Current understanding of the global cycling of carbon dioxide, methane, and nitrous oxide

To address the climate change caused by anthropogenic emissions of greenhouse gases into the atmosphere, it is essential to understand and quantitatively elucidate their cycling on the Earth's surface. This paper first presents an overview of the global cycling of three greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), followed by a description of their variations in the atmosphere. This paper then presents the recent global budgets of these greenhouse gases estimated using two different approaches, top-down and bottom-up. Discussions on our current knowledge regarding the global cycling of the three gases are also presented.

Transport and loss of nitrous oxide in soil water after forest clear-cutting

Whole-tree harvesting increased the concentration of nitrous oxide dissolved in soil water by two orders of magnitude over the concentration expected in equilibrium with the atmosphere. In contrast, the nitrous oxide content of soil water in an intact, second-growth forest was close to the expected theoretical value. Nitrous oxide, produced at active sites in the soil, dissolves in soil water and is transported to seeps and streams where it rapidly degasses from the solution and is released into the atmosphere. This loss of nitrous oxide after clear-cutting is not important to the nitrogen economy of the site; however, it may be important to the global atmospheric budget of nitrous oxide. Sources of nitrous oxide may have been overlooked because nitrous oxide emissions can be separated in time and space from the sites of the most intense production of nitrous oxide.

Nylon production: an unknown source of atmospheric nitrous oxide

Nitrous oxide in the earth's atmosphere contributes to catalytic stratospheric ozone destruction and is also a greenhouse gas component. A precise budgetary accounting of N(2)O sources has remained elusive, and there is an apparent lack of source identification. One source of N(2)O is as a by-product in the manufacture of nylon, specifically in the preparation of adipic acid. Characterization of the reaction N(2)O stoichiometry and its isotopic composition with a simulated industrial adipic acid synthesis indicates that because of high rates of global adipic acid production, this N(2)O may account for approximately 10 percent of the increase observed for atmospheric N(2)O.

Climate impact of increasing atmospheric carbon dioxide

The global temperature rose by 0.2 degrees C between the middle 1960's and 1980, yielding a warming of 0.4 degrees C in the past century. This temperature increase is consistent with the calculated greenhouse effect due to measured increases of atmospheric carbon dioxide. Variations of volcanic aerosols and possibly solar luminosity appear to be primary causes of observed fluctuations about the mean trend of increasing temperature. It is shown that the anthropogenic carbon dioxide warming should emerge from the noise level of natural climate variability by the end of the century, and there is a high probability of warming in the 1980's. Potential effects on climate in the 21st century include the creation of drought-prone regions in North America and central Asia as part of a shifting of climatic zones, erosion of the West Antarctic ice sheet with a consequent worldwide rise in sea level, and opening of the fabled Northwest Passage.

Changes in stratospheric ozone

The ozone layer in the upper atmosphere is a natural feature of the earth's environment. It performs several important functions, including shielding the earth from damaging solar ultraviolet radiation. Far from being static, ozone concentrations rise and fall under the forces of photochemical production, catalytic chemical destruction, and fluid dynamical transport. Human activities are projected to deplete substantially stratospheric ozone through anthropogenic increases in the global concentrations of key atmospheric chemicals. Human-induced perturbations may be occurring already.

Theoretical studies for structures and energetics of Rgn-N2O (Rg=He, Ne, Ar) clusters

Minimum-energy structures of the Rg(2)-N(2)O (Rg=He, Ne, Ar) clusters have been determined with ab initio MP2 optimization, whereas the minimum-energy structures of the Rg(n)-N(2)O clusters with n = 3-7 have been obtained with the pairwise additive potentials. Interaction energies and nonadditive three-body effects of the Rg(2)-N(2)O ternary complex have been calculated using supermolecule method at MP4 and CCSD(T) levels. It was found from the calculations that there are two minima corresponding to one distorted tetrahedral structure and one planar structure for the ternary complex. The nonadditive three-body effects were found to be small for Rg(2)-N(2)O complexes. Our calculations also indicated that, for He(n)-N(2)O and Ne(n)-N(2)O clusters, the first six He and Ne atoms form the first solvation ring around the middle nitrogen of the N(2)O monomer, while for Ar(n)-N(2)O, the first five Ar atoms form the first solvation ring.

Effects of xenon on acetylcholine release in the rat cerebral cortex in vivo

BACKGROUND

We have reported previously the effects of several anaesthetics on cholinergic activity in the central nervous system (CNS). In this study, we report the effects of xenon on cholinergic cell activity.

METHODS

Using in vivo brain microdialysis, we measured acetylcholine (ACh) release in the rat cerebral cortex in vivo during xenon anaesthesia.

RESULTS

Xenon induced an initial increase in ACh release, followed by a gradual decrease. The level of Ach release at 40 min of xenon administration was significantly higher than the control.

CONCLUSIONS

Xenon activates CNS cholinergic cell activity followed by development of acute tolerance.

Nitrous oxide formation in the Colne estuary, England: the central role of nitrite

Nitrate and nitrite concentrations in the water and nitrous oxide and nitrite fluxes across the sediment-water interface were measured monthly in the River Colne estuary, England, from December 1996 to March 1998. Water column concentrations of N(2)O in the Colne were supersaturated with respect to air, indicating that the estuary was a source of N(2)O for the atmosphere. At the freshwater end of the estuary, nitrous oxide effluxes from the sediment were closely correlated with the nitrite concentrations in the overlying water and with the nitrite influx into the sediment. Increases in N(2)O production from sediments were about 10 times greater with the addition of nitrite than with the addition of nitrate. Rates of denitrification were stimulated to a larger extent by enhanced nitrite than by nitrate concentrations. At 550 microM nitrite or nitrate (the highest concentration used), the rates of denitrification were 600 micromol N.m(-2).h(-1) with nitrite but only 180 micromol N.m(-2).h(-1) with nitrate. The ratios of rates of nitrous oxide production and denitrification (N(2)O/N(2) x 100) were significantly higher with the addition of nitrite (7 to 13% of denitrification) than with nitrate (2 to 4% of denitrification). The results suggested that in addition to anaerobic bacteria, which possess the complete denitrification pathway for N(2) formation in the estuarine sediments, there may be two other groups of bacteria: nitrite denitrifiers, which reduce nitrite to N(2) via N(2)O, and obligate nitrite-denitrifying bacteria, which reduce nitrite to N(2)O as the end product. Consideration of free-energy changes during N(2)O formation led to the conclusion that N(2)O formation using nitrite as the electron acceptor is favored in the Colne estuary and may be a critical factor regulating the formation of N(2)O in high-nutrient-load estuaries.

Evaluation of the PhysioFlex closed-circuit anaesthesia machine

The concentrations of nitrous oxide, sevoflurane and oxygen in the circle system of a closed-circuit anaesthesia machine, the PhysioFlex, were measured in seven patients. During anaesthesia, the settings for each gas were changed and their concentrations recorded. At the induction of anaesthesia, it took 80-510s (median 190s) for the end-tidal sevoflurane concentration to reach 2.0%, and 920-2640s (median 1500s) for the oxygen in the breathing circuit to reach 30%. At this time, the nitrous oxide concentration was 60+/-3% (mean+/-SD). During anaesthesia, it took 90-480s (median 140s) for the end-tidal sevoflurane concentration setting to decrease from 3.0 to 1.0%, and 90-400s (median 110s) to return from 1.0 to 3.0%. When the inspired oxygen was increased from 30 to 50%, circuit concentrations reached equilibrium in 40-60s (median 40s), and when decreased from 50% back to 30%, equilibrium took 310-470s (median 450s). During recovery from anaesthesia, inspiratory sevoflurane concentration took 40-70s (median 50s) to decrease to 0.2%. The PhysioFlex provided adequate control of sevoflurane and oxygen concentrations, but not of increasing nitrous oxide concentrations.

Rotational Spectroscopic Investigation of the Weak Interaction between CO and N(2)O

Pure rotational spectra of the CO-N(2)O complex are reported for the first time. Spectra of five isotopomers, i.e., (12)C(16)O-(14)N(14)NO, (13)C(16)O-(14)N(14)NO, (13)C(18)O-(14)N(14)NO, (13)C(16)O-(15)N(14)NO, and (13)C(16)O-(14)N(15)NO, were measured using a pulsed molecular beam Fourier transform microwave spectrometer. Rotational and quartic centrifugal distortion constants as well as the nuclear quadrupole coupling constants of the terminal and the central (14)N nuclei were determined. Comparisons of the nuclear quadrupole coupling constants with the corresponding values of the N(2)O monomer and with those of other N(2)O-containing complexes indicate significant electronic charge redistribution at the central nitrogen atom upon complex formation. Structural parameters based on the experimental rotational constants were derived. With the isotopic data available, the orientations of the CO and N(2)O subunits in the complex could be determined. Copyright 1999 Academic Press.

Study of the Rotational Spectrum of the Ne-N2O van der Waals Dimer with a Fourier Transform Microwave Spectrometer

Rotational spectra of six isotopomers of the van der Waals dimer Ne-N2O, namely 20Ne-14N14NO, 22Ne-14N14NO, 20Ne-14N15NO, 22Ne-14N15NO, 20Ne-15N14NO, and 22Ne-15N14NO, were measured in the frequency range between 5 and 18 GHz using a pulsed beam cavity Fourier transform microwave spectrometer. The spectra are those of prolate asymmetric rotors and are in accord with a T-shaped structure of the complex. Both a- and b-type transitions were measured. Nuclear quadrupole hyperfine patterns of the rotational transitions due to the 14N nuclei were observed and analyzed. The rotational and centrifugal distortion constants were determined, as well as the quadrupole coupling constants chiaa and chibb, for both terminal and central 14N nuclei. The distance from the center of mass of the N2O subunit to the Ne atom and the angle between this distance and the N2O axis were derived from the rotational constants. The structural parameters indicate that the Ne atom is on average closer to the O atom than to the terminal N atom. Copyright 1998 Academic Press.

The effect of xenon on spinal dorsal horn neurons: a comparison with nitrous oxide

We compared the effects of xenon (Xe) on the spinal cord dorsal horn neurons with those of nitrous oxide (N2O) in cats anesthetized with chrolarose and urethane. We assessed the potency of both anesthetics by the inhibition of wide dynamic range neuron responses evoked by cutaneous noxious (pinch) stimulation to a hindpaw. During 70% Xe inhalation, the responses of 7 of 11 neurons to pinch stimulation were suppressed. N2O, 70%, suppressed it in 8 of 11 neurons. The potency of Xe and N2O was compared in six neurons that were suppressed by both anesthetics. After 20 min of Xe inhalation, the response to pinch was suppressed to 49.5% +/- 8.2% (mean +/- SE), while N2O, 70% in oxygen, suppressed it to 45.9% +/- 7.9%. The difference between N2O and Xe was not significant. We conclude that Xe and N2O suppress the spinal cord dorsal horn neurons to a similar degree.