The new standards set the ceiling for the maximum allowable eight-hour average ozone level at 160 ppb, or parts per billion, for residential, commercial, cultural, industrial and rural areas and 100 ppb for areas such as nature reserves and scenic areas. The maximum permissible one-hour average ozone level for these two groups of areas is 200 ppb and 160 ppb, respectively.

Some cities, including Beijing, began to publish real-time PM2.5 readings to the public last year. However, most cities have not yet published real-time readings of their ozone levels.

Beijing began publishing real-time PM 2.5 levels in January 2012, later adding readings of ozone and carbon monoxide for the first time in May of last year on a trial basis. Real-time readings of these pollutants are accessible to the public on the website of Beijing Municipal Environmental Monitoring Center.

Ozone is not emitted directly into the air, but is instead created through chemical reactions between other pollutants such as nitrogen oxides and volatile organic compounds, and these reactions can take place in the presence of strong sunlight and high temperature.

Emissions of these two pollutants tend to mainly come from industrial facilities and electric utilities as well as exhaust fumes from motor vehicles, gasoline vapor and chemical solvents.

Although its presence is less apparent, ozone is more hazardous to human health than PM2.5, and more difficult to control, said Zhao Hualin, Director of the Pollution Prevention and Control Department at the MEP.

"Since ozone is produced by chemical reactions between nitrogen oxides and volatile organic compounds when they are exposed to ultraviolet light, we must simultaneously reduce nitrogen oxides and volatile organic compound emissions in order to reduce ozone levels," said Tang Xiaoyan, a Peking University professor and academician with the Chinese Academy of Engineering.

Hard to control

The 2012 State of China's Environment report released by the MEP showed that emissions of nitrogen oxides in China continued to rise until 2011, and then dropped by 2.77 percent in 2012, exceeding 23 million tons in the year.

Although emissions of nitrogen oxides have been regulated by the government, volatile organic compounds are not included in emissions reduction targets of the 12th Five-Year Plan, China's national social and economic development initiative for 2011-15, Tang said.

The 12th Five-Year Plan set goals to cut four of the major air pollutants, including sulfur dioxide and nitric oxide, by more than 10 percent each.

"Compared with the reduction of nitric oxide emissions, volatile organic compound emissions are more difficult to abate," Tang said.

China has published several standards for volatile organic compound emissions, including standards for pollutants discharged during oil storage, certification standards for oil-based coatings, and mandatory standards for toxic substances contained in interior decoration materials.

But Tang warned that a wide range of sources can emit volatile organic compounds, with various industries emitting different compositions of these pollutants. Even in one production process, if different raw materials and production techniques are used, different types and concentrations of volatile organic compounds will be discharged.

For example, chemical products can emit volatile organic compounds during their production, storage and use. "Currently, many local governmental departments have not yet realized how hazardous volatile organic compounds are. In believing that chemical plants are less polluting than power and steel plants, they have given the green light to many chemical engineering projects without realizing the consequences," Tang said.

Recently, the Chinese Government announced a plan to invest 1.7 trillion yuan ($277 billion) to combat air pollution in the next five years. The investment will mainly go toward helping highly polluted areas and areas with particularly high levels of PM2.5, said Wang Jinnan, Vice President of the Chinese Academy for Environmental Planning.

"Volatile organic compounds, ozone and PM2.5 are interrelated," said He Kebin, a professor at the Tsinghua University's School of Environment, adding that when ozone concentration is high, more nitric oxide and sulfur dioxide will be turned into PM2.5 pollutants.

If all pollutants are not effectively reduced together, reduction in one type of emission can be offset by an increase in another, He said.

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