Authors:	Appel, Oliver Köllner, Franziska Dragoneas, Antonis Hünig, Andreas Molleker, Sergej Schlager, Hans Maohnke, Christoph Weigel, Ralf Port, Max Schulz, Christiane Drewnick, Frank Vogel, Bärbel Stroh, Fred Borrmann, Stephan
Title:	Chemical analysis of the Asian tropopause aerosol layer (ATAL) with emphasis on secondary aerosol particles using aircraft-based in situ aerosol mass spectrometry
Online publication date:	13-Apr-2023
Year of first publication:	2022
Language:	english
Abstract:	Aircraft-borne in situ measurements of the chemical aerosol composition were conducted in the Asian tropopause aerosol layer (ATAL) over the Indian subcontinent in the summer of 2017, covering particle sizes below ~ 3 μm. We have implemented a recently developed aerosol mass spectrometer, which adopts the laser desorption technique as well as the thermal desorption method for quantitative bulk information (i.e., a modified Aerodyne AMS), aboard the high-altitude research aircraft M-55 Geophysica. The instrument was deployed in July and August 2017 during the StratoClim EU campaign (Stratospheric and upper tropospheric processes for better Climate predictions) over Nepal, India, Bangladesh, and the Bay of Bengal, covering altitudes up to 20 kma:s:l: For particles with diameters between 10 nm and ~3 μm, the vertical profiles of aerosol number densities from the eight research flights show significant enhancements in the altitude range of the ATAL. We observed enhancements in the mass concentrations of particulate nitrate, ammonium, and organics in a similar altitude range between approximately 13 and 18 km (corresponding to 360 and 410K potential temperature). By means of the two aerosol mass spectrometry techniques, we show that the particles in the ATAL mainly consist of ammonium nitrate (AN) and organics. The single-particle analysis from laser desorption and ionization mass spectrometry revealed that a significant particle fraction (up to 70% of all analyzed particles by number) within the ATAL results from the conversion of inorganic and organic gas-phase precursors, rather than from the uplift of primary particles from below. This can be inferred from the fact that the majority of the particles encountered in the ATAL consisted solely of secondary substances, namely an internal mixture of nitrate, ammonium, sulfate, and organic matter. These particles are externally mixed with particles containing primary components as well. The single-particle analysis suggests that the organic matter within the ATAL and in the lower tratosphere (even above 420 K) can partly be identified as organosulfates (OS), in particular glycolic acid sulfate, which are known as components indicative for secondary organic aerosol (SOA) formation. dditionally, the secondary particles are smaller in size compared to those containing primary components (mainly potassium, metals, and elemental carbon). The analysis of particulate organics with the thermal desorption method shows that the degree of oxidation for particles observed in the ATAL is consistent with expectations about secondary organics that were subject to photochemical processing and aging. We found that organic aerosol was less oxidized in lower regions of the ATAL (< 380 K) compared to higher altitudes (here 390–420 K). These results suggest that particles formed in the lower ATAL are uplifted by prevailing diabatic heating processes and thereby subject to extensive oxidative aging. Thus, our observations are consistent with the concept of precursor gases being emitted from regional ground sources, subjected to rapid convective uplift, and followed by secondary particle formation and growth in the upper troposphere within the confinement of the Asian monsoon anticyclone (AMA). As a consequence, the chemical composition of these particles largely differs from the aerosol in the lower stratospheric background and the Junge layer.
DDC:	624 Ingenieurbau und Umwelttechnik 624 Civil engineering 660 Technische Chemie 660 Chemical engineering
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-8834
Version:	Published version
Publication type:	Zeitschriftenaufsatz
Document type specification:	Scientific article
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Journal:	Atmospheric Chemistry and Physics 22
Pages or article number:	13607 13630
Publisher:	Copernicus GmbH EGU
Publisher place:	Katlenburg-Lindau
Issue date:	2022
ISSN:	1680-7375
Publisher DOI:	10.5194/acp-22-13607-2022
Appears in collections:	JGU-Publikationen