Chemical analysis of the Asian Tropopause Aerosol Layer (ATAL) with emphasis on secondary aerosol particles using aircraft based in situ aerosol mass spectrometry

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 summer 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 km a.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 km and 18 km (corresponding to 360 K 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 and organics. The single particle analysis from laser desorption and ionizaton 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 analyses suggest that the organic matter within the ATAL and in the lower stratosphere (even above 420 K) can partly be identified as organosulfates, in particular glycolic acid sulfate, which are known as components indicative for secondary organic aerosol formation. Also, 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 ageing. 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 diabatic heating processes and thereby subject to extensive oxidative ageing. 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. As a consequence the chemical composition of these particles largely differs from the aerosol in the lower stratospheric background and the Junge layer.