Eruptions of the Yellowstone volcanic system have included the two largest volcanic eruptions in North America in the past few million years; the third largest was at Long Valley in California and produced the Bishop ash bed.
On the diesel Passat, the TDI badge stands for Turbocharged Direct Injection, which isn't really all that helpful as the gasoline-powered Passat is also turbocharged and also uses direct injection. Volkswagen attempts to clear this up by calling this Passat the Passat TDI Clean Diesel.
The Audi Q7 TDI has about a 6 gallon (23 liter) adblue tank capacity which should last over 10,000 miles. The Q7 body is longer than the Touareg so it has 2 tanks: a 15.5 liter passive reservoir under the car and a 7 liter active tank next to the diesel fuel filler. Here's an interesting cutaway of the Q7 system.