HONGKONG is dirtier than it was a year ago, according to the Government's latest air quality statistics. And the Environmental Protection Department suspects that the Castle Peak Power Station could be partly to blame. ''The sulphur dioxide and nitrogen dioxide levels have gone up since last June,'' said Principal Environmental Protection Officer Tsui Wing-sing. ''And when that happens we want to find out the reason and monitor it carefully.'' He said the levels of some pollutants were higher in the Kwai Chung area than elsewhere - which combines with other evidence to suggest the emissions might be coming from China Light and Power's Castle Peak Power Station. ''But we cannot know for sure without a thorough analysis of the data,'' Mr Tsui said. The statistics for June, released yesterday, show Hongkong's pollution levels have still remained within normal pollution level fluctuations. At high levels, sulphur dioxide can lead to respiratory disease, reduced lung functions and a higher mortality rate, Mr Tsui said. ''At the moment we do not believe it is causing extra health problems for the population - but we are certainly keeping an eye on it. ''We are moving to a point where power stations are required to lower the sulphur content in their fuel. ''It is going to happen very soon, and China Light and Power has already agreed to the principle of using cleaner fuels.'' The worst day for nitrogen dioxide pollution - which mainly derives from vehicle exhausts - was June 6. The analysis revealed that low wind speed and abundant sunshine had encouraged the oxidation of nitric oxide. Sulphur dioxide reached its peaks at Kwai Chung on June 1, 5, 7 and 22. Preliminary analysis indicated that a single large emission source - possibly the power station - together with prevailing wind conditions might have accounted for the first three peaks, with nearby industry accounting for the last.