Sitting in a Shenzhen traffic jam on a bad air day, surrounded by buses and vans belching smoke, it might seem simple to pinpoint the source of the smog through which buildings just a block away are barely visible. But, the experts say, it is not so simple. In fact, the suspended respirable particulates - scientific jargon for the tiny bits of soot and other matter that are a key ingredient of smog - you are inhaling may have been generated days or weeks earlier and many kilometres away. Amid the debate on the reasons for Hong Kong's deteriorating air quality there is one factor most people agree on: discovering the source is notoriously difficult. 'The problem is that air pollution has no boundary. It is just like we are all living inside a glass box and the pollutants are accumulating over our heads and eventually intermingling without possible traces of their origins,' said Ho Kin-chung, of the Open University's environment programme. According to another scientist, the Pearl River Estuary may be a vast pollution sink, where pollutants from around the delta gather before spreading across the region. According to Guangdong reports, pollutant concentrations rose between 13 and 17 per cent in the province last year, while sulfur dioxide emissions passed the 1 million tonne mark for the first time. By the mainland's relatively relaxed standards, the number of poor air-quality days increased last year from 35 to 41 in Guangzhou, two to 15 in Shenzhen and 17 to 34 in Foshan. The rise has coincided with its surging economic growth, which last year reported a 13.6 per cent rise in gross domestic product and 21 per cent in industrial production - the highest in eight years. The good news, however, is that if Guangdong ever gets its air quality act together, it will not have to contend with foul air drifting in from other parts of the mainland. Unlike other regions such as the rival Yangtze River Delta, where breakneck development has battered the environment , the province is hemmed in by mountains. These mountain ranges - which once inspired the old proverb, 'the mountains are high and the emperor is far away' - mean that, while its citizens choke on their own filth, they are probably not choking on anyone else's. That is little comfort to Hong Kong, which continues to experience severe air pollution despite reducing emissions by 48 per cent from 1992 to 2002, the most recent year covered by an air quality study in the delta region. The report showed that Guangdong's and Hong Kong's share of the emissions were roughly in line with population - 87 per cent to 13 per cent - but in area, Hong Kong, little more than a two-hundredths of Guangdong's size, spewed out much more. The report also gauged the emission levels of various key polluting sectors and found power generation was the main culprit, particularly in sulfur dioxide and nitrogen dioxide. However, critics said the report failed to explain how the bulk of pollutants from both sides contribute to poor regional air quality. One smog ingredient, ground-level ozone, is formed by a photochemical reaction between primary pollutants such as nitrogen dioxide and volatile organic compounds in strong sunlight. Scientists say this secondary pollution can move across the border after its formation or is formed locally from key constituent pollutants. 'The situation varies differently depending on weather conditions and the amount of pollutants in the air,' said Alexis Lau Kai-hon, assistant professor at the University of Science and Technology. Excess ozone was to blame when the air pollution index at Tung Chung hit a record 201 in the mid-afternoon on September 14. On that occasion the gaseous pollutant was believed to have drifted directly from the delta region. Professor Lau said the Pearl River estuary could be a giant sink where pollutants gathered over the centre of the river mouth, intermingling before spreading further. 'Unlike the usual perception that pollutants are directly blown from the mainland, the dynamics are three-dimensional rather than two-dimensional,' he said. Pollutants in the region including those from Hong Kong tended to rise vertically first and then drift into the river estuary where cooler temperatures might lead them to fall and gather. This was graphically illustrated on November 3 last year when pollutants, mainly airborne particles, were captured in a satellite image spreading from the estuary, causing one of Hong Kong's most extreme air pollution days. Edward Chan Yue-fai, of Greenpeace, said cross-boundary pollution was too often blamed for Hong Kong's air woes, with the city's contribution to it being shrugged off.