For the mitigation of urban heat island and urban noise island: two simultaneous sides of urban discomfort

Urban environment well-being has become a crucial public issue to face, given the huge concentration of population and climate change-related hazards at a city scale. In this view, Urban Heat Island (UHI) is now very well-acknowledged to be able to produce a serious threat to populations around the world and compromise human well-being due to aggressive overheating, exacerbated by anthropogenic actions. The same anthropogenic actions are also responsible for other discomfort causes such as noise pollution, which has also been demonstrated to heavily impact societal life and health conditions in urban systems. Both these phenomena typically co-exist in terms of space and time coincidence, and they both may be mitigated by means of smart adaptive and multifunctional surfaces including urban pavements and building envelopes. This review bridges the gap between only-thermophysical analysis about UHI mitigation and only-acoustics analysis of urban noise pollution, here defined as Urban Noise Island (UNI). To this aim, the key physics background of mitigation techniques is presented and the most innovative and promising solutions for counteracting UHI and UNI are described, with the final purpose to foster research and innovation towards more livable cities through a multiphysics and holistic view.


Introduction
In recent years, the steep rise of urbanization, a phenomenon referring to the population shift towards urban areas, has posed numerous challenges for the preservation of fair living standards for citizens around the world [1]. In 2016, 54% of the world population was reported as urban dwellers [2], while projections forewarn that in 2100 this fraction may be increased up to 85% [3]. At the same time, it is well documented that urban sprawl resulted in many cases of serious environmental issues within cities, which in turn put dwellers health in high-risk [4].
Among all environmental repercussions of urbanization, urban overheating is generally considered as the preeminent one [5][6][7]. Therefore, the typically high inner-city ambient temperature as compared to adjacent rural areas, i.e. Urban Heat Island (UHI), is a well-reported phenomenon occurring in many metropolitan areas. Being inherently interconnected with the ongoing Global Climate Change (GCC), UHI is a present-day issue accountable, in many cases, for inferior living standards occurrences within urban areas [8][9][10]. Heat-related mortality and morbidity, expansion of air pollutants, high CO 2−eq emissions to the atmosphere, indoor/outdoor thermal discomfort, are some of the effects of UHI [9][10][11]. Yet, UHI effects are additionally magnified when interplay with heatwaves that increasingly occur within the last decades. [12][13][14]. In fact, more than 500 million urban dwellers are expected to undergo extreme heatwave conditions once per five years [15]. Additionally, UHI implications may be perplexed even more with respect to diurnal and weekly temperature cycles which may vary according to the latitude and the time of day [16].
In view of that, release of anthropogenic heat [17][18][19], urban surfaces covered with dark-colored conventional materials [20,21], complex concentrated urban structure, decreased sky view factor [22,23] and alterations of urban evapotranspiration and convection efficiency [24] have been reported as the main drivers for UHI. In fact, metropolitan regions are mainly covered by pavements and buildings. For instance, McPherson [25] reported that pavements and buildings cover 50% and 25% respectively of the commercial urban area of San Francisco, US while according to Akbari and Rose, paving surfaces may usually cover 35-40% of the total urban area [26]. As a consequence, the materials implemented in the urban infrastructure significantly regulate urban microclimate [27][28][29].
Still, urban overheating is not the only repercussion related to urbanization. The increased population density in urban areas inevitably led to another critical issue, such as high levels of anthropogenic noise [64]. The dense urban planning, mirroring the style of modern architectures, together with the exponentially increased need for urban transportation resulted in excessive noise incidences within cities [65]. As a result, cities are significantly louder than rural areas and thus can be regarded as Urban Noise Islands (UNI).
Urban noise is indeed observed in both developed and developing countries [66]. Already since 1994, a rough quarter of the total EU population had been reported as exposed to high transportation noise levels [67]. In 2011, one out of three European urban dwellers was reported distressed by urban noise incidences during its everyday routine [68]. This fraction rose up to 65% by 2013 [69]. More specifically, citizens were reported to systematically experience daily noise levels exceeding the upper limits (53 dB during day-time, 45 dB during night-time) established by the World Health Organisation [70]. Recent studies have moreover forewarned about the interconnection between high urban noise levels and severe impacts to humans' well-being such as stress [71] disturbed sleep [72], hearing loss [73] and increase of heart attack or stroke risk [74,75].
In view of that, various measures have been promoted for counteracting UNI. The majority of them focus on the mitigation of transportation noise since in most cases, it is liable for up to 80% of the total noise pollution in urban areas [76]. Low noise vehicle engines and tires, electric vehicles, effective traffic management, greenery solutions, reduction of traffic density, speed reduction, an overhaul of public transportation, and so forth [77][78][79] are among the main heretofore implemented solutions.
Nevertheless, unlike UHI, urban noise is not a well-documented phenomenon as it is. For instance, although several studies have highlighted UNI as a harmful problem and have proposed mitigation techniques on a small scale, according to the best of authors' knowledge, only a limited number studies has investigated holistic strategies for the mitigation of urban noise with respect to the mesoscale urban geometry and the build environment in particular [80,81].
Since, however, the built environment dominates urban areas, there is a growing interest concerning materials incorporated in terms of further UNI mitigation potential. Under this scenario, novel or natural materials with noise mitigation qualities, e.g. sound absorption and control, have been investigated by a number of researchers [82][83][84]. Studies conducted in this scientific field have implemented noise absorbent materials that can be divided into two main categories; porous absorbers and resonance absorbers [85,86]. The former is characterized by a wide absorption frequency range while the latter by a narrow range, particularly in the low frequencies.
As a result, porous absorbers have been used into the urban environment especially in paving infrastructure for attenuating road-traffic noise which typically dominates within cities [83,87]. In addition, porous media have been tested and found promising in terms of improving energy efficiency of buildings [88]. Moreover, within the last decades, advanced metamaterial structures have been developed with superior sound absorption, e.g. total sound absorption within a desired but rather narrow frequency range [89,90]. However, their application is mainly at a theoretical level yet.
Within this framework, it becomes evident that urban materials and surfaces, in particular, may regulate urban microclimate and the corresponding multi-physical parameters. Thus, novel cross-cutting material-based measures could be taken for developing a more resilient and adaptive built infrastructure. Under this framework, novel smart materials, capable of multifunctionally improving urban environmental quality, could be considered as the next generation of urban smart skins.
In this regard, the main objective of this review study lays on a scientific challenge, which according to the best of the authors' knowledge, has not been faced yet: the multiphysics analysis of smart materials incorporated into the urban environment that can tackle both UHI and UNI at the same time and in the same spatial context, i.e. the anthropogenically affected modern urban systems. Therefore, in this study, for the first time, a systematic review was carried out for presenting the scientific advances and techniques aiming to mitigate UHI and UNI which, out of the box, are characterized by different physical qualities. Nevertheless, their ongoing intensification has its origins on the materials incorporated into the urban environment. To that end, further on, this paper aims to discuss and suggest, under a conceptual framework, urban smart multifunctional surfaces that could conjoin simultaneously UHI and UNI offsetting qualities.

Methods and design
The review methodology of this study follows a systematic approach. The Systematic Review (SR) methodology is a research design for bringing together and critically analyzing results/findings from a list of strategically selected published studies. SRs originally emerged for collating outcomes of various experimental studies, especially ones related to health science field [91]. Yet, currently, their use is expanding towards evidence synthesis-based research methods aiming to give consolidate outputs, e.g. pinpoint scientific gaps/challenges and thus pave the way for future advancements of the relevant field. The main advantage of SRs compared to generic literature reviews lays on their quality of minimizing biases and random errors owing to a specific and predefined methodology typically consisting of the following 4 sub-steps: (a) Define a scientific question that needs to be answered. (b) Define explicit inclusion/exclusion criteria for the studies relevant to the topic to be reviewed. (c) Perform a critical review of the selected studies. (d) Conclude to research gaps/challenges and expound on future paths to be followed Under this framework, at first, the following scientific question was defined as the main stimulus of this critical review study: • Research Question: How can UHI and UNI be simultaneously mitigated by the same urban component?
Consequently, the following sub-questions were also defined in order to further structure the analysis.
(a) What are the recent advances of applied material science in terms of UHI mitigation and relevant applications? (b) What are the main advantages/disadvantages of the above solutions in terms of UHI mitigation? (c) What are the recent advances of applied material science in terms of UNI mitigation and relevant applications?
(d) What are the main advantages/disadvantages of the above solutions in terms of UNI mitigation?
Scopus and Web of Science (WoS) were chosen as the primary web-search engines for the presented analysis. In addition, Google Scholar was utilized for identifying (i) grey literature (e.g. governmental documents and standards) related to the research questions, (ii) the work of key researchers according to the author's point of view and (iii) referenced publications found in previously reviewed publications within the presented review. The web search query was carried out within November 2019-February 2020.
Afterward, specific search terms related to UHI and UNI were determined and combined through the Boolean operators OR and AND for application to the scholarly databases (figure 1). The outcomes of both web academic databases were then merged and checked for duplicates. Subsequently, eligibility criteria were applied to the remaining publications for their exclusion/inclusion in the analysis (figure 1). In order to capture the historical continuity of the relevant advances, no time-frame exclusion criteria were applied. Yet, the vast majority (87%) of the reviewed studies come from 2006 to 2020 (figure 2). Furthermore, it should be noted that studies utilizing numerical models were considered for review only if the models were validated.
Finally, three screening steps were followed. Within the first step, studies for both UHI and UNI were included/excluded by reading the corresponding abstracts, while during the second step by reading the whole manuscript. Specific attention was given to journal articles since they were peer-reviewed. Within the last screening step, articles were retrieved through snowball selection [92]. In more detail, 12 and 6 publications concerning UHI and UNI respectively were not found directly within the results of the keywords search. Instead, they were identified through the reference lists of publications selected through the second screening test and their outcomes were considered of relevant significance. In the end, 62 journal articles, three conference articles, six technical reports and one book were reviewed in terms of UHI and 71 journal articles concerning UNI (table 1). One article was reviewed in terms of both UHI and UNI and therefore the overall number of the manuscripts included in the analysis was 142.

UHI mitigation
Hundreds of scientific studies have investigated techniques and synergies aiming at mitigating the hazardous effects of UHI on urban habitats [93,94]. Furthermore, UHI is a well-investigated phenomenon not only at neighborhood or city scale but also on a global scale [95]. In fact, UHI may act synergistically with global climate change and therefore its mitigation is considered of high importance not only by the scientific community but also by local and global authorities and policymakers [96]. This chapter provides a snapshot of the scientific advances, utilized to date for cooling the cities [94].

Highly reflective materials
The first step towards the mitigation of UHI came off with the introduction of Cool Materials (CMs) (table 2). CMs' main attribute is high solar reflectance (also referred to as 'albedo' (a)) together with high emittance in the infrared spectrum [97]. This reduces the solar energy gains of the surface, and increases the longwave radiative heat dissipation efficiency, and thus low surface temperatures can be achieved.
Towards the perspective of white CMs, in 1968, Givoni and Hoffman [98] compared grey colored building facades with white counterparts in terms of the superficial temperature. They showed that whitecolored facades were approximately 3 • C cooler under unventilated conditions. Later on, Berg and Quinn [99] measured the temperature of surfaces with different albedo values. They reported that an albedo value equal to 0.55 can result in a surface temperature almost equal to the ambient one. On the contrary, surfaces with a lower albedo (0.15) were almost 11 • C warmer as compared to ambient temperature. The significance of the albedo value was reported also in the study of Taha et al [100]. They showed that an elastomeric coating characterized by albedo equal to 0.72 was up to 45 • C cooler than a conventional black coating (albedo of 0.08). Santamouris [104] reported that white-colored pavements, that naturally have higher albedo than dark-colored counterparts, can be up to 18 • C cooler than conventional asphalt pavements. In the extensive study of Doulos et al [103] the thermal properties of 93 different paving materials were infield investigated. Results showed that the materials' thermal balance is significantly related to their sunlight reflectivity and infrared emissivity. Moreover, it was concluded that the lighter the color of the surface, the higher the albedo and consequently the lower the surface temperature.
Within this framework, Synnefa et al [33] showed that a cool coating placed on a white concrete tile can decreased its surface temprature by 4 • C and 2 • C during daytime and nightime respectively, under typical hot summers conditions. Similarly, Stathopoulou et al [32] reported that incorporating CMs into the paving and building infrastructure can significantly counteract UHI. CMs have been widely investigated as roof components. For instance, Akbari et al [101] reported that an increase of roof albedo by 40% may result in a reduction of roof temperature up to 22.2 • C. Moreover, Akridge [102] reported a roof temperature reduction by 33 • C through the implementation of a high-albedo acrylic coating on a single floor building. Likewise, Pisello and Cotana [34] reported an up to 4.7 • C indoor temperature reduction by implementing cool clay tiles on the roof of a noninsulated residential building in Italy. In addition, they showed that the winter penalty was negligible. Similarly, a minor winter penalty as compared to the annual energy savings concerning a cool roof application was reported also by Ramamurthy et al [105]. In more detail, they monitored the yearlong surface temperature and the corresponding heat flux of various roof structures with different albedo values on a test site located in Princeton, New Jersey, USA. Results also showed that during the summer period the surface temperature of white and black roofs ranged from 10 • C to 48 • C, and 10 • C to 80 • C respectively.
The main determinant of heat loss during winter period was found to be the insulation thickness of the sub-layers.   [100] 1992 superficial temperature of surfaces with various albedo white (a = 0.72) was 45 • C cooler than black (a = 0.08) Akbari et al [101] 1998 impact of roof albedo in buildings' cooling demand increase in roof 's albedo Akridge [102] 1998 high-albedo acrylic coating vs. conventional roof 33 • C decrease of roof peak temperature Doulos et al [103] 2004 thermal properties of 93 commonly used pavement materials "cold" materials = smooth and light colored and/or made of marble/mosaic/stone Synnefa et al [33] 2004 reflective coatings vs. white tile coatings were cooler up to 4 • C under hot summer conditions Santamouris et al [97] 2008 low cost cool coating using lime vs. they highlighted that the overall mitigation potential could be substantially enhanced with the implementation of cool materials in the pavement and road infrastructure. Ramamurthy et al [112] performed yearlong simulations with Princeton Roof Model concerning a cool roof application in the New York City Metropolitan region. They showed that a highly reflective (albedo = 0.7) roof membrane can substantially decrease the annual energy demand. They also found out that the corresponding winter penalty is negligible since peak heating periods throughout winter period occur typically during night-time when albedo impact is trivial.
Zhang et al [95] utilized the version 1.2.0 Community System Model and perform an extensive study in terms of urban, continental and global scale. They reported, that except for some Africa and Mexico regions, a statistically significant annual and global mean temperature decrease varying from 1.6 to 1.2 • C can be succeed. Another implementation of the Weather Research and Forecasting (WRF) mesoscale model concluded that an elevated albedo in Terni's urban structure may lead to a temperature decrease up to 2 • C during both daytime and nightime [113]. Further focus on the ageing issues of high albedo surfaces was given in the microclimatic study of Tsoka et al [114] through the implementation of ENVImet for an urban district in the city of Thessaloniki. They reported that even though cool materials could reduce surface temperatures up to 9 • C, a 10% reduction of their solar reflectance due to ageing could lead to a 50% reduction of their UHI mitigation potential. In another review study of the same scientific group [115] it was concluded that synergistic mitigation techniques in-between cool materials and trees can reduce the air temperature and the mean radiant temperature up to 2 • C and 15 • C respectively. UHI is particularly responsible for an increasing trend of heat-related mortality within built-up areas, especially during heatwave periods [116,117]. Within this framework, Macintyre and Heaviside [118] used the WRF regional weather model version 3.6.1 for evaluating the counteracting potential of an urban modification based on a cool roof intervention towards UHI within West Midlands, UK, and the consequent impacts to heat-related mortality. They found out that UHI is liable up to 40% of heat-related mortality during the summer period, while an extensive cool roof (albedo = 0.7) implementation could lead to a seasonal offset of 18%. This offset fraction was found even larger during heatwave periods, i.e. 25%. Additionally, they showed that for heatwave periods, cool roofs could decrease the downtown 2 m height air temperature up to 3 • C, with an average decrease of 0.5 • C.

Cool membranes
A number of studies aiming to counteract UHI effect focused on the development of cool roof membranes (table 4). For instance, Pisello et al [119], developed and analyzed both in-lab and in-field, cool membranes, characterised by solar reflectance equal to 85%. They emphasized that cloudy weather conditions, are a critical challenge for obtaining reliable results [119]. Additionally, Pisello et al [120] tested cool roof membranes and complementary cool facades on a university campus test building in Italy. This modification was found to decrease indoor temperature up to 3.1 • C during cooling period.
Also, Phase Change Materials (PCM) have been embedded into cool roof membranes for optimizing indoor thermal comfort and reducing energy demand of the building [40]. In the study of Pisello et al [41], various alternative PCM concentrations were investigated for ameliorating ageing properties of cool roof membranes, while Saffari et al [43] conducted numerical simulations in order to determine the ideal PCMs melting temperature.

Natural cool materials
Several more eco-friendly or sustainable solutions have been reported as alternative passive cooling techniques, and, hence, capable of mitigating UHI (table  5). Pisello et al [35] quantified the cooling potential of several relatively inexpensive gravels implemented on both roof and pavement structures. By performing comparative experiments both in-lab and in-field, it was concluded that the highest reflectance of solar radiation (62%) was attributed to the grits with the finest grain size.
Castaldo et al [36] endorsed the aforementioned findings and concluded that the finest size stones preserve the higher indoor thermal comfort together with decreased energy demand. On a global scale, effective implementation of cool stone aggregates may also significantly counterbalance CO 2−eq emissions [36]. Moreover, natural stones have been reported to produce a significant evaporative cooling potential. For instance, Gonçalves et al [121] showed experimentally that natural stones can have in some cases a higher drying rate than the evaporation rate of a free water surface. In addition, evaporative cooling of natural stones implemented into build environment can protect built heritage from dampness and salt crystallization incidences.

Cool colored materials
In many cases, due to aesthetic or other reasons such as cultural heritage preservation, specific appearance should be preserved through colored materials and finishings. For that reason, scientists, apart from white high albedo materials, also developed cool colored alternatives (table 6). The principal idea behind cool colored materials lies in their ability to highly reflect the near infrared (NIR) part of the solar energy spectrum. In fact more than 50% of the incoming solar global radiation is included within the NIR wave-range [122]. As a result a cool material that (i) absorbs in the visible part for having its color, highly (ii) reflects in the NIR part and (iii) reemits in the infrared part, can remain cooler than a Table 3. Microclimatic studies on high albedo materials.

Study
Year Purpose Outcomes Rosenfeld et al [106] 1995 albedo modification in Los Angeles peak summertime temperature reductions between 2-4 • C with albedo increase of 0.13 Rosenzweig et al [107] 2006 impact of cool surfaces on New York microclimate light colored surfaces can decrease air temppperature up to 1.6 • C Synnefa et al [111] 2008 impacts of large-scale increases in surface albedo on ambient temperature   [41] 2017 optimize durability of polyurethane cool roof waterproof membrane with PCMs durability optimization up to 25 wt%.
Pisello et al [120] 2017 cool coatings on differently oriented building envelope surfaces a = 0.51 may improve indoor thermal comfort by 4.4 • C Saffari et al [43] 2018 impacts of large-scale increases in surface albedo on ambient temperature an optimized melting temperature of PCMs in cool membranes can reduce energy need without compromising durability conventional colored counterpart that absorbs also in the NIR part and overall stores more energy. One of the first studies under this framework was performed by Levinson et al [123]. They developed several cool pigments of various colors able to absorb less than 10% of the NIR energy. Therefore, when these pigments are combined with the appropriate NIR background, they can function as a cool coating. In a study conducted by Synnefa et al [45], a practical substitution of conventional asphalt with cool colored asphalt base materials led to an average 5 • C decrease of ambient temperature. Likewise, Levinson et al [124] installed a near-infrared non-white reflective coating and reported a roof surface temperature decrease by 5-14 • C. As a result, Levinson et al [125] pointed out the importance of increasing NIR reflectance for enhanced mitigation of UHI. Moreover, Doya et al [37] reported an 1.5 • C decrease of surface temperature on street facades by implementing cool selective paint. Similar results were reported by researchers attempting to apply cool color strategies in historical/historic build environments. In order to maintain the building's external heritage, Rosso et al [127] implemented cool colored cement base materials which were found to decrease surface temperatures up to 8 • C and lower building energy demand as well. Equivalently, in a study of Rosso et al [38] concrete based materials characterized by significant NIR reflectance were implemented in historical buildings Pisello et al [35] 2014 characterization of low-cost grave coverings for roofs/paving finest grain size has the highest solar reflectance (62%), highly reflective stone decreased its temperature by 5.5 • C compared to the commonlyused gravel Castaldo et al [36] 2015 experimental and numerical analysis of low-cost grave coverings for roofs/paving depending on the grain size albedo can change up to 24%, global scale implementation: equivalent carbon emission offset of 4400 tCO2−eq Gonçalves et al [121] 2015 experimental evaluation of natural stone and ceramic brick as building components limestones' drying rate was lower than the evaporation rate from a free water surface Nano-scale based materials for building environment implementation are currently gaining popularity among academics due to their excellent optical properties. Recently, Xie et al [130] developed ten cool pavement coatings incorporating pigments including inorganic metal oxides of various particle sizes. The novel cool colored coatings were found able to highly reflect the NIR radiation, i.e. 95%, while preserving at the same time a fair solar reflection fraction, i.e. 60%. Optimization techniques related to the particle size of the pigments are ongoing as well. For instance, Song et al [126] researched the effect of various particle size distributions specifically to titanium dioxide rutile pigments for cool colored coatings. The distribution effect was found indeed significant; an increased particle size leads to enhanced NIR reflection. Nonetheless, implementation of high-reflective pavement into urban environment is not trivial. In fact, their application may lead to thermal discomfort and/or glaring issues, e.g. when implemented in completely unshaded areas [131,132]. Hence, their application should take place with respect to corresponding microclimate and boundary conditions and synergistically act with other passive cooling techniques and urban components such as trees and greenery.

Retroreflective materials
Another passive cooling technique for counteracting UHI, are the Retroreflective (RR) materials [133]. Unlike white and colored cool materials whose mechanism is based on diffuse reflection, RR are specifically designed to reflect the incident solar radiation directly back to its source [134]. Thus they preserve their cooling potentiality even if high rise structures are located in the packed surrounded area [135] (table  7).
In this context, Rossi et al [54] reported that RR can be even more effective in densely built areas as compared to traditional cool coatings. In order to further evaluate the impact of RR, Akbari and Touchaei, [136] developed a model that can estimate the hourly reflectance of directional reflective materials as a function of zenith and azimuth angles. RR materials' UHI mitigation potential was moreover noted by Han et al [55]. They examined the performance of a bio-inspired RR by means of dynamic simulation. The proposed RR as found capable of substantially decreasing the temperature of facing building surfaces and pedestrian air volumes.
Yuan et al [56] investigated RR in terms of ageing. They found that a prototype of RR including glass covering could maintain its initial reflectance and retro reflectance values (0.81 and 0.44 respectively) for over a year. Further in-lab and/or simulation comparative investigations concluded to RR superiority against conventional reflective materials concerning small scale application [56,137].
An interesting comparison between RR and Diffuse Reflective (DR) material was carried out by Quin et al [138]. Walls enhanced with RR materials were found to maintain lower temperatures than walls with DR materials. However,their performance was substantially deteriorated when the incident angle of the sunlight on the surface exceeded 40 • . A similar penalty with respect to the incident angle was reported also in the study of Yuan et al [139]. Further, Rossi et al [140] showed through an experimental campaign that RR surfaces can contribute towards the reduction of the energy kept inside urban canyons. In addition, they calculated and compared the energies remaining inside an urban canyon due to (i) RR and (ii) DR surfaces with similar global reflectance. A RR surface may indeed lead up to 37% decrease of the remaining energy. However, as they pointed out, this result should not be considered as a representative one when it comes to real scale applications and moreover glare and light pollution issues should be further evaluated. A novel RR technique was introduced by Sakai and Iyota, [141]. They constructed RR materials able to reflect the incident light only during the summer period and thus capable of reducing cooling load and simultaneously rejecting the winter penalty. A similar approach was also proposed by Manni et al [142] who numerically investigated the potential application of selective RR materials into the urban canopy. The proposed RR materials were optimized according to the corresponding climate zone and latitude and were found to decrease the incident to urban surface radiation up to 50%.

Photoluminescent materials
Photoluminescent materials have been suggested as an alternative cooling method for mitigating UHI due to their intrinsic property to reject incident radiation not only by reflection but also by photoluminescence [48,49] (table 8). Photoluminescence is a subcategory of luminescence and refers to the light emission from a matter, owing to the precedent absorption of photons. More analytically, when a molecule absorbs a photon in the visible region, it subsequently excites one of its electrons to a higher electronic excited state and then radiates a photon (i.e. releases energy), as the electron returns back to a lower energy state. Therefore, unlike conventional materials, photoluminescent materials, are characterised by a twofold rejection mechanism concerning incident radiation.
Photoluminescent materials were first proposed as a passive cooling technique in the study of Berdahl et al [49]. They investigated the fluorescent cooling of a ruby crystal (aluminum oxide doped with chromium) and proposed it as a robust fluorescent solution. Unlike the passive NIR-reflectance of the conventional cool colored materials, ruby was characterized by a deep red and near-infrared efficient emissivity as an active phenomenon. In fact, they overlaid white paint with a layer of ruby crystal which thus remained cooler than the reference, i.e. a conventional red coating, by up to 6.5 • C. In the same study laser materials like ND-doped YAG, the cadmium pigments CdS, CdSe and their alloy fluorescence were also suggested as remarkable fluorescence examples. Apart from the fluorescent materials which rapidly re-emit solar radiation, another subcategory of photoluminescent materials is the phosphorescent ones. These materials are characterized by a longlasting emission of light, i.e. up to several hours later. While a small number of studies have investigated their lightning potential in road infrastructure [143,144] their cooling potential has been investigated only in the study of Kousis et al [50]. In Table 7. Retro-reflective materials.

Study
Year Purpose Outcomes Akbari and Touchaei [136] 2014 development of a model for the hourly reflectance of directional reflective materials Summer period: accurate estimation of mean hourly heat absorption and energy saving less than 40 W/m 2 error for peak heat absorption, Winter period:less than 20% error for energy saving estimation 22 kJ/m 2 and 9 W/m 2 error for mean hourly and peak heat absorptions Rossi et al [54] 2015 optic behaviour of RR materials in terms of angular reflectance for several inclination angles of solar radiation higher cooling potential than traditional diffusive coatings Han et al [55] 2015  [141] 2017 evaluation of novel high-reflective and ordinary RR materials high-reflective RR: reflects light only during summer but is fragile Ordinary RR: weak retroreflectivity but it can withstand distortion Manni et al [142] 2018 application of optimized selective RR materials up to 50% decrease of the incident to urban surfaces radiation more detail, they developed five concrete-based pavement fields with embedded phosphorescent components based on strontium aluminate with dysprosium and europium dopants and perform a monitoring campaign during the summer period for evaluating their cooling potential. They showed that the phosphorescent-based pavements can achieve up to 3.3 • C lower superficial temperature than a cool concrete field. Moreover, they concluded that further addition of phosphorescent components could lead to higher decrease in superficial temperature. The results of Berdahl et al [49] and Kousis et al [50] suggest that the unique reflection-reemission mechanism, known as Effective Solar Reflectance, of photoluminescent materials is a promising technique for passive cooling applications and should be further exploited. Under this scenario, optimization of the experimental techniques for quantitatively measuring ESR is deemed necessary.

Thermochromic materials
Depending on the microclimatic characteristics of the implementation area cool materials could result to increased heating demand during the winter period [122,145]. Under this framework, academics launched the "Thermochromic'' Materials (table   9), i.e. materials capable to alter their color and thus their optical properties when their temperature reaches a predefined transition temperature value [146,147]. As a result, a decreased heating/cooling demand during the cold/hot periods can be ensured [148].
Within this framework, Ma et al [149] placed thermochromic layers on a building facade and investigated their thermal behavior as well as their effect on indoor temperature. They reported that thermochromic coatings significantly reflected the incident solar radiation when the ambient temperature exceeded the transition temperature and as a result, interior thermal comfort was achieved. Karlessi et al [150] concluded to the same inference by benchmarking thermochromic coatings with both cool and conventional coatings during a coolingdemand period. Moreover, the thermochromic coating was found to maintain a lower mean surface temperature on a daily basis than the two counterparts. In this regard, Perez et al [51] developed a novel thermochromic mortar able to raise its reflectance in the visible spectrum as temperature rise, while simultaneously retaining constant values of reflectance in near-infrared area. Similarly, Hu and Yu [52] showed that a roof with thermochromic properties

Study
Year Purpose Outcomes Berdahl et al [49] 2016 development and test of fluorescent cool-colored materials 6.5 • C lower surface temperature than the conventional paint Levinson et al [48] 2017 Test methods for measuring ESR developed a computer-controlled rotary apparatus that measures ESR with a repeatability of about 0.02 Kousis et al [50] 2020 in-field monitoring of phosphorescent-based paving fields during summer period up to 3.3 • C lower surface temperature than cool concrete, phosphors delay peak surface temperature can decrease heating/cooling demand and in parallel increase energy and cost savings up to 40.9% and 47.7% respectively. In another study implementing in thermochromic materials for roof applications, Fabiani et al [53], confirmed the winter penalty reduction potential both experimentally and numerically (by implementing the Princeton Urban Canopy model). Moreover, they showed that a superior stabilization of heat flux and temperature gradients can be ensured within each season, resulting in a higher quality thermal comfort for the building occupants. Thermochromic solutions have been utilized in the pavement field as well. In the study of Yu and Hu [152] the thermal behavior of a thermochromic asphalt binder was examined. The novel binder was found able to maintain higher surface temperature during winter period in comparison with conventional asphalt and therefore was proposed as a snow deferring mechanism in cold climates. Generally speaking, however, thermochromic materials, are prone to photodegradation issues [150] that can significantly deteriorate their performance. For that reason, optimized thermochromic solutions have been developed with integrated red filter coverings [151]. Even though the coverings were found to delay the photodegradation further relevant research is needed for a large real-life implementation of thermochromics into the built environment.

Advanced nanoscale thermochromic and radiative cooling materials
Recently, nano-scale thermochromic solutions, based on Quantum Dots, Plasmonic, and Photonic structures were proposed as the next generation of cool urban materials in the study of Garshasbi and Santamouris [148] (table 10). Due to their non-bright nature, nano-scale materials can assert a pedestrian's visual comfort. In addition, since their properties are based on molecular rearrangement and nanoscale optical effects, they could counteract the photodegradation issues apparent on intermediate-scale materials [148] Under this scenario, Garshabi et al [153], investigated the mitigation potential of a surface coated with Quantum Dots with high emittance in the infrared wave-range. The results showed that the QDs coated sample was able to maintain a 10 • C lower temperature than the reference sample. Quantum dots capped with TOP/TOPO/AET have been found to increase their photoluminescence intensity with temperature and vice versa, an effect known as thermal anti-quenching [154]. Therefore QDs are considered as a promising thermoresponsive cooling solution that needs to be further investigated [148]. At the same time, another promising technique for cooling applications, especially in hot and arid climates, is radiative cooling [155], i.e. structures that can highly reflect solar radiation while they behave almost as a black body within the atmospheric window wave-range (8-13 µm) [156,157]. In recent years, plasmonic [158] and photonic [159] based prototypes have been tested for their radiative cooling potential. In their pioneering work, Raman et al [159], merged a photonic crystal structure and a thermal emitter and achieved sub-ambient temperature by 4.9 • C during the hot hours of the day. In another study concerning radiative cooling techniques, Zou et al [158], developed a plasmonicbased optical metasurface capable of decrease its temperature 7.4 • C below the ambient temperature during day-time. It should be noted, however, that even though several studies have demonstrated exceptional cooling properties concerning radiative cooling mechanisms, little attention has been given to a possible implementation into the built environment. This can be of course explained by their relatively complex nature which makes their implementation non cost-effective.

Sustainable materials for mitigating UHI
In parallel, apart from developing novel materials for mitigating the UHI, research efforts have been attempted for maintaining a more eco-friendly approach (table 11). In fact, urban environment has been found responsible for almost 55% of the worldwide emitted greenhouse gases [160]. Under this scenario, Zinzi and Fasano [161] developed and tested a cool pigment made with a mixture of milk and vinegar. This novel pigment was found to decrease its surface temperature up to  [53] 2019 evaluation of thermochromic roof with an Urban Canopy Model annual stabilization of heat flux and temperature gradients, enhanced thermal comfort of occupants  [153] 2019 QDs-based coating for UHI mitigation 10 • C lower temperature than the reference 20 • C compared to conventional enamel coating. In the study of Ferrari et al [162], a roof engobe was developed by commonplace materials, i.e. recycled glass and alumina for substituting conventional roof tiles in historical city-centers. The novel engobe was found to have an albedo value as high as 0.9. Also, waste bio-oils have been tested for paving application. Kousis et al [163], developed 4 novel paving binders made with waste palm oil and animal fat. They showed that the novel binders reflect more than 50% of incoming solar radiation within 750-1600 nm, i.e. significantly higher that than typical asphalt and cement components. In addition, a binder made with animal fat was found to maintain up to 2 • C lower superficial temperature during the hottest hours of a summer day than the cool binder reference.

UNI mitigation
UNI mitigation strategies should focus on the main components of the Urban structure, such as the building envelope and pavements and the corresponding materials and finishing. The vast majority of the urban surfaces around the world consist of materials that can hardly absorb the incident soundwaves and thus intensify UNI by excessively propagating them. Recent advances in the field of acoustics, however, have set the foundations for urban components that can fairly mitigate noise and therefore moderate UNI. In this chapter, the recent advances, novelties, and strategies aiming to mitigate UNI and/or effectively control incident soundwaves are presented.  [162] 2013 engobe manufactured by recycled glass and alumina albedo equal to 0.9 was achieved Kousis et al [163] 2020 development of cool paving binders with waste bio-oils (i)solar reflectance > 50% within 750-1600 nm (ii) 2 • C lower surface temperature than cool reference during hot-hours of day

Porous materials
Porous absorbents are popular for reducing excessive indoor or outdoor noise incidences [165]. Their porous nature leads to thermal and viscous phenomena when the sound wave is propagated inside them. As a result, the noise wave energy is effectively dissipated. Nevertheless, their absorption abilities are undermined at low noise frequencies [83,85,166]. The energy conservation of the incident soundwave to the porous material can be described by [83]: E i where is the total incident sound energy, E r is the sound energy of reflection, E a is the sound energy of absorption, E t is the sound energy of transmission. Properties such as low-priced value and pliable nature made porous sound absorbers particularly popular among noise abatement tactics in urban environment [83,87,88]. Porous materials are generally divided in two main subcategories; (a) porous fibers and (b) porous foams. Their performance in terms of sound absorption is very much affected by materials' porosity (distribution and morphology of internal pores) and installation geometry, as well as the size and the position with respect to sources and receivers [167]. The main metric of sound absorption is the sound absorption coefficient which is defined as the fraction of absorbed to incident sound wave energy [168] and typically is represented by a:

Porous fibrous materials
Fibrous absorbing materials consist of chains of fibrous assemblies and depending on their nature can be classified as natural, inorganic, synthetic, metallic and nanofibers. Their main advantages compared with foam absorbers is their lightweight, elasticity and aesthetic appearance. Sound absorption properties of fibrous materials are determined by characteristic impedance Z c [169]: where, ρ e is the effective density, K e the bulk modules.

Natural porous fibers
In the study of Ersoy and Ku [170] industrial Tea-Leaf-Fibre (TLF) was investigated in terms of sound absorption. They showed that depending on the backing thickness of the TLF, its performance is comparable with, or even better than, polyester and polypropylene based non-woven fibre in terms of acoustic absorption potential. Similarly, Ismail et al [171] measured the sound absorption coefficient of four different thicknesses of Aregna pinnata. The results showed that 40 mm thickness is capable of 0.75-0.90 sound absorption coefficient within the range of 2000 -5000 Hz. In another study conducted by Fatima and Mohanty [172], jute and its composites were proposed, as another eco-friendly and cost-effective noise attenuation counterpart of glass fibre materials (table 12). Likewise in the study of Putra et al [173], natural waste fibers from paddy were proposed as a substitution of synthetic glass wool. Under the same scenario, sound-absorbing materials made of kenaf, wood, hemp, coconut, cork, cane, cardboard, and sheep wool were examined by Berardi and Iannace [174] with promising results at medium and high frequencies. Asdrubali et al [175] reviewed several natural unconventional sound absorbing materials for building implementation such as reed (a> 0.5 above 300 Hz), bagasse (a> 0.5 above 1000 Hz), wood panel manufactured with the addition of 10% rice straw wood (a> 0.5 above 1900 Hz) as well as recycled materials such as polyethylene terephthalate (a> 0.6 above 500 Hz) and bats made of recycled denim (a> 0.95 at 125 Hz). Waste bio-oils have also been tested for their sound absorption capability. In the study of Kousis et al [163] four novel paving binders were developed with palm oil and animal fat and used for developing corresponding paving specimens. Results from impedance tube showed that all novel specimens significantly outperformed typical asphalt and cements counterparts in terms of sound absorption.
Peng et al [176] studied the acoustic behavior of a material made of wood fiber and polyester fiber and concluded that by modifying the cavities length, the absorption peak can be modulated towards the desired frequency range. The importance of air cavity gaps for enhancing sound attenuation at lower frequencies was pointed out also in the study of Hee
Fabiani et al [179] 2018 performance of green roof substrates the higher the RH the more the absorbance moves to higher Hz Putra et al [180] 2018 extracted pineapple-leaf fibres aaverage = 0.9 above 2000 Hz Santoni et al [181] 2019 hemp fibrous materials a > 0.5 above 500 Hz and a > 0.8 above 900 Kousis et al [163] 2020 paving specimens including binder made with animal fat or palm oil aaverage > 0.6 within 500-1500 Hz (road-traffic noise) et al [178] concerning a material based on oil palm empty fruit bunch fibres. Also, materials consisted of pineapple-leaf fibres and hemp fibres have been proposed as good sound absorbers [180,181]. Similarly, Asdrubali et al investigated common reed-based panels for building envelopes and reported an overall poor absorbing performance , mainly due to limited porosity [177]. Another study carried out by Fabiani et al [179] focused on the acoustic performance of organic-based materials for green roof implementation within the Mediterranean region. Results showed that low relative humidity values help towards better sound absorption properties.

Inorganic porous fibers 4.1.1.3. Metal fibers
Inorganic fiber porous materials, such as fibrous metals, have attracted researchers' interest mainly due to their ability to maintain their properties under extreme conditions e.g. high temperatures and high levels of pressure [182,183] (table 13). Therefore, various studies have investigated several metal-based sound-absorbing structures such as sintered fibrous metals [184], stainless steel fibrous felt materials [185,186] and copper fibers [187]. Meng et al [184] examined rigid-backed sintered fibrous stainless steel samples of different thicknesses in terms of sound absorption. They concluded that the smaller the diameter the higher the sound absorption coefficient which in fact can reach values almost equal to 1 for frequencies higher than 1000 Hz. Similarly, Qingbo et al [185] showed that by decreasing the mean pore size of porous stainless steel fibrous felt materials their sound absorption properties can be significantly improved within the frequency range of 50-3500 Hz. In addition, an increase of 14.3% regarding sound absorption performance was reported for samples prepared at a sintering temperature below 850 • C. In another study concerning stainless steel metal fibrous materials, it was pointed out that a gradient pore structure is superior as compared to the regular pore ones in terms of sound absorption behavior [186].
Copper fibers have also been utilized in the study of Chen et al [187] for developing tri-dimensional reticulated porous material with fine sound absorption properties from 800 to 4400 Hz, i.e. sound absorption coefficient values reaching up to 0.94. It should be noted, however, that due to their fine mechanical properties under extreme conditions, fibrous metal materials are generally proposed for implementation in the field of aeronautics [183].

Study
Year Purpose Outcomes Bo and Tianning [182] 2009 porous sintered fiber metal a > 0.9 within 1700-6400 Hz Sun et al [183] 2010 effect of high temperatures on absorbing properties of fibrous metal materials a > 0.8 within 1500-6500 Hz Meng et al [184] 2015 sintered fibrous metals a > 0.7 within 1500-6500 Hz Qingbo et al [185] 2015 Porous stainless steel fibrous felt materials a > 0.6 within 2000-3000 when sintered at 850 • C Zhu et al [186] 2016 optimization of metal fiber porous materials a gradient pore structure significantly improves sound absorption compared to regular porous samples Chen et al [187] 2017 tri-dimensional reticulated porous material made of copper fibers a > 0. 5 [188] and found to have an average sound absorption coefficient of 0.77 within 0-4000 HZ. ). Other studies scrutinized the acoustic performance of polyester and polypropylene nonwoven selvages [189], recyclable flame retardant nonwoven [190] islands-in-the sea fibers [191], high-loft nonwoven fibers [192]. The dependence of sound absorption coefficient on different fiber blend ratios and bulk densities has been examined as well [193]. However, the result showed that in general, materials exhibit little sound absorption properties in the low and medium frequency range which is the most critical for humans health (table 14).

Nanofibrous additions
Nanofibrous additions have been found to optimize the sound absorption mechanism of traditional acoustic materials in the low and medium frequency range (table 15). This was reported in the study of Xiang [194] wherein perforated, panel, foam and fiber-based materials were enhanced with electrospun polyacrylonitrile nanofibrous membranes. Similarly, Kucukali-Ozturk et al [195] reported sound absorption coefficient values up to 0.7 in the low and mid-frequency values by placing electrospun polyacrylonitrile-based nanofibers on specimens. Substantial sound attenuation properties within 400-900 Hz were reported also in the study of Chang et al [196] who launched a novel methodology for creating 3D nanofibrous networks. Similarly, in other relevant studies, it was demonstrated that electrospun piezoelectric Polyvinylidene fluoride (PVDF) membranes may not only substantially absorb noise at low frequencies, but also convert sound energy to electric potential [197,198].

Porous foams
Porous foams absorbers are consisting of interconnected cellular structures and depending on their chemical composition can be classified as organic, inorganic and hybrid. They are widely used in the urban environment due to their low manufacture cost.

Porous Hybrid Foams
Wu et al [199] reported a significant aound attenuation at low frequencies by implementing graphene foam (GF)/carbon nanotube (CNT)/poly (dimethyl siloxane) (PDMS) composites. In fact, they achieved over 30% sound absorption in the frequency range of 100-1000 Hz and 70% in the range of 100-200 Hz. Porous metal foams have also been examined for their sound absorption capabilities. Bai et al [200] examined 5 different porous metal absorbers; original porous metal, compressed porous metal, compressed and microperforated porous metal panel, microperforated spring steel panel, and microperforated uncompressed porous metal. It was noted that the compressed and microperforated porous metal panel absorber with a cavity length of 20 mm is capable for an average 59.69% sound absorption in the range of 100-6000 Hz, i.e. 25.70% higher than the original porous metal absorber (table 16).

Porous Organic Foams
Flexible polyurethane foams (FPF) have been widely regarded as a fine example of organic foam with significant sound absorption properties [201,202] (table 17). Moreover, it has been reported that the sound absorption properties of flexible polyurethane foams can be further enhanced with the addition of fluorine-dichloroethane and triethanolamine [203]. Other flexible polyurethane foams alterations have also been proposed as good sound absorbers, due to the inclusion of uretonimine contents [204], high molecular weight copolymer polyol [205], wood fibers [206] and high molecular weight isocyanate contents [207].
On the other hand, one of the first efforts aiming to construct environmentally friendly porous foams was carried out by Mosanenzadeh et al [208]. They reported that better sound absorption

Study
Year Purpose Outcomes Lou et al [189] 2005 polyester and polypropylene nonwoven selvages a > 0.5 above 1184 Hz Kosuge et al [190] 2005 A flame retardant nonwoven fabric made with para-aramid fibre and polyester fibre sintered fibrous metals when attaching para-aramid paper with less than 30 cc/sec/cm 2 of permeability to nonwoven fabric, a above 2000 Hz was better than that of glass wool and above 0.6 Suvari et al [191] 2013 spunbonded nonwovens made with Nylon 6 (PA6) and polyethylene a > 0.1 above 4500 Hz Zhu et al [193] 2015 Thermally Bonded Nonwovens a > 0.5 above 1250 Hz Suvari et al [192] 2016 Thermally bonded high-loft nonwovens with minimum thickness a > 0.5 within 2000 -6000 Hz for 1575 g/m 2 samples thickness range 10-50 mm Xiang et al [194] 2011 electrospun polyacrylonitrile (PAN) nanofibrous membranes placed on BASF foam and glass fibers BASF foam/Glass fiber improved both a > 0.5 above 1900 Hz Kucukali-Ozturk et al [195] 2015 Polyacrylonitrile nanofibers electrospun on spacer-knitted fabrics by varying deposition amount and surface coating arrangement a = 0.7 at the low and medium frequency range Chang et al [196] 2016 3D nanofibrous networks, featured with neutralization and self-assembly of electrospun nanofibers a > 0.5 above 410 Hz Wu and Chou [197] 2016 porous piezoelectric PVDF materials a > 0.5 above 900 Hz Wu and Chou [198] 2016 electrospun polyvinylidene fluoride/graphene membranes a > 0.5 above 700 Hz

Study
Year Purpose Outcomes Wu et al [199] 2017 graphene foam/carbon nanotube/ poly(dimethyl siloxane) composites a = 0.7 at 100-150 Hz Bai et al [200] 2019 absorbers fabricated by the compression and microperforation of porous metal aaverage = 0.6 within 100-6000 Hz performance can be achieved, by substituting the non-recycling polyurethane with polypropylene as a recyclable thermoplastic polymer, and polylactide as a bio-based thermoplastic polymer made from renewable resources. Another novel graded bio-based foam structure capable of enhancing the noise attenuation in wider frequency range as compared to uniform foams has been introduced also by Mosanenzadeh. [209].

Porous inorganic foams
Inorganic foams (table 18) are typically used for sound absorption applications in harsh environments (e.g. aerospace and marine industry) [210]. Towards this direction, acoustic absorption properties of foams based on open-cell Al alloy [210], alumina and alumina/trifunctional epoxy composites [211], open-cell metallic [82], porous zeolite [212] and Si 3 N 4 ceramics [213] have been examined. Results showed promising results concerning sound absorption, especially in high frequencies. For example, Ke et al [210] increased the sound absorption of opencell Al alloy foams with graded pore size by introducing an air gap behind the foam. The resulted absorption coefficient was higher than 0.5 within 300 to 1100 Hz. Cuiyun et al [212] fabricated porous zeolite specimens through sintering process by mixing zeolite powder with polymer foam particles which were found capable of absorbing more than the 50% of the incident soundwave above 850 Hz. Similar results were reported by Ligoda et al [211] concerning alumina foams within the frequency range above 1200 Hz, while Zhai et al reported a sound absorption coefficient higher than 0.9 within 1500-6000 Hz for an open-cell IN625 metallic foam of 50 mm thickness. Lee et al [202] 1991 Flexible Polymeric Foams a > 0.9 at 1000 Hz Sung et al [201] 2007 optimized flexible polyurethane foam with various plate-like fillers a > 0. 6

above 2000 Hz
Mosanenzadeh et al [208] 2013 polymeric open cell foams from polypropylene(PP) and polylactide (PLA) resin a > 0.5 above 3000 Hz for PP a > 0.6 above 2000 Hz for PP Mosanenzadeh et al [209] 2015 graded bio-based foam structure a > 0.6 above 2000 Hz Chen et al [203] 2015 Additive components for the optimization of Polyurethane Foams a > 0.6 above 700 Hz whith foaming agent 141b Sung et al [204] 2016 Effect of isocyanate molecular structures in fabricating flexible polyurethane foams the higher level of uretonimine content the lower the value of a Sung and Kim [207] 2017 Effect of high molecular weight isocyanate contents on manufacturing polyurethane foams a > 0.5 above 1000 Hz and a > 0.9 around 2000 Hz Sung et al [205] 2018 flexible polyurethane foams including high molecular-weight copolymer polyol a > 0.5 above 750 Hz Choe et al [206] 2018 wood fibers to enhance the sound absorption coefficient of flexible polyurethane composite foams a > 0.5 above 1000 Hz and a > 0.9 around 2000 Hz  [213] 2017 Highly porous Si3N4 ceramics a > 0.5 above 1200 Hz Ligoda et al [211] 2017 alumina foam/tri-functional epoxy resin composites vs. alumina foam Alumina foam: a > 0.5 above 1200 Hz Alumina/epoxy composite: a > 0.5 within 2000-3100 Hz Zhai et al [82] 2018 open-cell metallic foams a > 0.9 within 1000-6000 Hz

Resonant sound absorbers
Resonant absorbers can ensure fair sound absorption properties at low to mid frequencies, where porous absorbers usually fail to achieve significant absorption due to the internal resonance effect [85]. More specifically, their resonant frequency can be tuned to a specified value in which sound absorption will be maximized [166]. This property however, makes them efficient in terms of sound absorption only for a narrow frequency band [85,214,215]. There are two common forms of resonant absorbers: membrane/panel absorber and Helmholtz absorber. For a membrane/panel absorber, the mass is a vibrating sheet of membrane or panel made from various materials. The spring is usually provided by the resilient boundary of the membrane or panel or air enclosed in the cavity. In the case of a Helmholtz absorber, the mass is a plug of air in the opening of a perforated sheet and the spring is usually provided by air enclosed in the cavity. (table 19) consists of a thin sheet panel perforated with a lattice of sub-millimetre apertures which creates high acoustic resistance and low acoustic mass reactance. Both are necessary for broadband sound absorption without further using additional porous material [216]. In the past decades, high manufacturing cost [215] of MPP was a critical disadvantage for their wide implementation. Nevertheless, recently, several recyclable materials have been proposed for MPPs manufacture leading to their sound absorption properties exploitation [217]. Therefore, many scientists tend to regard them as superior solutions than traditional porous materials [218]. The acoustic impedance of a micro-perforated panel, with either circular or slit cross-section, can be approximated as [216,219]:

Membrane/panel absorber
where t is panel thickness, d is perforation diameter, η is the dynamic viscosity coefficient, ω is the angular frequency, ρ 0 is the density of air, and k c and k s are the perforation constants for the circular and slit crosssection respectively. Cobo and Espinosa [215] introduced a novel fabrication technique of MPPs with good performance in mid to high-frequency range, i.e. absorption coefficient values above 0.8 within the frequency range of 1600-2600 Hz. Aiming to magnify the noise attenuation in lower frequencies, Li et al [220,221] proposed the use of perforated panel with extended tubes in unison with a porous material. The designed structure was found capable of significantly absorbing soundwaves from 300 to 1100 Hz (sound absorption coefficient values varied form 0.6 to almost 1). Similarly, Bucciarelli et al [222] developed a multilayer microperforated panel able to highly absorb sound in lower frequencies. Their study demonstrated a sound-absorbing structure comprised of six plastic micro-perforated panels with constant absorption properties over 90% from 400-2000 Hz. Another novel structure with perforated honeycomb-corrugation hybrid sandwich panels with fair noise reduction performance in low frequencies has been proposed by Tang et al [223]. By implementing the Simulated Annealing algorithm, they demonstrated an average sound absorption coefficient equal to 0.75 in the range of 500-1000 Hz together with fine mechanical properties.

Helmholtz resonator
The Helmholtz Resonator (HR) (table 20), which consists of a cavity communicating with an external duct through an orifice, is a well-known device to reduce noise centralized in a narrow band at its resonance frequency [224]. For a single neck Helmholtz resonator, the resonant frequency, f r is given by: where L' and A are the length and the area of its neck respectively, V is its volume, and ν is the soundspeed. The acoustic impedance is given by: where R a is the acoustic resistance of the absorber, which is related to the porous material used in the cavity and/or the viscous loss along the neck. X a is the acoustic reactance of the absorber and can be calculated with respect to the mechanical reactance of the system by: where M a is the mass of the air inside the neck, ω is the angular frequency,and K a is the acoustic stiffness of the cavity.
Under this scenario, Chen et al [225] demonstrated a great improvement in terms of transmission loss by adding HRs into a duct. This was succeeded in specific frequencies, i.e 28 dB and 18 dB for 250 Hz and 500 Hz respectively, by placing specially designed band-pass filters. In a study of Tank [226] HRs sound absorption properties were enhanced by introducing a neck tapering technique. He found out that the deeper the tapering the higher the sound absorption coefficient which can reach values above 0.8 in the frequency range of 80-100 Hz. Similarly, Esteve and Johnson [227] demonstrated an overall decrease of 7.7 dB in the 50-160 Hz range by coupling distributed vibration absorbers and HRs into a cylindrical enclosure.
HRs have also been proposed for window implementation [228]. The proposed double glazed window structure was found capable of decreasing the transmitted sound pressure levels within 50-120 Hz. In general, HRs have been further utilized for tackling the low-frequency penalty of sound absorption MPPs. For instance, Park [229] backed MPP with HRs and succeeded an overall reduction of 7.9 dB within 25-630 Hz. Similar approaches have been reported from several scientific groups with successful results opening the way for further application of resonant sound absorbents [84,230] in terms of noise mitigation.

Metamaterials
The latest frontier in the evolution of acoustic absorption materials is the development of acoustic metamaterials. Unlike natural porous and resonant absorbers, acoustic metamaterials are artificial materials with advanced acoustic properties compared to their natural counterparts. For example, while effective mass density and bulk modulus of conventional materials are always positive, when it comes to metamaterials their values can be also negative [231][232][233].
In general, the effective mass is described by [234]: where D eff is the effective density, V is the total volume, ω 0 is the resonant frequency, ω is the angular frequency of the time harmonic excitation, and M 0 and m are the mass of the matrix and the solid components, respectively. Moreover, in the framework of Effective Medium Theory (EMT), the effective bulk modulus of fluid and solid composite structures B eff is [235]:   [230] 2016 microperforated panel mounted with helmholtz resonators a > 0.7 with 220-1000 Hz where B 1 and B 2 refer to the modulus of the matrix and solid components, respectively. When the damping of the system is negligible, the effective bulk modulus of a Helmholtz resonator can be written as [236]: where B is the bulk modulus of air, ω sh is the cutoff frequency of the side hole, and ω is the angular of the harmonic excitation. As a result, acoustic metamaterials can achieve superior manipulation and control of the incident sound waves. In other words, they can conceal an object in terms of acoustics or redirect a soundwave to a desired direction opening simultaneously the way for further energy harvesting applications [89,237,238].

Metasufaces-Membrane type metamaterials
Various studies have proposed acoustic metasurfaces capable to accomplish total absorption in a narrow band of low frequencies range [89,90] (table  21). However, the membrane-type acoustic metamaterials (MAMs) have raised moreover the scientific interest mainly due to their compact and lightweight nature [239]. The first MAM was introduced by Yang et al [240] and was characterized by negative dynamic mass, i.e. acceleration and spatially mean average force have opposite phase and hence materials comprising infinitesimal components can exhibit dynamics that deviate from Newton's second law [241], and almost perfect reflectance of acoustic waves within the 200-300 Hz frequency range.
In the study of Lee et al [242], one-dimensional acoustic metamaterial with negative effective density using an array of very thin elastic membranes was found to highly block soundwaves from 0 to 735 Hz. Further on, Yang [243] developed a thin lightweight acoustic attenuation panel characterised by sound transmission loss greater than 40 dB in the broader range of 50-1000 Hz. Likewise, Mei et al [244] produced an elastic membrane decorated with asymmetric rigid platelets able to ultimately absorb sound at specific frequencies in-between the range of 100-1000 Hz.

Metaporous structures
Furthermore, metamaterials have been developed for enhancing the sound absorption properties of porous structures, i.e. metaporous material (table 22). For instance, Yang [245] increased the sound absorption of a homogeneous porous layer by embedding rigid partitions and achieved a sound absorption coefficient higher than 0.7 above 2000 Hz. The same was reported by both studies of Zhou et al [246] and Yang et al [247], wherein the designed porous metasurface outperformed the conventional porous one in terms of acoustic absorption since the sound absorption coefficient reached values over 0.9 at frequencies higher than 1000 Hz.

Critical discussion
UHI and UNI, can be independently mitigated through a win-win approach: incorporation of smart skin with advanced physical properties into the built environment. However, coupling these properties in multifunctional surfaces than can act simultaneously towards the mitigation of both UHI and UNI is still a challenge and must be further investigated both in theory and practice. With regards to UHI, several passive material-based cooling techniques have been reported for being capable of substantially rejecting incident solar radiation. The main key performance indicators of the cool materials are typically the following: • Magnitude of temperature abatement.
Nonetheless, the momentum of each cooling strategy is interconnected with the distinct physical and structural particularities of the implementation area; i.e. topography, meteorological profile, broader morphology, urban planning and anthropogenic heat [93]. In this regard, it is reported that white materials with high albedo values may substantially decrease their temperature during the summer period and can be cost-effective (table 23). However, they are subject to glaring issues due to their high reflectance in the visible region of the spectrum and moreover to inevitable ageing problems. As a result, in many cases, their implementation is prevented, especially to pedestrians and road pavements. The glaring penalty was partially solved with the introduction of the cool colored and fluorescent materials due to their property of highly reflecting in the NIR spectrum. However, both undergo significant ageing issues. Similarly, thermochromic materials are vulnerable to ageing issues, however they have the main advantage of eliminating the winter penalty and decreasing the yearlong energy demand. Thus tackling the ageing issue of materials is, to date, of high importance. A potential solution however could be given through the further implementation of nanoscale thermochromic materials owing to their nature, though they still cannot be considered as a cost effective solution. On the contrary, an inexpensive solution excelling at the same time in terms of environmental impact are the natural or sustainable cool materials such as cool stones and grains.
In parallel, recent advances in noise mitigation techniques lead to proficient sound absorption on the audio frequency spectrum (table 24). Each sound absorption technique is characterized by different properties and thus it should be incorporated in the urban environment accordingly. For instance, porous materials are an ideal solution for exterior urban areas (i.e. pavements, building envelopes) owing to their fine sound absorption in a wide frequency range, their cost effectiveness and their low environmental impact since their components can be natural or organic. On the other hand, resonant absorbers excel in low frequencies which makes them ideal for indoors implementation. However, synergistic interactions in-between resonant and porous absorbers should be further investigated for urban applications. At the same time, metamaterials, can be regarded as the next generation of sound absorption materials since they are capable of not only totally absorbing sound but also redirecting the incident sound wave. That said, their mainly theoretical to date investigation and their high cost impede their urban application for the moment. However, further investigation should be carried out for cost effective metamaterial solutions in terms of sound absorption, since they could be pertinent for urban applications such as noise barriers.
Future focus should be given on merging rejection of solar radiation with sound absorption properties into one urban component (table 25). Under this scenario, urban materials comprising multiple layers should be developed. Mitigation mechanisms should be independently tuned within each layer which would synergistically act without compromising each other properties. Concerning UHI mitigation mechanism, special focus should be given to the external/upper layer of the surface. This layer directly interacts with incident radiation an hence its optical properties may regulate the overall thermal performance of the system. The UNI mitigation mechanism of the proposed material should be developed into the internal/bottom layer and hence will not affect solar rejection performance. The key attributes for not  [244] 2012 thin elastic membranes decorated with designed patterns of rigid platelets almost 99% absorption at 164 Hz and 645 Hz Ma et al [89] 2014 metasurface that employs a novel hybrid resonance Total absorption at 255, 309 and 420 Hz Li and Assosuar [90] 2016 combination of a perforated plate and a coiled coplanar air chamber total absorption around 125 Hz Langfeldt et al [239] 2017 perforated membrane-type acoustic metamaterials increased transmission loss over 25 dB within 100-1000 Hz  [247] 2016 A set of rigid partitions of varying lengths inserted in a hard-backed porous layer a > over 0.9 above 1000 Hz Zhou et al [246] 2017 numerical study for a 2D acoustic metasurface fabricated using porous material a > over 0.8 above 500 Hz Porous sound absorbers, e.g. natural fibers, inorganic foams, in unison with natural or sustainable cool materials should be considered as future solutions for tackling both UNI and UHI within pedestrian areas, cycle footpaths, parking lots, old town centers and so forth. In fact, an external layer comprising particles of an approximate size of 0.3-0.7 cm can function as good UHI mitigation mechanism that simultaneously allows the penetration of incident ambient sound-waves. Subsequently, a bottom layer developed with grains of an approximate size of 1.5-2.4 cm can ensure low flow resistivity (10,000-30,000 Ns/m 4 ) and hence mitigate noise. Depending on the desired UNI mitigation magnitude, the porosity of the bottom layer should be well tuned above  20%. Moreover, the thickness of the layer should be optimized according to the application. Concerning the absorption of sound-waves within low to mid frequencies, that mainly affect human health, an approximate thickness of 10 cm should be considered as reference [248]. Further increase of the thickness can lead to increased sound absorption, but within a narrower range at low frequencies.
The addition of cool-colored or photoluminescent particles into the external layer could minimize glare incidences, enhance the cooling potential, visibility and safety and/or satisfy aesthetic prerequisites. However, dispersing cool colorants either photoluminescent or not is not trivial [50]. Qualitative and quantitative dispersion assessments should be carried out through microscopy (electron and force),scattering (x-ray, neutron, and light), chemical spectroscopic methods, electrical and dielectric characterization, and mechanical spectroscopy, especially when nano-scale components are dispersed into polymer matrices [249]. In fact, parameters such as the dispersion technique, the size and, the center-to-center distance of the colorant particles may regulate the thermo-optical properties of the layer and hence material's temperature. However, reflective pavements should be developed with prudence and in accordance with the corresponding urban environment. Else, they may contribute towards increased mean radiant temperature and hence to pedestrian's thermal discomfort [132]. On a similar approach, porous metal inorganic fibers/foams matched with modified asphalt including either cool pigments or photoluminescent components could be utilized in heavy traffic road pavements forming an inexpensive and environmentally benign solution. In that case, sound absorption should be specifically tuned within 500-1500 Hz which typically corresponds to the road-traffic noise. Moreover, a fine polishing of the surface may reduce the texture impact of tires.
Porous materials together with cool membranes or natural cool stones could be also placed on building facades and roofs. Under this scenario, sound absorption mechanism should be tuned into a wider frequency-range since community noise, e.g. noise originated from aircrafts, railways and motor vehicles, typically ranges between 50 Hz and 5000 Hz. The addition of an air gap behind the sound absorptive layer could increase sound absorption especially at mid and higher frequencies [250]. Also, sound absorption at low frequencies should be taken into consideration, especially with regards to office buildings. In fact, low-frequency noise originated from heating/cooling systems, ventilation systems, and fans or pumps in particular, together with indoor thermal discomfort have resulted in the Sick Building Syndrome (SBS) in many workplaces [251]. To that end, a layer comprising resonant sound absorbers embedded into porous matrices could moderate the low frequency penalty of porous structures, and thus back a cool facade on office buildings.
Nano-scale thermochromics embedded in or placed above either porous foams or fibers can be further exploited for more advanced UHI and UNI mitigation solutions incorporated in facades or roofs of buildings. For example, adaptive structures that seasonally retune their UHI mitigation properties could be developed by dispercing cool, photoluminescent, photonic and so forth, nano-particles within a temperature responsive matrix, such as strain sensible polymers, biomimetic stretchable polymer opals or PCMs. Their cooling potential could be enhanced more if they are backed by a sublayer of selective absorption/emission spectra. In addition, the photocatalytic properties of zinc-oxide or titanium and silicon dioxide [252,253] could be implemented in all proposed components for tackling air pollution in the lower heights of the urban environment, enhancing the cooling effect and adding self-cleaning properties. It should be noted that the present analysis focused on material-based solutions comprising mainly artificial components. Thus greenery and tree-based solutions were excluded from the analysis. However, since these type of solutions have been found capable of mitigating either UHI or UNI, they should be regarded and investigated as a supplementary component towards the moderation of urban discomfort [59].