Healthy breathing is the lifeline of each living body, so what about cities? Chinese scientists have been discovering the secret of "urban respiration" by observing and evaluating its variations in an industrial metropolis.

"The 'urban respiration' is a prospective study. It is not only a scientific issue but also vital to the sustainable development of every city, country, and region," says Huang Jianping, an academic with the Chinese Academy of Sciences and professor at Lanzhou University in Gansu province.

The team established a high-precision atmospheric oxygen observation platform and carried out continuous observation in Lanzhou, an industrial powerhouse in Northwest China.

The team conducted quantitative estimation on urban respiration and pioneered a new research field of the oxygen cycle effects on urban health.

The study results have been published as the cover story of the journal of Environmental Science & Technology.

Essential for the survival of almost all living things on Earth, oxygen is one of the most critical gaseous components of the atmosphere. However, excessive consumption of oxygen threatens human life and health.

"Like a person, a city also breathes, inhaling oxygen and exhaling carbon dioxide," Huang says. "Populous and dense urban areas are housing more people and consuming more fossil fuels."

They initiated the field study in Lanzhou, with a population exceeding 4.4 million and the provincial capital of Gansu. The city sits amid semiarid mountainous terrain with a river running through it. It has little wind and rain, which limits atmospheric dispersion and leads to a stable accumulation of pollutants.

Researchers built a high-precision oxygen concentration observation platform on the top of a 22-floor building in downtown Lanzhou. The air-intake port faces the busy 10-lane Tianshui Street, which neighbors the railway station and is significantly affected by human activities.

"The urban center of Lanzhou has a unique terrain and at its narrowest is 1 kilometer wide. We explored to what extent people's respiration affects the oxygen concentration in an urban atmosphere," says Liu Xiaoyue, a PhD student at the College of Atmospheric Sciences of Lanzhou University.

Researchers made unremitting efforts in adjusting equipment, combing through data, and finding a correction method of atmospheric oxygen observation data suitable for the platform.

"With persistent effort, our observation data gained recognition from our peers globally," says Huang.

They quantitatively break down urban respiration into two parts: human respiration and fossil fuel combustion.

The study results show that declines in atmospheric oxygen were associated with deteriorated air quality and robust anticorrelations between oxygen and gaseous oxides.

Generally speaking, the burning of fossil fuels is the primary cause of the oxygen-concentration drop in Lanzhou, while plant photosynthesis is the primary source of oxygen, according to Huang.

The study provides new insights into oxygen and energy consumption, pollutant emission, and urban atmospheric transport processes. "In the longer term, we hope to promote a big science project on urban respiration and call on a global effort to find solutions," Huang adds.