Time-varying constraints on atmospheric hydroxyl derived from methyl chloroform observations
Dr Stephen Montzka, National Oceanic and Atmospheric Administration, USA
Atmospheric measurements of CH3CCl3 have provided important constraints on hemispheric- to global-scale mean hydroxyl radical concentrations ([OH]) and their changes over time. Constraints on [OH] add to our understanding of changes in the global methane mole fraction because the main loss of atmospheric methane is by hydroxyl radical oxidation. Atmospheric emissions and mole fractions of CH3CCl3 have declined over the past 20 years by factors of 100 or more, thus affecting the accuracy and precision of derived global [OH]. Here this talk explores the time-dependencies of measurement precision, network limitations, and emission uncertainties from 1998 to 2020, and will assess their influence on estimates of [OH] variability and trends. The results, derived from analyses of surface and aircraft observations, suggest that the period from 1998 to 2015–2017 was optimal for constraining [OH] from the analysis of CH3CCl3 measurements because of the reduced influence of emissions. Subsequently, however, continued observations reveal a growing influence of poorly constrained emissions. Given the uncertain origin of these residual emission and limitations on independent methods for estimating emission magnitudes, constraints on [OH] as derived from CH3CCl3 observations have become much less certain since 2017.
Observational constraints on the global methane budget
Dr Xin Lan, National Oceanic and Atmospheric Administration, Global Monitoring Laboratory and University of Colorado Boulder, USA
Dr Ed Dlugokencky, National Oceanic and Atmospheric Administration, USA
Atmospheric CH4 is arguably the most interesting of the anthropogenically-influenced, long-lived greenhouse gases. It has a diverse suite of sources, each presenting its own challenges in quantifying emissions, and while its main sink, atmospheric oxidation initiated by reaction with hydroxyl radical (OH), is well-known, determining the magnitude and trend in this and other smaller sinks remains challenging. This talk provides an overview of the state of knowledge of the dynamic atmospheric CH4 budget of sources and sinks determined from measurements of CH4 and δ13CCH4 in air samples collected predominantly at background air sampling sites. While nearly 4 decades of direct measurements provide a strong foundation of understanding, large uncertainties in some aspects of the global CH4 budget still remain. More complete understanding of the global CH4 budget requires significantly more observations, not just of CH4 itself, but other parameters to better constrain key, but still uncertain, processes like wetlands and sinks.
Atmospheric δ13C(CH4): an honest assessment from the closed-toed shoes in the lab
Sylvia Englund Michel, INSTAAR, University of Colorado Boulder, USA
Since 2007 there has been a decrease in the stable carbon isotope delta value of atmospheric methane. Researchers at many laboratories, including the Stable Isotope Laboratory at INSTAAR, University of Colorado, have spent significant effort validating that this signal is real and not an artefact of sampling, calibrations, or the analytical process, which involves sample extraction, separation, combustion, and detection. Comparisons among laboratories of co-located samples have been valuable in validating trends, and also bringing attention to potential analytical problems within labs. Merging datasets allows complementary sampling networks to be used in modelling efforts, but also requires careful consideration of each laboratory’s ties to VPDB and uncertainty thereof. Through an ongoing round-robin comparison, offsets between labs have been assessed, and progress has been made toward unified methane-in-air reference materials. Despite the challenges of measurement, stable isotopes of atmospheric methane have proven to be a valuable tool in testing hypotheses about changing sources and sinks of methane, and further collaborations will continue to improve the global data set.
The isotopic insight
Dr Rebecca Fisher, Royal Holloway, University of London, UK
Records of the stable isotopic composition of atmospheric methane (δ13C and δ2H) can help identify changes in sources and sinks that have caused the continuing growth of the atmospheric methane burden. The unexpected upturn in the amount of methane in the air in 2007 was concurrent with a negative shift in δ13C, reversing the sustained positive trend of the preceding 200 years. Changes in methane mole fraction and its carbon isotopic composition have eliminated some hypotheses for the rise in methane, but global modelling is still too ill-constrained to fully solve the problem. There are currently few long-term records of δ2H, but a larger network of measurements of this isotope would help constrain global inversions.
The isotopic composition of the methane emitted from different source categories needs to be well known, taking into account regional variability in the isotopic signatures, for example latitudinal variability in wetlands. Detailed ground-based measurements are used to identify the signature of individual sources, and regional signatures of the bulk emissions from a source region can be identified using measurements from aircraft or fixed sites.
Using satellite measurements to interpret changes in atmospheric methane
Professor Paul Palmer, University of Edinburgh, UK
Surface observations have recorded large and incompletely understood changes to atmospheric methane this century. Their ability to reveal the responsible surface sources and sinks is limited by their geographical distribution, which is biased towards the northern midlatitudes. Data from Earth-orbiting satellites designed specifically to measure atmospheric methane have been available since 2009 with the launch of the Japanese Greenhouse gases Observing SATellite. This presentation will assess what we have learnt from these satellite data, with a particular focus on recent years that include some of the largest global atmospheric growth rates of methane since surface observations began in the 1980s.
Evaluation of the UK’s methane emissions using atmospheric data: current capability and future directions
Professor Matthew Rigby, University of Bristol, UK
Over the last three decades, the UK has reported a steady decline in methane emissions, with recent levels being around half of the 1990 value. Because of the availability of long-term observations in Ireland and the Netherlands since the 1980s, and the construction of a national monitoring network since 2012, it is possible to evaluate these inventory reports using atmospheric methane measurements. This 'top-down' evaluation has revealed that the reported emission decline of around 1%/yr over the last decade is consistent with atmospheric observations, but the reduction between 1990 and 2010 is not. Because of the relatively high density of observations in the last decade, it is becoming possible to discern which sources are responsible for the recent decline. In contrast, because the earlier 1990s and 2000s measurements are more remote, it is not possible to determine where the discrepancy with the inventory originates. To improve sector-level source evaluation, novel measurement and modelling approaches are being developed to discern methane emissions from the major source sectors using ethane and methane isotopologues. The role that these techniques could play in measuring our path to net-zero in the future will be discussed, both for the UK and elsewhere.