Reframing urban green infrastructure for particulate matter mitigation: a systematic review and functional planning framework for high-density cities

Rapid urbanization and increasing particulate matter (PM) pollution have heightened public health risks in high-density urban environments, highlighting the need for effective nature-based solutions. Although urban green infrastructure (UGI) is widely recognized for its potential to improve air quality, existing classification systems, particularly those based on development purposes, remain inadequate for capturing the functional mechanisms necessary for effective PM mitigation. This limitation hinders the development of context-sensitive planning strategies that account for spatial conditions, emission sources, and human exposure. To address this gap, this study develops a PM-oriented functional planning framework for high-density cities by reclassifying UGI according to PM mitigation functions rather than conventional purpose-based categories. Drawing on 8 Korean institutional/policy documents and a systematic review of 139 academic studies (124 international and 15 Korean), four functional classification criteria, namely development purpose, proximity to emission sources, receptor vulnerability, and physical structure, were identified and used to establish an evidence-informed typology comprising ten PM-oriented UGI types. These types were contextualized and operationalized within the Korean urban system as a representative model for complex, high-density urban development. The findings indicate that type-specific strategies emphasizing deposition, blocking, and dispersion are essential for effective PM mitigation. Structurally diverse vegetation can enhance particle deposition in residential areas, whereas optimized vegetation porosity improves performance in roadside environments. The proposed framework offers a decision-support tool for policymakers and urban planners; however, further empirical validation is required to assess its effectiveness and applicability across diverse urban contexts.