Stratospheric aerosols contain significant amounts of metals released as satellites burn-up during re-entry, mass spectrometry high above Alaska has shown. While this fraction is expected to increase dramatically in the coming decades, the impact of these additional metals on atmospheric chemistry is unknown.
Aerosols in the upper atmosphere primarily comprise sulfuric acid from the condensation of oxidized carbonyl sulfide or sulfur dioxide with water vapour. These aerosols sometimes nucleate on metallic nanoparticles, which can naturally form by meteor ablation. ‘The meteors burn up very high in the atmosphere – like 100km,’ explains atmospheric scientist Daniel Murphy of the National Oceanic and Atmospheric Administration in Boulder, Colorado. Metals like iron and magnesium are vaporised from the meteors, and as the atoms undergo Brownian motion and cool, they condense into nanoparticles less than 100nm in size.
In the new research, Murphy and colleagues studied aerosols 19km above Fairbanks, Alaska in February and March this year using a specially-designed mass spectrometer mounted in the nose of NASA’s WB-57 high-altitude aircraft. As air descends during winter in polar regions, it becomes especially useful for sampling stratospheric composition from even higher altitudes. ‘We detect when a particle comes by seeing a flash of light as it crosses a continuous laser beam,’ says Murphy. ‘Then we quick hit it with a pulse from an excimer laser and look at the ions that come off – so it’s a tiny fraction of a second between when the particle’s in the outside world and when we analyse it.’
About 50% of the sulfuric acid aerosols contained metal cores, and the researchers found clear evidence of contamination from spacecraft debris in some of these. Approximately 10% showed unexpectedly high aluminium levels, and some contained exotic elements. Particles that showed niobium and hafnium did so in a ratio that matched the C-103 alloy in some rocket extensions. The correlation between enrichments of lithium and aluminium could be explained by the increased using of Al–Li alloys in spacecraft bodies and of lithium-ion batteries. Most of the stratospheric copper, a metal rare in meteors, is already thought to come from spacecraft.
The effects of this are unknown, but it seems certain the contamination will increase. ‘People are anticipating a huge, huge increase in re-entry events because of this constellation of low earth-orbiting satellites,’ says Murphy. ‘There’s thousands of satellites being launched now, and tens of thousands being planned … so if it’s 10% now that contain these metals, it’s very likely to be a much higher percentage in the near future.’
‘We don’t know whether there are any ways these metals could change these sulfuric acid particles, and in some ways that’s the uncomfortable part of the story,’ he adds. One possibility is that the metals could potentially affect the ways in which aerosols form clouds. ‘I think people in the laboratory are going to start looking into how much you change the properties of a sulfuric acid particle when there’s a little bit of aluminum and copper in it.’
Atmospheric chemist Sergey Nizkorodov of University of California, Irvine in the United States, who was not involved in the work, is impressed. ‘People knew that metals are produced in the upper atmosphere by breaking up meteorites: they considered natural meteorites obviously, but this is the first study to the best of my knowledge that looks at the decomposition of man-made objects, and apparently there is enough of them to make a difference. I think it’s a really good [study],’ he says. ‘Aluminum is a pretty benign metal but there is also copper they mention and other transition metals that could potentially have interesting chemistry related to the ozone layer or formation of clouds … I think it should be investigated, for sure.’
D M Murphy et al, PNAS, 2023, DOI: 10.1073/pnas.2313374120