Reducing PM2.5 and O3 through optimizing urban ecological land form based on its size thresholds

Abstract

Optimizing the size and form of urban ecological land (UEL) is an effective approach to addressing PM_2.5-O₃ composite pollution in China. However, existing strategies are usually proposed based on the impact of one type of UEL on individual pollutants, while overlooking UEL forms’ different pollution reduction effects across its size intervals. This study identifies UELs (including forest, shrub, grassland, water, and wetland) of 1068 counties within the Yangtze River Economic Belt (YREB) and calculates their size and form metrics. Then the cross-sectional threshold regression model is used to analyze the threshold effect of UEL size on fitting models of pollutant concentrations. Finally, quadrant analysis is extended to categorize counties and provide differentiated planning strategies. The conclusions show: (1) UEL size presents a triple threshold effect on PM_2.5 concentrations at 4.302%, 8.055%, and 23.742%, and a single threshold effect on O₃ concentrations at 3.275%. Size and form metrics are not always significant across UEL size intervals. (2) Counties are categorized into 6 types based on their primary pollutants and UEL sizes, showing spatial clustering within each type. (3) With size increasing, dispersed and irregular UEL form helps more in reducing PM_2.5, while O₃ reduction prefers aggregated one, thereby the evolutionary UEL planning strategy is proposed.