After three versions and much discussion, here’s the latest, corrected version of the answer.

Planes vs Volcano: Who's emitting the most CO2?

UPDATE 3 – 20th April – : More new info and some shame for us. According to leading geologists, Eyjafjallajoekull is emitting between “150,000 and 300,000” tons of CO2 a day (source). Despite the attentions of the Icelandic vulcanologists and detailed research, our calculations were apparently off by a factor to 10. Many apologies for this error. The volcano *is* belching huge gobs of CO2 into the atmos. Arguably, still less than the amount that would’ve been emitted by the grounded planes. We’ve corrected the diagram. Thanks to all the commenters who helped us refine and correct our calcs.

UPDATE 2 – 18th April –: We’ve been sent some new figures from the Nordic Volcanological Institute of the University of Iceland (thanks Nicole!). They’ve measured the CO2:SO2 ratio as 5:1 (source). So Eyjafjallajoekull is emitting an estimated 15,000 tons of CO2 a day – twice our original estimate. We’ve updated the diagram accordingly.

UPDATE 1 – April 17th – : A few people have asked so here’s how we estimated the CO2 emissions of Eyjafjallajoekull.

We couldn’t find a direct CO2 emissions figure for the icelandic volcano but we did find an emissions figure for Sulphur Dioxide (SO2) – 3,000 tons a day (Source)

When Mount Pinatubo in the Philippines erupted in 1991, it emitted 42 million tons of CO2 (source) and 17 million tons of SO2 (source). That’s a CO2-to-SO2 ratio of 2.47:1

Applying that ratio to the 3,000 tons of SO2 emitted by Eyjafjallajoekull gives us the figure of 7,412 tons of CO2 per day.

Clearly, this is a ballpark figure and it’s likely to be more complex. If any vulcanologists are passing by and would like to enhance or correct our numbers, please help yourselves to the data!