THE air blowing from East Asia is about 10 times dirtier than that over the Pacific Ocean and poses a long-term threat in terms of climate change, a NASA mission has found. The United States space agency is in Hong Kong this week to conduct tests for a global study on climate change and is working with the Royal Observatory and the Hong Kong Polytechnic. Dr Robert McNeal, who is managing the study for NASA, said pollution levels soared as their specially equipped DC-8 plane approached Hong Kong from Guam. They hit a cold front blowing from China that contained levels of nitrogen oxides 12 times higher than over the Pacific, and carbon monoxide levels nine times higher, he said. These pollutants interfere with the atmosphere's ability to rid itself of greenhouse gases, which can lead to global warming. An earlier NASA visit, in 1991, found 20 per cent of nitrogen oxides over the Pacific were from industrial activity - despite being thousands of kilometres from any human settlement - and the level is feared to be rising. Dr McNeal said: ''We're seeing clear evidence of long-term, persistent pollution loads and we expect that to increase as human activity increases.'' The region's phenomenal economic and population growth rates were a problem in this respect, he said. ''Pollution is the result of people plus economic activity. The interest here [in Asia] is that both are growing at a significant rate,'' he said. The NASA plane has already visited California, Wake Island and Guam and will make two flights from Hong Kong, today and Sunday, to take further readings before flying on to Japan and Alaska. The flights are timed for when the outflow of air from the continent is at a peak, while the 1991 visit took place when the outflow was at a minimum. Data collected on these flights, together with data from the Atlantic Ocean and equator, will provide a baseline so scientists in future can measure how much the climate has changed. The only other measurements have been piecemeal, but significant. The depletion of the ozone hole over Antarctica and steadily increasing carbon dioxide levels at a Hawaii-based NASA station set up in the 1960s were clear evidence of human impact on the atmosphere, Dr McNeal said. In addition to the flights, ground-based stations are taking long-term measurements of background air quality. The Hong Kong Polytechnic has set up a station at Cape D'Aguilar to measure pollutants blowing in from the ocean and China, and the Royal Observatory is providing meteorological data. Other stations are in Taiwan, Lin An in China, Korea, Japan, Hawaii, Alaska and Midland Island. The polytechnic has pumped $8 million into its station which will keep it running for four years, but is seeking outside funding to improve facilities and extend the life of the project.