## 3 Results and Discussion
### Temporally Dependence of Pollutant Concentrations

Table 12: Cross-correlation values between Roma Green Band and suburbs or province stations of [PM\({}_{2,5}\)] in 2015.  
values (from \(-1.4\%\) to \(-6.2\%\)) denoting an average decrease every year after year of the pollutant concentration in these seasons. Different results are for winter in which the range has positive values (from \(1.3\%\) to \(-3.2\%\)) denoting that the average decrease every year after year isn't for all the pollutant concentrations. Focusing the attention on these aspects, the only pollutant that has an average increase year after year in winter is [PM\({}_{2.5}\)] that has a value of \(1.3\%\). As a matter of fact, in Figure 2 it is possible to notice that the trend line in winter is quite constant year after year despite the one of spring-autumn that decrease. Regarding the [O\({}_{3}\)] the results are an increase of the level year after year of about \(4.4\%\) and \(6.2\%\) for winter and spring-autumn cases respectively. Figure 2: Average pollutant concentrations trend of Rome over the decade 2006–2015 of winter (blue line) and spring-autumn (red line). Dashed lines are referred to as trend lines. The presence of heating systems turned on in winter denotes a higher pollutant emission. As a matter of fact, in Figure 2 the blue line is always over the red line despite the ozone concentration. This result affirms that the heating systems in cities have an important contribution to the pollutant level. Furthermore, the emission from these systems leads to a more irregular variation unlike in spring and autumn due to the variability of climate during the years (colder or less cold winter leads to a different use of heating systems). The higher pollutant concentration in winter than in spring and autumn is due also to the cooling air masses that promotes the stagnation at low altitude. So, the air temperature inside cities can influence the pollutant concentration at the pedestrian level. For this reason, the urban heat island phenomenon plays an important role, that is the increase of urbanization lead to the change of climatic conditions [53]. In particular, the increase of waterproof surfaces and the build-up area in cities lead to the increase of the air temperature in comparison with rural areas. High temperatures in summer improve the natural convention phenomenon that increases the mixing with clean air and the consequent dispersion of the pollutant concentration. It is important to remember that the more the pollutants stay on the street level, the more people are affected. Focusing on 2015, to quantify the differences of urban traffic and building heating systems sources, the mean pollutant concentration of the period spring-summer-autumn, in which there is only urban traffic, was removed from the data. The results of this analysis are shown in Figure 3 in which the data set were represented in a normalized form in order to have zero mean and unitary standard deviation defined as follows:  
\[\left[\hat{x_{i}}\right]=\frac{\left[x_{i}\right]-\left\langle\left[x_{i} \right]\right\rangle}{\sigma_{\left[x_{i}\right]}} \tag{1}\]  
The highest values can be found in December, not only for the role of the heating systems but also for a bigger presence of traffic because of Christmas. It is useful to clarify that the values in Figure 3 are the results of the difference of the pollutant concentration in time by time with the mean values in spring, summer, and autumn in order to decurtate the pollutant level from the urban traffic sources. In this way, the values shown in Figure 3 should show only the contribution of the heating systems. Of course, the high traffic during Christmas can't be eliminated just by the mean value of spring, summer, and autumn. So,  
Figure 3: Normalized pollutant concentrations of Rome in 2015. Values are referred to the subtraction of the pollutant concentration in time by time with the mean values in spring, summer, and autumn in order to decurtate the pollutant level from the urban traffic sources.