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PubMed2006

Phenol production in benzene/air plasmas at atmospheric pressure. Role of radical and ionic routes.

Benzene can be efficiently converted into phenol when it is treated by either corona or dielectric barrier discharge (DBD) plasmas operating at atmospheric pressure in air or mixtures of N(2) and O(2). Phenol produced by corona discharge in an atmospheric pressure chemical ionization source (APCI) has been detected as the corresponding radical cation C(6)H(5)OH(+*) at m/z 94 by an ion trap mass spectrometer. On the other hand, phenol has been observed also as neutral product by gas chromatography-mass spectrometry analysis (GC-MS) after treatment in a DBD plasma. Experiments aimed at shading light on the elementary processes responsible for benzene oxidation were carried out (i) by changing the composition of the gas in the corona discharge source; (ii) by using isotopically labeled reagents; and (iii) by investigating some relevant ion-molecule reactions (i.e. C(6)H(6)(+*) + O(2), C(6)H(5)(+) + O(2)) via selected guided ion beam measurements and with the help of ab initio calculations. The results of our approach show that ionic mechanisms do not play a significant role in phenol production, which can be better explained by radical reactions resulting in oxygen addition to the benzene ring followed by 1,2 H transfer.

Air↗

PubMedDate not supplied

Direct atmospheric pressure coupling of polyacrylamide gel electrophoresis to mass spectrometry for rapid protein sequence analysis.

Using laser desorption-atmospheric pressure chemical ionization we describe a novel approach for coupling mass spectrometry to polyacrylamide gel electrophoresis. In contrast to other approaches, the method allows for the direct sampling of a polyacrylamide gel-embedded protein without the addition of any exogenous matrixes and is performed at atmospheric pressure. After electrophoresis and enzymatic digestion, the gel is analyzed at AP by photons that desorb neutral peptide molecules, followed by corona discharge ionization in the gas-phase, and subsequent mass analysis. Our experimental results demonstrate the method to (1) rapidly identify electrophoresed proteins via "peptide fingerprinting" using protein databases, (2) detect single-amino acid polymorphisms, and (3) has potential for sub-picomole sensitivity while still maintaining in situ gel desorption-ionization at ambient conditions.

Atmospheric Pressure↗

PubMed1999

Photo-induced fractionation of water isotopomers in the Martian atmosphere.

The history and size of the water reservoirs on early Mars can be constrained using isotopic ratios of deuterium to hydrogen. We present new laboratory measurements of the ultraviolet cross-sections of H2O and its isotopomers, and modeling calculations in support of a photo-induced fractionation effect (PHIFE), that reconciles a discrepancy between past theoretical modeling and recent observations. This supports the hypothesis that Mars had an early warm atmosphere and has lost at least a 50-m global layer of water. Likely applications of PHIFE to other planetary atmospheres are sketched.

Atmosphere↗

PubMed1995

Photochemistry of Triton's atmosphere and ionosphere.

The photochemistry of 32 neutral and 21 ion species in Triton's atmosphere is considered. Parent species N2, CH4, and CO (with a mixing ratio of 3 x 10(-4) in our basic model) sublime from the ice with rates of 40, 208, and 0.3 g/cm2/b.y., respectively. Chemistry below 50 km is driven mostly by photolysis of methane by the solar and interstellar medium Lyman-alpha photons, producing hydrocarbons C2H4, C2H6, and C2H2 which form haze particles with precipitation rates of 135, 28, and 1.3 g/cm2/b.y., respectively. Some processes are discussed which increase the production of HCN (by an order of magnitude to a value of 29 g/cm2/b.y.) and involve indirect photolysis of N2 by neutrals. Reanalysis of the measured methane profiles gives an eddy diffusion coefficient K = 4 x 10(3) cm2/s above the tropopause and a more accurate methane number density near the surface, (3.1 +/- 0.8) x 10(11) cm-3. Chemistry above 200 km is driven by the solar EUV radiation (lambda < 1000 angstroms) and by precipitation of magnetospheric electrons with a total energy input of 10(8) W (based on thermal balance calculations). The most abundant photochemical species are N, H2, H, O, and C. They escape with the total rates of 7.7 x 10(24) s-1, 4.5 x 10(25) s-1, 2.4 x 10(25) s-1, 4.4 x 10(22) s-1, and 1.1 x 10(24) s-1, respectively. Atomic species are transported to a region of 50-200 km and drive the chemistry there. Ionospheric chemistry explains the formation of an E region at 150-240 km with HCO+ as a major ion, and of an F region above 240 km with a peak at 320 km and C+ as a major ion. The ionosphere above 500 km consists of almost equal densities of C+ and N+ ions. The model profiles agree with the measured atomic nitrogen and electron density profiles. A number of other models with varying rate coefficients of some reactions, differing properties of the haze particles (chemically passive or active), etc., were developed. These models show that there are four basic unknown values which have strong impacts on the composition and structure of the atmosphere and ionosphere. These values and their plausible ranges are the CO mixing ratio fco = 10(-4)-10(-3), the magnetospheric electron energy input (1 +/- 0.5) x 10(8) W, the rate coefficient of charge-exchange reaction N2(+) + C k = 10(-11)-10(-10) cm3/s, and the ion escape velocity Vi approximately equal to 150 cm/s.

Atmosphere↗

PubMed1987

Concepts for collection of aerosols in Titan's atmosphere.

In this paper we consider design of instruments for collection of aerosols during entry in Titan's atmosphere. Major constraints on designs are small sample collection time, low aerosol column density, and the need to collect 1-10 micrograms of aerosol for gas chromatographic analysis. Thus it is important to maximize aerosol collection through collector design, which includes consideration of various types of collectors and maximizing the collection efficiency of a given type of collector. Sampling systems discussed include inertial impactors, filters, electrostatic devices, and multistage instruments. Aerosol sampling is reviewed in the context of high-altitude (200-70 km) and low-altitude (60-30 km) regions of Titan's atmosphere.

Aerosols↗

PubMed2001

A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles.

To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (<1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1-8 Myr) and early Permian (275-290 Myr) times. But for most of the Mesozoic era (65-250 Myr), CO2 levels were high (1,000-2,000 p.p.m.v.), with transient excursions to even higher CO2 (>2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records, but suggest that CO2 reconstructions based on carbon isotope proxies may be compromised by episodic outbursts of isotopically light methane. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr.

Atmosphere↗

PubMed1995

Evolution of water reservoirs on Mars from D/H ratios in the atmosphere and crust.

Ancient fluvial networks on the surface of Mars suggest that it was warm and wet over three billion years ago. Surface features resembling massive outflow channels provide evidence that, even more recently, the martian crust contained the equivalent of a planet-wide reservoir of water several hundred metres deep. But arguments based on the isotopic fraction and present-day escape rate of hydrogen in the martian atmosphere require only 0.5 metres of crustal water today and about six metres in the past. An additional constraint on the evolution of the isotopic composition of martian water has recently been obtained from measurements of the deuterium to hydrogen ratio of hydrous minerals in the SNC meteorites--meteorites that almost certainly originated on Mars. Here I show that these new data require that the modern crustal reservoirs of martian water must be quite large, at least several metres in global-equivalent depth. The deuterium enrichment of the present martian atmosphere then implies that the reservoir of crustal water on ancient Mars was several hundred metres deep, consistent with the geological evidence.

Atmosphere↗

PubMed2002

Atmospheric CO2 from fossil plant cuticles.

Plants respond to changes in atmospheric carbon dioxide levels by regulating the number of stomata in their leaves. In his reconstruction of a continuous, 300-million-year record of atmospheric CO2, Retallack bases his curve on stomatal counts of fossil plant cuticles taken from published micrographs. However, the preservation of cuticles from Permian times is generally too fragmentary for the stomatal index to be reliably determined, the micrographs used could have biased the results, and there are important errors in the supplementary data - all of which cast doubt on the Permian part of Retallack's record.

Atmosphere↗

PubMed2002

The effect of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2.

Most global biogeochemical processes are known to respond to climate change, some of which have the capacity to produce feedbacks through the regulation of atmospheric greenhouse gases. Marine denitrification-the reduction of nitrate to gaseous nitrogen-is an important process in this regard, affecting greenhouse gas concentrations directly through the incidental production of nitrous oxide, and indirectly through modification of the marine nitrogen inventory and hence the biological pump for CO2. Although denitrification has been shown to vary with glacial-interglacial cycles, its response to more rapid climate change has not yet been well characterized. Here we present nitrogen isotope ratio, nitrogen content and chlorin abundance data from sediment cores with high accumulation rates on the Oman continental margin that reveal substantial millennial-scale variability in Arabian Sea denitrification and productivity during the last glacial period. The detailed correspondence of these changes with Dansgaard-Oeschger events recorded in Greenland ice cores indicates rapid, century-scale reorganization of the Arabian Sea ecosystem in response to climate excursions, mediated through the intensity of summer monsoonal upwelling. Considering the several-thousand-year residence time of fixed nitrogen in the ocean, the response of global marine productivity to changes in denitrification would have occurred at lower frequency and appears to be related to climatic and atmospheric CO2 oscillations observed in Antarctic ice cores between 20 and 60 kyr ago.

Atmosphere↗

PubMed2002

Triassic-Jurassic atmospheric CO2 spike.

I question the claim by Tanner et al. that atmospheric CO2 levels remained constant across the Triassic-Jurassic boundary on the grounds of problems with stratigraphic completeness and contamination with atmospheric methane. Because methanogenic CH4 has a light isotope composition and oxidizes readily to CO2, methane-clathrate dissociation and oxidation events cannot be detected by palaeobarometers that use the carbon-isotope composition of palaeosol carbonate.

Atmosphere↗

PubMed2003

An extended upper atmosphere around the extrasolar planet HD209458b.

The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed. Together with radial velocity measurements, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption. Here we report the detection of atomic hydrogen absorption in the stellar Lyman alpha line during three transits of HD209458b. An absorption of 15 +/- 4% (1sigma) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.

Atmosphere↗

PubMed2003

The long-term carbon cycle, fossil fuels and atmospheric composition.

The long-term carbon cycle operates over millions of years and involves the exchange of carbon between rocks and the Earth's surface. There are many complex feedback pathways between carbon burial, nutrient cycling, atmospheric carbon dioxide and oxygen, and climate. New calculations of carbon fluxes during the Phanerozoic eon (the past 550 million years) illustrate how the long-term carbon cycle has affected the burial of organic matter and fossil-fuel formation, as well as the evolution of atmospheric composition.

Atmosphere↗

PubMed2005

Low European methyl chloroform emissions inferred from long-term atmospheric measurements.

Methyl chloroform (CH3CCl3, 1,1,1,-trichloroethane) was used widely as a solvent before it was recognized to be an ozone-depleting substance and its phase-out was introduced under the Montreal Protocol. Subsequently, its atmospheric concentration has declined steadily and recent European methyl chloroform consumption and emissions were estimated to be less than 0.1 gigagrams per year. However, data from a short-term tropospheric measurement campaign (EXPORT) indicated that European methyl chloroform emissions could have been over 20 gigagrams in 2000 (ref. 6), almost doubling previously estimated global emissions. Such enhanced emissions would significantly affect results from the CH3CC13 method of deriving global abundances of hydroxyl radicals (OH) (refs 7-12)-the dominant reactive atmospheric chemical for removing trace gases related to air pollution, ozone depletion and the greenhouse effect. Here we use long-term, high-frequency data from Mace Head, Ireland and Jungfraujoch, Switzerland, to infer European methyl chloroform emissions. We find that European emission estimates declined from about 60 gigagrams per year in the mid-1990s to 0.3-1.4 and 1.9-3.4 gigagrams per year in 2000-03, based on Mace Head and Jungfraujoch data, respectively. Our European methyl chloroform emission estimates are therefore higher than calculated from consumption data, but are considerably lower than those derived from the EXPORT campaign in 2000 (ref. 6).

Atmosphere↗

PubMed2005

Impacts of orbital forcing and atmospheric carbon dioxide on Miocene ice-sheet expansion.

The processes causing the middle Miocene global cooling, which marked the Earth's final transition into an 'icehouse' climate about 13.9 million years ago (Myr ago), remain enigmatic. Tectonically driven circulation changes and variations in atmospheric carbon dioxide levels have been suggested as driving mechanisms, but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult. Here we present high-resolution climate proxy records, covering the period from 14.7 to 12.7 million years ago, from two complete sediment cores from the northwest and southeast subtropical Pacific Ocean. Using new chronologies through the correlation to the latest orbital model, we find relatively constant, low summer insolation over Antarctica coincident with declining atmospheric carbon dioxide levels at the time of Antarctic ice-sheet expansion and global cooling, suggesting a causal link. We surmise that the thermal isolation of Antarctica played a role in providing sustained long-term climatic boundary conditions propitious for ice-sheet formation. Our data document that Antarctic glaciation was rapid, taking place within two obliquity cycles, and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change.

Antarctic Regions↗

PubMed2002

Isotopic processes in atmospheric chemistry.

Trace gases are responsible for the atmospheric processes underlying air pollution and global change. Analysis of the distribution of stable isotopes in these gases is a way of gaining additional knowledge that may not otherwise be available. This approach requires knowledge of the isotopic signature of emissions sources and chemical reactions. This review focuses on the chemical physics of the processes underlying the distribution of stable isotopes in atmospheric gases.

Air Pollutants↗

PubMed2004

A multiresidue method for the analysis of phenols and nitrophenols in the atmosphere.

A method using GC-MS and derivatization with N-(t-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA) was developed for the analysis of 20 phenolic compounds in atmospheric samples (gas and particles). Air sampling was carried out using a Hi-Vol sampler with glass fibre filter and XAD-2 resin at a flow rate of 60 m(3) h(-1). The particle and gas phases were collected separately over a period of 4 h. Samples were Soxhlet extracted, evaporated to dryness under nitrogen and refilled with acetonitrile. 100 microl of these extracts were derivatized with 100 microl of MTBSTFA at 80 degrees C for 1 h under strong stirring. Phenolic compounds were injected into a GC-MS in splitless mode and quantified as their TBDMS derivatives in the SIM mode. Mass spectral analysis of the derivatives of the 20 compounds studied indicates that the spectra are highly specific showing an ion at [M - 57]+ which is useful for structure confirmation or analysis at low levels using selected ion monitoring. Quantification limits varied between 5 microg l(-1) and 10 microg l(-1) which correspond to 20 pg m(-3) and 40 pg m(-3) for 250 m(3) of air sampled. This method was successfully applied to atmospheric samples.

Acetamides↗

PubMed2005

Simplification of the representation of the organic component of atmospheric particulates.

We present an analysis of recent field data to investigate the variation in the organic component of atmospheric aerosol and its behaviour in the moist environment. In all locations the degree of oxygenation of the organic material increases with photochemical age, as does the particulate hygroscopicity. These changes will generally occur in spatial scales comparable to a single cell in global models at representative boundary layer wind speeds. Using ADDEM, a new model of the equilibrium state of multicomponent aerosol, we show that inorganic component changes must be responsible for the increase in particulate hygroscopicity with photochemical age. It is suggested that a common representation of nearfield and background organic aerosol composition is sufficient to describe the behaviour of organic components in a variety of field experiments; nearfield small mode organics being dominated by a combustion-derived unoxidised signature, whilst the background accumulation mode is more oxygenated and dominates in air masses with a photochemical age of more than a couple of days. This representation may be used within the sub-saturated regime to predict the behaviour of ambient particulates in the moist atmosphere. Whether a similar common representation can be used for cloud activation prediction in supersaturated environments, or for investigation of gas-to-particle partitioning, should be investigated.

Aerosols↗

PubMed2006

An improved dual channel PERCA instrument for atmospheric measurements of peroxy radicals.

This paper describes a new dual-channel PEroxy RadiCal Amplification (PERCA) instrument, which has been designed to improve the time resolution and signal to noise and to reduce the interference caused by variations in ambient ozone concentrations. The instrument was run at the Weybourne Atmospheric Observatory (WAO), North Norfolk, during WAOWEX (Weybourne Atmospheric Observatory Winter Experiment) in January/February 2002 and INSPECTRO (Influence of clouds on the spectral actinic flux in the lower troposphere) in September 2002. The performance of the instrument is assessed and compared to that of a single channel instrument. In particular, it is shown how the precision is greatly improved in fluctuating background ozone conditions. In addition the improved time response of the instrument allows changes in peroxy radical concentrations to be related to rapid changes in nitric oxide concentrations and the ozone photolysis frequency, j(O(1)D).

Air Pollutants↗