Green groups want to narrow the differences in environmental standards between the SAR and the mainland. Mr Tung said co-operation with the Guangdong authorities would be strengthened through a new joint working group on sustainable development and environmental protection. Air pollution in Hong Kong is believed to be aggravated by cross-border trucks filling up with the mainland's lower standard diesel, which is high in sulphur dioxide content. Friends of the Earth assistant director Plato Yip Kwong-to said a common standard should be established to minimise cross-border pollution. 'Even if we can't have a common standard quickly, we should gradually narrow the gap,' Mr Yip said. But Green Power executive director Man Chi-sum said that the mainland would find it hard and too expensive to upgrade standards quickly. The green groups urged Hong Kong investors in the region to follow the SAR standard voluntarily. Eddy Li Sau-hang, president of the Hong Kong Economic and Trade Association, said a common standard was unnecessary. He said the mainland already had stringent environmental laws but there were problems with enforcement and it was constrained by limited resources. 'How could they raise their standards of diesel fuel if they don't have money to import cleaner fuel?' Mr Li asked.