Hekla
I can see a problem with a Hekla theory. When it erupted in 2000, it produced a significant decline in the ozone layer in the North, believed to have been produced by chemicals such as chlorine.
However, its large eruptions, pumping out enormous quantities of silicon dioxide, would have to propel this dust into the stratosphere for it to inhibit solar radiation from reaching the surface. This would be very different from many other large volcanic eruptions, like Mount Rinjani that produce vast amounts of sulfur dioxide.