@Beagle.
I don't know how neutral the assumptions the Environmental Association are, but from the Grauniad: According to the Environmental Transport Association, by the end of today the flight ban will have prevented the emission of some 2.8m tonnes of carbon dioxide since the first flights were grounded.
The volcanic eruption has released carbon dioxide, but the amount is dwarfed by the savings. Based on readings taken by scientists during the first phase of Eyjafjallajokull activity last month, the website Information is Beautiful calculated the volcano has emitted about 15,000 tonnes of CO2 each day.