Tropospheric ozone is produced via the photochemical reaction of volatile organic compounds, carbon monoxide, and nitrogen oxides in the presence of sunlight. Over the past 20 years, serious ozone pollution has been found in the most highly populated and industrialized city clusters in China, such as the Beijing–Tianjin–Hebei, Yangtze River Delta, Pearl River Delta, and Sichuan Basin regions.

To control ozone pollution is a great challenge because of the diverse range of precursors and the nonlinear relationship between ozone concentrations and these precursors. Accurately determining the main sources of ozone is therefore the key to the formulation of a reasonable and efficient ozone control strategy.

A recent article published in Atmospheric and Oceanic Science Letters by the research team led by Prof. ZHANG Meigen from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, has summarizes the approaches and main conclusions of studies on the sources of ozone and its precursors for both regional and sectoral sources in China, including back-trajectory analyses and ozone source apportionment based on the observation-based method (OBM) and emissions-based method (EBM).

"The OBM avoids the uncertainties of the emission inventories required in the EBM. However, in practical applications, results based on the OBM are limited to a fixed time and fixed site, without the regional results obtained, due to the high requirements for the accuracy and representativeness of the observational data," said Prof. ZHANG.

The results of ozone source apportionment over China suggest that in addition to contributions from local and surrounding areas, super-regional (outside the simulated area) transport also makes a significant contribution to the high concentrations of ozone in China, indicating the importance of strengthening regional defense and control measures. Mobile and industrial sources are the major sources of surface ozone, and this is where future control actions should be focused.

"Existing ozone sensitivity analysis and source apportionment are mostly limited to a single ozone pollution episode or one particular month, with few long-term studies," said Prof. ZHANG. "In the future, more long-term research should be carried out to provide references for the development of emission reduction strategies to achieve long-term ozone attainment in China."