Effectiveness of green roofs and walls to mitigate atmospheric particulate matter pollution in a semi-arid climate
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Date
2021
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Abstract
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.
Description
Tesis (Doctor in Engineering Sciences)--Pontificia Universidad Católica de Chile, 2021