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We present an observing system simulation experiment examining the potential benefits of new methane isotopologues measurements for global- and national-scale source and sink inversions. New measurements are expected in the coming years, using quantum cascade laser spectroscopy with sample preconcentration, that will allow observations of delta C-13 - CH4 and delta D - CH4 at approximately hourly intervals and higher precision than previously possible. Using model-generated 'pseudo-data', we predict the variability that these new systems should encounter in the atmosphere, and estimate the additional uncertainty reduction that should result from their use in source and sink inversions. We find that much of the delta-value variability from seasonal to daily timescales should be resolvable at the target precision of the new observations. For global source estimation, we find additional uncertainty reductions of between 3-9 Tg/year for four major source categories (microbial, biomass burning, landfill and fossil fuel), compared to mole fraction-only inversions, if the higher end of the anticipated isotopologue-measurement precisions can be achieved. On national scales, we obtain average uncertainty reductions of similar to 10% of the source strength for countries close to high-frequency monitoring sites, although the degree of uncertainty reduction on such small scales varies significantly (from close to 0% to almost 50%) for different sources and countries.
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