Characterization of refractory aerosol particles collected in the tropical UTLS within the Asian Tropopause Aerosol Layer (ATAL)

Abstract

Abstract. Aerosol particles with diameters larger than 40 nm were collected during the flight campaign StratoClim2017 within the Asian Tropopause Aerosol Layer (ATAL) of the 2017 Monsoon Anticyclone above the Indian subcontinent. A multi-impactor system was installed on board of the aircraft M-55 Geophysica, which was operated from Kathmandu, Nepal. The size and chemical composition of more than 5000 refractory particles/inclusions of 17 selected particle samples from 7 different flights were analyzed by use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray microanalysis (EDX). Based on chemical composition and morphology, the refractory particles were assigned to the particle groups: extraterrestrial, silicates, Fe-rich, Al-rich, Hg-rich, other metals, C-rich, soot, Cl-rich, and Ca-rich. Most abundant particle groups within the refractory particles are silicates and C-rich (nonvolatile organics). In samples taken above the tropopause extraterrestrial particles are becoming increasingly important with rising altitude. The most frequent particle sources for the small (maximum in size distribution DP-max = 120 nm) refractory particles carried into the ATAL are combustion processes at ground (burning of fossil fuels / biomass burning) and the agitation of soil material. The refractory particles in the ATAL represent only a very small fraction (< 2 % by number for particles > 40 nm) of the total aerosol particles which are dominated by species like ammonium, sulfate, nitrate, and volatile organics. During one flight additionally a large number of very small (DP-max = 25 nm) cinnabar particles (HgS) were detected. These particles are most likely generated directly on ground by coal combustion in Northeastern India or Southern China. These findings show that coal burning is an important source for the entry of refractory particles and in particular mercury into the ATAL respectively in the upper troposphere/ lower stratosphere (UTLS) region.