Chloromethanes in the North American Troposphere and Lower Stratosphere Over the Past Two Decades

Aircraft observations of the four chloromethanes: carbon tetrachloride (CCl₄), methyl chloride (CH₃Cl), dichloromethane (CH₂Cl₂), and chloroform (CHCl₃), collected over North America between 2000 and 2022, were used to evaluate their vertical distributions and temporal trends in the atmosphere. We examine the vertical profiles, from the surface to the lower stratosphere (LS), of these increasingly important contributors to ozone-depleting chlorine in both altitude and potential temperature space. Airborne chloromethane trends were compared with those measured at long-term, ground-based monitoring stations. Below 20 km altitude, CCl₄ trends were decreasing at all levels studied in the North American atmosphere (−1.1 ppt yr⁻¹). CHCl₃ and CH₂Cl₂ airborne observations were comparable to ground network measurements: CHCl₃ increased between 2000 and 2018 and then decreased leading to a negligible trend over the 22 years studied and CH₂Cl₂ has been increasing at all levels in the troposphere (+2.41 ppt yr⁻¹, 2000–2022, <20 km).