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

Abstract

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.

Abundant Nitrate and Nitric Acid Aerosol in the Upper Troposphere and Lower Stratosphere

“…Simulated nitrate aerosols in ATAL are mostly composed of condensed HNO 3 particles, which can form near the cold tropopause and evaporate at warmer altitudes. Note that the measured variability of aerosol concentration in ASM on the flight‐to‐flight basis during the StratoClim field campaign is large because the dynamics of the anticyclone is highly variable (Appel et al., 2022). For example, the 25th and 75th percentiles of the nitrate concentration measured during the entire StratoClim field campaign are about 0.5–1.5 μg m −3 in the standard air for ATAL region and about 0.006–0.27 μg m −3 in the standard air between 10 and 14 km (Appel et al., 2022).…”

“…Aerosol compositions including nitrate, sulfate, organics and ammonium for particles with diameters between 0.11 and 3.5 μm were measured in real time by ERICA. Details of the instrument can be found in Hünig et al, 2022 andAppel et al (2022).…”

“…The infrared limb‐sounding indicates that these nitrate particles are in the solid phase over the ASM region, which can potentially induce heterogeneous ice nucleation in the upper troposphere (Wagner et al., 2021). The in situ aerosol mass spectrometry measurements from the STRATOspheric and upper tropospheric processes for better CLIMate predictions (StratoClim) field campaign found that the mass concentration of nitrate is similar to organics in the UTLS of the ASM (Appel et al., 2022). The recent Cosmics Leaving OUtdoor Droplets experiment shows that under low temperatures (below 258 K), ammonia and nitric acid vapor can nucleate to form AN aerosols in the multi‐acid solutions (HNO 3 ‐H 2 SO 4 ‐NH 3 ) with a nucleation rate which is orders of magnitude higher than the pure HNO 3 ‐NH 3 or H 2 SO 4 ‐NH 3 solutions (Wang et al., 2022).…”

“…Figure 1. (a) Vertical distribution of the aerosol compositions measured by the Europe Research Council Instrument for the Chemical Composition of Aerosols (ERICA) instrument during the StratoClim field campaign in August of 2017(Appel et al, 2022;Höpfner et al, 2019). Nitrate, organics and sulfate are denoted by the color-coded stacked regions.…”

Global Distribution of Asian, Middle Eastern, and Saharan Dust Simulated by CESM1/CARMA

“…In the meantime, a high occurrence of cirrus clouds is found by satellites (Sassen et al, 2008;Nazaryan et al, 2008), and dust particles might play an important role in cirrus formation in the ASM region. Recent airborne in-situ measurements suggest that the ASM tropopause aerosol layer is composed of mostly sulfate, organics, and nitrate (Hopfner et al, 2019;Appel et al, 2022). The budget of dust particles near the tropopause (~100 hPa) and at cirrus altitudes (e.g., 500-200 hPa) remains unquantified.…”