How the Asian Summer Monsoon Affects Aerosol Properties in the UTLS Region Across the Northern Hemisphere (Invited)

The Asian summer monsoon (ASM) circulation is characterized by fast convective transport of boundary layer emissions combined with an upper-level anticyclonic confinement of these air masses. In this way, the so-called Asian tropopause aerosol layer (ATAL) is formed, each year re-occurring from June to August, extending horizontally from the Mediterranean Sea (down to North Africa) to eastern China (down to Thailand) [1]. The ASM-trapped aerosol air can become distributed to higher latitudes, preferentially via east- and westward eddy shedding [2]. Motivated by the limited knowledge of the ATAL, the following three in-situ airborne missions were conducted in recent years: the StratoClim mission (Stratospheric and upper tropospheric processes for better climate predictions) in summer 2017 out of Kathmandu (Nepal), the Asian Summer Monsoon Chemical & CLimate Impact Project (ACCLIP) in summer 2022 from Osan (South Korea), and the PHILEAS mission (Probing High Latitude Export of air from the Asian Summer Monsoon) in summer/autumn 2023 from Oberpfaffenhofen (Germany) and Anchorage (Alaska). We thus had the unique opportunity to collect data over a broad geographical range - from the center of the anticyclone to the eastern Mediterranean Sea as well as in the western and northeastern Pacific region. The individual research aircraft were equipped with instrumentation to study aerosol formation processes and physicochemical characteristics of the ATAL. The presentation will focus on the main outcomes of the aforementioned missions.

We have evidence that the ATAL is mainly driven by upper-level secondary formation of aerosol containing ammonium nitrate, sulfate, and organic matter [3-5]. In contrast, the vertical transport of aerosol ingested into the convection from the Asian boundary layer, including black carbon and mineral dust, plays an exceedingly minor role for the presence of the ATAL [3, 6, 7]. We frequently observed new particle formation above the cloud convective tops and subsequent growth of these particles as they were being slowly lifted within the anticyclonic motion [8-12]. The availability of precursor gases that are relevant for particle formation can partly be explained by the vertical uplift of ammonia from agricultural emissions in South Asia and anthropogenic emission of SO₂ as well as NO­_X from lightning [4]. Along with this, observations during ACCLIP demonstrate the significance of East Asian summer monsoon convection as an effective transport pathway of precursors from East Asia to the upper troposphere [13]. We have evidence that the ATAL composition, in terms of ammonium, sulfate, and organic matter, shows variability along with the origin of precursor gases in South and East Asia [14,15]. During PHILEAS, we studied the long-distance transport and the fate of the specific aerosol originating from the ATAL. We found the Asian monsoon as an important source for ammonium nitrate and organic matter in the extratropical lower stratosphere (up to 400 K and 70 °N) in boreal summer and autumn [16]. Taken together, the ASM-influenced air affects aerosol number, size, and composition as well as aerosol acidity in the upper troposphere and lower stratosphere across the Northern hemisphere. With this knowledge, we should consider effects on aerosol radiative forcing, heterogenous chemistry, and ozone depletion.

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[2] S. B. Honomichl and L. L. Pan, Journal of Geophysical Research: Atmospheres, 125(13), (2020).

[3] O. Appel, F. Köllner, A. Dragoneas, et al., Atmos. Chem. Phys., 22(20), 13607-13630, (2022).

[4] M. Höpfner, J. Ungermann, S. Borrmann, et al., Nat. Geosci., 12, 608-612, (2019).

[5] D. Murphy and G. Schill: Measurements with PALMS-NG onboard the WB-57 during ACCLIP.

[6] M. Ebert, R. Weigel, S. Weinbruch, et al., Atmos. Chem. Phys., 24, 4771-4788, (2024).

[7] J. Schwarz: Measurements with SP2 onboard the WB-57 and the GV-HIAPER during ACCLIP.

[8] R. Weigel, C. Mahnke, M. Baumgartner, et al., Atmos. Chem. Phys., 21, 11689-11722, (2021).

[9] C. Mahnke, R. Weigel, F. Cairo, et al., Atmos. Chem. Phys., 21, 15259-15282, (2021).

[10] C. Williamson and C. Brock: Measurements with NMASS onboard the GV-HIAPER during ACCLIP.

[11] L. Ziemba and M. Brown: Measurements with PUTLS onboard the WB-57 during ACCLIP.

[12] M. Fujiwara: Ground-based lidar measurements from Fukuoka (Japan) during ACCLIP.

[13] L. L. Pan, E. Atlas, S. B. Honomichl, et al., P. Natl. Acad. Sci., 121 (12), (2024).

[14] O. Eppers, P. Brauner, F. Köllner, et al.: Measurements with ERICA onboard the GV-HIAPER during ACCLIP.

[15] W. Smith, R. Ueyama, and S. Honomichl: Trajectory-derived Convective Influence for ACCLIP Airborne In Situ Observations. Version 1.1. UCAR/NCAR - Earth Observing Laboratory. https://doi.org/10.26023/DP1P-C32K-YJ05, (2023).

[16] F. Köllner, F. Ekinci, O. Eppers, S. Johansson, J. Ungermann, M. Höpfner, et al.: Measurements with ERICA and GLORIA onboard the HALO during PHILEAS.