The Ministry of Ecology and Environment has vowed consistent efforts will be made to ramp up ozone-related monitoring, as it strives to figure out the sources and generation mechanisms of the hazardous gas amid a grim control situation.

China started to monitor ozone in 2013. The average density of the gas from May to September in the past five years stood at 150 micrograms per cubic meter, Jiang Huohua, deputy director of the ministry's department of monitoring, said in a news conference on May 26.

Excessive concentrations of ozone were present about 11 percent of the time over these five months, but on most occasions only mild pollution occurred, he said.

Ozone is imperceptible, so it has no effect on visibility. However, the gas can be hazardous to people's health and plant growth when its concentration is high, he added.

"Ozone has become primary air pollutant in summer in China," he noted.

The ozone layer, which is about 20 kilometers above the ground, helps to shield the Earth from harmful ultraviolet radiation. Near the ground, however, even relatively low amounts can cause lung damage and result in shortness of breath.

High concentrations usually occur during hot periods. Sunlight and high temperatures can trigger chemical reactions between volatile organic compounds and nitrogen oxides that lead to the formation of ozone. Vehicle exhaust, for example, is one of the major contributors to the two types of substances.

Some key regions for air pollution control have experienced meteorological conditions that are favorable for the generation of ozone since March, Jiang said. Because of the conditions, the average density of ozone from January to April in the country's 339 major cities reached 127 mcg/cubic m, up by 8.5 percent from the same period last year.

The increase in the density in Chengdu, Sichuan province, Chongqing municipality and the city cluster in the middle reaches of the Yangtze River, even exceeded 20 percent, he continued.

"The situation for ozone pollution control is not optimistic," he said. "The ministry is confronted with heavy tasks."

He said the ministry has intensified its efforts for ozone monitoring. Aside from synergizing the monitoring of PM2.5 particulate matter and ozone, the ministry has ramped up the monitoring of volatile organic compounds and nitrogen oxides to figure out the changes in their densities, major sources and generation mechanisms, which can feed back into the control work.

Measures have been rolled out in all of the 339 major cities to learn about the total emissions of volatile organic compounds. To date, 244 of them have automated, internet-connected monitoring stations.

Monitoring work has also been ramped up in enterprises and industrial parks with high emissions. Stations have been set up at roads, ports, airports and railway stations as the ministry ratchets up monitoring of nitrogen oxides.

Insisting on a problem-oriented work approach, the ministry will shore up the weak spots by further expanding the monitoring network and beefing up the connectivity of monitoring data, he said.

He said the ministry will step up its endeavor to analyze monitoring data so that ozone pollution control can be carried out in a more precisely targeted manner.