Methods to quantify particle air pollution removal by urban vegetation: A review

Among the ecosystem services provided by urban forests, the air quality amelioration is particularly relevant. The high level of air pollution in modern cities and the indirect involvement of particulate matter (PM) in the spread of COVID-19 have exacerbated the air quality issue worldwide. However, in the estimation of urban vegetation effectiveness in particle air pollution removal, there is a lack of a standard procedure. Different methods are used for this purpose, making the comparison across different studies difficult. Therefore, there is a need of an extensive review, aimed at: i) identifying the existing direct methods to quantify this ecosystem service, ii) assessing their pros and cons, accuracy and reliability, sustainability, and iii) laying the foundations to create a standard method, commonly and universally recognized. We identified and meticulously assessed five main direct metrics: the gravimetric method (G, 40%), aerosol monitor (AM, 20.5%), wind tunnels and deposition chambers (WT&CH, 19.5%), Scanning Electron Microscopy (SEM, 14%) and Saturation Isothermal Remanent Magnetization (SIRM, 6%). This work provides a crystal picture and a critical framework of the last thirty years literature on this topic and lays the foundations to create a common and shareable approach to quantify the air PM mitigation potential of the urban vegetation. This will be useful to guide researchers and urban planners in shaping greener, healthier, and more sustainable cities.