## 1 Introduction  
In the last few years, the control of air quality has become a very current topic. The impact of pollutants on the environment has been neglected for years. It is a cause of growing concern for the awareness of the risks to human health related to the characteristics of the air we breathe. According to [1], in Europe, 790,000 deaths per years are caused by environmental air pollution; one of the main culprits is particulate matter (PM). PM is the most important pollutant in urban areas. It is composed of a complex mixture of solid and liquid particles of organic and inorganic substances suspended in the air [2]. In addition to being of natural origin, PM can also be produced from anthropogenic sources, such as domestic heating, industrial processes, and obviously traffic. Moreover, it can be directly emitted into the atmosphere, called primary aerosol, or can react with other substances present in the air, leading to the formation of secondary aerosol. PM is characterized by long persistence times in the atmosphere and can therefore be transported even at a great distance from the emission point, considerably increasing the PM impact in a large part of the population. Health risks due to PM are related to the particle size; PM\({}_{10}\) (aerodynamic diameter \(\leq\) 10 \(\upmu\)m) can penetrate the upper part of the respiratory system up to the bronchi, while PM\({}_{2.5}\) (aerodynamic diameter \(\leq\) 2.5 \(\upmu\)m) can reach the lungs. Some studies reveal a correlation between the mortality for respiratory and cardiovascular diseases and the ultrafine PM [3; 4; 5]. The World Health Organization reveals that 30% of respiratory disease is caused by a high level of airborne PM [6]. Recent epidemiological episodes caused by COVID-19 have shown a high mortality rate in areas of Northern Italy that was attributed to the PM acting as a vector of the virus. However, very recent studies [7; 8; 9] have shown that, despite the fact that a direct correlation cannot be found, PM can be considered a dangerous pollution source. In particular, due to the high mortality rate recorded in the north of Italy, which is more polluted than the south, it cannot be excluded that the high rate of airborne pollution could be a co-factor. According to INEMAR (INventario EMissioni ARia) ARPA Lombardia (Agenzia Regionale per la Protezione dell'Ambiente) data, in Lombardy, one of the most polluted region of Italy, about 50% of primary PM\({}_{10}\) emission is generated from non-industrial combustion (such as domestic heating), 20% from vehicles, 10% from industrial combustion, and the remaining part from other polluting sources [10]. People living in strongly polluted areas are subjected to very high levels of pollutants daily and are therefore more prone to suffer from respiratory and lung diseases favoring the attack by very aggressive viruses [11]. In addition, high concentrations of PM have negative effects not only on human health but also on economic and energy production levels. In recent years, in fact, the attention has focused on how airborne PM affects the production of energy through solar panels [12; 13; 14]. PM is responsible for 780 MW and 7400 MW of solar power reduction in India and China, respectively [15], with consequences also on the sector of investments in renewable energy. It is evident that PM influences society in various ways, thus it is necessary to reduce PM concentrations in cities, especially to minimize citizens' exposure and health risks. Filters are the conventional system widely adopted for PM entrapment. The materials used for their production are petroleum based (such as polystyrene or polypropylene). This makes the filter expensive and not competitive on the market. In addition, these materials have bad impacts on environment. As demonstrated in [16], the production of 1 kg of these polymers generates emissions in a range from 30 to 150 kg CO\({}_{\text{2 eq}}\) and requires high energy--more than 100 MJ.