P. Bergamaschi

Three decades of global methane sources and sinks

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain.

Inverse modeling of global and regional CH4 emissions using SCIAMACHY satellite retrievals

Methane retrievals from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument onboard ENVISAT provide important information on atmospheric CH_4 sources, particularly in tropical regions which are poorly monitored by in situ surface observations. Recently, Frankenberg et al. (2008a, 2008b) reported a major revision of SCIAMACHY retrievals due to an update of spectroscopic parameters of water vapor and CH_4. Here, we analyze the impact of this revision on global and regional CH_4 emissions estimates in 2004, using the TM5-4DVAR inverse modeling system. Inversions based on the revised SCIAMACHY retrievals yield ∼20% lower tropical emissions compared to the previous retrievals. The new retrievals improve significantly the consistency between observed and assimilated column average mixing ratios and the agreement with independent validation data. Furthermore, the considerable latitudinal and seasonal bias correction of the previous SCIAMACHY retrievals, derived in the TM5-4DVAR system by simultaneously assimilating high-accuracy surface measurements, is reduced by a factor of ∼3. The inversions result in significant changes in the spatial patterns of emissions and their seasonality compared to the bottom-up inventories. Sensitivity tests were done to analyze the robustness of retrieved emissions, revealing some dependence on the applied a priori emission inventories and OH fields. Furthermore, we performed a detailed validation of simulated CH_4 mixing ratios using NOAA ship and aircraft profile samples, as well as stratospheric balloon samples, showing overall good agreement. We use the new SCIAMACHY retrievals for a regional analysis of CH_4 emissions from South America, Africa, and Asia, exploiting the zooming capability of the TM5 model. This allows a more detailed analysis of spatial emission patterns and better comparison with aircraft profiles and independent regional emission estimates available for South America. Large CH_4 emissions are attributed to various wetland regions in tropical South America and Africa, seasonally varying and opposite in phase with CH_4 emissions from biomass burning. India, China and South East Asia are characterized by pronounced emissions from rice paddies peaking in the third quarter of the year, in addition to further anthropogenic emissions throughout the year.

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

We extend the analysis of a global CH_4 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 CH_4 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 CH_4 distribution are consistent between SCIAMACHY observations and model simulations. However, the analysis suggests that SCIAMACHY CH_4 retrievals may have some bias that depends on latitude and season (up to ∼30 ppb). Large enhancements of column averaged CH_4 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.

Inverse modeling of the global CO cycle: 1. Inversion of CO mixing ratios

A three-dimensional modeling study on atmospheric carbon monoxide is presented, based on the TM2 model. A Bayesian inverse technique is applied to optimize the agreement between model and observational data, including a priori source information as regularization term. Using the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory data set for CO mixing ratios at 31 globally distributed sites, a posteriori CO budgets can be derived, which allow the model to reproduce the observations at most sites within two standard deviations of monthly mean values. Use of different spatiotemporal emission distributions for terpenes (Global Emissions Inventory Activity, ∼80% of emissions in the tropics; Hough [1991], ∼70% of emissions in the extratropical Northern Hemisphere) showed a large impact on calculated a posteriori source strengths and on the modeled partitioning among individual CO sources. In order to reproduce the interhemispheric gradient of observed CO mixing ratios, a ratio between total sources in the Northern Hemisphere and those in the Southern Hemisphere of ∼1.8 is required. While it is obvious that this asymmetry is mainly due to CO emissions from technological sources, the inversion results suggest that either (1) the global technological CO source strength is higher (∼800 Tg CO/yr) than present inventory based estimates or (2) CO from terpenes or vegetation (or additional sources with dominant emissions in the Northern Hemisphere) have a significant impact on the northern hemispheric mixing ratios. Further sensitivity studies showed that a posteriori results slightly depend on biomass burning seasonality (shifted by 1 month), but they are virtually identical for the two different OH fields (CH4-nonmethanehydrocarbons chemistry vs. CH4-Only chemistry). Inversion results, however, were sensitive to model wind fields used (based on meteorological observations of 1987 and 1986, respectively), mainly due to stations near source regions. Use of a reduced set of stations resulted in virtually identical a posteriori source strengths for both model wind fields. The analysis is extended in the companion paper which considers the additional information on the CO budget provided by measurements of the stable isotope ratios (13C/12C, 18O/16O).

Tropical methane emissions: A revised view from SCIAMACHY onboard ENVISAT

Methane retrievals from near-infrared spectra recorded by the SCIAMACHY instrument onboard ENVISAT hitherto suggested unexpectedly large tropical emissions. Even though recent studies confirm substantial tropical emissions, there were indications for an unresolved error in the satellite retrievals. Here we identify a retrieval error related to inaccuracies in water vapor spectroscopic parameters, causing a substantial overestimation of methane correlated with high water vapor abundances. We report on the overall implications of an update in water spectroscopy on methane retrievals with special focus on the tropics where the impact is largest. The new retrievals are applied in a four-dimensional variational (4D-VAR) data assimilation system to derive a first estimate of the impact on tropical CH_4 sources. Compared to inversions based on previous SCIAMACHY retrievals, annual tropical emission estimates are reduced from 260 to about 201 Tg CH_4 but still remain higher than previously anticipated.

Global column‐averaged methane mixing ratios from 2003 to 2009 as derived from SCIAMACHY: Trends and variability

After a decade of stable or slightly decreasing global methane concentrations, ground-based in situ data show that CH_4 began increasing again in 2007 and that this increase continued through 2009. So far, space-based retrievals sensitive to the lower troposphere in the time period under consideration have not been available. Here we report a long-term data set of column-averaged methane mixing ratios retrieved from spectra of the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) instrument onboard Envisat. The retrieval quality after 2005 was severely affected by degrading detector pixels within the methane 2ν_3 absorption band. We identified the most crucial problems in SCIAMACHY detector degradation and overcame the problem by applying a strict pixel mask as well as a new dark current characterization. Even though retrieval precision after the end of 2005 is invariably degraded, consistent methane retrievals from 2003 through 2009 are now possible. Regional time series in the Sahara, Australia, tropical Africa, South America, and Asia show the methane increase in 2007–2009, but we cannot yet draw a firm conclusion concerning the origin of the increase. Tropical Africa even seems to exhibit a negative anomaly in 2006, but an impact from changes in SCIAMACHY detector degradation cannot be excluded yet. Over Assakrem, Algeria, we observed strong similarities between SCIAMACHY measurements and ground-based data in deseasonalized time series. We further show long-term SCIAMACHY xCH_4 averages at high spatial resolution that provide further insight into methane variations on regional scales. The Red Basin in China exhibits, on average, the highest methane abundance worldwide, while other localized features such as the Sudd wetlands in southern Sudan can also be identified in SCIAMACHY xCH_4 averages.

Carbon 13 and D kinetic isotope effects in the reactions of CH4 with O(1 D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane

Measurements of the 13C and D kinetic isotope effects (KIE) in methane, 13CKIE = k(12CH4)/k(13CH4) and DKIE = k(12CH4)/k(12CH3D), in the reactions of these atmospherically important methane isotopomers with O(1D) and OH have been undertaken using mass spectrometry and tunable diode laser absorption spectroscopy to determine isotopic composition. For the carbon kinetic isotope effect in the reaction with the OH radical, 13CKIEOH = 1.0039 (±0.0004, 2σ) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (±0.0009, 2 σ). For DKIEOH we found 1.294 (± 0.018, 2σ) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O(1D) 13CKIEO(1D), was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by DKIEO(1D) = 1.06. Both values are approximately independent of temperature between 223 and 295 K. The room temperature fractionation effect 1000(KIE-1) in the reaction of O(1D) with 12CH4 versus CH4 is thus ≈ 13‰, which is an order of magnitude greater than the previous value of 1‰. In combination with recent results from our laboratory on 13CKIE and DKIE for the reaction of CH4 with Cl, these new measurements were used to simulate the effective kinetic isotope effect for the stratosphere with a two-dimensional, time dependent chemical transport model. The model results show reasonable agreement with field observations of the 13CH4/12CH4 ratio in the lowermost stratosphere, and also reproduce the observed CH3D/CH4 ratio.

High-precision direct measurements of 13 CH 4 / 12 CH 4 and 12 CH 3 D/ 12 CH 4 ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer

Measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane (CH(4)) sources provide important information about the global CH(4) budget as well as about CH(4) production and consumption processes occurring within the various sources. As an alternative to the conventional mass spectrometer (MS) technique, which requires conversion of CH(4) to CO(2) and H(2), we have developed a tunable diode laser absorption spectrometer (TDLAS), which permits rapid direct measurements of the (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios. An intercomparison between TDLAS and MS techniques for samples from natural wetlands, landfills, and natural gas sources resulted in a mean deviation of Δδ(13)C = 0.44‰ and ΔδD = 5.1‰. In the present system the minimum mixing ratios required are 50 parts in 10(6) by volume (ppmv) CH(4) (sample size 2 µmol CH(4)) for direct δ(13)C measurements and 2000 ppmv (sample size 80 µmol CH(4)) for direct δD measurements. These mixing-ratio limits are adequate for most CH(4) source characterization studies without requiring sample preconcentration.

Inverse modeling of European CH4 emissions 2001-2006

European CH4 emissions are estimated for the period 2001-2006 using a four-dimensional variational (4DVAR) inverse modeling system, based on the atmospheric zoom model TM5. Continuous observations are used from various European monitoring stations, complemented by European and global flask samples from the NOAA/ESRL network. The available observations mainly provide information on the emissions from northwest Europe (NWE), including the UK, Ireland, the BENELUX countries, France and Germany. The inverse modeling estimates for the total anthropogenic emissions from NWE are 21% higher compared to the EDGARv4.0 emission inventory and 40% higher than values reported to U.N. Framework Convention on Climate Change. Assuming overall uncertainties on the order of 30% for both bottom-up and top-down estimates, all three estimates can be still considered to be consistent with each other. However, the uncertainties in the uncertainty estimates prevent us from verifying (or falsifying) the bottom-up inventories in a strict sense. Sensitivity studies show some dependence of the derived spatial emission patterns on the set of atmospheric monitoring stations used, but the total emissions for the NWE countries appear to be relatively robust. While the standard inversions include a priori information on the spatial and temporal emission patterns from bottom-up inventories, a further sensitivity inversion without this a priori information results in very similar NWE country totals, demonstrating that the available observations provide significant constraints on the emissions from the NWE countries independent from bottom-up inventories.

Carbon kinetic isotope effect in the reaction of CH4 with Cl atoms

The carbon kinetic isotope effect in the reaction between Cl and CH4 (KIECl) has been measured using tunable diode laser absorption spectroscopy to determine 13CH4/12CH4 ratios. Cl atoms were generated by the irradition of Cl2 in static mixtures of Cl2/CH4/N2 or Cl2/CH4/N2/O2. Both methods resulted in a (KIECl) of 1.066±0.002 at 297 K. The KIECl displayed a slight temperature dependence, increasing to 1.075±0.005 at 223 K. This result suggests a significant influence of the title reaction on the stratospheric CH4 isotopic composition and may help to resolve discrepancies between measurements of stratospheric 13CH4/12CH4 profiles and laboratory measurements of KIEOH.