Green-blue-grey infrastructure (GBGI) offers environmental benefits in
urban areas, yet its impact on air pollution is under-researched, and the
literature fragmented. This review evaluates quantitative studies on GBGI's
capability to mitigate air pollution, compares their specific pollutant
removal processes, and identifies areas for further investigation. Of the 51
GBGI types reviewed, only 22 provided quantitative pollution reduction data.
Street trees and mixed-GBGI are the most studied GBGIs, with efficacy
influenced by wind, GBGI type vegetation characteristics, and urban
morphology. Negative percentages denote worsening air quality, while
positive reflect improvement. The 22 different GBGI grouped into eight main
categories provide an average (±s.d.) reduction in air pollution of 16±21%,
with substantial reduction shown by linear features (23±21%), parks
(22±34%), constructed GI (14±25%), and other non-sealed urban areas
(14±20%). Other individual GBGI reducing air pollutants include woodlands
(21±38%), hedges (14±25%), green walls (14±27%), shrubland (12±20%),
green roofs (13±23%), parks (9±36%), and mixed-GBGI (7±23%). On average,
GBGI reduced PM1, PM2.5, PM10, UFP and BC by 13±21%, 1±25%, 7±42%,
27±27% and 16±41%, respectively. GBGI also lowered gaseous pollutants
CO, O3 and NOx by 10±21%, 7±21% and 12±36%, on average, respectively.
Linear (e.g., street trees and hedges) and constructed (e.g., green walls)
features can impact local air quality, positively or negatively, based on the
configuration and density of the built environment. Street trees generally
showed adverse effects in street canyons and beneficial outcomes in openroad
conditions. Climate change could worsen air pollution problems and
impact GBGI effectiveness by shifting climate zones. In Europe and China,
climate shifts are anticipated to affect 8 of the 22 GBGIs, with the rest
expected to remain resilient. Despite GBGI's potential to enhance air quality,
the meta-analysis highlights the need for a standardised reporting structure
or to enable meaningful comparisons and effectively integrate findings
into urban pollution and climate strategies.