Continental and marine source regions contributing to the outflow of the Asian summer monsoon anticyclone during the PHILEAS campaign in summer 2023

Abstract. During the PHILEAS aircraft campaign, which probed the export of air from the Asian summer monsoon anticyclone (ASMA), observations were carried out from Germany and Alaska during August and September 2023. The flights from Germany sampled the western part of the ASMA and its western outflow, whereas filaments that had separated at its eastern flank and were transported across the Pacific were observed from Alaska. The aircraft measurements were combined with Lagrangian transport simulations. Our results show that air masses within the ASMA and its outflow are characterized by a mixture of different continental and marine sources. In the western part of the ASMA and its western outflow, enhanced pollutants and greenhouse gases such as CH₂Cl₂ , aerosol, CH₄ , and H₂O were detected, indicating sources in South Asia. In filaments separated at the ASMA’s eastern flank, additional marine air masses from the western Pacific, characterized by enhanced CH₂Br₂ and low O₃ , were found. Thus, air masses from two different surface regions and with different chemical compositions are mixed in filaments separated at the ASMA’s eastern flank. Our findings show that the chemical composition of the ASMA’s outflow is highly variable and depends on altitude, surface emissions, mixing of air masses from different source regions, and the interplay between the ASMA and tropical cyclones. The Asian summer monsoon is an important pathway for transporting short-lived ozone-depleting and radiatively active substances into the stratosphere.