Chemical analysis of the Asian tropopause aerosol layer (ATAL) with emphasis on secondary aerosol particles using aircraft-based in situ aerosol mass spectrometry
Authors: Appel Oliver , Köllner Franziska , Dragoneas Antonis , Hünig Andreas , Molleker Sergej , Schlager Hans , Mahnke Christoph , Weigel Ralf , Port Max , Schulz Christiane , Drewnick Frank , Vogel Bärbel , Stroh Fred , Borrmann Stephan 
Published: October 2022

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 410 K 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 stratosphere (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. Additionally, 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.