Since 2013, the Chinese government has been enforcing strict policies on emissions in an effort to reduce fine particulate air pollution, known as PM 2.5, a major problem in Chinese cities. While the campaign has been successful in drastically reducing the level of fine particulates, a new study published on 31 December 2018 in Proceedings of the National Academy of Sciences has suggested that the rapid reduction in PM 2.5 achieved by China may have led to an increase in harmful ground-level ozone pollution in megacities, notably Beijing and Shanghai. (1).
The so-called Air Pollution Action Plan succeeded in making significant improvements to air quality by setting PM 2.5 targets for between 2013 and 2017 for the most affected regions in the country. Impressively, PM 2.5 concentrations in eastern China have fallen by nearly 40 per cent since the programme started, mainly by reducing the number of cars on roads in cities and the number of fossil fuel power plants by replacing coal with natural gas. Beijing achieved a drop of 35 per cent ― surpassing its 33 per cent target ― which meant an annual average PM 2.5 level of 58 µg/m³. Although none of China’s megacities has yet to reach the annual average PM 2.5 level of 10 µg/m³ recommended by the World Health Organization.
Nonetheless, owing to the huge amount of data collected over the past five years by air quality monitoring stations in China, researchers from Harvard’s John A. Paulson School of Engineering and Applied Sciences and the Nanjing University of Information Science & Technology (NUIST) have now been able to asses the overall effects of reducing PM2.5. Surprisingly, they discovered that the rapid reduction of PM 2.5 has considerably changed the chemistry of the atmosphere. The researchers showed that when less particulate matter is available to soak up the radicals involved in generating ozone pollution, this leads to the production of even more ozone.
Whereas ozone in the stratosphere ― the “good” ozone layer ― provides protection from the harmful UV rays of the sun, ozone in the lower atmosphere, or troposphere, can be harmful to breathe and can damage crops, plants, and trees by reducing photosynthesis, stunting growth or weakening them. This “bad” ozone is the main component in smog. Unlike the ozone layer, ground-level ozone is created by a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOC) from combustion gases, chemicals, and many other sources.
Despite the dramatic decrease in particulates over the past 5 years, smog-inducing ozone and fine particles are still responsible for an average 1.1 million premature deaths and the loss of 20 million tonnes of rice, wheat, maize, and soybean each year, according to another recently published study (2). Industrial activity remains the largest contributor to PM 2.5 pollution, followed by residential and commercial burning of coal, particularly for heat during the winter months.
China has recently narrowed its targets for reducing ozone-forming pollutants and fine particles for cities as part of a three-year action plan for 2018 to 2020 to “win the war for blue skies.” Based on this new study, the task of further reducing pollution in cities will have to be a careful balancing act which focuses on further measures to reduce NOx and VOC emissions alongside the PM 2.5 targets to limit the previously unforeseen and unexpected increase in ozone pollution.
(1) Li, K. et al. Anthropogenic drivers of 2013–2017 trends in summer surface ozone in China. Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1812168116
(2) Hou, X. et al. Impacts of transboundary air pollution and local emissions on PM2.5 pollution in the Pearl River Delta region of China and the public health, and the policy implications. Environmental Research Letters (2018). DOI: 10.1088/1748-9326/aaf493