Façade Greening for Healthy Urban Air: An Umbrella Review on Particulate Matter Reduction, Challenges, and Future Directions

Abstract

Particulate matter (PM) exposure in urban areas is a continuous threat to human health and requires sustainable solutions for effective reduction. The aim of this study is to provide a comprehensive overview of façade greening and its effects on PM and to identify knowledge gaps. We systematically screened, analyzed, and re-evaluated previous reviews according to the criteria established for an umbrella review. As a result, this study covers and discusses common measurement methods, influencing factors, the effectiveness of façade greening, leaf characteristics, and plant species from different subtopics. The main findings are that inconsistent measurement methods limit the comparability of the analyzed reviews, and different climatic conditions affecting the use of plant species lead to different international effects. Despite the existing studies reporting a positive effect on the reduction of PM, comparability is difficult due to methodological differences and external influences. Given the complexity, interdisciplinary collaboration with experts is essential to increase the effectiveness of façade greening for PM accumulation for healthy urban air. We suggest that this umbrella review provides a foundation for future research and practical implementations, such as a suitable plant selection or façade design, by integrating the results.

1. Introduction

Air pollution is considered one of the greatest environmental threats to human health, is responsible for millions of premature deaths, and also represents a significant economic burden [1]. Vulnerable populations, such as children, the elderly, and people with existing health conditions, are at a higher risk of adverse health effects from PM exposure. Traffic-related pollution from busy roads is responsible for a significant proportion of asthma cases and coronary heart disease [2]. Air pollution is a complex mixture of gaseous compounds and solid components such as PM—which is the main point of investigation in this work. Anthropogenic sources can include road traffic, industrial sites, power stations, and heating systems [3]. PM can be distinguished by different aerodynamic particle sizes of PM₁₀, PM_2.5, and PM₁ (i.e., PM with an aerodynamic diameter of less than 10 µm, 2.5 µm, and 1.0 µm) [4]. Across European cities, chronic exposure to PM_2.5 and smaller particles causes the largest health problems [2,5]. The smaller PM fraction is particularly dangerous as these particles can penetrate deeply into the lungs and enter the bloodstream, causing systemic health effects. It has been shown that a significant reduction in PM levels would also result in a large monetary gain due to savings on health expenses, absenteeism, and intangible costs such as well-being, life expectancy, and quality of life [5]. On this basis, ensuring clean air is an important political, economic, and public interest [1]. Green infrastructures (GIs) are natural or semi-natural systems that provide a range of ecosystem services and benefits to urban environments. One of the key benefits of GI is its ability to reduce ambient temperatures in urban areas, which helps to mitigate the urban heat island (UHI) effect. The UHI effect is a phenomenon where urban areas experience higher temperatures than surrounding rural areas due to the heat absorbed and emitted by buildings and pavement [6]. This can lead to a range of negative impacts on human health, including heat-related illnesses, as well as increased energy demand and air pollution. GI such as trees, green roofs, and façade greenings can help to mitigate the UHI effect by providing shade and evapotranspiration, which cools the air and reduces the amount of heat absorbed by buildings and pavement, which evidently increases thermal comfort in people spending time in natural environments such as forests [6]. GI can also provide stormwater and greywater management functions by absorbing and filtering rainwater, reducing the amount of runoff, and improving water quality [7,8].

In addition to benefits such as heat reduction or water management, the use of vegetation in an urban, populated environment can be an effective cleaning method for reducing air pollutant concentrations [9]. However, their effectiveness depends on different factors, such as the location of the vegetation in relation to the environment [10] or the plant species [11]. In fact, if GI is used improperly, the concentration of pollutants may increase in a particular area whose air exchange is disturbed by vegetation [12,13]. In regard to air cleaning properties, the provided ecosystem services vary depending on species richness [14]. Plants exert direct positive effects on air quality by reducing air pollutant concentrations and associated detrimental health effects. Leaves can physically bind PM through microstructural leaf traits such as the density of leaf trichomes, leaf wettability, surface roughness, and epicuticular waxes. Previous studies also showed that leaf surface structure can have a crucial impact on PM accumulation [15,16,17,18,19].

Besides street trees, hedges, etc., GI includes green building features such as façade greening (equivalent to vertical greening systems) and green roofs. Façade greenings offer the significant advantage that their space requirements in cities are comparatively small, and vertical surfaces bring with them large-scale potential for greening. Façade greening includes different systems whose nomenclatures are not always used in the same way. A distinction is made between two main types of greening, ground-based and wall-based systems [20], whereas the terms green façade, green wall, and living wall [20,21] are also used depending on the system. The main characteristic, besides the system type, is the resulting plant selection. Ground-based systems primarily use climbing plants with or without climbing aid, and wall-based systems use perennials. In this paper, the term “façade greening” covers all vertical greening systems previously mentioned.

In addition to the aforementioned benefits, façade greenings also provide services to reduce PM concentrations in different urban environments and can help to improve the ambient air. As research on PM and façade greening is still scarce, the purpose of this paper is to summarize the existing literature in a new context and, in particular, to provide a consistent fundamental basis for future work. In contrast to existing reviews, which are often limited to specific aspects such as environmental conditions, specific road profiles, or GI in general [22], this review synthesizes these different aspects related to green façades and presents a synthesis. Although the focus is on green façades, this article also includes measurement methods and leaf characteristics relevant to all forms of GI and makes links to their relevance in the context of façade greening.

The significance of this study is that it provides a more comprehensive overview and highlights the key research gaps that should be investigated in future studies. The state-of-the-art was identified and discussed, and recommendations for science and practice were reviewed and reformulated. Furthermore, this study contributes to improving and implementing sustainable solutions and possibilities on their basis. The results of this study should provide decision-makers with valuable insights that will enable them to effectively implement and adapt sustainability solutions.

2. Methods

2.1. Search Strategy

To identify relevant literature and achieve the aim of this study, this umbrella review was prepared according to the guidelines of PRISMA [23] and the procedure of Aromataris et al. [24]. Review papers with different research focuses related to PM and façade greening were identified and analyzed for the current state-of-the-art. This review follows three stages before the inclusion (4) of individual papers/publications: (1) identification, (2) screening, and (3) eligibility (see Figure 1).

To identify relevant literature, review articles relating to façade greening and particulate matter were searched for using a defined search string in common search engines (see Section 2.2). As shown in

Table 1. Categories of individual keywords and their synonyms combined into one search string.

Source and Type of Air Pollution	Pollutant	Type of Planting
Particulate matter, PM, PM2.5, PM10	Particulate matter composition	Facade greening
Dry deposition	Road dust composition	Living wall*s
Air pollution	Particulate matter characterization	Green wall*s
PM capture	Road dust characterization	Vertical green
Deposition		Living wall system*s
Local pollution exposure		Living wall plant*s
Airborne particulate matter		Green wall plant*s

, the keywords provide a comprehensive framework for the subsequent formulation of a targeted search term. To avoid ignoring relevant literature, synonyms for keywords were used in three groupings, as shown in Table 1.

After the identification was completed, reviews were screened in a second step, selection criteria were defined (see Section 2.3), and it was decided whether they were relevant to this study (see Section 2.4). After the screening was completed, the review articles were further checked for their eligibility (see Section 2.5) and then integrated into this study after all previous criteria were met (see Section 2.6).

2.2. Identification of Review Publications

To summarize the current research, common search engines such as WebOfScience, Scopus, and Google Scholar were used to search for published reviews on the topic of façade greening and PM that were both in English language and submitted to peer review. After categorization using Boolean search methods, the individual search terms were used to form a search string to research in the different databases and search engines.

The following search string was used: “particulate matter composition” OR “dry deposition” OR “road dust composition” OR “particulate matter characterization” OR “road dust characterization” OR “particulate matter” OR “air pollution” OR “PM” OR “deposition” OR “local pollution exposure” OR “PM_2.5” OR “PM₁₀” OR “airborne particulate matter” OR “PM capture” OR “street” AND “facade greening” OR “living wall*s” OR “green wall*s” OR “vertical green” OR “living wall system*s” OR “living wall plant*s” OR “green wall plant*s” AND “review” OR “meta-analysis” NOT “indoor”.

To achieve more targeted results, publications on indoor greening were previously excluded, and the search was limited to review articles. The search terms and keywords with common synonyms were categorized into source and type of air pollution, pollutant, and type of planting, as shown in Table 1. In the identification phase of potential review publications, five papers were removed for inaccurate titles and abstracts as another reason for exclusion. The results of the literature research were reviewed during the period from 17 October 2022 to 15 November 2022.

2.3. Selection Criteria

The studies found were selected via a two-step process (see Section 2.4). For this purpose, inclusion and exclusion criteria were previously defined. Literature reviews were included when the publication dealt with façade greening and PM and focused on outdoor areas. Thus, all review articles that summarized the air-filtering performance of indoor plants were excluded, as well as studies that did not investigate green façades or focused primarily on other air pollutants, as well as duplicates from different searches.

2.4. Screening

Firstly, the title and the abstract were screened. The first step of the inclusion/exclusion criteria was to exclude duplicates and non-peer-reviewed articles and retain possible titles and abstracts relevant to the topic. Secondly, the full text was analyzed. Remaining literature reviews were included in the eligibility test.

2.5. Eligibility

The publications considered suitable were further assessed for their eligibility. The criteria for suitability were that the studies were (1) written in English, peer-reviewed, and contained literature summaries only (with no publication period restrictions); (2) they were about PM on outdoor façade greening; and (3) they had at least one of the following parameters: information on measurement methodology of PM on plants, external influences, effectiveness of green façades in PM accumulation, leaf characteristics, species investigated, and recommendations for practice and/or further research. Summaries that mentioned but did not investigate green façades as PM filters in more detail were excluded.

2.6. Inclusion

Publications that met the aforementioned criteria were included in this study. The data obtained by the method were analyzed in a further step and categorized in relation to individual specific research foci, with the aim of identifying differences or parallels in a specific research area, such as leaf characteristics or methodologies in this overarching research area of PM and façade greening.

2.7. Data Analysis

In the next step, the content of the included review papers was analyzed. In addition to general information such as publication year, topic, journal, and study design, the number of included studies on façade greening and PM was evaluated. Figure 2 provides a summarized overview of the individual stages and the methodological approach of this umbrella review. Based on the content’s results, the three categories (1) green infrastructures, (2) street canyon/open road, and (3) leaf traits were identified. To provide a more comprehensive overview of the topic, the content was divided into these categories, which considered the different facets of façade greening and its effects on PM mitigation. Thus, different research perspectives can be integrated to provide a more complete picture of the topic that extends the focus of the individual reviews. The information on measurement methods, influencing parameters, plant species, and leaf characteristics, as well as the effectiveness of façade greening and resulting recommendations, was extracted in detail and summarized, as shown in Figure 2. The combination of content from review publications out of different research foci and topics aims to link the basic research gaps and state-of-the-art and analyze commonalities or differences to provide meaningful recommendations for practice and further research.

Figure 2 shows a flow chart starting with the analysis, research foci of the used content, the categorization of the relevant content, the research results, and the recombining of the results from the individual foci into one result up to the synthesis from the results of the literature research.

3. Results

3.1. Study Description

Most of the five studies out of 24 reviews integrated into this paper did not deal exclusively with green façades in terms of content but generally targeted GI in urban areas. Nevertheless, the integrated studies met the previously established criteria of this review and have a sufficient component of the core topic. The only review integrated into this paper that focused exclusively on green façades is by Ysebaert et al. [25]. The other integrated studies included information on green façades in some parts, as shown in

Table 2. Review papers included in this umbrella review, with information about publication year, topic, and journal.

Authors (Year of Publication)	Topic	Journal
Abhijith et al. (2017) [13]	GI in street canyons and open roads	Atmospheric Environment
Corada et al. (2021) [26]	Leaf traits on GI	Environmental Pollution
Tomson et al. (2021) [27]	Overview of GI in street canyons	Environment International
Wróblewska et al. (2021) [28]	Plant effectiveness of GI for ambient PM removal	Environmental Sciences Europe
Ysebaert et al. (2021) [25]	General overview of green walls and living walls	Urban Forestry & Urban Greening

. This review focused exclusively on green façades and included only studies that could fulfill the criteria for this work described in Section 2. To ensure that no relevant studies were left out, all stages corresponding to the PRISMA guideline were carefully run through and checked. Table 2 shows the five papers included in the review. Without any restriction on the publication year, the included studies ranged from 2017 to 2021 and were consequently published in the last five years.

Table 3. Table of contents of the five included review papers and categorization of the six identified aspects of the study.

Review	Measuring Methods	Influencing Factors	Effectiveness of Façade Greening	Leaf Traits	Investigated Plant Species	Implications
Abhijith et al. (2017) [13]		+	+		+	+
Corada et al. (2021) [26]	+	+		+	+	+
Tomson et al. (2021) [27]	+	+	+			+
Wróblewska et al. (2021) [28]	+	+	+		+	+
Ysebaert et al. (2021) [25]	+	+	+	+	+	+

+ = content available in the category

shows the categorization of the previously defined subsections of the selected publications and the content of each review article. Six categories were created to span an umbrella around the topic of façade greening and PM to provide the following state-of-the-art and other basic recommendations for further research and practice.

3.2. Methodologies Used for the Determination of PM Binding Capacity and Elemental Composition

The analytical techniques used for the determination of particle number concentrations and elemental composition of PM on plant parts do not depend on factors such as the investigated plant species, location, or degree of exposure. Therefore, all commonly used techniques from the selected review articles were discussed.

Tomson et al. [27] described four main approaches for the evaluation of PM removal efficiency:

• In situ measurement of PM concentrations in ambient air or the use of passive air samplers;
• Assessment of PM deposition on leaf surfaces, typically by gravimetric methods;
• Microscopic assessment of PM deposition on leaf surfaces;
• Assessment of impacts on PM elemental composition by GI.

The approaches come with different limitations. To complement PM deposition assessment using gravimetric methods or saturation isothermal remanent magnetization (SIRM), leaf traits had to be identified using other techniques [26]. In some studies, different approaches were combined, e.g., microscopic assessment of leaf surfaces and elemental analysis by using scanning electron microscopy (SEM) or environmental scanning electron microscopy (ESEM) analyses in conjunction with energy-dispersive X-ray spectroscopy (EDS) in (E)SEM-EDS systems. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was reported as a commonly applied technique for multi-elemental analysis [27].

Using different methods found contradictory results in measurements of different tree species [28]. The gravimetric method was primarily used in the review publication of Corada et al. [26] to determine PM accumulation. SEM was mainly used to determine leaf traits [26]. Explicitly for façade greening, primarily the methods (E)SEM-EDS and gravimetric methods, including adaptations of these, were applied [25,26].

Wróblewska et al. [28] reported on the difficulty of comparability due to different experimental setups and atmospheric parameters, as well as seasonal characteristics. In addition to this, unequal units in the different measurement methods were discussed, which then led to different statements about PM accumulation for the same species. Listed measured parameters were foliar retention Cp (μg cm⁻²), deposition velocities (cm s⁻¹), daily/annual retention efficiency (mg m⁻² d⁻¹), number density (particles mm⁻²), and saturation isothermal remanent magnetization (SIRM, µA).

Table 4. Summary of analytical techniques and approaches to the study of particulate matter and challenges in assessing the deposition of PM on plant parts.

Analytical Techniques	Additional Information/Limitations
In situ measurement of PM concentrations in ambient air or passive air samplers	Variability in experimental setups and atmospheric parameters
Gravimetric methods typically by PM deposition on leaf surfaces	Requires complementary techniques for leaf trait identification
Microscopic assessment of PM deposition on leaf surfaces	May involve SEM or ESEM analyses in conjunction with EDS for elemental analysis
Combined approaches	Contradictory results among different species

provides a conclusive overview of the analytical methods found.

3.3. Description of the Possible External Influencing Factors for PM Accumulation in Plants

An improvement in air quality by vegetation and PM was determined by different processes and distinguished by deposition and dispersion. Deposition processes were divided into dry and wet deposition, which are relevant for plants and PM. Wet deposition is the transfer of particles by precipitation to plant parts, whereas dry deposition is the transfer of particles by gravity, diffusion, impaction, and interception processes [25].

The dry deposition was presented in Wróblewska et al. [28] and Tomson et al. [27] and was described as a temporary removal of particles from the air, thus providing a reduction in ambient air concentrations. The deposition was influenced by various factors and was described in the following formula:

Deposited amount (g m⁻²) = LAI × v_d × c × t

(1)

The individual parameters used to calculate the deposited amount are the leaf area index (LAI), the deposition velocity (v_d), the concentration of pollution (c), and the time (t).

$$v_{\mathrm{d}}=\frac{1}{R_{tot}}=\frac{1}{R_a}+\frac{1}{R_b}+\frac{1}{R_c}$$

(2)

The deposition velocity is calculated based on the aerodynamic resistance (R_a), boundary resistance (R_b), and surface resistance (R_c) and thus includes plant-specific and meteorological parameters [27,28].

Besides micro- and macrostructures of the plants, factors such as precipitation, wind speed, and humidity, as well as the distance to the pollution source and, in the case of GI, road geometry, play an important role [13]. Ysebaert et al. [25] and Corada et al. [26] report climatic conditions or short-term changes that can affect plant health and thus PM accumulation, such as heat and drought stress, as well as also seasonality.

In addition, plant structures can influence dispersion by adding mechanical turbulence and acting as a physical barrier to airflow. Model studies showed that vegetation thickness, i.e., leaf area density (LAD), and deposition velocity are crucial parameters influencing PM accumulation [27].

3.4. Effectiveness of Façade Greening in the Accumulation of PM

Wróblewska et al. [28] reported a particular dependence of air purification performance on the location. The greatest impact was obtained in small-scale geometry, such as street canyons with a low prevailing wind velocity [28], and depended on humidity and LAI [13]. Vegetation alters the flow pattern, changes pressure and drag forces, and thus acts as a momentum sink. The resulting air turbulence promoted the deposition of particles on the leaf surface [25].

Reductions of PM₁₀ of up to 60% were reported, as well as reductions of PM_2.5 without giving a percentage. It is also positively noted that behind a green wall at pedestrian height (1.4 m), a positive effect on PM reduction was achieved, which decreased with increasing height. It was indicated that façade greenings should not be placed near tall buildings due to the effect described above [28]. Tomson et al. [27] reported that façade greening has been shown to be effective in removing PM, especially PM₁₀. In open streets, façade greening—in this particular case, a vegetated wall—acts as a barrier, increasing PM concentrations on the street through air recirculation while reducing them significantly behind compared to vegetated-only structures. In addition, ultra-fine particle (UFP) concentrations were reduced by up to 50% on the downwind side compared to no barrier [13,27]. It was summarized that for different plant species of a green façade, on average (122.1 ± 6.9) × 107 particles of PM₁, 8.24 ± 0.72 × 107 particles of PM_2.5, and 4.45 ± 0.33 × 107 particles of PM₁₀ were accumulated related to one square meter of façade [27].

Consequently, washing off the particles resulted in their deposition in the growing medium [25,28]. In addition, ultra-fine particles (PM_0.1) that are difficult to wash off could be taken up via the stomata of the leaves [25].

Table 5. Summary and overview of the main results on the effectiveness of façade greening to accumulate PM.

Environmental Effects	Additional Information
Impact of location	Air purification varies depending on the location; vegetation changes the flow patterns and increases the deposition of particles
PM reduction	Positive effects behind vegetation, decreasing with height
Street dynamics with vegetated barriers	Vegetated barriers increase street PM but reduce concentrations behind
Particle accumulation in façade greenings	Variable for different particle sizes; wash off is difficult for ultra-fine particles

provides a summarized overview of the results described above.

3.5. Impact of Individual Leaf Characteristics of Macro- and Micromorphology of Façade Greening Plants to Increase Particle Deposition

Ysebaert et al. [25] reported in their work on plant macrostructures that differences in wall leaf area index (WLAI) and leaf size were observed in green wall (GW) plants, similar to differences observed for trees and shrubs. PM deposition was increased by complex heterogeneous structures, which generated more turbulence than linear homogeneous structures [25].

Unlike macrostructures, reports on the impacts of microstructures were inconsistent with what was reported in previous investigations for trees and shrubs. Thus, for example, rough leaf surfaces, grooves and furrows, and even wax layers on leaves did not necessarily have the highest PM deposition values as in the case of trees and shrubs. Similarly, there were conflicting conclusions about the presence of trichomes, where individual studies had found no significant difference. However, the stomatal density and usability of leaves were found to have a positive effect on PM uptake for the examined species [25].

Corada et al. [26] summarized the micromorphological leaf traits to remove airborne PM referring to GI in general. In a certain difference to the previous finding, further studies show positive effects for trichomes, wax, and roughness and contradictory results for leaf size and leaf shape. Additional contradictions or no clear consensus could be found for stomatal distribution and stomatal density, leaf area index and adaxial surface, in contrast to previous results [26].

3.6. Investigation of Façade Greening Plant Species and Their Potential Effectiveness for PM Accumulation

The most commonly investigated plant species of façade greening is Hedera helix [13,25,27]. Using different methods, significant differences in PM accumulation were found in the Hedera helix for different traits, such as the number of particles on the abaxial and adaxial surface, exposure of the leaves to the PM source, and leaf surface and wax layer. Only the level of leaf sampling and the season were documented as characteristics of non-significant differences.

Conflicting results were also found for PM accumulation over time, which were thought to be due to differences in rainfall and time [25]. In addition, different measurement methods were used when comparing the results [25,27].

The study by Tomson et al. [27] summarizes that field studies primarily used evergreen species (85%) in order to be able to investigate year-round performance, which limits comparability. Results on effective species, less effective species, and conflicting results between species are presented in

Table 6. Overview of plant species investigated for façade greening and their potential effectiveness in PM accumulation.

Effective Species		Less Effective Species	Conflicting Results
Trees/Shrubs	Perennials/Fern/Grass
1,2,4Berberis buxifoliaa,b,f	1Aptenia cordifoliaa,b2,d	1Blechnum spicanta1,b,h	1Bergenia cordifoliaa,b,g
1 Buxus sempervirensa,b,f	3Carex caryophylleaa2,b,h	3Festuca rubraa2,b,h	1,4Philodendron Xanadua,b2,g,*
1Hebe albicansa,b2,f	1,4Chlorophytum comosum variegatuma,b2,g	1Lonicera kamtschaticaa2,b,f
1Hebe youngiia,b2,f	1 Erigeron karvinskianusa,b,g	1 Luzula niveaa,b,h
1,4,5Hedera helixa,b,e,f	4 Geranium macrorrhizuma1,b,g	1Persicaria amplexicaulisa2,b,g
1,2,4Juniperus chinensisa,b,f	1Helleborus x sterniia1,b2,g	1 Phlomis fruticosaa,b,f
5Lianas speciese	4Nephrolepsis exaltata bostoniensisa,b2,h	1 Phyllitis scolopendriuma,b,h
1,4Nematanthus glabraa,b2,f	3Polygonum avicularec	4 Neomarica gracilis *
1Sophora japonicaa2,b,f	1 Sedum palmeria,b1,d
1,3,4Thymus vulgarisa,b,f	1Sedum spuriuma,c,d
1 Trachelospermum jasminoidesa,b1,e,f	3Sisymbrium loeseliic
1,4Veronica vernicosaa,b1,f	1,4Spathiphyllum wallisiia,b2,g

¹ Tomson et al. [27], ² Corada et al. [26], ³ Wróblewska et al. [28], ⁴ Ysebaert et al. [25], and ⁵ Abhijith et al. [13]. ^a = evergreen; ^a1 = wintergreen; ^a2 = summer green; ^b = frost resistant; ^b1 = limitedly frost resistant; ^b2 = not frost resistant; ^c = annual plant; ^d = succulent plant; ^e = climbing plant; ^f = tree/shrub; ^g = perennial/herbaceous; ^h = fern/grass; * less effective with higher atmospheric PM concentrations.

. In addition to Juniperus chinensis and Berberis buxifolia, the study by Corada et al. [26], which compared SEM studies only, identified Berberis x media as a third species effective for PM accumulation in comparison to Tomson et al. [27]. Wróblewska et al. [28] stated that non-woody species accumulated less PM than shrubs in comparison. Thymus vulgaris, Carex caryophyllea, Polygonum aviculare, and Sysimbrium loeselii were found to be effective herbaceous plants, whereas Festuca rubra showed the least effectiveness [28].

3.7. Implications for Practice and Research—Knowledge Gaps and Significance of Studies

All included publications call for more research in the area of façade greening and PM. Recommendations for future research and for the application in practice were nevertheless provided. More wind tunnel studies and validated model studies should contribute to a better understanding and close knowledge gaps on key parameters such as type, residence time, and pollution time [25].

Based on the results, recommendations on proper plant selection with different species were provided. In addition, tolerance to chemical stressors such as volatile organic compounds (VOC) should be considered in plant selection [28]. Despite the existence of a significant gap in knowledge about the effectiveness of façade greening for PM reduction, they offer an alternative to street trees and can be used as a potential PM reduction measure in existing infrastructures where space constraints prevail [27].

Differences in parameters usually make the comparability of previous research challenging. Standardization of vegetation density to make further recommendations for practice is essential. Nevertheless, there is an overall reduction in air pollutants in the presence of GI. Due to the complexity of street canyons, pilot modeling is recommended for a better understanding of how to maximize PM reduction through GI. It is recommended that research should also be expanded to include relationships between vegetation and climate zones on air quality and should increasingly target green façades and green roofs as they take up less space. The use of these GI could also be applied to building surfaces such as bridges, overpasses, retaining walls, and noise barriers [13].

Furthermore, there should be more collaboration between botanists, architects, landscape architects, and urban planners, as well as air pollution experts, when establishing GI in urban areas to integrate crucial meteorological and external parameters such as location and seasonality into the planning process. In addition to the existing research gaps, plants should be considered in their entirety, including branches and stems, which offer further potential to accumulate PM from the air in the short or long term, especially because individual leaf characteristics may only make a small contribution to PM reduction. It is considered important to provide standardized methods, in particular, the method for measuring PM on the leaf surface, as well as background information on sampling [26].

4. Discussion

The objective of this article was to provide a comprehensive overview of previous research on façade greening and PM, addressing several subtopics. By systematically synthesizing information from multiple review publications, we summarized standard measurement methods, factors influencing the effectiveness of façade greening, leaf traits, plant species investigated, and the impact of façade greening as a PM accumulator. The resulting findings revealed the current state-of-the-art but also various knowledge gaps that need to be discussed to provide recommendations for research and practice.

4.1. Methodologies Used for the Determination of PM Binding Capacity and Elemental Composition

As the results have shown, the comparison between individual studies is challenging, both due to the different methodologies used, such as imaging analysis and gravimetric measurements, and due to the large number of units presented. In some cases, there are contradictory results for the same investigated species, depending on the method used. This represents a major problem of the current research situation and impedes conclusive meta-analyses [26,27,28].

As described in Corada et al. [26], leaf property studies were primarily conducted using SEM measurements in the study of green façades. The advantage of this imaging method is that particle sizes can be distinguished [26,29], whereas gravimetric or SIRM methodologies can only measure the total amount of PM on leaves. These methodologies are well-suited to determine individual leaf characteristics that have a positive or less positive effect on the accumulation of PM [26]. Since these methods only investigate a very small section of leaves, it is difficult to obtain or estimate the overall performance of a façade using these methods. In addition, with these measurements, the resuspension of particles becomes difficult to determine.

Nevertheless, the results on leaf traits and the applied methodology provide a first overview and show tendencies for further investigations. The traits already published should be further investigated using a uniform method and considering influencing parameters since, for example, residence times and ambient concentrations can play a crucial role.

Regarding the determination of PM elemental composition, the analytical techniques used in the five included studies comprise ICP-OES as well as (E)SEM-EDS [27]. Methods based on inductively coupled plasma mass spectrometry (ICP-MS) were not mentioned in the included review articles but are able to provide more detailed insights, especially into the contents of less abundant elements due to strongly increased sensitivity and thus lower limits of detection [30,31].

In combination with laser ablation (LA), spatial resolution analyses are possible, and samples can be analyzed directly without sample preparation [32]. Laser-induced breakdown spectroscopy (LIBS) is another promising technique for the elemental characterization of PM [33]. While interferences hamper the analysis of major elements such as H, C, N and O with ICP-MS strongly, they become accessible with LIBS. Combined LA-ICP-MS/LIBS systems have been used for simultaneous trace element analysis and depth profiling [34]. This tandem technique might also deliver valuable information when applied to leaf samples, which is to be investigated in future studies.

4.2. Possible External Influencing Factors for PM Accumulation on Plants

The results of this study emphasize that, alongside diverse methodologies, the intricacy of this subject encompasses numerous influencing factors, each carrying variable importance depending on the context. Factors such as total leaf area, leaf area index (LAI), deposition velocity, pollution concentration, time, precipitation, wind speed, humidity, and plant structure collectively contribute to the overall understanding and should be consistently considered in their entirety [13,25,27,28]. In addition, there are plant variabilities during a vegetation period, but also stress situations, such as heat and drought, that can change the plant structure and consequently lead to changes in wind turbulence [25,26].

To determine basic data on the effectiveness of façade greening, model and wind tunnel studies [35,36] are used in addition to field studies, as well as, for example, climatic chamber studies. These studies benefit from the advantage of controlling influencing factors [37], such as wind speed, temperature, and humidity, in larger or smaller parts. This is also demonstrated in the field study of Viecco et al. [38], where tests are performed under controlled environmental conditions in a room, followed by gravimetric procedures to measure particle concentration [39].

Furthermore, computational fluid dynamic (CFD) studies can provide basic data on this topic. In general, CFD modeling works with simplified representations of the plant structure. A combination of modeling to determine real drag coefficients of the airflow, including a comparison with real vegetation, for example, seems to be a more reasonable and faster way to adjust to geometric conditions [40,41]. Nevertheless, CFD modeling also has its limitations when transferred to real façades [42], and at the same time, the data are not extensively available, as these would have to be checked on a plant-specific basis.

In summary, the results of this umbrella review show that the external influencing parameters are numerous and partly depend on the selected façade greening system and the plants themselves. The dynamics of the vegetation alone make it difficult to draw general conclusions about the accumulation of PM. A possible solution could be model and wind tunnel studies, as well as climate chamber investigations in combination with measurements on real façade greening systems. For subsequent studies, a variety of external influences should be included in the measurements to make them more comparable.

4.3. Effectiveness of Façade Greening in the Accumulation of PM

The results regarding the effectiveness of façade greening to accumulate PM indicate a correlation between PM accumulation and the previously mentioned factors, including location and wind speed [28], humidity and LAI [13], or air turbulence generated due to its own geometry [25]. Consequently, stated percentages and averages [27] on the effectiveness of façade greening for PM reduction are generally very difficult to transfer to another location.

In addition to the dependencies already mentioned, there is a variety linked to different greening systems. A modeling study by Santiago et al. [43] using CFD showed that the particle concentrations on vegetated walls, compared to trees, are reduced in the vicinity of a building due to the lower residence time. As a result, trees and shrubs were evaluated as more effective GI. However, for deep street canyons, the advantage of vegetated walls over trees is that they do not reduce street ventilation and thus also improve air quality in general [43]. To determine an increase in the effectiveness of greened façades in relation to non-greened façades, comparisons with conventional façades, such as wood, plaster, and glass, should be investigated in addition to measurements of ambient concentrations. Individual studies indicated an average reduction of 25% in PM_2.5 and a reduction of 37% in PM₁₀ compared to non-green surroundings [10]. The previously mentioned comparability due to influencing factors and different methodologies should not be disregarded in this context.

Because of the restricted number of review publications on vertical greening, a statement on the effectiveness of the reduction of PM concentration is limited. Furthermore, individual studies show little or no comparison to alternatives and should be extended in this regard. Nevertheless, the results showed effectiveness in reducing PM, which should be continued as a basis for further studies with different systems and different plant species.

4.4. Impact of Individual Leaf Characteristics of Macro- and Micromorphology of Façade Greening Plants to Increase Particle Deposition

As shown in the results, it remains questionable whether individual leaf surface structures have a major impact on PM accumulation, which was also evident in the summaries investigated. While some studies show an increased effectiveness of individual features, others do not provide such an indication.

In general, and with respect to GI, including trees, shrubs, green roofs, and green façades, field studies of the effectiveness of individual leaf or plant characteristics can show positive effects on PM accumulation. These generally showed that small leaves [44], roughness factors [45], trichomes on leaves [46], waxes [26], and stomata showed enhanced effects on PM accumulation. In addition, complex leaf structures [47], lanceolate [27], pinnate [48], lobed [49], ovate [26], and obovate [27] leaf shapes have shown positive effects.

In addition, plants usually have combinations of these rather than a single trait in their structure, which argues against assessing a specific trait and instead favors examining the entire leaf structure, including all individual traits [50]. However, studies on trait combinations that have a positive effect on particle deposition in plants also proved to be challenging due to variability linked to vegetation macrostructures and environmental parameters [51]. It would be important to include not only the individual surface structures but also the entire above-ground parts of plants in the studies since these lead to an external influence that cannot be neglected when turbulence is generated in different ways.

Furthermore, macrostructural characteristics such as LAI and LAD have been assessed as being relevant to PM accumulation performance [47] by influencing wind flows as well as the residence time of particles [4]. For GI in general, high LAI [4,27,49,52,53,54] and LAD [4,17,52,55] are reported to have a positive impact on macrostructure. In combination with LAI and LAD, other plant physiological parameters could be included, which could have an influence on PM accumulation. For example, an assessment scheme for the vitality of herbaceous vegetation [8] or the condition of the plant in general could be included.

As shown in the results section of this study, results found for trees and shrubs were partially contradicted in studies on façade greening [25,26]. Impacts of individual traits could not be verified due to the limited data availability. The challenge lies in the comparison between the large number of different plant species used for façade greening in different systems worldwide. Nevertheless, a link to the existing substantial data situation for trees and shrubs could be expected but should be investigated in more detail for herbaceous vegetation and climbing plants. Consequently, it can be stated that macro- and microstructures are not always clear parameters but nevertheless appear to have an influence, as another individual study on herbaceous vegetation for façade greening shows [56].

4.5. Investigation of Façade Greening Plant Species and Their Potential Effectiveness for PM Accumulation

Hedera helix has been identified as the only plant that has been investigated more extensively than others, although individual studies have reached different conclusions [25].

This result, in turn, indicates the importance of the external influencing factors and the use of uniform measurement methods. To identify differences between plant species, the same environmental conditions would need to be produced [37]. An extension of the already investigated parameters should include the air pollution tolerance index (APTI) in future measurements. A field study shows, for example, that the plant species Carpobrotus edulis and Rosmarinus officinalis had the highest tolerance to air pollution, while Kochia prostrata was very sensitive to it [29]. Such values should also be included in future planning, as in addition to a high ability to absorb PM, it is also important that the selected plants can withstand pollution and thus have a high APTI value [57].

Challenges in the comparability of investigated species are associated with the experimental sites. The selection of plants for façade greening is primarily determined by (micro-) climatic conditions [58]. This has the consequence that not only internationally different plants are investigated but also nationally different plants are used depending on the location, exposition, and, additionally, the façade greening system used. Furthermore, due to the lack of frost resistance, not all species already investigated can be used in other climates. Nevertheless, façade greening plants and their correct selection have the ability to accumulate PM [59].

4.6. Implications for Practice and Research, Knowledge Gaps, and Significance of Studies

The results of the articles reviewed highlight the need for further research in the field of façade greening and its effects on particulate matter (PM). The recommendations emphasize the identification of knowledge gaps regarding key parameters and suggest that these gaps be explored through methods such as wind tunnel experiments and pilot modeling studies [13,25]. For the selection of appropriate plants, future studies are recommended to consider the air filtering traits of the whole plant [26], as well as their tolerance to stress [28]. Vertical greening is suggested as a space-saving measure for the reduction of air pollution with high potential [27]. Standardization of the applied measurement methods was provided as another important recommendation for future research [26]. Based on the present state of research, clear recommendations for practical application, especially in plant selection, are premature. Nevertheless, it is recommended to include the current findings in planning and to extend them to the already better-investigated benefits of GI. These include potential cooling effects or an increase in biodiversity [58]. Furthermore, the increase in biodiversity (respectively, the investigation of polycultures compared to monocultures) also showed an increase in PM deposition [60].

5. Conclusions

The aim of this study is to provide a more comprehensive and new scope of the existing literature on the topic of façade greening and PM, including various sub-topics, to identify gaps in knowledge and to make recommendations for future projects. This could be achieved and discussed in detail using the selected methodology of an umbrella review and the investigation of previous reviews. Our umbrella review covers various aspects of the relationship between façade greening and PM. We found that the methods used to determine PM binding capacity and elemental composition are diverse and may affect the accuracy of results, including particle size distribution. External factors such as climate, traffic, and building characteristics can also influence PM accumulation in plants. The variability of external factors, like wind conditions and relative humidity or wash-off events, affecting particle deposition across studies presents a challenge for the comparative analysis of different studies. A further and additional consequence of the previous conclusions is that the effectiveness of façade greening in reducing PM is difficult to assess due to variations in plant species and façade greening systems used. Our analysis shows that the impact of individual leaf characteristics on PM deposition is inconclusive, and further research is needed to support the validity. This knowledge gap is primarily based on the numerous measurement methods and influencing factors and should be considered in future studies. This highlights the need for modeling compared to real facade investigations to address this knowledge gap and increase the reliability of the results.

From a public health perspective, exposure to PM can cause a range of adverse health effects, including respiratory and cardiovascular problems, reduced lung function, asthma, lung cancer, and premature death. Thus, mitigating PM exposure can have significant health and economic benefits, including improved respiratory and cardiovascular health, reduced healthcare costs, increased worker productivity, and reduced environmental damage. Incorporating nature-based solutions into urban planning and design can help to create more sustainable, resilient, and livable cities.

Due to the quite complex topic of façade greening and PM, extensive interdisciplinary work should be carried out. Planning and implementing different façade greenings require interdisciplinary collaboration among urban planners, landscape architects, engineers, ecologists, and other stakeholders to ensure that the GI is designed, installed, and maintained to maximize its benefits and minimize its risks. Future empirical studies should incorporate the findings of this study, such as already established methods like (E)SEM-EDS, leaf characteristics, the measurement of environmental parameters, and the inclusion of model studies in comparison with real facades. The indications that can be seen from the limited data available can already be used to extend the known benefits of façade greening as a sustainable solution for reducing PM.