## 3 Results  
Evaluation with pH optical sensors has been accomplished for the four seasons of a full year from February 2019 to November 2019. Monitoring covered four different exhibition halls in which 12 positions were checked. A total of 236 recordings were made. In exhibition halls, the sensor positions were inside showcases as well as under the hall free environment. In addition, two sensors were installed outdoors for comparison purposes. One of them was installed in the main facade (West orientation) and another one in the rear facade (East orientation). The main facade is near Paseo de la Castellana, an important avenue in downtown Madrid with heavy road traffic. The rear facade is somehow protected from pollution and is very close to a backyard without road traffic. Figure 3 shows some sensor positions. Winter evaluation (Table 1) was carried out from 4 February up to 18 February. Six recordings were done for each sensor from the day after installation up to the end of evaluation when sensor recordings were stabilized. Outdoor sensors were installed on 6 February and they were exposed up to 18 February. Figure 4 summarizes the average results obtained. Although the initial pH values before sensor sensitization were plotted in Figure 4, these values are not taken into account for average calculations. Sensor number 4 was lost during the evaluation period; therefore, its recordings of 14 and 18 February are missing. As can be seen in Figure 4, all indoor sensors are stabilized after the first 24 h of exposition, which confirms, as expected, their high sensitivity and fast response time. However, sensors installed outdoors showed a decreasing habit that indicates an intensely acidic pH as well as insufficient exposition time. Probably they would reach stabilization after more days of exposition. Figure 3: Some sensor positions. (**a**) Under the hall free environment. (**b**) Inside a showcase. (**c**) Outdoor in the main MNCN facade. The second evaluation was undertaken from 20 May up to 27 May. Four recordings were taken for each sensor from the day after installation up to the end of evaluation. Sensor positions were the same as for the former evaluation. pH average results obtained are shown in Figure 5. In this case, an initial stabilization of pH recordings from outdoor sensors can be observed. Summer evaluation was carried out from 23 July up to 31 July. Four measurements were recorded from the day after installation to the evaluation ending. All sensor positions were changed except those of showcase V18 near the MNCN entrance and the two sensors installed outdoors. Figure 6 shows the results obtained, and Table 2 summarizes the new sensor positions. Figure 4: pH average results obtained during the evaluation carried out on February 2019. Numbers at the bottom of the X axis are the sensors numbers. Figure 5: pH average results obtained during the evaluation carried out on May 2019. Numbers at the bottom of the X axis are the sensor numbers. Finally, the fourth evaluation corresponding to fall was carried out from 25 November up to 29 November. Sensor positions were the same as in the former summer evaluation (Table 2). Three recordings were taken for each sensor, and the corresponding results are summarized in Figure 7. It should be noted that in this evaluation the pH values from outdoor sensors reached stabilization during the evaluation period.