Sub-micrometer refractory carbonaceous particles in the polar stratosphere

Katharina Schütze, James Charles Wilson, Stephan Weinbruch, Nathalie Benker, Martin Ebert, Gebhard Günther, Ralf Weigel, Stephan Borrmann
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> 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<span class="thinspace"></span> = <span class="thinspace"></span>3872; SEM<span class="thinspace"></span> = <span
more » ... n class="thinspace"></span>330) were analyzed from these samples, which were collected mostly inside the polar vortex in the altitude range between 17.3 and 19.9<span class="thinspace"></span>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<span class="thinspace"></span>nm were refractory in the electron beams. Carbonaceous particles containing additional elements to C and O comprised from 72 to 100<span class="thinspace"></span>% 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<span class="thinspace"></span>(mg<span class="thinspace"></span>air)<sup>−1</sup> and varied between 0.65 and 2.3<span class="thinspace"></span>(mg<span class="thinspace"></span>air)<sup>−1</sup>. <br><br> 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<span class="thinspace"></span>±<span class="thinspace"></span>0.06<span class="thinspace"></span>nm (mean value<span class="thinspace"></span>±<span class="thinspace"></span>standard deviation). Carbon and oxygen are the only detected major elements with an atomic O∕C ratio of 0.11<span class="thinspace"></span>±<span class="thinspace"></span>0.07. Minor elements observed include Si, S, Fe, Cr and Ni with the following atomic ratios relative to C: Si∕C: 0.010<span class="thinspace"></span>±<span class="thinspace"></span>0.011; S∕C: 0.0007<span class="thinspace"></span>±<span class="thinspace"></span>0.0015; Fe∕C: 0.0052<span class="thinspace"></span>±<span class="thinspace"></span>0.0074; Cr∕C: 0.0012<span class="thinspace"></span>±<span class="thinspace"></span>0.0017; Ni∕C: 0.0006<span class="thinspace"></span>±<span class="thinspace"></span>0.0011 (all mean values<span class="thinspace"></span>±<span class="thinspace"></span>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. <br><br> 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. <br><br> 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.</p>
doi:10.5194/acp-17-12475-2017 fatcat:277smsrsvnhc5c3vwsultrktmu