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At least 721 records · Page 40Linked to original sources
PubMed2001

Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere.

Northern mid-latitude forests are a large terrestrial carbon sink. Ignoring nutrient limitations, large increases in carbon sequestration from carbon dioxide (CO2) fertilization are expected in these forests. Yet, forests are usually relegated to sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen. Here we present evidence that estimates of increases in carbon sequestration of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic. In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilizing at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus, fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2. Assessment of future carbon sequestration should consider the limitations imposed by soil fertility, as well as interactions with nitrogen deposition.

Atmosphere↗
PubMed1995

Chemical and thermal response of Jupiter's atmosphere following the impact of comet Shoemaker-Levy 9.

In July 1994, the collisions of the fragments of comet Shoemaker-Levy 9 with Jupiter resulted in dramatic changes in the planet's atmosphere. Observations of the events suggest that the composition and thermal properties of the atmosphere were considerably modified at the impact sites, with the changes persisting for times lasting from minutes to weeks (see, for example, refs 1-4). Here we report observations of the impact sites at millimetre wave-lengths, which reveal strong emission lines associated with carbon monoxide, carbonyl sulphide and carbon monosulphide. The abundance of carbon monoxide in the jovian atmosphere is normally very low; carbonyl sulphide and carbon monosulphide, on the other hand, have not hitherto been detected. We find that the largest fragments (G and K) each produced approximately 10(14) g of carbon monoxide, 3 x 10(12) g of carbonyl sulphide and 3 x 10(11) g of carbon monosulphide, most probably by shock-induced chemical reactions. Our observations also place firm constraints on the thermal response of Jupiter's stratosphere to the impacts.

Atmosphere↗
PubMed2002

A short-term sink for atmospheric CO2 in subtropical mode water of the North Atlantic Ocean.

Large-scale features of ocean circulation, such as deep water formation in the northern North Atlantic Ocean, are known to regulate the long-term physical uptake of CO2 from the atmosphere by moving CO2-laden surface waters into the deep ocean. But the importance of CO2 uptake into water masses that ventilate shallower ocean depths, such as subtropical mode waters of the subtropical gyres, are poorly quantified. Here we report that, between 1988 and 2001, dissolved CO2 concentrations in subtropical mode waters of the North Atlantic have increased at a rate twice that expected from these waters keeping in equilibrium with increasing atmospheric CO2. This accounts for an extra 0.4-2.8 Pg C (1 Pg = 10(15) g) over this period (that is, about 0.03-0.24 Pg C yr(-1)), equivalent to 3-10% of the current net annual ocean uptake of CO2 (ref. 3). We suggest that the lack of strong winter mixing events, to greater than 300 m in depth, in recent decades is responsible for this accumulation, which would otherwise disturb the mode water layer and liberate accumulated CO2 back to the atmosphere. However, future climate variability (which influences subtropical mode water formation) and changes in the North Atlantic Oscillation (leading to a return of deep winter mixing events) may reduce CO2 accumulation in subtropical mode waters. We therefore conclude that, although CO2 uptake by subtropical mode waters in the North Atlantic--and possibly elsewhere--does not always represent a long-term CO2 sink, the phenomenon is likely to contribute substantially to interannual variability in oceanic CO2 uptake.

Atlantic Ocean↗
PubMed2004

Evidence from massive siderite beds for a CO2-rich atmosphere before approximately 1.8 billion years ago.

It is generally thought that, in order to compensate for lower solar flux and maintain liquid oceans on the early Earth, methane must have been an important greenhouse gas before approximately 2.2 billion years (Gyr) ago. This is based upon a simple thermodynamic calculation that relates the absence of siderite (FeCO3) in some pre-2.2-Gyr palaeosols to atmospheric CO2 concentrations that would have been too low to have provided the necessary greenhouse effect. Using multi-dimensional thermodynamic analyses and geological evidence, we show here that the absence of siderite in palaeosols does not constrain atmospheric CO2 concentrations. Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O2 concentrations and pH conditions in well-aerated soils have favoured the formation of ferric (Fe3+)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H2-rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO2 concentration was more than 100 times greater than today, causing the rain and ocean waters to be more acidic than today. We therefore conclude that CO2 alone (without a significant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.

Atmosphere↗
PubMed2005

The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe.

Saturn's largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellite's surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.

Argon↗
PubMed2006

Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing.

Since the mid-nineteenth century the Earth's surface has warmed, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation. An important aspect of this tropical circulation is a large-scale zonal (east-west) overturning of air across the equatorial Pacific Ocean--driven by convection to the west and subsidence to the east--known as the Walker circulation. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century.

Air Movements↗
PubMed2003

Tropical rain forest tree growth and atmospheric carbon dynamics linked to interannual temperature variation during 1984-2000.

During 1984-2000, canopy tree growth in old-growth tropical rain forest at La Selva, Costa Rica, varied >2-fold among years. The trees' annual diameter increments in this 16-yr period were negatively correlated with annual means of daily minimum temperatures. The tree growth variations also negatively covaried with the net carbon exchange of the terrestrial tropics as a whole, as inferred from nearly pole-to-pole measurements of atmospheric carbon dioxide (CO(2)) interpreted by an inverse tracer-transport model. Strong reductions in tree growth and large inferred tropical releases of CO(2) to the atmosphere occurred during the record-hot 1997-1998 El Niño. These and other recent findings are consistent with decreased net primary production in tropical forests in the warmer years of the last two decades. As has been projected by recent process model studies, such a sensitivity of tropical forest productivity to on-going climate change would accelerate the rate of atmospheric CO(2) accumulation.

Atmosphere↗
PubMed2002

Low atmospheric CO(2) levels during the Permo- Carboniferous glaciation inferred from fossil lycopsids.

Earth history was punctuated during the Permo-Carboniferous [300-250 million years (Myr) ago] by the longest and most severe glaciation of the entire Phanerozoic Eon. But significant uncertainty surrounds the concentration of CO(2) in the atmosphere through this time interval and therefore its role in the evolution of this major prePleistocene glaciation. Here, I derive 24 Late Paleozoic CO(2) estimates from the fossil cuticle record of arborsecent lycopsids of the equatorial Carboniferous and Permian swamp communities. Quantitative calibration of Late Carboniferous (330-300 Myr ago) and Permian (270-260 Myr ago) lycopsid stomatal indices yield average atmospheric CO(2) concentrations of 344 ppm and 313 ppm, respectively. The reconstructions show a high degree of self-consistency and a degree of precision an order of magnitude greater than other approaches. Low CO(2) levels during the Permo-Carboniferous glaciation are in agreement with glaciological evidence for the presence of continental ice and coupled models of climate and ice-sheet growth on Pangea. Moreover, the Permian data indicate atmospheric CO(2) levels were low 260 Myr ago, by which time continental deglaciation was already underway. Positive biotic feedbacks on climate, and geotectonic events, therefore are implicated as mechanisms underlying deglaciation.

Atmosphere↗
PubMed1989

Effects of atmospheric ammonia on chick embryo development.

1. Two experiments were carried out in which fertile, White Leghorn eggs were individually subjected to atmospheric ammonia, for 24 h before incubation, in order to examine the effects on embryo development. 2. In experiment 1, development was unaffected by 0.35 mg/l (493 microliters/l) atmospheric ammonia and slightly reduced by 0.7 mg/l (986 microliters/l) while 7 mg/l (9,860 microliters/l) produced 100% mortality. In experiment 2, embryo development was unaffected by 0.35 mg/l atmospheric ammonia, but severely reduced by 3.15 mg/l (4,437 microliters/l).

Ammonia↗
PubMed1989

Modified atmosphere packaging of fruits and vegetables.

Modified atmospheres (MA), i.e., elevated concentrations of carbon dioxide and reduced levels of oxygen and ethylene, can be useful supplements to provide optimum temperature and relative humidity in maintaining the quality of fresh fruits and vegetables after harvest. MA benefits include reduced respiration, ethylene production, and sensitivity to ethylene; retarded softening and compositional changes; alleviation of certain physiological disorders; and reduced decay. Subjecting fresh produce to too low an oxygen concentration and/or to too high a carbon dioxide level can result in MA stress, which is manifested by accelerated deterioration. Packaging fresh produce in polymeric films can result in a commodity-generated MA. Atmosphere modification within such packages depends on film permeability, commodity respiration rate and gas diffusion characteristics, and initial free volume and atmospheric composition within the package. Temperature, relative humidity, and air movement around the package can influence the permeability of the film. Temperature also affects the metabolic activity of the commodity and consequently the rate of attaining the desired MA. All these factors must be considered in developing a mathematical model for selecting the most suitable film for each commodity.

Atmosphere↗
PubMed2003

Determination of atmospheric nitrogen input to Lake Greenwood, South Carolina: part 2--Gaseous measurements and modeling.

The Reedy River branch of Lake Greenwood, SC, has repeatedly experienced summertime algal blooms, upsetting the natural system. A series of experiments were carried out to investigate atmospheric nitrogen (N) input into the lake. N was examined because of the insignificant phosphorus dry atmospheric flux and the unique nutrient demands of the dominant algae (Pithophora oedogonia) contributing to the blooms. Episodic atmospheric measurements during January and March 2001 have shown that the dry N flux onto the lake ranged from 0.9 to 17.4 kg N/ha-yr, and on average is caused by nitric acid (HNO3; 31%), followed by nitrogen dioxide (NO2; 23%), fine ammonium (NH4+; 20%), coarse nitrate (NO3-; 16%), fine NO3 (5%), and coarse NH4+ (5%). Similar measurements in Greenville, SC (the upper watershed of the Reedy River), showed that the dry N deposition flux there ranged from 1.4 to 9.7 kg N/ha-yr and was mostly caused by gaseous deposition (40% NO2 and 40% HNO3). The magnitude of this dry N deposition flux is comparable to wet N flux as well as other point sources in the area. Thermodynamic modeling showed low concentrations of ammonia, relative to the particulate NH4+ concentrations.

Atmosphere↗
PubMed2005

Beryllium-7 measurements in the Houston and Phoenix urban areas: an estimation of upper atmospheric ozone contributions.

Natural radionuclides have been proposed as a means of assessing the transport of ozone (O3) and aerosols in the troposphere. Beryllium-7 (7Be) is produced in the upper troposphere and lower stratosphere by the interaction of cosmogenic particles with atmospheric nitrogen and oxygen. 7Be has a 53.29-day half-life (478 keV gamma) and is known to attach to fine particles in the atmosphere once it is formed. It has been suggested that O3 from aloft can be transported into rural and urban regions during stratospheric-tropospheric folding events leading to increased background levels of O3 at the surface. 7Be can be used as a tracer of upper atmospheric air parcels and the O3 associated with them. Aerosol samples with a 2.5-microm cutoff were collected during 12-hr cycles (day/night) for a 30-day period at Deer Park, TX, near Houston, in August-September of 2000, and at Waddell, AZ, near Phoenix, in June-July of 2001. A comparison of 7Be levels with 12-hr O3 averages and maxima shows little correlation. Comparison of nighttime and daytime O3 levels indicate that during the day, when mixing is anticipated to be higher, the correlation of 7Be with O3 in Houston is approximately twice that observed at night. This is consistent with mixing and with the anticipated loss of O3 by reaction with nitric oxide (NO) and dry deposition. At best, 30% of the O3 variance can be explained by the correlation with 7Be for Houston, less than that for Phoenix where no significant correlation was seen. This result is consistent with the intercept values obtained for 7Be correlations with either O3 24-hr averages or O3 12-hr maxima and is also in the range of the low O3 levels (25 ppb) observed at Deer Park during a tropical storm event where the O3 is attributable primarily to background air masses. That is, maximum background O3 level contributions from stratospheric sources aloft are estimated to be in the range of 15-30 ppb in the Houston, TX, and Phoenix, AZ, area, and levels above these are because of local tropospheric photochemical production.

Aerosols↗
PubMed1992

Assessment of multiple markers of environmental tobacco smoke (ETS) in controlled, steady-state atmospheres in a dynamic test chamber.

Controlled test atmospheres of sidestream and mixed mainstream/sidestream tobacco smoke were produced in a dynamic test chamber of 16.9 m3 volume. University of Kentucky 1R4F research cigarettes were automatically smoked by a Borgwaldt smoking machine. Different target levels of smoke were attained by varying the number of cigarettes being smoked (one or two cigarettes continuously) and dilution airflow through the chamber (1.6 to 7.4 m3/min). For each level, a "smoke index" was calculated as the number of cigarettes being smoked divided by the dilution flow rate. Ultraviolet light-absorbing particulate matter (UVPM); aerosol counts and size distribution; and concentrations of CO, NO2, NH3, formaldehyde, acetaldehyde, acrolein, and nicotine were determined in the test atmospheres and background dilution air. Parameters exhibiting the highest correlations with smoke index (r2 > 0.98) included UVPM, aerosol counts, formaldehyde, and acetaldehyde. The other parameters were also highly correlated with r2 > 0.9 in each case. Differences in concentration for sidestream and mixed mainstream/sidestream atmospheres were statistically significant for acetaldehyde, formaldehyde, and CO; in each case the level was higher in mixed mainstream/sidestream smoke. These results demonstrate the utility of employing multiple markers for assessment of environmental tobacco smoke levels and for dichotomizing the contributions of sidestream and mainstream smoke to these levels. The generation factors developed in this work can be used for estimating required ventilation rates for attainment of target air quality conditions in smoking rooms.

Air Pollutants↗
PubMed2003

Emission of burning emulsified diesel oil with sodium sulfate in salty atmospheric air.

The effects of sodium sulfate in fuel oil and salty atmospheric air on the emission characteristics of furnaces or boilers burned with emulsified diesel oils are considered in this study. An industrial cylindrical furnace made of stainless steel associated with an automatic oil-fired burner was used for the emission measurements. Both neat diesel oil and emulsified diesel oil with distilled water were used as the tested oils. A homogenizing and emulsifying machine was employed to stir the diesel oil and sodium sulfate powder into a homogeneous oil mixture, and to prepare emulsions of micro-droplets of water dispersed in diesel oil. The experimental results showed that the existence of sodium chloride in atmospheric air enhanced SO2 formation. The use of emulsified diesel oil with 300-ppm sodium sulfate as fuel reduced the burning gas temperature and NOx emission while increased O2 emission. Moreover, the presence of sodium chloride in atmospheric air hindered the completeness of the combustion process and thus resulted in lower burning efficiency and larger excess oxygen emission.

Air↗
PubMed1993

Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter.

Polycyclic aromatic hydrocarbons (PAHs) are important components of the interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze. Titan tholin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionization mass spectrometry. A range of two- to four-ring PAHs, some with one to four alkylation sites are identified, with net abundance approximately 10(-4) g g-1 (grams per gram) of tholins produced. Synchronous fluorescence techniques confirm this detection. Titan tholins have proportionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chrysene, prominent in some discussions of interstellar grains, is found in Jupiter tholins. Solid state 13C NMR spectroscopy suggests approximately equal to 25% of the total C in both tholins is tied up in aromatic and/or aliphatic alkenes. IR spectra indicate an upper limit in both tholins of approximately equal to 6% by mass in benzenes, heterocyclics, and PAHs with more than four rings. Condensed PAHs may contribute at most approximately 10% to the observed detached limb haze layers on Titan. As with interstellar PAHs, the synthesis route of planetary PAHs is likely to be via acetylene addition reactions.

Ammonia↗
PubMed1984

Photochemistry of the atmosphere of Titan: comparison between model and observations.

The photochemistry of simple molecules containing carbon, hydrogen, nitrogen, and oxygen atoms in the atmosphere of Titan has been investigated using updated chemical schemes and our own estimates of a number of key rate coefficients. Proper exospheric boundary conditions, vertical transport, and condensation processes at the tropopause have been incorporated into the model. It is argued that he composition, climatology, and evolution of Titan's atmosphere are controlled by five major processes: (a) photolysis and photosensitized dissociation of CH4; (b) conversion of H to H2 and escape of hydrogen; (c) synthesis of higher hydrocarbons; (d) coupling between nitrogen and hydrocarbons; (e) coupling between oxygen and hydrocarbons. Starting with N2, CH4, and H2O, and invoking interactions with ultraviolet sunlight, energetic electrons, and cosmic rays, the model satisfactorily accounts for the concentrations of minor species observed by the Voyager IRIS and UVS instruments. Photochemistry is responsible for converting the simpler atmospheric species into more complex organic compounds, which are subsequently condensed at the tropopause and deposited on the surface. Titan might have lost 5.6 x 10(4), 1.8 x 10(3), and 4.0 g cm-2, or the equivalent of 8, 0.25, and 5 x 10(-4) bars of CH4, N2, and CO, respectively, over geologic time. Implications of abiotic organic synthesis on Titan for the origin of life on Earth are briefly discussed.

Atmosphere↗
PubMed1989

Paleosols and their relevance to Precambrian atmospheric composition

Various paleosols have been reported from within the Witwatersrand and Ventersdorp Supergroups, South Africa. They were studied in an attempt to constrain the amount of oxygen available in the atmosphere during deposition of the gold- and uranium-bearing Witwatersrand conglomerates. The majority of these horizons do not have any physical characteristics of paleosols, and none of them have a chemistry consistent with weathering, suggesting they have been subjected to modification by later alteration processes. A similar chemistry, indicating overprinting of any original soil chemistry, has been reported from paleosols elsewhere (Elliot Lake region, Canada; Hekpoort basalt, South Africa), but it does not appear that significant cognizance has been taken of this fact when using these paleosols to determine the composition of the Precambrian atmosphere. It is concluded here that characteristics previously attributed to Precambrian weathering in an oxygen-deficient atmosphere are better explained by post-burial, hydrothermal alteration along lithological contacts.

Aerosols↗
PubMed1980

An atmospheric pressure ionography system suitable for mammography.

Ionography is a possible replacement for silver halide based imaging systems in diagnostic radiology. This type of imaging depends upon the ionisation produced in a gas by incident x-radiation. The ions formed are collected on a plastic foil and the image is made visible by a suitable development process. The physical design properties which can affect sensitivity and image quality of an ionography system are outlined in this paper. Based upon the requirements of adequate sensitivity and resolution an atmospheric pressure ionography system is described which has been designed primarily for use in mammography. Freon 13B1 gas was chosen as the x-ray absorbing medium because it contains a relatively high atomic number atom (bromine, Z = 35) and also has a high density. It is a relatively cheap gas, has a high breakdown potential and has satisfactory resolving capability, particularly when employed at atmospheric pressure with low energy x-ray beams. The performance of the system is discussed in terms of sensitivity and resolution. Problems associated with an atmospheric pressure system are outlined and possible solutions described. Preliminary clinical results obtained with the system are also presented.

Atmospheric Pressure↗