Find in Library

<p>The <span class="inline-formula">¹³</span>C isotopic ratio of methane, <span class="inline-formula"><i>δ</i>¹³</span>C of <span class="inline-formula">CH₄</span>, provides additional constraints on the <span class="inline-formula">CH₄</span> budget to complement the constraints from <span class="inline-formula">CH₄</span> observations. The interpretation of <span class="inline-formula"><i>δ</i>¹³</span>C observations is complicated, however, by uncertainties in the methane sink. The reaction of <span class="inline-formula">CH₄</span> with Cl is highly fractionating, increasing the relative abundance of <span class="inline-formula">¹³CH₄</span>, but there is currently no consensus on the strength of the tropospheric Cl sink. Global model simulations of halogen chemistry differ strongly from one another in terms of both the magnitude of tropospheric Cl and its geographic distribution. This study explores the impact of the intermodel diversity in Cl fields on the simulated <span class="inline-formula"><i>δ</i>¹³</span>C of <span class="inline-formula">CH₄</span>. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the <span class="inline-formula"><i>δ</i>¹³</span>C of <span class="inline-formula">CH₄</span> to the diversity of Cl output from chemical transport models. We find that <span class="inline-formula"><i>δ</i>¹³</span>C is highly sensitive to both the amount and geographic distribution of Cl. Simulations with Cl providing 0.28&thinsp;% or 0.66&thinsp;% of the total <span class="inline-formula">CH₄</span> loss bracket the <span class="inline-formula"><i>δ</i>¹³</span>C observations for a fixed set of emissions. Thus, even when Cl provides only a small fraction of the total <span class="inline-formula">CH₄</span> loss and has a small impact on total <span class="inline-formula">CH₄</span>, it provides a strong lever on <span class="inline-formula"><i>δ</i>¹³</span>C. Consequently, it is possible to achieve a good representation of total <span class="inline-formula">CH₄</span> using widely different Cl concentrations, but the partitioning of the <span class="inline-formula">CH₄</span> loss between the OH and Cl reactions leads to strong differences in isotopic composition depending on which model's Cl field is used. Comparing multiple simulations, we find that altering the tropospheric Cl field leads to approximately a 0.5&thinsp;‰ increase in <span class="inline-formula"><i>δ</i>¹³</span><span class="inline-formula">CH₄</span> for each percent increase in how much <span class="inline-formula">CH₄</span> is oxidized by Cl. The geographic distribution and seasonal cycle of Cl also impacts the hemispheric gradient and seasonal cycle of <span class="inline-formula"><i>δ</i>¹³</span>C. The large effect of Cl on <span class="inline-formula"><i>δ</i>¹³</span>C compared to total <span class="inline-formula">CH₄</span> broadens the range of <span class="inline-formula">CH₄</span> source mixtures that can be reconciled with <span class="inline-formula"><i>δ</i>¹³</span>C observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of <span class="inline-formula">CH₄</span> sources from <span class="inline-formula"><i>δ</i>¹³</span>C observations.</p>