In China, people breathe ozone-laden air two to six times more often than people in the United States, Europe, Japan, or South Korea, according to a new international study published in Environmental Science & Technology Letters. By one metric—total number of days with daily maximum average ozone values (8-hour average) greater than 70 ppb—China had twice as many high ozone days as Japan and South Korea, three times more than the United States, and six times more than Europe.

Urban surface ozone level trends in China (red), Japan (purple), Europe (orange), and the United States (blue) from 1980−2017 for one ozone metric. For Japan, the EU, and the US, only urban sites with records for more than 25 years (1980−2014) are included. For China, sites in 74 major cities with continuous observations from 2013−2017 are included. The inset shows ozone trends in Beijing (red) and Los Angeles (blue). Note that ozone in Beijing today is comparable to Los Angeles in the 1990s.

We find that in the most populous urban regions of Eastern and Central China, there are more than 60 days in a calendar year with surface ozone levels exceeding the Chinese national ozone air quality standard.

—Lin Zhang of Peking University, lead author

Many countries regulate ozone because of the damage the pollutant does to plants and people.

In the United States, for example, the current health-based standard for ground-level ozone, set by the US Environmental Protection Agency, is 70 ppb (based on the maximum daily 8-hour average). The Chinese national ozone air quality standard is a daily maximum 8-hour average greater than 160 micrograms per cubic meter, equivalent to about 80 ppb.

Ground level ozone is most commonly formed when volatile organic compounds and nitrogen oxides react in the atmosphere in the presence of sunlight. The burning of fossil fuels and biomass burning (from crop clearing or forest fires) are major sources of volatile organic compounds and nitrogen oxides. Since the 1990s, tighter controls on emissions of those ingredients have lessened ozone pollution in many European and US cities. But the extent of surface ozone pollution in China hasn’t been widely recognized, in part because there were so few Chinese monitoring sites before 2012, according to the researchers.

For this study, the researchers used data from a relatively new network of 1,600 ozone monitors in China and a massive new global ozone database to quantify ozone levels in China and compare those to the levels in other countries.

The new report shows that China has higher ozone pollution levels than all nations with ozone monitors, including the United States, Europe, Japan, and South Korea. Every ozone metric the researchers looked at rose continuously in China over the last five years.

Present-day ozone levels in major Chinese cities are comparable to US levels in the 1980s and 1990s, when emission controls were just beginning to have an impact on reducing ozone levels there.

For the past several years, wintertime haze pollution has been the main public concern in China and the focus of government action on air pollution, according to Zhang. The Chinese government has implemented stringent emission control measures to improve air quality: Since 2013, the Action Plan on Air Pollution Prevention and Control has reduced the concentration of primary air pollutants and particulate matter an average of 35 percent for 74 major cities.

The researchers think the harmful effects of surface ozone pollution are much less recognized.

Many people in China do not realize that we may suffer severe ozone pollution under a typical blue sky in summer days. The emerging severity of ozone pollution in China now presents a new challenge for emission control strategies.

—Lin Zhang

The database the researchers used in this analysis is part of the Tropospheric Ozone Assessment Report (TOAR), a series of global assessments of ozone pollution and its relevance to people, plants, and climate.

Resources

• Xiao Lu, Jiayun Hong, Lin Zhang, Owen R. Cooper, Martin G. Schultz, Xiaobin Xu, Tao Wang, Meng Gao, Yuanhong Zhao, and Yuanhang Zhang (2018) “Severe Surface Ozone Pollution in China: A Global Perspective” Environmental Science & Technology Letters 5 (8), 487-494 doi: 10.1021/acs.estlett.8b00366