Effectiveness of green roofs and walls to mitigate atmospheric particulate matter pollution in a semi-arid climate

Air pollution is an atmospheric phenomenon by which particles (solid/gas) contaminate the environment. The World Health Organization (WHO) considers air pollution as a substantial environmental risk for health, specifically for cities. Reducing the levels of air pollution can decrease morbidity related to strokes, lung cancers, and chronic and acute lung diseases, including asthma. Industries such as construction, transportation, and consumption of fossil fuels, in other sectors, have contributed to increasing pollutant emissions to the urban environment. Pollutants, such as atmospheric particulate matter (PM), are considered highly harmful to people's health. Long-term exposure to PM is statistically associated with respiratory morbidity and mortality. Thus, many cities have focused on improving urban air quality using different strategies, such as the implementation of green roofs (GRs) and green walls (GWs). Although there have been significant advances in research on the effect of GRs and GWs on urban air quality, there are still research gaps. These gaps related to identifying vegetation that favors the capture of PM, establishing strategies to enhance the PM capture capacity of different vegetation, and quantifying the impact of implementation of GRs and GWs at urban scales. The main objective of this research is to analyze the effectiveness of GRs and GWs on mitigating air pollution by PM10 and PM2.5 in improving urban air quality. The variability in capturing PM for GRs and GWs plants is evaluated and the influence of species biodiversity in capturing PM is investigated. The impact of GRs and GWs layouts on PM capture and concentrations in a highly dense urban area with a Mediterranean climate is studied. In particular, GRs and GWs layouts, considering coverage and building heights where GRs and GWs are located, are two aspects of urban morphology analyzed. The research methodology consists of quantifying the PM capture capacity for nine GRs and GWs species as monocultures and polycultures to investigate how PM capture varies among plants and due to biodiversity. Two methods are used to evaluate the PM capture, gravimetric analysis and decay curve. Based on the results of PM capture, a validated ENVI-met model is used to assess how urban morphology and GRs and GWs coverage influence PM capture and concentrations at a neighborhood scale of Santiagos’ downtown. The results in monocultures, GRs and GWs show that PM capture is highly species type dependent. PM2.5 capture ranged from 0.09 μg·cm-2∙h-1 for Sedum Spurium P to 1.32 μg·cm-2∙h-1 for S. Album. Moreover, it found that biodiversity significantly increases the PM2.5 capture compared to monocultures. In four of the five species studied, the PM2.5 levels captured by the vegetation was higher in polycultures. Moreover, the results from ENVI-met modeling show that priority should be given to GRs for buildings lower than 10 m height to decrease PM2.5 concentrations at pedestriam level. For GWs, the PM2.5 abatement is favorable in all building configurations. In addition, the combined use of GRs and GWs can reduce up to 7.3% of PM2.5 in Santiago’s downtown compared to a base case scenario without GRs and GWs. In conclusion, GRs and GWs are a valuable strategy to improve urban air quality, thus they should be implemented as a complement to other air quality mitigation strategies in large cities. S. Album outperforms the other species evaluated in capturing PM2.5 either as monoculture or polyculture. It was also found that biodiversity enhance the PM2.5 capture of GRs and GWs. It is recommended the polycultures of L. Spectabillis, Lavandula Angustifolia and S. Album for GRs and Sedum Palmeri, S. Album and Sedum Spurium P for GWs to maximize the effectiveness of GRs and GWs to capture PM2.5. On the other hand, the implementation of GWs has a greater impact on PM2.5 abatement than GRs due to their proximity to the emission source. It is suggested to implement GRs on buildings up to 10 m height and coverage between 50% and 75%. For GWs, a coverage of 25% is recommended. These results provide scientific support for the inclusion of GRs and GWs in public policies and urban development plans in order to improve urban air quality. Moreover, the methodologies used in this research can also be applied to other species and urban morphologies to identify the optimum combinatory of vegetation species and layouts to capture PM in the urban environment.