I agree, if the vis is bad, it may be difficult to tell the difference between your average industrial gunk and God's clean Iclandic stuff.
Well, I've viewed the sky from ground level a lot of times and seen only stark blue. Then went up in the aircraft to be confronted with a massive viz problem due to an inversion at, say, FL50. So I'm not a believer in "what I can't see isn't there". And if the Met Office says it's unsafe to fly I tend to stay on the ground. (Easy decision since I didn't intend to fly today or tomorrow anyway.)
But on the other hand, I find the methods that are employed by the met observers rather crude. Heck, even in the very official SIGMET straight from the VAAC specialists at the Met Office they define vast areas of airspace where a bit of the cloud may have ended up, and then write this:
RMK: ASH CONCENTRATIONS WITHIN THE INDICATED AREAS ARE UNKNOWN
Come on folks, for cloud, visibility and the like we have all sorts of norms and standards. But shutting virtually all of the Northern Europe airspace down because of an unknown concentration of ash? That sounds a bit last century to me. Are they really not able to specify/measure something along the lines of a certain number of grams of ash per m3, and are engine manufacturers really not able to tell what concentration is harmful to engines/windshields/paintwork and what concentration is not?
If this eruption, just like last time, is going to last for two years we are going to have to come up with something a little more clever than this.