I'm pretty sure that the important bit here was the quality of those particles, not their quantity.

The study was designed to detect aerosols covered with "meteor dust" left behind by space rocks that burned up upon entry. Instead, the plane detected high levels of metallic elements contaminating the floating molecules, none of which could be explained by meteors or other natural processes.

The discovery "represents the first time that stratospheric pollution has been unquestionably linked to reentry of space debris," researchers wrote in the statement.

In total, the study identified 20 different metallic elements that do not naturally occur in Earth's atmosphere, including silver, iron, lead, magnesium, titanium, beryllium, chromium, nickel and zinc.

The team suspects that the main source of the pollution is rocket boosters that are ejected by rockets shortly after they clear the upper atmosphere, then fall back to Earth.