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

Abstract. Background ozone (O3) represents the baseline concentrations in the absence of local anthropogenic emissions and is critical for understanding and mitigating tropospheric O3 pollution. Accurate estimation of background O3 constrains the maximum achievable benefits of precursor emissions control and informs effective air quality management. This review synthesizes the definition and estimation methods for background O3, including in situ measurement, statistical analysis, numerical modeling, and integrated methods. A meta-analysis of background O3 in China from 1994 to 2020 reveals pronounced spatial variability, with concentrations ranging from 33 ppb in the Northeast China to 48 ppb in the Northwest China, and a national mean of 41 ppb, accounting for 79 % of the tropospheric maximum daily 8 h average O3. Methodological discrepancies are evident for background O3: in situ and statistical methods yield higher estimates, whereas integrated approaches produce lower yet more consistent values. Placed in a global context, background O3 levels in China are medium-to-high and exhibit an increasing trend. This review highlights the need for integrated estimation methods to improve accuracy, underscores the international collaboration to address 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, it provides critical insights for atmospheric chemistry and air pollution control in China and beyond.