A comprehensive review of tropospheric background ozone: definitions, estimation methods, and meta-analysis of its spatiotemporal distribution in China

Abstract. Background ozone (O3) refers to O3 concentrations that remain unaffected by direct local anthropogenic emissions, critical for comprehending tropospheric O3 pollution, as it defines the baseline levels without local anthropogenic emissions. Accurately estimating background O3 is essential for determining the maximum achievable reductions in O3 through anthropogenic precursor emissions control and for developing effective air quality management strategies. This review synthesizes the definition and estimation methods for background O3, including in situ measurement, statistical analysis, numerical modeling, and integrated method. A meta-analysis of the spatiotemporal distribution of background O3 across China from 1994 to 2020 reveals substantial spatial variability, with the highest concentrations in the Northwest region (48 ppb) and the lowest in the Northeast and Central regions (~33 ppb). The national average background O3 concentration is approximately 40 ppb, contributing 77 % to the tropospheric maximum daily 8-hour average ozone. Estimation methods show notable discrepancies: in situ measurement and statistical analysis methods yield higher estimates, while integrated method provide lower yet more consistent values. On a global scale, background O3 concentrations in China are ranked medium-to-high and exhibit an increasing trend. This review, from a global perspective, highlights the need for integrated estimation methods to improve accuracy, underscores the importance of international collaboration in addressing long-range pollutant transport, and calls for further research on the interactions between background O3 and climate change. By advancing the understanding of background O3 dynamics, this study provides critical insights for atmospheric chemistry research and air pollution control efforts in China and beyond.