Air quality over the Pearl River Delta region improved last year as levels of three major pollutants dipped - but one worrying pattern was a worsening of the harmful ozone pollutant, an official report showed. The average annual concentration of sulphur dioxide, nitrogen dioxide and breathable suspended particulates recorded in 16 monitoring stations in the region over Hong Kong and Guangdong declined by 11 per cent, 8 per cent and 11 per cent respectively when compared with 2013. While this was good news, the report noted that the increase of ozone gas showed that photochemical smog pollution in the region had yet to improve. A green group and an expert on air pollution said while the report showed "positive signs", air quality in the region was still below the World Health Organisation's standard. The average concentration of ozone, formed by photochemical reaction of oxygen, nitrogen oxides and volatile organic compounds in the air under sunlight, was 57 micrograms per cubic metre last year, up 5.6 per cent from 2013. The level stayed unchanged in 2012 and 2013. "The ozone could have come from somewhere farther, and got blown to the Pearl River Delta region," said Professor Lau Kai-hon, of the University of Science and Technology's environment division. He added that weather conditions, such as light intensity, would also affect the ozone concentration. Ozone is a major component of photochemical smog that not only reduces visibility but also threatens human health when exposure is prolonged and intense. The head of Clean Air Network, Kwong Sum-yin, called on the governments in the region to find out the ozone source. But she conceded it would be difficult as ozone is a secondary pollutant. While both agreed it was a positive sign that the level of three major air pollutants had dropped, they said it was still not enough as the levels were still high when compared with the WHO standard. Professor Lau said: "It is a positive sign, but the government could not be complacent because of it." The Environmental Protection Department said Hong Kong and Guangdong would embark on a mid-term review and study the emission reduction targets for 2020, "so as to continue improving regional air quality, including the alleviation of the ozone problem". The Air Quality Monitoring Network began its operation in 2006 with only 16 monitoring stations in Hong Kong and Guangdong. The network was expanded last September to include Macau, bringing the total monitoring stations to 23. However, the department said the three months' data was not sufficient for annual trend analysis.