A DFS air traffic authority spokeswoman said the agency, which opened Germany's airspace at 11 am Wednesday, had based its decisions on information provided by the German Weather Service (DWD).
The DWD in turn used data from Britain's Volcanic Ash Advisory Centres (VAAC) and computer simulations to estimate how the data applied to German air space, she said.
“Today, we see no contamination,” she said.
The DFS has consistently relied on the VAAC data and computer modelling in deciding whether to allow flights - an approach that has been criticised by airlines. Only on Monday did another agency, the German Aerospace Centre (DLR) gather raw data from German airspace to determine how much ash was actually in the air.
The DLR's report said the ash cloud was visible as a “brown layer” and measurements showed the air above Leipzig at about 4,000 metres altitude showed a contamination of 60 microgrammes of ash particles per square metre - a level comparable to dust from the Sahara Desert that sometimes blows into the German atmosphere.
The report stated the test plane suffered no damage, but did not offer an assessment of the danger of this level of contamination – and the actual threshold at which ash becomes dangerous remains unclear.
Airlines, particularly Germany's biggest carrier Lufthansa and second biggest, Air Berlin, spent days criticising the reliance on computer simulations, as tens of thousands of flights across Europe were cancelled and losses for airlines mounted.
Lufthansa spokesman Klaus Walther told Bild am Sonntag newspaper: “The flight ban, which is completely based on computer calculations, is causing economic damage in the billions. This is why, for the future, we demand that dependable measurements must be available before a flight ban is imposed.”
On Wednesday, a Lufthansa spokesman, Thomas Jachnow, dismissed suggestions the airline was engaging in a double standard by now accepting the DFS's ruling without complaint.
“Our decision depends on the decisions of the DFS. We followed the rules before and we're following them now,” he told The Local. “We were simply asking the question from the beginning whether one computer simulation was enough to say something about the amount of volcanic ash over Germany.”
Jachnow said test flights made by Lufthansa in partnership with the Max Planck Institute showed no damage to engines or equipment.
The results of the flight's contamination tests would be released in coming days, he added.
Air Berlin did not respond immediately to a request for comment on how the DFS justified the ban.
Meanwhile a report by the International Air Transport Association (IATA) said the several days of grounded European flights from volcanic ash had cost airlines $1.7 billion in lost sales alone.