Utilizing tropospheric CO isotope observations from a low-latitude Atlantic sampling network to constrain the oxidative chlorine sink

The isotopic composition of CO can be used to detect enhanced oxidation of methane by atomic chlorine due to the strong kinetic isotope effect related to this reaction (KIE_CH4+Cl = 66 per mil). Importantly, this detection method has demonstrated the presence of a large ground-level North Atlantic chlorine source for the years 1996-1997, linked to the geographic distribution of iron-rich Sahara dust within the marine boundary layer (Mak et al., 2003; van Herpen et al., 2023). Here, we present 2023-2024 d¹³C_CO and d¹⁸O_CO data from an air sampling network established across the low-latitude Atlantic Ocean, including bi-weekly measurements from Tenerife (IEO and IZO), Cape Verde (CVAO), Barbados (RPB), and northern Brazil (ATTO). In addition, the network includes intermittent flask samples taken aboard commercial shipping vessels as they complete trans-Atlantic routes. Our analysis supports the existence of a large chlorine sink of methane in dust-associated regions, which varies seasonally. Underestimates in the occurrence of chlorine oxidation propagate to isotope-constrained top-down global methane models, shifting predicted contributions away from fossil fuels and towards biological sources. Ultimately, our results provide an opportunity to reconcile missing chlorine sources, which may have significant implications for global methane source estimations.