"Some days we can definitely tell that the air has come from China," said Kim Holmen, research director of the Norwegian Polar Institute, at the station which has spectacular views over fjords, mountains and glaciers of Spitsbergen island.

The good news from a barrage of sensors is that many of the worst air pollutants, some of them linked to cancers or acid rain, have declined because of clean air laws in recent decades.

But greenhouse gases are surging and other pollutants are building up again even in a wilderness 1,200 km (750 miles) from the North Pole and 1,000 km from the nearest towns and factories in Russia and Norway.

A polluting haze that can blur the view in the Arctic springtime has thickened since around the late 1990s, perhaps because of more forest fires caused by climate change or rising pollution from Asia, led by China's boom, scientists say.

"The Arctic haze is increasing," said Lars Otto Reiersen, head of the Arctic Monitoring and Assessment Programme in Oslo. But the haze is still not as thick as in the 1980s.

Zeppelin, on a 474 metre (1,555 ft) high mountain ridge, is one of about a dozen stations in remote spots from Hawaii to Antarctica that dissect the atmosphere in a UN network. It is named after Count Ferdinand von Zeppelin, a German Arctic explorer better known for building airships.

DETECTIVE WORK

"The air is always mixing but you can do some detective work: the particles are slightly different in the United States, Russia, China, Europe or India," Holmen said.

Emissions from cars, for instance, have a different chemical signature according to national gasoline blends. Israel is alone in using a type of pesticide on its orange trees.

More ghoulishly, funeral pyres in some Asian countries release toxic mercury from fillings in the teeth of the deceased. If detected, the mercury means air did not come from Europe, North America or Japan where crematoriums have filters.

"Most of the particles we see come from Europe and Russia," Holmen said of measurements at the site, reached by a tiny cable car. "About 20 percent are from elsewhere."

Clambering up a ladder onto a snow-covered roof crowded with high-tech air-sniffing sensors, Holmen noted the clock to make sure scientists would disregard all measurements when people were outside and disrupt readings.

"When we are out here it has an immediate impact on carbon dioxide levels," he told visitors, as a chill wind blew from the Pole. People emit the gas when breathing.

A recent spike in some readings was explained after a scientist spotted the tracks of an Arctic fox in snow nearby.

One of the clearest trends at Zeppelin is a rise in greenhouse gases, at the highest in more than 650,000 years according to studies of air bubbles trapped in ancient ice.

Carbon dioxide levels reached about 390 parts per million this year against 270 ppm before the Industrial Revolution of the 18th century ushered in wide use of fossil fuels. Warming is widely expected to bring more heatwaves, floods and rising seas.

Most greenhouse gases come from Europe and North America but the rise is quickening, perhaps again pointing to growth in developing nations led by China. But greenhouse gases are invisible and the thickening of the Arctic haze is a puzzle.

FOREST BLAZES

"There was an improvement in the transparency of the Arctic atmosphere until 6-8 years ago and then it started to worsen again," Reiersen said.