Articles | Volume 17, issue 20
Atmos. Chem. Phys., 17, 12475–12493, 2017
Atmos. Chem. Phys., 17, 12475–12493, 2017

Research article 19 Oct 2017

Research article | 19 Oct 2017

Sub-micrometer refractory carbonaceous particles in the polar stratosphere

Katharina Schütze1,2, James Charles Wilson3, Stephan Weinbruch1, Nathalie Benker1, Martin Ebert1, Gebhard Günther4, Ralf Weigel2, and Stephan Borrmann2,5 Katharina Schütze et al.
  • 1Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Darmstadt, Germany
  • 2Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Mainz, Germany
  • 3Department of Mechanical and Materials Engineering, University of Denver, Denver, CO 80208, USA
  • 4Institute for Energy and Climate Research (IEK-7), Research Center Jülich, Jülich, Germany
  • 5Partikelchemie, Max-Planck-Institut für Chemie, Mainz, Germany

Abstract. Eleven particle samples collected in the polar stratosphere during SOLVE (SAGE III Ozone loss and validation experiment) from January until March 2000 were characterized in detail by high-resolution transmission and scanning electron microscopy (TEM/SEM) combined with energy-dispersive X-ray microanalysis. A total of 4202 particles (TEM  =  3872; SEM  =  330) were analyzed from these samples, which were collected mostly inside the polar vortex in the altitude range between 17.3 and 19.9 km. Particles that were volatile in the microscope beams contained ammonium sulfates and hydrogen sulfates and dominated the samples. Some particles with diameters ranging from 20 to 830 nm were refractory in the electron beams. Carbonaceous particles containing additional elements to C and O comprised from 72 to 100 % of the refractory particles. The rest were internal mixtures of these materials with sulfates. The median number mixing ratio of the refractory particles, expressed in units of particles per milligram of air, was 1.1 (mg air)−1 and varied between 0.65 and 2.3 (mg air)−1.

Most of the refractory carbonaceous particles are completely amorphous, a few of the particles are partly ordered with a graphene sheet separation distance of 0.37 ± 0.06 nm (mean value ± standard deviation). Carbon and oxygen are the only detected major elements with an atomic O∕C ratio of 0.11 ± 0.07. Minor elements observed include Si, S, Fe, Cr and Ni with the following atomic ratios relative to C: Si∕C: 0.010 ± 0.011; S∕C: 0.0007 ± 0.0015; Fe∕C: 0.0052 ± 0.0074; Cr∕C: 0.0012 ± 0.0017; Ni∕C: 0.0006 ± 0.0011 (all mean values ± standard deviation).High-resolution element distribution images reveal that the minor elements are distributed within the carbonaceous matrix; i.e., heterogeneous inclusions are not observed. No difference in size, nanostructure and elemental composition was found between particles collected inside and outside the polar vortex.

Based on chemistry and nanostructure, aircraft exhaust, volcanic emissions and biomass burning can certainly be excluded as sources. The same is true for the less probable but globally important sources: wood burning, coal burning, diesel engines and ship emissions.

Recondensed organic matter and extraterrestrial particles, potentially originating from ablation and fragmentation, remain as possible sources of the refractory carbonaceous particles studied. However, additional work is required in order to identify the sources unequivocally.

Short summary
Stratospheric particles were collected in the polar stratosphere in winter 1999/2000. Besides the well-studied volatile particles from that region, the main findings of this study are stable carbonaceous particles in the sub-micrometer size range. In addition to carbon, many particles show the elements Si, Fe, Cr and Ni to a minor amount. Based on exclusion, carbonaceous material from IDPs and residues from meteoric ablation and fragmentation remain as the most probable sources.
Final-revised paper