CH4 Mixing Ratios Research Articles

Satellite chartography of atmospheric methane from SCIAMACHY on board ENVISAT: 2. Evaluation based on inverse model simulations

We extend the analysis of a global CH4 data set retrieved from SCIAMACHY (Frankenberg et al., 2006) by making a detailed comparison with inverse TM5 model simulations for 2003 that are optimized versus high accuracy CH4 surface measurements from the NOAA ESRL network. The comparison of column averaged mixing ratios over remote continental and oceanic regions shows that major features of the atmospheric CH4 distribution are consistent between SCIAMACHY observations and model simulations. However, the analysis suggests that SCIAMACHY CH4 retrievals may have some bias that depends on latitude and season (up to ∼30 ppb). Large enhancements of column averaged CH4 mixing ratios (∼50–100 ppb) are observed and modeled over India, Southeast Asia, and the tropical regions of South America, and Africa. We present a detailed comparison of observed spatial patterns and their seasonal evolution with TM5 1° × 1° zoom simulations over these regions. Application of a new wetland inventory leads to a significant improvement in the agreement between SCIAMACHY retrievals and model simulations over the Amazon basin during the first half of the year. Furthermore, we present an initial coupled inversion that simultaneously uses the surface and satellite observations and that allows the inverse system to compensate for the potential systematic bias. The results suggest significantly greater tropical emissions compared to either the a priori estimates or the inversion based on the surface measurements only. Emissions from rice paddies in India and Southeast Asia are relatively well constrained by the SCIAMACHY data and are slightly reduced by the inversion.

Effects of methane outgassing on the Black Sea atmosphere

Abstract. Methane in air and seawater was measured in the Eastern Black Sea during the 10–18 December 1999 BIGBLACK project cruise. The measurements allowed for the calculation of supersaturation ratios and methane fluxes across the air-sea interface. CH4 mixing ratios in air were generally in the 1.8–2.0 ppmv range, while surface (4 m depth) seawater concentrations varied from 4 to 93 ppmv. Above active seep areas, the water was supersaturated to around 500% with respect to the overlying atmosphere. Accordingly, flux densities varied greatly and were up to 3300 umol m−2 day−1. In the Sevastopol harbour, supersaturations up to around 3000%, similar to those at the Danube Delta, were observed, while in the Istanbul harbour supersaturations could not be determined because the very high values of water concentrations led to detector saturation. Simple modelling shows that the observed fluxes do not have any substantial impact on the methane content of the Black Sea atmosphere, as they would only raise its concentrations by less than 50 ppt. On the other hand, calculations performed as part of the CRIMEA project show that mud volcano eruptions could episodically raise the methane concentrations well above their regional background for several tens of kilometres downwind. These calculations, which also apply to mud volcano eruptions elsewhere on the globe, indicate that the spatial extend and the magnitude of the atmospheric perturbation is such that its observation might lie within the capabilities of existing satellite instrumentation such as SCIAMACHY on ENVISAT.

Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane

During the past 15 years the annual growth rate of tropospheric methane (CH4) has shown striking changes over 2–3 year periods, varying from +1% yr−1 to slightly negative values (−0.2% yr−1). These fluctuations are superimposed on an overall slowdown of the CH4 growth rate since the 1980s. Here we use our complementary measurement of other compounds (ethane, tetrachloroethene) to confirm the influence of biomass burning on large global CH4 pulses in 1998 and 2002–2003. Methane growth rate fluctuations also track ENSO indices, most likely via the influence of ENSO activity on large‐scale biomass burning. We also report the seventh year of near‐zero growth of global CH4 levels (Dec. 1998–Dec. 2005). The global CH4 mixing ratio was 1772 ± 1 ppbv in 2005, and CH4 increases of 118–376 ppbv between 2000–2020 (ten scenarios in the 2001 IPCC report, to levels around 1900+ ppbv by 2020, now appear quite unlikely.

Response of CH4 oxidation and methanotrophic diversity to NH4 + and CH4 mixing ratios

Methane oxidising activity and community structure of 11, specifically targeted, methanotrophic species have been examined in an arable soil. Soils were sampled from three different field plots, receiving no fertilisation (C), compost (G) and mineral fertiliser (M), respectively. Incubation experiments were carried out with and without pre-incubation at elevated CH4 mixing ratios (100 ml CH4 l−1) and with and without ammonium (100 mg N kg−1) pre-incubation. Four months after fertilisation, plots C, G and M did not show significant differences in physicochemical properties and CH4 oxidising activity. The total number of methanotrophs (determined as the sum the 11 specifically targeted methanotrophs) in the fresh soils was 17.0×106, 13.7×106 and 15.5×106 cells g−1 for treatment C, G and M, respectively. This corresponded to 0.11 to 0.32% of the total bacterial number. The CH4 oxidising activity increased 105-fold (20–26 mg CH4 g−1 h−1), the total number of methanotrophs doubled (28–76×106 cells g−1) and the methanotrophic diversity markedly increased in treatments with a pre-incubation at elevated CH4 concentrations. In all soils and treatments, type II methanotrophs (62–91%) outnumbered type I methanotrophs (9–38%). Methylocystis and Methylosinus species were always most abundant. After pre-incubation with ammonium, CH4 oxidation was completely inhibited; however, no change in the methanotrophic community structure could be detected.

Long‐term changes of methane and hydrogen in the stratosphere in the period 1978–2003 and their impact on the abundance of stratospheric water vapor

The long‐term changes of the stratospheric mixing ratio of CH4 over the period of 1978–2003 are derived from balloon‐borne data of H2, CH4 and N2O. The data were obtained by collecting whole air samples and subsequent gas chromatographic analyses. To eliminate the short‐term variability attributed to dynamical processes, the N2O mixing ratio is used as a proxy for altitude. A correlation analysis for the individual years is applied and the CH4 mixing ratios are interpolated to four different levels of N2O, corresponding to altitudes of approximately 17, 23, 26 and 30 km at midlatitudes. For the investigated period of 1978 to 2003 we find increases at the four levels of 207 ± 32 ppb, 159 ± 21 ppb, 140 ± 34 ppb and 111 ± 60 ppb, respectively. The CH4 trend has slowed down in recent years and is best fitted by a second‐order polynomial. The increase of CH4 can account for only 25–34% of the increase in stratospheric H2O of 1%/yr over the last decades as derived from previous studies. The simultaneously measured time series of stratospheric H2 mixing ratios shows that the contribution of stratospheric H2 to the H2O trend in the period 1988–2003 is minor.

Antisymmetric transport of middle stratospheric methane with respect to the equator

Halogen Occultation Experiment (HALOE) data are used to study the quasi-biennial variability of CH4 mixing ratio in the middle stratosphere. The results of EOF analysis indicate that quasi-biennial period is principal for the interannual variability of methane. Antisymmetry with respect to the equator is significant for the methane QBO by explaining 59.3% variance, while the symmetric component only explains about 30%. The antisymmetry is more significant in 10°–20° latitude than between 10°S and 10°N. Analyses of the vertical motions show that anomalies of annual cycle play an important role in the antisymmetric distributions. Whereas, the zonal wind QBO is still the most important dynamical effect for the interannual variability of CH4 between 10°S–10°N.

Simultaneous mass balance inverse modeling of methane and carbon monoxide

We perform a simultaneous mass‐balance inversion of atmospheric methane (CH4) and carbon monoxide (CO) using measurements from the NOAA/CMDL Cooperative Air Sampling Network and a model of tropospheric transport and background chemistry over the period 1990–2000. Our method has a spatial resolution of a semihemisphere and a temporal resolution of 1 month. The deduced CO sources show relatively low interannual variability except around the major biomass burning event in 1997–1998, when we calculate an anomalous emission between July 1997 and December 1998 of 270 Tg(CO). This is enough to suppress the modeled global air mass weighted hydroxyl radical (OH) concentration during this time by 2.2%, and account for 75% of the observed increase in CH4 mixing ratios during 1998. We compare our implied CH4/CO emissions factors with published biomass burning emissions factors, suggesting that the remainder of the increase in CH4 observed in 1998 is due to anomalously high biomass burning emissions, with CH4 emissions from wetlands showing a small negative anomaly in 1998.

The impact of anthropogenic chlorine emissions, stratospheric ozone change and chemical feedbacks on stratospheric water

Abstract. Mixing ratios of water (H2O) in the stratosphere appear to increase due to increased input of H2O and methane from the troposphere and due to intensified oxidation of CH4 in the stratosphere, but many of the underlying mechanisms are not yet understood. Here we identify and quantify three chemical mechanisms which must have led to more efficient oxidation of CH4 in the stratosphere over the past several decades: 1) The increase in stratospheric chlorine levels due to anthropogenic CFC emissions, 2) the thinning of the stratospheric ozone column and 3) enhanced OH levels in the stratosphere due to increasing H2O levels themselves. In combination with the increase in tropospheric CH4 mixing ratios and with solar cycle related variations of upper stratospheric ozone, these effects can explain about 50% of the additional conversion of CH4 to H2O as observed throughout the stratosphere. The relative contributions from the individual processes have varied over the past decades.

The effects of ammonia on the growth of large-scale patterned aligned carbon nanotubes using thermal chemical vapor deposition method

A systematic study on the effects of NH 3 on the growth of carbon nanotubes in atmospheric thermal-CVD was undertaken. Large-scale patterned aligned CNTs were prepared by thermal-CVD method in temperature of 900 °C. The effects of NH 3 on the growth stage were investigated. Patterned Ni-catalyst of thickness 10 nm was prepared by optical lithography and e-gun evaporation on p-Si(100) substrates lift-off process. Following a heating process, the CNTs were then grown at 900 °C by thermal-CVD method using argon (Ar), NH 3 and methane (CH 4) as the diluting, reactive and carbon source gases, respectively. In the experimental results, aligned CNTs could be synthesized with mixing ratio of CH 4/NH 3 larger or less than 1 varying the fraction of Ar dilution. The role of NH 3 was considered to enhance the catalyst activity by preventing it from being passivated. Furthermore, vertically aligned free-standing CNTs were observed at low CNT density when dealing the introduction of NH 3; meanwhile, a transition of growth characteristics from spaghetti-like to vertically-aligned was observed when the growth condition was switched from low NH 3 to high NH 3 ratio. Without Ar dilution, high NH 3 flow relative to CH 4 seemed to hamper the CNT growth as well as the degree of straightness and vertical alignment. A kinetics-based growth mechanism, either on carbon supply limited kinetics or on carbon diffusion limited kinetics, in combination with the proposed role of NH 3, can successfully explain the effect of NH 3 in different experimental conditions. Furthermore, the base growth mode was observed on all process conditions of thermal CVD in this work, and a model based on the effect of surface radius of curvature of catalyst particles on the surface energy based on broken bond model was adopted to explain the formation of bamboo-like structure and the correlation between the spacing of cone-shaped graphite and its straightness.

Large‐scale distribution of CH4 in the western North Pacific: Sources and transport from the Asian continent

Methane (CH4) mixing ratios in the northern Pacific Basin were sampled from two aircraft during the TRACE‐P mission (Transport and Chemical Evolution over the Pacific) from late February through early April 2001 using a tunable diode laser system. Described in more detail by Jacob et al. [2003], the mission was designed to characterize Asian outflow to the Pacific, determine its chemical evolution, and assess changes to the atmosphere resulting from the rapid industrialization and increased energy usage on the Asian continent. The high‐resolution, high‐precision data set of roughly 13,800 CH4 measurements ranged between 1602 ppbv in stratospherically influenced air and 2149 ppbv in highly polluted air. Overall, CH4 mixing ratios were highly correlated with a variety of other trace gases characteristic of a mix of anthropogenic industrial and combustion sources and were strikingly correlated with ethane (C2H6) in particular. Averages with latitude in the near‐surface (0–2 km) show that CH4 was elevated well above background levels north of 15°N close to the Asian continent. In the central and eastern Pacific, levels of CH4 were lower as continental inputs were mixed horizontally and vertically during transport. Overall, the correlation between CH4 and other hydrocarbons such as ethane (C2H6), ethyne (C2H2), and propane (C3H8) as well as the urban/industrial tracer perchloroethene (C2Cl4), suggests that for CH4 colocated sources such as landfills, wastewater treatment, and fossil fuel use associated with urban areas dominate regional inputs at this time. Comparisons between measurements made during TRACE‐P and those of PEM‐West B, flown during roughly the same time of year and under a similar meteorological setting 7 years earlier, suggest that although the TRACE‐P CH4 observations are higher, the changes are not significantly greater than the increases seen in background air over this time interval.

Carbon and hydrogen isotopic compositions of stratospheric methane: 1. High‐precision observations from the NASA ER‐2 aircraft

Measurements of δ13C and δD of atmospheric CH4 from whole air samples collected in the upper troposphere and lower stratosphere aboard the NASA ER‐2 aircraft during the SOLVE (2000), POLARIS (1997), and STRAT (1996) campaigns are reported. Samples cover latitudes from 1°S to 89°N and altitudes from 11 to 21 km, providing CH4 mixing ratios that range from 1744 to 716 ppbv. Measurements of isotope ratios were made by continuous‐flow gas chromatography isotope ratio mass spectrometry which provides high‐precision analyses on 60 ml aliquots of air. These measurements comprise the first upper atmosphere isotopic CH4 data set to date using this technique and the most extensive with respect to latitude and season in any case. Values of δ13C‐CH4 on the V‐PDB scale range from −47.28‰ near the tropical tropopause to −34.05‰ in the high northern latitude stratosphere. Values of δD on the V‐SMOW scale range from −90.9‰ to +26.4‰. Correlations of isotope ratios with CH4 mixing ratios show enrichment in the heavy isotopes as CH4 mixing ratios decrease due to kinetic isotope effects associated with oxidation by reaction with OH, Cl, and O(1D). Empirical fractionation factors are found to be highly dependent on the range of CH4 mixing ratio considered, increasing with decreasing mixing ratio. Systematic nonlinearity in a Rayleigh fractionation model suggests a range of stratospheric fractionation factors, αCstrat = 1.0108 ± 0.0004 to 1.0204 ± 0.0004 (2σ) and αHstrat = 1.115 ± 0.008 to 1.198 ± 0.008 (2σ), from high to low CH4 mixing ratio, respectively. The variation in α over the range in mixing ratios reflect changes in partitioning between CH4 sink reactions in different regions of the stratosphere. In Part 1, these new high‐precision observations are discussed and compared with other stratospheric and tropospheric isotope measurements. In Part 2 [McCarthy et al., 2003] the observations are compared with 2‐D model results, and implications for the kinetic isotope effects for reactions with OH, Cl, and O(1D) are discussed.

Six-year record of atmospheric carbon dioxide and methane at a high-altitude mountain site in Poland

Carbon dioxide and methane observations from the continental mountain station Kasprowy Wierch in the Tatra Mountains, southern Poland, are presented. They cover a six-year period from 1994 to 2000. Significant year-to-year variability of CO2 concentration was observed. The seasonal cycles 1996–1997 were similar, with a peak-to-peak amplitude of the selected and smoothed CO2 record of approximately 20 ppm and no significant increase of the annual mean values. For 1998 and 1999 large increases of the annual mean values by 3.3 and 4.0 ppm per year, respectively, were observed. This increase was accompanied by a reduction of the seasonal amplitude of the smoothed record to approximately 15 ppm in 1998 and 16 ppm in 1999. In 2000 the seasonal amplitude increased again to a value similar as in 1996/1997, whereas the mean annual value remained close to that recorded for 1999. Similar features can also be traced in the selected and smoothed CO2 record for Schauinsland station, Germany, located ca. 1000 km west of Kasprowy Wierch. These similarities strongly suggest that both stations are capturing the same large-scale European phenomena, most probably related to a disturbance of the CO2 cycle by the recent El Niño event. The mean CH4 mixing ratio at Kasprowy Wierch for the period 1996–1999 was about 30 ppb higher than over the Atlantic Ocean, confirming previous observations that the European continent is a net source of methane throughout the year. No significant seasonal cycle of methane has been observed at Kasprowy Wierch. The short-term changes of CO2 and CH4 are strongly correlated during winter months: the average monthly mean slope of the linear relationship between CH4 and CO2 was 10.7 ± 0.3 ppbCH4 per ppmCO2. During summer months this strong correlation breaks down. Diurnal changes of CO2 and CH4 mixing ratios observed at Kasprowy Wierch are typical for continental mountain sites, with a distinct minimum of CO2 during afternoon hours in summer and maximum during winter. For CH4, diurnal cycles have similar shape throughout the year, with a broad maximum during daytime. The mean peak-to-peak amplitudes for summer (July) were 4.5 ppm for CO2 and 30 ppb for CH4, whereas during winter (February) they diminished to 1.5 ppm and 10 ppb, respectively.

Seasonal variability of greenhouse gases in the lower troposphere above the eastern European taiga (Syktyvkar, Russia)

A three year long record of regular vertical aircraft profiling for continuous atmospheric CO2 mixing ratio measurements as well as for flask sampling to derive the climatology of other greenhouse gases (CH4, SF6 and N2O), is presented. Measurements were undertaken in the lower troposphere between 100 and 3000 m over the eastern European taiga about 100 km south east of the city of Syktyvkar(61°24′N, 52°18′E). From the continuous profiles mean CO2 mixing ratios were calculated for the atmospheric boundary layer (ABL) and for the “free troposphere” up to 3000 m. The amplitudes of the respective seasonal cycles are 22.1 ± 3.5 and 14.0 ± 2.1 ppm. ABL mixing ratios are generally larger than free tropospheric values during the winter period, and smaller during the summer due to the change of the continental biosphere from a source to a sink. The phasing of the seasonal cycles is slightly different between the two height intervals (by about 30 days), with the ABL extremes occurring earlier. Very abrupt concentration changes up to 8 ppm are observed in the free troposphere associated with changes in air mass origin. Mean CO2 mixing ratios derived from flask samples at 3000 m compare well with the respective integrated values measured in the continuous profiles above the ABL (ΔCO2 = 0.3 ± 1.6 ppm). CH4 mixing ratios also show a pronounced seasonality, and winter time vertical gradients correlate well with those of CO2. Similarly, SF6 vertical gradients are correlated with CO2 gradients possibly pointing to some anthropogenic origin of the boundary layer CO2 signal during winter. N2O and SF6 also show a slight seasonality with almost the same phasing. The main reasons for the seasonality of both gases are probably transport processes with a possible contribution from stratosphere/troposphere exchange.

Three years of trace gas observations over the EuroSiberian domain derived from aircraft sampling — a concerted action

A three-year trace gas climatology of CO2 and its stable isotopic ratios, as well as CH4, N2O and SF6, derived from regular vertical aircraft sampling over the Eurasian continent is presented. The four sampling sites range from about 1°E to 89°E in the latitude belt from 48°N to 62°N. The most prominent features of the CO2 observations are an increase of the seasonal cycle amplitudes of CO2 andδ13C–CO2 in the free troposphere (at 3000 m a.s.l.) by more than 60% from Western Europe to Western and Central Siberia. δ18O–CO2 shows an even larger increase of the seasonal cycle amplitude by a factor of two from Western Europe towards the Ural mountains, which decreases again towards the most eastern site, Zotino. These data reflect a strong influence of carbon exchange fluxes with the continental biosphere. In particular, during autumn and winter δ18O–CO2 shows a decrease by more than 0.5% from Orléans (Western Europe) to Syktyvkar (Ural mountains) and Zotino (West Siberia), mainly caused by soil respiration fluxes depleted inδ18O with respect to atmospheric CO2. CH4 mixing ratios in the free troposphere at 3000 m over Western Siberia are higher by about 20–30 ppb if compared to Western Europe. Wetland emissions seem to be particularly visible in July–September, with largest signals at Zotino in 1998. Annual mean CH4 mixing ratios decrease slightly from 1998 to 1999 at all Russian sites. In contrast to CO2 and CH4, which show significant vertical gradients between 2000 and 3000 m a.s.l., N2O mixing ratios are vertically very homogeneous and show no significant logitudinal gradient between the Ural mountains and Western Siberia, indicating insignificant emissions of this trace gas from boreal forest ecosystems in Western Siberia. The growth rate of N2O (1.2–1.3 ppb yr−1) and the seasonal amplitude (0.5–1.1 ppb) are similar at both aircraft sites, Syktyvkar and Zotino. For SF6 an annual increase of 5% is observed, together with a small seasonal cycle which is in phase with the N2O cycle, indicating that the seasonality of both trace gases are most probably caused by atmospheric transport processes with a possible contribution from stratosphere–troposphere exchange.

Seasonal variability of greenhouse gases in the lower troposphere above the eastern European taiga (Syktyvkar, Russia)

A three year long record of regular vertical aircraft profiling for continuous atmospheric CO2 mixing ratio measurements as well as for flask sampling to derive the climatology of other greenhouse gases (CH4, SF6 and N2O), is presented. Measurements were undertaken in the lower troposphere between 100 and 3000 m over the eastern European taiga about 100 km south east of the city of Syktyvkar(61°24′N, 52°18′E). From the continuous profiles mean CO2 mixing ratios were calculated for the atmospheric boundary layer (ABL) and for the “free troposphere” up to 3000 m. The amplitudes of the respective seasonal cycles are 22.1 ± 3.5 and 14.0 ± 2.1 ppm. ABL mixing ratios are generally larger than free tropospheric values during the winter period, and smaller during the summer due to the change of the continental biosphere from a source to a sink. The phasing of the seasonal cycles is slightly different between the two height intervals (by about 30 days), with the ABL extremes occurring earlier. Very abrupt concentration changes up to 8 ppm are observed in the free troposphere associated with changes in air mass origin. Mean CO2 mixing ratios derived from flask samples at 3000 m compare well with the respective integrated values measured in the continuous profiles above the ABL (ΔCO2 = 0.3 ± 1.6 ppm). CH4 mixing ratios also show a pronounced seasonality, and winter time vertical gradients correlate well with those of CO2. Similarly, SF6 vertical gradients are correlated with CO2 gradients possibly pointing to some anthropogenic origin of the boundary layer CO2 signal during winter. N2O and SF6 also show a slight seasonality with almost the same phasing. The main reasons for the seasonality of both gases are probably transport processes with a possible contribution from stratosphere/troposphere exchange.

Observation of methyl iodide clusters in gas phase by infrared cavity ring-down spectroscopy

Abstract Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the C–H stretching region by cavity ring-down spectroscopy. The dependence of the spectra on the mixing ratio of CH 3 I versus He and on the stagnation pressure has led to a tentative assignment of the absorption peaks to trimer up to pentamer, based on our previous study with matrix isolation technique (Chem. Phys. Lett. 343 (2001) 185). Ab initio calculations at the MP2 level for the trimer and tetramer have shown that two stable isomers exist for the tetramer whereas only one isomer is found to be stable for the trimer. The tentative assignment of the observed spectra has been in qualitative agreement with the results of the calculations. The structure of each isomer and its photochemical relevance are discussed.

Analysis and presentation of in situ atmospheric methane measurements from Cape Ochi‐ishi and Hateruma Island

In situ measurements of atmospheric methane (CH4) from the monitoring stations at Cape Ochi‐ishi (latitude 43°10′N, longitude 145°30′E) during the period from July 1995 to July 2000 and at Hateruma Island (latitude 24°03′N, longitude 123°48′E) during the period from January 1996 to January 2000 are presented. At each station a fully automated gas chromatograph equipped with a flame ionization detector measures the CH4 mixing ratios at a frequency of more than 82 air samples per day. Over the above observation periods, average growth rates of CH4 mixing ratios were 4.5 parts per billion (ppb) yr−1 for Ochi‐ishi and 4.7 ppb yr−1 for Hateruma, but there were considerable fluctuations in the instantaneous growth rate. The peak‐to‐peak amplitudes in the seasonal cycles determined from the smooth curve fits were 71 ppb for Ochi‐ishi and 94 ppb for Hateruma. These seasonal amplitudes are larger than those at the sampling network sites of the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory. Analysis of the back trajectories indicated that the air masses at both sites are transported from continental Asia in winter and from the Pacific Ocean in summer. Because there are strong CH4 sources in continental Asia but no strong sources in the Pacific Ocean, the seasonality of airflow patterns can explain the enhancement of the seasonal cycles. The significantly large seasonality, especially at Hateruma, is a result of air masses originating from a wide range of latitudes, the clear seasonality of airflows, and the steep latitudinal gradient of the CH4 mixing ratio.

Transects of atmospheric CO, CH4, and their isotopic composition across the Pacific: Shipboard measurements and validation of inverse models

Measurements of carbon monoxide, methane, and their isotopic composition are presented for samples collected on six ship voyages across the Pacific between New Zealand and the United States between 1996 and 1998. The data cover a latitude range from approximately 40°S to 40°N and clearly define significant latitudinal gradients in mixing ratios and isotopic composition and their seasonal variations. Observational data are compared with recent three‐dimensional inverse modeling studies of the global CO and CH4 distributions (Bergamaschi et al., 2000a, b, c) constrained by CO and CH4 mixing ratio data from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory network and additional isotope measurements from a set of five globally distributed stations. Thus the shipboard measurements represent a unique opportunity for independent validation of the inverse models. In general, we see a very good agreement between measurements and model results, both for CO and CH4 mixing ratios and their stable isotopic composition (δ13CO, δC18O, δ13CH4). This is in particular true for the simulation of mean latitudinal gradients of these quantities. For some voyages, however, significant deviations in the “fine structure” of the latitudinal profiles are apparent and are attributed to synoptic scale variations that are not captured by the model (for which a standard meteorology is applied). In most cases, deviations between observations and model results exhibit similar patterns for CH4 and CO, and the deviations in mixing ratios are linked with corresponding deviations in the isotopic signature. Furthermore, a clear correlation between CO and ethane mixing ratios is visible. Observations of 14CO show a general minimum around the equator and cycles of opposite phase in each hemisphere. Virtually identical 14CO minima were observed in both hemispheres in 1997, indicating similar OH levels. We also show model results for δCH3D and the mass‐independent isotope fraction in C17O (despite the absence of observational data for the ship voyages) in our discussion to illustrate the potential of these isotopic signatures for the understanding of the global cycles of CH4 and CO, respectively.

Effects of O2 and CH4 on presence and activity of the indigenous methanotrophic community in rice field soil.

The activity and distribution of methanotrophs in soil depend on the availability of CH4 and O2. Therefore, we investigated the activity and structure of the methanotrophic community in rice field soil under four factorial combinations of high and low CH4 and O2 concentrations. The methanotrophic population structure was resolved by denaturant gradient gel electrophoresis (DGGE) with different PCR primer sets targeting the 16S rRNA gene, and two functional genes coding for key enzymes in methanotrophs, i.e. the particulate methane monooxygenase (pmoA) and the methanol dehydrogenase (mxaF). Changes in the biomass of type I and II methanotrophic bacteria in the rice soil were determined by analysis of phospholipid-ester-linked fatty acid (PLFA) biomarkers. The relative contribution of type I and II methanotrophs to the measured methane oxidation activity was determined by labelling of soil samples with 14CH4 followed by analysis of [14C]-PLFAs. CH4 oxidation was repressed by high O2 (20.5%), and enhanced by low O2 (1%). Depending on the CH4 and O2 mixing ratios, different methanotrophic communities developed with a higher diversity at low than at high CH4 concentration as revealed by PCR-DGGE. However, a prevalence of type I or II populations was not detected. The [14C]-PLFA fingerprints, on the other hand, revealed that CH4 oxidation activity was dominated by type I methanotrophs in incubations with low CH4 mixing ratios (1000 p.p.m.v.) and during initiation of CH4 consumption regardless of O2 or CH4 mixing ratio. At high methane mixing ratios (10 000 p.p.m.v.), type I and II methanotrophs contributed equally to the measured CH4 metabolism. Collectively, type I methanotrophs responded fast and with pronounced shifts in population structure and dominated the activity under all four gas mixtures. Type II methanotrophs, on the other hand, although apparently more abundant, always present and showing a largely stable population structure, became active later and contributed to CH4 oxidation activity mainly under high CH4 mixing ratios.

N2O concentration and flux measurements and complete isotopic analysis by FTIR spectroscopy

We report the development and application of analytical techniques for atmospheric N2O based on Fourier transform infrared (FTIR) spectroscopy. Using mobile low resolution (1 cm−1) FTIR spectroscopy in the field, the technique delivers mixing ratio measurements of precision ±0.3 ppbv (0.3 nmol mol−1) N2O and in situ soil–atmosphere flux chamber measurements of fluxes less than 1 ngN m−2 s−1 (0.04 nmol N2O m−2 s−1) with a time resolution of 30 min. The method offers the additional advantages of being simultaneously able to measure CO2, CH4 and CO mixing ratios in air to high precision (±0.15 μmol mol−1, ±1 nmol mol−1, ±0.3 nmol mol−1, respectively). By a similar analysis procedure, but with laboratory-based high resolution (0.012 cm−1) FTIR spectroscopy, the N2O isotope ratios δ15N, δ18O and δ17O are determined simultaneously for a single sample, with current precision of ±1.0‰,±2.5‰ and ±4.4‰, respectively. FTIR also resolves the individual contributions of the 15N14N16O and 14N15N16O to overall δ15N. The resolution of these two isotopomers is not possible using conventional isotope ratio mass spectrometry (IRMS). We present laboratory results demonstrating precision, and N-positionally resolved δ15N and δ18O measurements of UV-photolysed N2O in which a distinct asymmetric 15N positional effect is observed.