## 1 Introduction
Leaves currently represent the most sustainable low-cost materials for PM reduction. The airborne PM entrapment by vegetation is mainly driven by the interactions between particles and plant surfaces. It is influenced by particles size and by plants morphology such as roughness, hair, and wax cover [17]. Literature reports many works about the PM capture by plants. Generally, the species used are: Field maple such as _Acer campestre_, European ash such as _Fraxinus_, London plane such as _Platanus_, and Common Ivy such as _Hedera Helix_ L., the widely used evergreen [18]. _Hedera Helix_ L. has shown relatively high PM densities in all size fractions due to the important role of leaf surface wax in removing particles. However, leaves alone are not able to capture such high quantities of PM and also have the disadvantage that most of them are not present during autumn and winter. Therefore, new approaches need to be found to address the problem. Given the great relevance of buildings in the urban context rather than green areas, the idea is to exploit these wide surfaces at our disposal by using porous materials to be applied on the buildings' facades. The advantage of these materials is not only the improvement of air quality but also the reuse of industrial by-products in the frame of circular economy principles. The capability of porous material, named SUNSPACE (\"SUstaiNable materials Synthesized from by-Products and Alignates for Clean air and better Environment\") silica fume (SF), to trap fine and ultrafine particles has already been demonstrated in different studies [16; 19; 20; 21; 22]. SUNSPACE SF synthesis involves the use of industrial by-products such as silica fume, deriving from ferro-silicon processing. The idea is to apply SUNSPACE SF as plaster on urban surfaces. Unfortunately, its dark color surely represents an aesthetic limit for the application as a plaster. In order to overcome this limit and to use sustainable materials to depollute urban air [16], a variation to the SUNSPACE SF formulation is here proposed. For this aim, the idea was to use a different raw material but with the aim to improve the sustainability of the construction sector. In particular, the use of secondary materials in building application is still limited. This work shows for the first time the use of bottom ash for the synthesis of a lighter colored porous material. This waste is the main residue produced from the incineration of municipal solid waste. It is estimated that 16 Mt of bottom ash are produced each year in Europe (EUROSTAT), posing a significant waste management challenge. The chemical composition of bottom ash is very similar to that of some cementitious and ceramic materials, providing a source of sustainable supply of secondary raw materials for a greener and more circular economy [23]. In this work, bottom ash was used instead of silica fume. The aim of this paper was to compare these hybrid porous materials (SUNPACE SF and SUNSPACE bottom ash (BA)) with cement and _Hedera Helix_ L. leaf. The last one was used as a reference due to its ability in airborne PM entrapment, as confirmed in different studies [24]. As discussed in literature, the small size and the large surface area of nanoparticles may cause adverse effects on environment and human health, particularly neurologically. As reported in literature, titanium dioxide nanoparticles are widely used in different applications such as paints, coating, building materials, wastewater treatment, additives in pharmaceutical, food colorants, and cosmetics [25; 26; 27]. Due to the complexity in the chemistry and the different available dimensions of real PM sources [22], a model experiment was settled. In particular, titanium dioxide was selected to simulate the aerosol PM dispersion, and a generator was used to generate the PM flux. Then, this work was devoted to study nanoparticles entrapment. The work was performed in the frame of Azure chemistry approach; indeed, it demonstrates that it is possible to use a waste material to remediate the environment [16].