Green groups claim that ash cloud has beneficial side effects

The strange thing is that all the "green groups" here seem to be affiliated with universities.

Iceland's erupting volcano has spewed plenty of ash but far less greenhouse gas than Europe's grounded aircraft would have generated.
Carbon dioxide emissions totalled 150,000 tonnes a day in the early days of the eruption, according to Durham University. That compares with 510,000 tonnes per day emitted when planes are flying as normal over the continent.
But experts cautioned it was hard to draw conclusions about the overall impact of pollution because more cars and buses were on the roads to help stranded travellers and the volcano is emitting a nasty cocktail of toxins. . . .

Here are some problems with the discussion.

1) If people didn't take planes, how many of them took alternative ways to get to their destinations. Where extra trains added? Did people take cars? With the British navy being used to get people home, how much emissions were created by that? (The aircraft carrier below sure looks like it emits CO2.) Will extra planes eventually be added to help get some people home?
2) It is strange to me that people care so much about CO2 emissions. What about SO2 emissions produced by the volcano? Or the soot particles that environmentalists have been so upset about from diesel vehicles?