## 3 Results and Discussion  
Figure 2 shows the graphic for different concentrations of CO\({}_{2}\) for an incidence angle close to 84\({}^{\circ}\) and, as for example, for 1000 m above sea level. Depending on the reflectance in the photodetector, we can detect the concentration of CO\({}_{2}\) in real time. It is essential to know the relative humidity and pressure of the air to know the refractive index of the mixture of the air (therefore the CO\({}_{2}\) concentration) so that the plasmonic sensor can accurately measure the refractive index and then know the CO\({}_{2}\) concentration. The sensitivity of our sensor can be calculated from the change in reflectance per unit of change in refractive index (see Table 2 with more detail and data). The sensor resolution depends upon the accuracy with which the monitored SPR parameter can be determined by the specific sensing device and, as such, is limited by sensor-system noise [31]. To calculate the resolution of the sensor, we divide the accuracy of the photodetector by its sensitivity. Considering an accuracy of 0.1% in the signal registered by the photodetector [32,33], we achieve resolutions of 5.15 \(\times\) 10\({}^{-5}\) RIU, 4.02 \(\times\) 10\({}^{-5}\) RIU, 4.54 \(\times\) 10\({}^{-5}\) RIU and 6.05 \(\times\) 10\({}^{-5}\) RIU for concentrations of 400 ppm, 450 ppm, 500 ppm, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Altitude (m)** &
\begin{tabular}{c} **Sensitivity (RIU\({}^{-1}\)) and** \\ **Resolution (RIU)** \\ \end{tabular} & **400–450 ppm** & **450–500 ppm** & **500–550 ppm** & **550–600 ppm** \\ \hline \multirow{3}{*}{0} & Sensitivity & 17.18 & 22.07 & 19.53 & 14.71 \\  & Resolution \(\times\) 10\({}^{-5}\) & 5.80 & 4.55 & 5.10 & 6.80 \\  & Sensitivity & 13.39 & 20.54 & 20.76 & 17.28 \\  & Resolution \(\times\) 10\({}^{-5}\) & 7.45 & 4.85 & 4.80 & 5.80 \\  & Sensitivity & 19.37 & 24.89 & 22.03 & 16.59 \\  & Resolution \(\times\) 10\({}^{-5}\) & 5.15 & 4.02 & 4.54 & 6.05 \\  & Sensitivity & 7.51 & 16.54 & 20.65 & 20.16 \\  & Resolution \(\times\) 10\({}^{-4}\) & 1.33 & 0.61 & 0.49 & 0.50 \\
2000 & Sensitivity & 6.01 & 14.75 & 19.70 & 20.45 \\  & Resolution \(\times\) 10\({}^{-4}\) & 1.67 & 0.68 & 0.51 & 0.49 \\
2500 & Sensitivity & 6.68 & 14.37 & 18.98 & 20.12 \\  & Resolution \(\times\) 10\({}^{-4}\) & 1.50 & 0.70 & 0.53 & 0.50 \\  & Sensitivity & 5.09 & 12.53 & 17.68 & 19.82 \\  & Resolution \(\times\) 10\({}^{-4}\) & 1.97 & 0.80 & 0.57 & 0.51 \\  & Sensitivity & 3.88 & 10.91 & 16.28 & 19.16 \\
3500 & Resolution \(\times\) 10\({}^{-4}\) & 2.58 & 0.92 & 0.62 & 0.52 \\  & Sensitivity & 3.02 & 9.56 & 14.92 & 18.28 \\
4000 & Resolution \(\times\) 10\({}^{-4}\) & 3.31 & 1.05 & 0.67 & 0.55 \\  & Sensitivity & 2.59 & 8.62 & 13.79 & 17.37 \\  & Resolution \(\times\) 10\({}^{-4}\) & 3.87 & 1.16 & 0.73 & 0.58 \\
5000 & Sensitivity & 2.36 & 7.89 & 12.79 & 16.43 \\  & Resolution \(\times\) 10\({}^{-4}\) & 4.24 & 1.27 & 0.78 & 0.61 \\
5500 & Sensitivity & 2.41 & 7.47 & 12.04 & 15.62 \\  & Resolution \(\times\) 10\({}^{-4}\) & 4.16 & 1.34 & 0.83 & 0.64 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The sensitivity and resolution values of the sensor as a function of altitude and gas concentration change. Figure 2: Reflectance curves as a function of the angle of incidence of the light in the prism and different concentration of CO\({}_{2}\). The height above sea level is 1000 m. The sensor works with an incidence angle of 83.67\({}^{\circ}\). 550 ppm and 600 ppm respectively. This resolution is more than adequate to accurately measure the CO\({}_{2}\) concentration. With a variation of the fifth decimal place, we can assure that the measurements of the refractive indexes are reliable, taking into account that the variations of the refractive indexes due to pressure, temperature, humidity and concentration are to the third decimal place. The sensor of [PERSON] et al. [17] has a resolution similar to ours, but uses infrared while our laser is in the range of the visible; it is also cheaper and more commercially viable.