Meteoritic materials within sulfate aerosol particles in the troposphere are detected with transmission electron microscopy

“When meteorites impact the Earth’s atmosphere, they fractionalize and ablate, generating aerosol particles containing extraplanetary materials. Meteoritic materials have been identified among aerosol particles using elemental markers (iron and magnesium); however, their shapes and mixing states are largely unknown. Here we demonstrate the presence of meteoritic materials collected by a research aircraft from the troposphere over the western Pacific using transmission electron microscopy. The distribution of meteoritic elements within individual particles coincided with sulfur, indicating that they were in forms of sulfates, i.e., water-soluble. Enhanced number fractions of sulfate particles with meteoritic materials were observed during tropopause-folding events, suggesting that they originated from the stratosphere. We also estimated the potential contributions of the Chelyabinsk meteorite event, which occurred 5 months prior to the sampling and represents the largest meteorite event in the past century. This study provides unique observational evidence for the linkage between extraplanetary materials and tropospheric aerosols.”

The elemental markers of meteoritic materials used in the SPMS analyses6,17, mainly Fe and Mg, were detected in our collected particles (Fig. 1 and Supplementary Fig. 2). Based on the elemental compositions, we first classified aerosol particles into mineral dust, sea salt, sulfate, or carbonaceous particles using scanning TEM analysis combined with energy-dispersive X-ray spectroscopy (STEM-EDS) (Supplementary Fig. 3). The overall number fractions of sulfate, sea salt, carbonaceous, and dust particles were 85.7%, 10.9%, 2.5%, and 0.4%, respectively, and the distribution of particle types exhibited a dependence on altitude, particularly for sea salt particles (Fig. 2). Next, we subclassified the sulfate particles with detectable amounts of Fe and Mg (>0.3 wt%) as sulfate particles with meteoritic materials, which constituted a number fraction of 2.8% of the measured particles. Although sea salt and mineral dust particles can contain Fe and Mg, we considered only particles associated with sulfates and excluded those mixed with sea salt and mineral dust particles. Anthropogenic Fe-bearing particles can also be a source of Fe; however, their contribution to the sulfate particles with meteoritic materials is limited in the current samples because the current samples were collected from the air with a negligible anthropogenic influence, as we discuss later, and Mg is commonly absent in anthropogenic Fe-bearing particles18,19. Additionally, Fe and Mg concentrations in the sulfate particles were well correlated (R2 = 0.6 with a linear regression slope of 0.14) (Fig. 3) and had a Mg/Fe ratio similar to that of stratospheric aerosol particles containing meteoritic material (0.2–0.3)20. These results suggest that meteoritic materials are the primary source of Fe and Mg in these sulfate particles. The major proportion of sulfate particles with meteoritic materials had area-equivalent diameters ranging from 630 to 1000 nm. These sulfate particles were systematically larger than other sulfate-containing particles (Supplementary Fig. 4). Note that the original meteoritic materials could be much smaller than the hosting sulfate particles, as the former occupy only a small portion of the latter, which is apparent from the average wt% of Fe and Mg of 3.8 ± 3.1 within the sulfate particles.

Fig. 1: TEM images of sulfate particles with and without meteoritic material and the corresponding elemental maps.
figure 1

Sulfate particles containing meteoritic material have S, Mg, and Fe, as well as Al, Cr, and Ni (particle c). These sulfate particles are surrounded by satellite structures, indicating that they were less neutralized and had a low viscosity when collected. Although these particles are mixtures of various elements, they were defined as sulfate based on (1) the abundance of S, (2) the round shapes and crystal structures, and (3) the beam sensitivity, all of which are consistent with sulfate particle properties reported in previous studies5354. The TEM samples were collected on a 5:21 UTC, July 17, 2013, b 5:24 UTC, July 18, 2013, and c 3:53 UTC, July 7, 2013.

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