O2 Mixing Ratio Research Articles

Methane oxidation activity in various soils and freshwater sediments: Occurrence, characteristics, vertical profiles, and distribution on grain size fractions

CH4 oxidation activities from various soils and freshwater sediments were measured at low (≤2 parts per million by volume (ppmv)) and high (≥1000 ppmv) CH4 mixing ratios. Most of the tested soils acted as sinks for atmospheric CH4. A correlation between the CH4 oxidation activity and the numbers of methanotrophs was only observed at high (1000 ppmv) CH4 mixing ratios. This indicates that the counted methanotrophs were not the bacteria which are oxidizing atmospheric CH4 (≤1.7 ppmv). The CH4 oxidation was due to prokaryotic microorganisms active only under oxic conditions. The CH4 oxidation activity decreased at O2 mixing ratios below 1–3% and was rather insensitive for the variation of O2 at mixing ratios >3%. Undisturbed, stratified soils, and freshwater sediments showed vertical profiles of CH4 oxidation activities with a distinct maximum. Sediments showed an exact correspondence between the number of methanotrophs and the maximum of CH4 oxidation both being localized at the surface sediment layer. The oxic soils showed maxima of CH4 oxidation activities generally located in subsurface layers. The maxima of CH4 oxidation activities were slightly shifted below the maxima of the numbers of methanotrophs indicating that the counted bacteria (incubation under 20% CH4) might not represent the active population which oxidizes atmospheric CH4. Plowed, agricultural soils showed no distinct maxima, neither of the CH4 oxidation activities nor of the numbers of methanotrophs. The grain size fractionation by centrifugation or wet sieving of slurries of two forest soils showed that the bulk (80–96%) of the CH4 oxidizing activity was attached to the smaller mineral fractions (clay, silt, fine sand) of these soils. Within the mineral fractions, greater particles had higher specific activities of CH4 oxidation than smaller particles.

Metabolism of nitric oxide byPseudomonas stutzeriin culture and in soil

Denitrification of nitrate with glucose as electron donor was measured in Pseudomonas stutzeri in batch culture and after inoculation into sterile soil. In closed culture, denitrification of 4 mM nitrate, NO and N2O with maximum concentrations of 2.2 mM, 126 nM and 169 nM, respectively. Denitrification experiments in soil were done in a flow-through system in which the soil was continuously flushed with N2. Denitrification of about 80 μg nitrate-N g−1dw soil (= 67mM) resulted in the intermediate accumulation of about 3–6 μg nitrate-N g−1dw soil (= 2.5–5.0 mM. The produced NO and N2O whcih were flushed out of the system amounted to 37–61% and 2–6% of the reduced nitrate, respectively. Both glucose and nitrate were required for denitrificatiom and for release of NO and N2O. NO consumption wasm studied in nitrate-exhausted soil using the flow-through system. NO consumption followed Michaelis-Menten kinetics. The apparent Vmax was about 15–42 amol NO h−1 cell−1, and the apparent Km was about 0.92 ppmv NO equivalent to 1.5 nM NO in the soil aqueous phase. At high soil water content (60–75% WHC), O2 mixing ratios between 0 and 20% in the gas phase had no short-term effect on the Vmax of NO consumption. At low soil water contents (10–30% WHC) however, the aerobic Vmax reached only about 20% of the aneorobic control. At high soil water contents only prolonged incubation (up to 96 h) under 20% O2 resulted also in decreased Vmax being about 30% of the anaerobic control The Km of NO consumption was unaffected by all these treatments. The NO reductase activity of P. stutzeri appeared to be relatively resistant towards O2 when the bacteria had been inoculated into soil, and was sufficient to explain NO uptake in oxic soils.

Simultaneous measurement of minor stratospheric constituents with emission far‐infrared spectroscopy

The vertical mixing ratio profiles of H2O, HDO, O3, heavy ozone, HF, HCl, HCN, and OH in the stratosphere are retrieved from high‐resolution far‐infrared emission spectra. These spectra were obtained with a balloon‐borne Fourier transform spectrometer in the spring of 1979 at 32°N. The nonlinear least squares fit method is used in the analysis. The results are obtained relative to the O2 mixing ratio, whose emission features can be found in the same spectra. The retrieval of OH together with other species should be particularly significant for the study of upper stratospheric photochemistry, although the errors are relatively large: 28% for OH and 16–19% for other species.

Role of sulfur atoms in microwave plasma etching of silicon

The Si etch rate in an (F2+O2) microwave plasma has been measured as a function of O2 mixing ratio at a fixed total pressure of 5.3×10−2 Pa. The etch rate significantly decreases with the mixing ratio. This etch rate decrease is due primarily to surface oxidation. When sulfur is added to the (F2+O2) plasma, the Si surface is much less oxidized and the etch rate increases by about a factor of 4. Such sulfur-containing species as S atoms react with O atoms or ions in the plasma and form O atom-containing species, such as SO2, SO+, SOF+, and SOF+2, thereby reducing the O atom and O+ ion concentrations in the plasma. As a result, the Si surface is scarcely oxidized, so that the etching reaction can easily proceed. Sulfur atoms inhibit surface oxidation and promote Si etching. Sulfur atoms contained in SF6, which is usually used in microwave plasma etching of Si, are expected to have the same role.

The evolution of the prebiotic atmosphere.

One-dimensional radiative convective and photochemical models are used to estimate the vertical temperature structure and composition of the earth's prebiotic atmosphere. Greatly enhanced CO2 levels (100-1000 times present) are required to keep the mean surface temperature above freezing in the face of decreased solar luminosity during the earth's early history. Such high CO2 partial pressures would have affected the atmospheric oxidation state by facilitating the photochemical production of soluble species including H2O2 and H2CO. Oxidation of ferrous iron in the oceans by H2O2 dissolved in rainwater should have kept the atmospheric H2 mixing ratio above 2x10(-4) and the ground-level O2 mixing ratio below 10(-11), regardless of the magnitude of the rate of volcanic release of reduced gases.

Effects of high CO2 levels on surface temperature and atmospheric oxidation state of the early Earth

One-dimensional radiative-convective and photochemical models are used to examine the effects of enhanced CO2 concentrations on the surface temperature of the early Earth and the composition of the prebiotic atmosphere. Carbon dioxide concentrations of the order of 100-1000 times the present level are required to compensate for an expected solar luminosity decrease of 25-30%, if CO2 and H2O were the only greenhouse gases present. The primitive stratosphere was cold and dry, with a maximum H2O volume mixing ratio of 10(-6). The atmospheric oxidation state was controlled by the balance between volcanic emission of reduced gases, photo-stimulated oxidation of dissolved Fe+2 in the oceans, escape of hydrogen to space, and rainout of H2O2 and H2CO. At high CO2 levels, production of hydrogen owing to rainout of H2O2 would have kept the H2 mixing ratio above 2x10(-4) and the ground-level O2 mixing ratio below 10(-11), even if no other sources of hydrogen were present. Increased solar UV fluxes could have led to small changes in the ground-level mixing ratios of both O2 and H2.

Atomic carbon in the atmosphere of Venus

The densities of atomic carbon in the Venusian thermosphere are computed for a model which includes both chemistry and transport. The maximum density of C is 2.8×107 cm−3 near 150 km for an assumed O2 mixing ratio of 1×10−4. Photoionization of atomic carbon is found to be the major source of C+ above 200 km, and resonance scattering of sunlight by atomic carbon may be the major source of the C I emissions at 1561 Å, 1657 Å, and 1931 Å. The computed C+ densities are found to be in substantial agreement with those measured by Pioneer Venus.

Measurements of the Venus lower atmosphere composition: A comparison of results

From the Pioneer Venus, as well as the Venera 11 and 12 spacecrafts, data on the chemical composition of the Venus atmosphere from about 700 km to the surface were obtained. In general, reasonable consistency can be found among the results from the seven gas analyzers that encountered Venus in December 1978. One of the more significant findings is the large excess abundance of primordial gases on Venus. The O2 abundances below 30 km must be very small, since none of the instruments detected O2 there. In and near the clouds the O2 mixing ratio may be as much as a few tens of parts per million. The CO mixing ratio is 20–30 ppm. The existence of COS, originally reported to be a major sulfur compound in the lowest part of the atmosphere, is doubtful there, and its mixing ratio at higher altitudes does not exceed a few parts per million. On the other hand, SO2 may exist below the clouds at a mixing ratio of the order of 100 ppm. In the clouds it is less than 10 ppm. A serious discrepancy in the data still exists for the case of the water vapor abundance. As a result of the cross‐checks provided by the number and types of instruments flown to Venus, confidence in the validity of these results on its atmospheric composition has progressed to a reasonable degree of certitude.

Photochemistry of SO2 in Venus' upper cloud layers

A model is presented for the distribution of photochemically active gases in and above the upper cloud layer of Venus. Attention is focused on reproducing the experimental results of the Pioneer Venus (PV) mission, especially the ultraviolet spectrometer results for the cloud top SO2 distribution. The reconciliation of these results with the photochemical oxidation and hydration of SO2 into sulfuric acid aerosol puts limits on the vertical mixing rates in this region, with a value of K around 2 × 104 cm²s−1 being preferred. A model with SO2 being the major sulfur bearing gas is consistent with the PV measurements. The measured SO2 is more than ample to produce the observed H2SO4 in the upper cloud region and balance the sulfur budget in this region. The ClOx chemistry above the clouds and the relatively large SO2 concentration in the cloud top region suppresses the odd hydrogen density and thus inhibits the catalyzed recombination of O2 with CO to form CO2. In conjunction with the slow eddy mixing, this leads to O2 mixing ratios at the cloud tops that are much larger than earth‐based measurements; however, the major features of our sulfur model are not dependent on the O2 abundance.

Oxygen levels in the prebiological atmosphere

A photochemical model of the early terrestrial atmosphere is used to find oxygen profiles as a function of the hydrogen and carbon monoxide outgassing rates from volcanoes and the rate of oxidation of the earth's crust. Steady state solutions for the amount of O2 in the atmosphere are possible only when the combined loss rate from all three processes can balance the production of oxygen from photodissociation of H2O, followed by escape of hydrogen to space. If limiting flux controls the escape rate, then the production rate of oxygen atoms depends linearly upon the water vapor mixing ratio at the tropopause. Evidence is presented which indicates that the hydrogen outgassing alone was sufficient to overwhelm the production of oxygen, leading to ground level O2 mixing ratios of the order of 10−12 times the present atmospheric level (PAL) or below. The O2 mixing ratio increases rapidly with altitude up to a peak of 10−5 PAL at 60 km owing to the presence of CO2. Present‐day concentrations of N2, CO2, and H2O are assumed in the calculation. The effect of possible temperature variations upon the H2O content is discussed.