Impact of Asian pollution on the UTLS derived from in situ observations of a wide range of trace gases during the HALO PHILEAS mission in autumn 2023

Due to fast industrial growth and a high population density East Asia has become one of the most polluted regions on Earth. In combination with the world’s largest convective system, the Asian summer monsoon (ASM), East Asia now is the most significant source region of pollutants entering the upper troposphere and lower stratosphere (UTLS). Thus, understanding their transport pathways and the corresponding time scales of their transport and mixing into the UTLS as well as their impact on the UTLS’s sensitive chemical composition is of crucial importance for precise climate predictions but is not yet fully achieved.

To tackle these questions we use in situ measurements of our multi tracer instrument HAGAR-V (High Altitude Gas Analyzer – 5 channel version) during the German research aircraft HALO mission PHILEAS in August/September 2023. Flights from Germany and from Alaska targeted plumes and filaments of ASM air masses in the UTLS above the Mediterranean, the North Pacific, Alaska and Canada. HAGAR-V measured a suite of 30 trace gases including very short-lived NMHCs (e.g. Benzene, C₂H₂, C₄H₁₀), halogenated VOC (e.g. CH₂Cl₂, CHCl₃, C₂Cl₄, CH₂Br₂), as well as longer-lived halocarbons (e.g. CH₃Cl, CH₃Br, CCl₄, Halons, HCFCs, and HFCs) every 120 s using in-flight gas chromatography and mass spectrometry. Further long-lived species, including the age-of-air tracer SF₆, were measured every 40 s (F12, SF₆) and every 80 s (F11, F113, H1211) using electron capture detection.

Tracer-tracer relations of species with different source regions and/or atmospheric lifetimes provide insight on sampled air mass origin, mixing, and transport times from the source region to the location of measurement. As shown by Lauther et al. (ACP, 2022) CH₂Cl₂ is an ideal anthropogenic tracer to identify air masses originating from the ASM region. In the UTLS we find increases of CH₂Cl₂ by up to 500 % compared to tropospheric background correlating well with other species like SF₆, HCFC22, CHCl₃, C₂Cl₄, C₂H₂, Benzene, and C₂H₅Cl. The latter three species have tropospheric lifetimes of days to weeks implying that such correlations suggest fast transport from the ASM region to the UTLS. Furthermore, up to 1 ppt enhancement of SF₆ in air masses originating from the ASM region suggest a significant ASM-induced negative bias in mean age of air derived from SF₆.

Tracer-tracer relations of ASM-enhanced short-lived tracers (e.g. CH₂Cl₂, CHCl₃) with long-lived tracers (e.g. F12, N₂O) indicate isentropic mixing of polluted air masses into the stratospheric background. In addition, species with more diverse source regions like CH₃Br (rural anthropogenic, biomass burning and oceanic), CHCl₃ (industrial, soil, and oceanic), CH₂Br₂ (mainly oceanic), or C₂H₂ (anthropogenic combustion and biomass burning) yield several different correlation slopes against CH₂Cl₂. These relations provide an empirical tool that, along with simulated surface origin tracers of the CLaMS (Chemical Lagrangian Model of the Stratosphere) model, further helps to distinguish the origin of the sampled air masses.