Himalayas of Nepal). Rain samples show a range of B concentration between 0.3 and 9.4 μg/L (average of 1.8 ± 1.7 μg/L), excluding two rains with higher B contents of 17 and 37.5 μg/L, most likely reflecting anthropic contamination. Snowpacks and lichens which average atmospheric deposition over periods of a few months to a few years show a smaller range of variation, from 0.1 to 2.3 μg/L (average 1.1 ± 0.8 μg/L) for snowpacks and from 1 to 25.9 ppm for lichens. The lichens have elemental ratios (such as B/Cl) similar to the average of rains, showing that they are good monitors of atmospheric B deposition without significant biofractionation of elements. This is also demonstrated by their halogen contents, which follow the systematics of the atmospheric distribution of these elements previously derived from the study of rains and atmospheric particles. Though individual rains do not show systematic decrease in B concentration with distance to the sea, this behavior is clearly shown by samples having longer integration times, snowpacks, and lichens. The snow and lichen data show that seawater is a major source of atmospheric B. Using the lichen data, the enrichment factor (normalized to Na) of marine air masses relative to seawater is estimated to be ∼13 (average B/Na of 5.6 × 10−3). This implies that the fractionation factor of atmospheric suspended marine particles relative to seawater (FX = (X/Na)particle/(X/Na)seawater) is 0.177. Thus B transfer from seawater to the atmosphere occurs mostly via degassing of sea salts, the residence time of gaseous B being estimated at 15.9‐fold that of particulate B. The distribution of B in the atmosphere between gaseous B and degassed sea salts can be used to model the large range of B concentrations measured in single rain events by selective removal of one of the two B components. In this model, unfractionated marine air masses can reach the continents with very variable B concentrations, between ∼0.2 and ∼20 μg/L.
Proceedings of a workshop on alkenone‐based paleoceanographic indicators
Authors: Eglinton Timothy I. , Conte Maureen H. , Eglinton Geoffrey , Hayes John M.
Published: February 2004

The papers in this theme derive from a workshop on alkenone‐based paleoceanographic indicators held at the J. Erik Jonsson Center of the National Academy of Sciences in Woods Hole, Massachusetts, October 3–6, 1999. The papers fall into two groups: “position papers” prepared in advance of the workshop by the leaders of six different working groups and “reports” prepared during and after the workshop by rapporteurs and members of the working groups. Their publication here is intended to provide a record of the discussions and thus to serve to the paleoceanographic community. An overview of the conference and its participants is also provided.
On the fate of past gas: What happens to methane released from a bacterially mediated gas hydrate capacitor?
Authors: Dickens Gerald
Published: February 2004

Gas hydrates and associated free gas in marine sediment may constitute a large capacitor of CH4 in the global carbon cycle with outputs controlled by water temperature at intermediate depths of the ocean. The best support for this concept previously has come from stable isotope records of benthic foraminifera that show pronounced negative δ13C excursions during certain brief intervals of deep to intermediate water warming. New work by Hinrichs [2001] demonstrates that such isotope excursions in the Santa Barbara Basin coincide with high accumulations of diplopterol, a biomarker for aerobic CH4 oxidation by bacteria. This is the first direct evidence for enhanced CH4 concentrations in the deep ocean during or immediately after bottom water warming and injection of 12C‐rich carbon. However, the formation of biomarkers indicates that a fraction of CH4 released during warming is oxidized to biomass and ΣCO2 in the water column. In contrast to most literature, the primary consequences of CH4 release from dissociated gas hydrate may be dissolved O2 depletion and carbonate dissolution rather than atmospheric warming.
Nitrous oxide emissions from forests and pastures of various ages in the Brazilian Amazon
Authors: Melillo J. M. , Steudler P. A. , Feigl B. J. , Neill C. , Garcia D. , Piccolo M. C. , Cerri C. C. , Tian H.
Published: February 2004

Nitrous oxide emissions from tropical forest soils are thought to account for 2.2–3.7 Tg N yr−1 of the total annual global production of 10–17 Tg N yr−1. Recent research suggests that clearing of tropical forest for pasture can increase N2O emissions but that the period of elevated emissions may be limited and fluxes from older pastures may be lower than from the original forest. Here we report N2O emissions from two land‐use sequences in the Brazilian Amazon's state of Rondônia. Each sequence includes a forest and a set of pastures of different ages. One sequence contains a newly created pasture that we studied intensively through its first 2 years, including forest cutting, burning, and the planting of forage grasses. Emissions from the newly created pasture were about two and one half times the forest emissions during the first 2 years (5.0 kg N2O‐N ha−1 yr−1 versus 1.9 kg N2O‐N ha−1 yr−1). Nitrous oxide fluxes from pastures older than 3 years were on average about one third lower than fluxes from uncut forest (1.4 kg N2O‐N ha−1 yr−1 versus 1.9 kg N2O‐N ha−1 yr−1). The best predictor of N2O flux across the chronosequences was the magnitude of the NO3 pool in the upper 10 cm of soil measured at the time of gas sampling. Using a simple cohort model combined with deforestation rates estimated from satellite images by Brazil's Instituto de Pesquisas Espaciais (INPE) for the period 1978 through 1997, we estimate that for the Brazilian Amazon the basin‐wide flux of N2O‐N from pasture soils was 0.06 Tg in 1997. This is ∼8% of the combined forest plus pasture flux of 0.78 Tg N2O‐N we estimate for the Brazilian part of the basin in 1997. In the absence of any forest‐to‐pasture conversion in the Brazilian part of the basin, we estimate that the basin‐wide flux of N2O‐N would have been only slightly larger: 0.80 Tg in 1997. Through a second modeling analysis we estimate that for the whole of the Amazon Basin, including parts of the basin outside of Brazil, the N2O‐N emissions from forests averaged 1.3 Tg yr−1 over the period 1978–1995.
Limitations of the equivalent CO2 approximation in climate change simulations
Authors: Govindasamy B. , Taylor K. E. , Duffy P. B. , Santer B. D. , Grossman A. S. , Grant K. E.
Published: February 2004

The “equivalent CO2” approximation often used to simulate the climatic effects of a suite of trace greenhouse gases is investigated using a recent version of the NCAR Community Climate Model. We performed present‐day and preindustrial equilibrium climate simulations. The climate sensitivity is lower by ∼20% in the equivalent CO2 case compared to the control case in which the individual trace gases were treated explicitly. This is reflected in similar percentage differences in global‐ and annual‐mean surface temperature, precipitation, precipitable water, and sea ice. The temperature changes are also different regionally in the tropical and subtropical troposphere and in the stratosphere. This difference in climate sensitivity originates from differences in the spatial pattern of radiative forcing. The equivalent CO2 forcing pattern differs from the control case forcing pattern for several reasons, but the dependence on temperature of the Planck emission spectrum appears to be fundamentally most important. The primary absorption bands of CH4 and N2O are found at wavelengths more sensitive to the temperature‐related shift of the wavelength of maximum emission than the absorption bands of CO2. This leads to stronger spatial variations in absorption by trace gases than by CO2. We conclude that because of differences in the pattern of radiative forcing, the equilibrium response of global climate to increases in trace gases is larger than the response to an “equivalent” increase in CO2, and the patterns of response are also different.
Chemical and dynamical contributions to ozone profile trends of the Payerne (Switzerland) balloon soundings
Authors: Weiss Andrea K. , Staehelin Johannes , Appenzeller Christof , Harris Neil R. P.
Published: February 2004

Long‐term changes in the vertical ozone distribution over Switzerland are examined for the period 1967–2000. A statistical trend analysis is performed accounting for chemical and dynamical variability. The tropopause pressure, the North Atlantic oscillation (NAO), the Arctic oscillation (AO), and the quasi‐biennial oscillation (QBO) are used as dynamical quantities. In addition, the solar effect and an ozone depletion factor (ODF), which describes the joint effect of stratospheric chlorine and aerosol loading, are included. A term is allowed for an unexplained linear trend. The various influences on ozone are estimated employing stepwise regression. Tropopause pressure and lower stratospheric ozone (10–19 km) variability are found to be strongly linked throughout the year. The NAO‐AO indices show a negative correlation with lower stratospheric ozone in winter‐spring. A positive correlation of NAO‐AO and ozone is found in the middle stratosphere (23–30 km) during late summer to early winter. There is a solar signal in the middle stratosphere which becomes strongly significant in summer. The QBO signal is prominent at the height of the ozone maximum in winter‐spring. The unexplained trends are much larger than the contributions of the ODF, suggesting that mechanisms other than midlatitude in situ chemistry are mostly responsible for the observed ozone loss. In the lowermost stratosphere (10–15 km), the observed ozone trends are found to be caused by dynamics. Above, the residual ozone trend (in percent) is independent of height and in the range of −2% to −5% per decade up to 30 km.
Vertical profiles of atmospheric electric parameters close to ground
Authors: Kulkarni Madhuri , Kamra A. K.
Published: February 2004

The ion‐aerosol balance equations have been solved for different mixing strengths and two types of ionization profile to get vertical profiles of atmospheric electric parameters close to the Earth's surface. The inclusion of surface radioactivity in the model causes the decrease in electric field to be more intense at lower levels than at higher levels, an increase in the asymptotic value of the field, and an increase in the space charge density gradient. When the mixing of atmosphere is very weak, a reverse electrode effect is observed under the condition of enhanced ionization due to trapped radon, with the decrease in ion densities above 1 m. However, with an increase in eddy diffusivity, the positive ion concentration decreases, and the negative ion concentration increases with increasing height above the diffusion layer. Further increase in eddy diffusivity causes the profiles to become almost similar to those for which the ionization is constant with height. Even with the inclusion of radon radioactivity, relatively higher diffusivity keeps the nature of the profile of the negative ions similar to that for the constant ionization case. On the contrary, there is a drastic change in the profile of positive ions because of the strong downward gradient of positive ions in the diffusion sublayer. The profile of positive ions is more sensitive to the radon radioactivity and that of negative ions to the strength of turbulence. Further, the asymptotic value of aerosol concentration is reached before that of small ions when mixing is very low. Our results are in reasonable agreement with the earlier theoretical and experimental studies.
An assessment of hygroscopic growth factors for aerosols in the surface boundary layer for computing direct radiative forcing
Authors: Im Jung‐Sun , Saxena Vinod K. , Wenny Brian N.
Published: February 2004

Aerosol optical properties in the southeastern United States were measured at two research sites in close horizontal proximity but at different altitudes at Black Mountain (35.66 °N, 82.38 °W, 951 m msl) and Mount Gibbes (35.78 °N, 82.29 °W, 2006 m msl) to estimate the direct radiative forcing in the lowest 1 km layer of the troposphere during the summer of 1998. Measurements of light scattering and light absorption at ambient relative humidity (RH) are categorized by air mass type (polluted continental, marine with some continental influence, continental) according to 48‐hour back‐trajectory analysis. At a wavelength of 530 nm the average total scattering coefficient (σsp) measured at the valley site was 1.46×10−4 m−1 for polluted continental air masses, 7.25×10−5 m−1 for marine air masses, and 8.36×10−5 m−1 for continental air masses. The ratio of σsp at the mountain site to σsp at the valley site was 0.64, 0.58, and 0.45 for polluted continental, marine, and continental air masses, respectively. The hygroscopic growth factor (σsp(RH = 80%)/σsp(RH = 30%)) was calculated to be almost a constant value of 1.60±0.01 for polluted continental, marine, and continental air masses. As the RH increased from 30% to 80%, the backscatter fraction decreased by 23%. On the basis of these measurements, direct radiative climate forcing (ΔFR) by aerosols in the lowest 1 km layer of the troposphere was estimated. The patterns of ΔFR for various values of RH were similar for the three air masses, but the magnitudes of ΔFR(RH) were larger for polluted continental air masses than for marine and continental air masses by a factor of about 2 due to higher sulfate concentration in polluted continental air masses. The average value of ΔFR(RH = 80%)/ΔFR(RH = 30%) was calculated to be almost a constant value of 1.45±0.01 for all three types of air masses. This implies little dependence of the forcing ratio on the air mass type. The averaged ΔFR for all the observed ambient RHs, in the lowest 1 km layer during the 3‐month summer period, was −2.95 W m−2 (the negative forcing of −3.24 W m−2 by aerosol scattering plus the positive forcing of +0.30 W m−2 by aerosol absorption) for polluted continental air masses, −1.43 W m−2 (−1.55 plus +0.12) for marine air masses, and −1.50 W m−2 (−1.63 plus +0.14) for continental air masses. The ΔFR for polluted continental air masses was approximately twice that of marine and continental air masses. These forcing estimates are calculated from continuous in situ measurements of scattering and absorption by aerosols without assumptions for Mie calculations and global mean column burden of sulfates and black carbon (in g m−2) used in most of the model computations.
Lower‐Tropospheric Ozone (LTO) derived from TOMS near mountainous regions
Authors: Newchurch M. J. , Liu X. , Kim J. H.
Published: February 2004

Using Total Ozone Mapping Spectrometer (TOMS) version‐7 level‐2 clear‐sky (reflectivity ≤ 20%) ozone measurements corrected for aerosol effects and sea‐glint errors, we derived Lower Tropospheric Ozone (LTO) west and east of the Andes, the Mexican and Rocky Mountains, the mountains in Africa and the Arabian Peninsula, New Guinea, and the Himalayan Mountains. The derived results agree reasonably well with the seasonality of LTO from ozonesonde observations at Boulder, Cristobal, Fiji, Java, and Tahiti. The LTO seasonality found in the biomass burning seasons characterized by the ATSR World Fire Atlas west and east of the Andes (23°S–2°N), east of the Mexican Mountains (15°–23°N), South Sudan (6°–14°N), South Africa (30°–28°S), and west of New Guinea is consistent with the influence of biomass burning on the formation of tropospheric ozone in these regions. The significant El Niño influence on LTO west of New Guinea is evident throughout several El Niño cycles. The spring maximum in ozone west of the Mexican Mountains, in western China, and west of the Andes (32°–23°S) is consistent with a stratospheric intrusion source. East of the Mexican Mountains (23°–30°N), both west and east of the Rocky Mountains, in north Sudan and Iraq, and in western China, high concentrations of ozone are found in these continental and coastal regions which are affected by anthropogenic sources. The maximum ozone in these regions usually occurs in the summer due to photochemical ozone production. A summer LTO minimum occurs in coastal regions west of the Andes and west of Mexico, due to ozone destruction in low NOx and high H2O marine environment. A summer minimum also occurs in south Sudan in the rainy season. The LTO in the northern tropics of South America (4°–10°N), Africa (1°S–2°N), and east of New Guinea (7°–3°S) experiences little seasonal variation.
Experimental determination of scattering matrices of randomly oriented fly ash and clay particles at 442 and 633 nm
Authors: Muñoz O. , Volten H. , de Haan J. F. , Vassen W. , Hovenier J. W.
Published: February 2004

We present measurements of the scattering angle distribution of the whole scattering matrix for randomly oriented particles of three mineral samples: fly ash, green clay, and red clay at 442 and 633 nm. Fly ash consists of aggregates of nearly spherical particles while green clay and red clay particles represent irregular compact particles. We compare the measured results for fly ash with an experimentally determined average scattering matrix which is based on measurements for a broad selection of irregular mineral aerosol particles. We find that the scattering matrix of our polydisperse sample of aggregates of nearly spherical particles differs considerably from that of compact particles. In addition, the angular distribution of the elements of the scattering matrix (except F22(θ)/ F11(θ)) for fly ash particles seem to be dominated by the single monomers. The effects of small differences in composition on the scattering behavior have also been studied by comparing our experimental results for green clay particles with those obtained by Volten et al. [2001] for red clay particles at the same wavelengths.
Transport of smoke from the Central American fires of 1998
Authors: Rogers Christopher M. , Bowman Kenneth P.
Published: February 2004

During the spring of 1998, smoke produced by biomass burning in Central America was transported northward, where it eventually affected the continental United States. To quantify this event, this study analyzes the presence of aerosols using the Total Ozone Mapping Spectrometer (TOMS) aerosol index. Unusually large amounts of UV‐absorbing aerosols were present over parts of Central America during 1998 compared to a climatology created from the 13‐year Nimbus 7 TOMS data set (1979–1992). The role of transport is studied by computing trajectories for air parcels initialized in the area with the most widespread fires. Comparison of the TOMS aerosol index maps and the parcel trajectories indicates that the trajectories adequately represent the smoke transport. Analysis of the TOMS data, the meteorological observations, and the trajectories indicates that the source region of the smoke is influenced by two prevailing transport regimes: one northward and one westward. The transport alternates between the two flow patterns. Statistical analysis of the transport shows that May 1998 and the climatology contain similar patterns of northward and westward flow regimes. The northward flow regime in 1998, however, is among the strongest of the 20‐year period analyzed. The combination of unusually large smoke production and stronger than normal northward transport led to significant smoke concentrations over large areas of the central and southern United States.
Extinction coefficients and properties of Pinatubo aerosol determined from Halogen Occultation Experiment (HALOE) data
Authors: Lee K.‐M. , Park J. H. , Massie S. T. , Choi W.
Published: February 2004

Relative extinction coefficients as a function of wavelength are determined for stratospheric aerosols from the Mount Pinatubo eruptions in June of 1991, using the Halogen Occultation Experiment (HALOE) data at latitudes 20°–55° in the northern hemisphere from November 1991 to February 1992. Extinction coefficients at each of the eight ELALOE channels are obtained from the ratio of two transmittance profiles of consecutive occultation measurements separated by 25° longitude, one loaded with aerosols larger than the other during the early stage of aerosol dispersion after the eruptions. These coefficients are compared to theoretical Mie calculation values. Composition and a single mode particle size distribution are derived as a function of altitude. The retrievals indicate that the weight percentage of H2SO4 for 45 occultation cases is larger than the equilibrium value by about 5 wt %, while the size distribution parameters are within the range of those measured in situ at Laramie, Wyoming.
AGCM hindcasts with SST and other forcings: Responses from global to agricultural scales
Authors: Shah Kathryn Pierce , Rind David , Druyan Leonard , Lonergan Patrick , Chandler Mark
Published: February 2004

Multiple realizations of the 1969–1998 time period have been simulated by the GISS AGCM to explore its responsiveness to accumulated forcings, particularly over sensitive agricultural regions. A microwave radiative transfer postprocessor has produced the AGCM lower tropospheric, tropospheric, and lower stratospheric brightness temperature (Tb) time series for correlations with microwave sounding unit (MSU) time series. AGCM regional surface air temperature and precipitation were also correlated with GISTEMP temperature data and with rain gage data. Seven realizations by the AGCM were forced solely by observed sea surface temperatures. Subsequent runs hindcast January 1969 through April 1998 with an accumulation of forcings: observed sea surface temperatures (SSTs), greenhouse gases, stratospheric volcanic aerosols, stratospheric and tropospheric ozone, and tropospheric sulfate and black carbon aerosols. Lower stratospheric Tb correlations between the AGCM and the MSU for 1979–1998 reached as high as 0.93 globally given SST, greenhouse gases, volcanic aerosol, and stratospheric ozone forcings. Midtropospheric Tb correlations reached as high as 0.66 globally and 0.84 across the equatorial, 20°S–20°N band. Oceanic lower tropospheric Tb correlations were less high at 0.59 globally and 0.79 across the equatorial band. Of the sensitive agricultural areas considered, Nordeste in northeastern Brazil was simulated best with midtropospheric Tb correlations up to 0.80. The two other agricultural regions, in Africa and in the northern midlatitudes, suffered from higher levels of non‐SST‐induced variability. Zimbabwe had a maximum midtropospheric correlation of 0.54, while the U.S. Corn Belt reached only 0.25. Hindcast surface temperatures and precipitation were also correlated with observations, up to 0.46 and 0.63, respectively, for Nordeste. Correlations between AGCM and observed time series improved with addition of certain atmospheric forcings in zonal bands but not in agricultural regions encompassing only six AGCM grid cells.
A new methodology for homogenization of 19th century marine air temperature data
Authors: Chenoweth Michael
Published: February 2004

A new data set of over 43,000 surface marine air temperature (MAT) records covering the years 1795–1843 is described and a new methodology proposed for the homogenization of historical MAT data. The data consist of observations made at 1200 LT, as this was overwhelmingly the most common observation time in use before 1850. Simultaneous sea surface temperature (SST) is present in only 3.6% of records. For this reason, extension of historical marine temperature fluctuations back to the early 19th century will need to rely on MAT data. Nonstandard thermometer exposures and observing practices introduce nonclimatic biases into the data. Here I propose new correction schemes to account for the major sources of bias in the data. Similar principles are applicable to post‐1850 data as well. The seeming disadvantage of nonstandard thermometer exposures and local noon observation time actually provides information that is used to adjust and homogenize the data. Unscreened thermometers absorb seasonally and geographically varying short wave radiation and give daytime temperatures that are too high. Calculation of grid box mean temperature anomalies, ΔTa, using a modern nighttime climatology highlights these biases. Ship‐to‐ship variations in thermometer calibration and performance are measured and adjusted using tropical ΔTa as a reference. Individual ships have large biases (in excess of 1°C) which can dominate observed patterns of temperature anomalies in individual years. Using data from low latitudes, a regression equation is produced that predicts ΔTa as a function of net radiation. Predicted ΔTa at higher latitudes is then compared to observed values of ΔTa in the historical record to detect biases arising from internally generated ship's heat. The advantage to this approach is that SST data are not needed to correct MAT data, thereby preserving both as independent marine temperature records. Finally, a correction for nonstandard thermometer exposure is made. This last correction is made by comparing historical and modern noon minus nighttime temperature differences. The difference between the two is considered to be due to nonstandard thermometer exposure in the historical data. Since the local MAT diurnal range is assumed to be independent of secular changes in temperature the historical adjusted nighttime marine air temperature can be compared to modern climatologies.
Observations of new particle production in the atmosphere of a moderately polluted site in eastern England
Authors: Harrison Roy M. , Grenfell J. Lee , Savage Nick , Allen Andrew , Clemitshaw Kevin , Penkett Stuart , Hewitt C. Nicholas , Davison Brian
Published: February 2004

Measurements of particle number density and Fuchs surface area, together with a range of gaseous pollutant concentrations, have been made in June 1995 at a coastal site in eastern England which receives air from a range of polluted and less polluted origins. Periods of enhanced local particle production were identified and found to be associated predominantly with relatively polluted air sectors. An examination of the factors contributing to homogeneous nucleation and hence new particle production suggests that those most important at this location are probably the production of hydroxyl radicals and the availability of ammonia. A numerical modeling study calculating characteristic timescales for new particle production and for condensation onto existing aerosol surfaces is able to predict periods of new particle production. The model suggests that oxidation of dimethyl sulphide and sulphur dioxide and homogeneous nucleation of sulphuric acid and water, probably in combination with ammonia, are the source of new particles at this site.
Effective radiative properties of bounded cascade nonabsorbing clouds: Definition of the equivalent homogeneous cloud approximation
Authors: Szczap Frédéric , Isaka Harumi , Saute Marcel , Guillemet Bernard , Ioltukhovski Andrey
Published: February 2004

In the present study we investigated the radiative properties of inhomogeneous nonabsorbing clouds under the Equivalent plane‐parallel Homogeneous Cloud Approximation (EHCA), by using the one‐dimensional (1‐D) bounded cascade inhomogeneous clouds. The effective optical depth was defined under the EHCA by requiring the identity of the radiant flux components of the radiation budget between the inhomogeneous clouds and their equivalent homogeneous counterparts. Such requirement provides a rational framework to define the effective optical depth of the inhomogeneous nonabsorbing clouds. We analyzed the dependency of the effective optical depth on the horizontal scale of averaging and solar incidence angle and specified the conditions under which an inhomogeneous cloud segment could be treated as a plane‐parallel homogeneous cloud. A parameterization of the effective optical depth was proposed as a function of the mean optical depth and a relative cloud inhomogeneity parameter. Finally, we compared the EHCA with the effective thickness approximation, both based on the definition of the effective optical depth, and discussed the difference between their respective effective optical depths.
Prairie and arctic areal snow cover mass balance using a blowing snow model
Authors: Pomeroy J. W. , Li L.
Published: February 2004

Algorithms to calculate the threshold wind speed and the effect of exposed vegetation on saltation and to describe vertical profiles of humidity in blowing snow, permit the calculation of point blowing snow transport and sublimation fluxes using standard meteorological and landcover data or simple interfaces with climate models. Blowing snow transport and sublimation fluxes can be upscaled to calculate open environment snow accumulation by accounting for their variability over open snow fields, increase in transport and sublimation with fetch, and the effect of exposed vegetation on partitioning the shear stress available to drive transport. Blowing snow fluxes scaled in this manner were used to calculate snow mass balance and to simulate seasonal snow accumulation at a southern Saskatchewan prairie and an arctic site. Field measurements at these sites indicated that from 48% to 58% of snowfall was removed by blowing snow before melt began. Simulations suggest that the ratios of snow removed and sublimated by blowing snow to that transported were 2∶1 and 1∶1 at the prairie and arctic sites respectively. The resulting methodology was capable of estimating winter season mass balances for these snowpacks that compared well with snowfall and snow accumulation measurements.