Transport of anthropogenic CH₂Cl₂-rich air from Asia across the North Pacific

Abstract. Dichloromethane (CH₂Cl₂) is emitted mainly anthropogenically and suspected to delay stratospheric ozone recovery. We present in detail the first retrievals of mesoscale 2D distributions of CH₂Cl₂ from infrared limb observations. The airborne remote sensing instrument GLORIA-AB (Gimballed Limb Observer for Radiance Imaging of the Atmosphere, hereinafter “GLORIA”) was deployed aboard the German research aircraft HALO during the PHILEAS campaign in August and September 2023 to study the outflow of the Asian Summer Monsoon (ASM). During long-range flights from Alaska, GLORIA observed filaments with elevated CH₂Cl₂ above the North Pacific, Canada and Alaska. Filaments with horizontal extents along the flight track of up to ~1000 km and CH₂Cl₂ volume mixing ratios of locally up to ~400 pptv were observed in the free troposphere, corresponding to levels more than six times higher than the northern hemispheric background for that season. Moderate CH₂Cl₂ enhancements were detected locally also in air masses with characteristics of the extra-tropical lowermost stratosphere. We applied new high-resolution CH₂Cl₂ spectroscopic data, resulting in ~11 % lower volume mixing ratios than in previous analyses using commonly available absorption cross sections. The GLORIA measurements are consistent with in situ observations on HALO by HAGAR-V and GhOST. Using back-trajectory analysis and surface-origin tracers by the CLaMS model, observed filaments are traced back to the ASM region and potential source regions are identified. Mesoscale filaments are shown to rapidly transport substantial amounts of CH₂Cl₂ from the ASM across the North Pacific into the Northern Hemisphere Upper Troposphere and Lower Stratosphere (UTLS).