## 2 Materials and Methods
### Type of Pollutant  
**Carbon monoxide (CO)**[48] is a colorless, odorless gas emitted from combustion processes. Nationally and particularly in urban areas the, majority of CO emissions to ambient air come from mobile sources. CO can cause harmful health effects by reducing oxygen delivery to the body's organs (like heart and brain) and tissues. At extremely high levels, CO can cause death. **Nitrogen dioxide (NO\({}_{2}\))**[48] is one of a group of highly reactive gasses known as \"oxides of nitrogen,\" or \"nitrogen oxides (NO\({}_{\text{X}}\)).\" NO\({}_{2}\) is used as the indicator for the larger group of nitrogen oxides. NO\({}_{2}\) comes from emissions by cars, trucks and buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone, and fine particle pollution, NO\({}_{2}\) is linked with a number of adverse effects on the respiratory system. **Sulfur dioxide (SO\({}_{2}\))**[48] is one of a group of highly reactive gasses known as \"oxides of sulfur.\" The largest sources of SO\({}_{2}\) emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%). SO\({}_{2}\) is linked with a number of adverse effects on the respiratory system. **Particulate matter (PM)**[48] is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles. The size of particles is directly related to their potential for causing health problems. Once inhaled, these particles can affect the heart and lungs and cause serious health effects. There are two categories of PM:  
* \"Inhalable coarse particles,\" such as those found near roadways and dusty industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter. * \"Fine particles,\" such as those found in smoke and haze, are 2.5 micrometers in diameter and smaller. These particles can be emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries and automobiles react in the air. With reference to **NO\({}_{\text{2}}\), SO\({}_{\text{2}}\), and (PMs)** there is general agreement in the scientific literature that they are the main agent responsible for the damage encountered on monuments and historical buildings in urban areas [34]. Atmospheric composition is of unquestionable importance in the study of the damage produced on building materials of artistic interest since it directly influences the species characteristics and entity of the degradation mechanism occurring on the cultural heritage. \begin{table}
\begin{tabular}{c c c c} \hline
**Pollutant** & **Concentration** & **Averaging Period** & **Permitted Excess Each Year** \\ \hline PM\({}_{10}\) & 50 \(\upmu\)g/m\({}^{3}\) & 24 h & 35 \\ PM\({}_{10}\) & 40 \(\upmu\)g/m\({}^{3}\) & 1 year & - \\ PM\({}_{2.5}\) & 25 \(\upmu\)g/m\({}^{3}\) & 1 year & - \\ NO\({}_{2}\) & 200 \(\upmu\)g/m\({}^{3}\) & 1 h & 18 \\ NO\({}_{2}\) & 40 \(\upmu\)g/m\({}^{3}\) & 1 year & - \\ SO\({}_{2}\) & 350 \(\upmu\)g/m\({}^{3}\) & 1 h & 24 \\ SO\({}_{2}\) & 125 \(\upmu\)g/m\({}^{3}\) & 24 h & 3 \\ O\({}_{3}\) & 120 \(\upmu\)g/m\({}^{3}\) & Maximum daily 8 h mean & 25 days averaged over 3 years \\ CO & 10 mg/m\({}^{3}\) & Maximum daily 8 h mean & - \\ C\({}_{6}\)H\({}_{6}\) & 5 \(\upmu\)g/m\({}^{3}\) & 1 year & - \\ \hline \end{tabular}
\end{table} 
Table 3: Directive 2008/50/EC standards of the main pollutant. **Ozone (O3)**[48] is not emitted directly into the air, but it is created by chemical reactions between oxides of nitrogen (NOX) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC. Breathing ozone can trigger a variety of health problems, particularly for children, elderly people, and people of all ages who have lung diseases such as asthma.