The concentration of harmful ozone gas in the Pearl River Delta region is at a seven-year high, raising questions over the validity of a Hong Kong government report that suggested there was a general decline in pollution in the city. Annual results from the Pearl River Delta Regional Air Quality Monitoring Network, released on Thursday, recorded average ozone concentrations at 58 micrograms per cubic metre last year, the same as in 2017, and the worst since 2011. The network collates data from 23 monitoring stations in Hong Kong, Guangdong, and Macau, and is part of a joint effort by the governments of the three places to improve air quality in the region. While the concentration levels of air pollutants of sulphur dioxide (SO2) and respirable suspended particulates (PM10) had decreased by 81 per cent and 36 per cent respectively since 2006, the 2018 ozone concentration levels had gone up by 21 per cent over the same period. Ozone is formed through a reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the air and under sunlight. It is a main component of photochemical smog, and at elevated levels can cause respiratory diseases. The gas is the cause of the most days where there have been “serious” health risk warnings in air quality for Hong Kong, and it can soar when a typhoon looms. A Hong Kong Environment Bureau spokesman said the local and Guangdong governments had launched a study on emission reduction targets and concentration levels of regional air pollutants with a view to formulating long-term goals. Environmentalist Angus Wong Chun-yin, of World Green Organisation, said: “As ozone is a secondary pollutant, it would require efforts to lower the emissions of NOx and VOCs in order to control the concentration levels of ozone.” Wong urged closer cooperation between Hong Kong and Guangdong. Loong Tsz-wai, of the green group Clean Air Network, criticised the Hong Kong government for dragging its feet. “The Shenzhen government [in Guangdong province] has been imposing aggressive targets to cut emissions, like promoting the use of electric cars,” he said. The regional report coincided with a separate emission report published on Thursday by the Hong Kong government, which details the emissions by different sectors in 2017. The report attributed the increase in hill fires to the increase in emissions in some pollutants, including PM10, PM2.5, and carbon monoxide. There were 991 hill fires in 2017, up from 537 in 2016. Excluding hill fires, all pollutants’ emissions recorded a drop, most notably a 9.3 per cent drop in PM2.5 emissions, and an 8.8 per cent drop in PM10 emissions over the same period. According to the report, Hong Kong’s total emissions in 2017 were 16,160 tonnes of SO2, 84,960 tonnes of NOx, 4,020 tonnes of PM10, 3,120 tonnes of PM2.5, 25,520 tonnes of VOCs and 57,110 tonnes of CO. “Due to the ongoing control measures for power plants, vessels and motor vehicles, the emissions of SO2, NOx, PM10 and PM2.5 in 2017 were reduced by 7 to 9 per cent compared to the 2016 levels, and have been reduced by 34 to 80 per cent since 2001,” the department said. But Loong argued: “Presenting the emissions in weight may not give the public the full picture. Because it is the concentration levels that really matter.” As an example, Loong cited roadside pollution and added: “While the total emissions of NOx may have dropped, if the pollutant concentrates on areas where many people live or work, one can hardly say the air quality has improved.” He also noted the PM10 and PM2.5 emissions from power plants had showed a slight increase in 2017, and he saw no significant improvements until after next year, when coal-fired generation would be slashed from about half of the fuel mix to 20 per cent, to be replaced by relatively cleaner gas-fired power plants.