I have also seen speculation that it was a very large bolide strike. That would have blown ash far higher into the upper atmosphere to make it last for a decade. What do the Greenland ice cores show?? Tambora did have a major impact on US NE climate in 1816 and elsewhere in the world too. I had also wondered why the empire of Charlemagne did not continue after his death. Then I read in a book about nutrition that in the hundred years after his death in 814 there were 33 famines some lasting 2 and 3 years. Two causes were ergot fungus on rye in northern Europe, and another fungus on wheat to the south because Arabs had introduced the barberry plant which is the alternate host to something that kills wheat. Below is an article on all kinds of these plant killers.
http://www.botany.hawaii.edu/faculty/wong/BOT135/Lect08.htm