Air pollutant removal by urban vegetation: A meta-analysis of gaps in research and environmental management

Abstract

Air pollutant removal by urban vegetation is a type of nature-based solution (NbS), and has received growing attention by researchers and decision-makers across the world. However, the effectiveness of botanical pollutant removal in cities requires more synthesis to inform urban planning and management. In this study, we quantified the effectiveness of the removal of typical air pollutants – particle matter (PM), nitrogen oxide (NOx), sulfur dioxide (SO2) and ozone (O3) – by synthesizing global field-measurement studies based on a Bayesian meta-regression approach. The results revealed that urban vegetation can mitigate the growth of air pollutant concentrations, with reduction rates of 16.5～26.7% for PM, 13.9～36.2% for NOx, and 20.5%～47.8% for SO2. However, they failed to significantly mitigate ground-level O3, corresponding to an increase of 5.1～25.9%. The variability in effect sizes was mainly influenced by the distance to nearest highway, ambient concentration, relative humidity and green coverage. To link the above effects of botanical pollutant removal to environmental management, a brief questionnaire was then conducted in one of case-study cities (Shenzhen, China) to investigate environmental managers’ views on the removal effect of urban vegetation. The feedbacks showed that most respondents supposed positive impacts of urban vegetation on all four air pollutants, which was at odds with our findings with respect to O3. Finally, concerning the case study of Shenzhen, some suggestions were proposed to improve the nature-based solutions (NbS) for air pollution mitigation, highlighting the important role of urban vegetation for public health in global cities.