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.