Planet Earth Online - Ash Fall

Kat -   In April 2010, a  previously dormant volcano in Iceland, whose name I shall not try and pronounce, released a massive ash cloud that closed most Europe's air space and disrupted millions of people's travel plans.  Planet Earth podcast presenter Sue Nelson was one of those people stranded abroad, so she welcomed the chance to meet Oxford University's Professor David Pyle for whom this cloud had an academic silver lining...

David -   I haven't previously worked on Iceland but in this case it was the opportunity of having the ash cloud move from Iceland across the UK and then out across northern Europe, it gave us a once in a lifetime opportunity.

Sue -   Let's hope it's once in a lifetime!

David -   Once in 50 years maybe - an opportunity to collect fine particles of ash that have travelled between 1000 and 2000 km from the point of emission and before it actually reaches the ground.  So we can use our observations to test our understanding of how we think these processes happen.

Sue -   How did you sample the ash then, did you simply come up to rooftops like where we are now and literally wet your finger and put your finger out?  Because while I was stuck in Portugal,  I was getting texts and emails from people saying 'my car's covered in ash'.

David -   We have a lab full of filters and pumps which we normally take into the field and in this case, the field area was Oxford and the best place to do it was the top of the department of Earth Sciences.

Sue -   And you used a sort of larger version of the pump that you've brought up to the roof with us now, which I've got to say is spectacularly unimpressive in that - well, it's a large battery pack about half the size of a bag of sugar, with a couple of big crocodile clips.  Otherwise it could be one of those paddles people use to give emergency cardiac arrest treatment.  So how does it work?

David -   This is an air-sampling pump - I'll just turn it off so you can hear.  We use this air-sampling pump to suck air at about 20 litres a minute through a series of filters and it's a way of collecting direct samples from the atmosphere.

Sue -   And do you have any of these particles still here in the laboratory?

David -   Yes we do.

Sue -   Well in that case I think we definitely ought to go and see them...

The lab contains filters, each with evenly spaced dots, showing a range of different size particles collected on the roof.  These continue to be part of ongoing studies into the properties of ash particles.  But David felt it was easier for me to observe the ash from a slightly more low-tech method of collection.

David -   These are actually just two pieces of sellotape which a colleague used to collect the volcanic ash from their car windscreen.

Sue -   Is that the large - well compared to what else is on that bit of sellotape - the large dark grain that almost looks like a black sugar or salt grain?

David -   No.  In fact those large grains will probably be bits of wood or something that's just carried around in the atmosphere and it's kind of the pale brown background material.

Sue -   Oh wow!  So the stuff that I just thought was general background fluff is actually all the ash.

David -   That's right, yes.  I don't think we've ever had an opportunity quite like this where we've had observations and samples collected from the volcano itself during the eruption, we've had satellite measurements of how the ash cloud then moves through the atmosphere from the volcano and then thousands of kilometres downwind.

Sue -   It's sort of an evolving experiment, isn't it?

David -   That's right, yes.  So we have a great opportunity to use the measurements in the laboratory to work out whether the ash particles landed as single grains or as clots of grains all caught together.

Sue -   Why do you need to know that?

David -   It's actually very important both for understanding the hazard to aircraft and also for understanding whether there might be any tiny effect in terms of human health.  Very fine particles, between 2.5 and 5 microns or so, are actually small enough to be inhaled into the lungs.  If those very fine particles have actually clumped together in the ash cloud, that clump of particles will behave as if it were a larger particle and it will then land on the ground after 1000km rather than after 5000km.  And once it's landed on the ground there's a chance that the fine particles will then separate and you'll have a higher than expected concentration of very fine particles at ground level.  And this is one of the things we want to investigate.

Kat -   That was Professor David Pyle from the University of Oxford talking to Sue Nelson about the ongoing investigation of ash particles collected from the troublesome Icelandic volcano eruption last year.