Answer: El Chichón was the first eruption to have its atmospheric effects.
The eruption was really small, but was studied by modern instruments, so it helped to understand better.
it released an unusually large volume of aerosols. Ash contained up to 2 weight percent sulfate. Anhydrite crystals were also in the deposits. Anhydrite is rare in volcanic rocks.
A good result appeared between gas flow and soil temperature. That may offer potential for monitoring in the future.