## 3 Results and Discussion
### Modeling \(H_{2}\) Effect under Altitude and Sea Level Conditions  
The initial phase of the study focused on model calibration to reproduce the observed conversion efficiencies for CO and HC during steady-state tests conducted under warm sea-level and cold high-altitude conditions, as described in [16]. The calibration tests are detailed in Tables 2 and 3. In particular, Table 2 displays the experimental data for warm sea-level conditions (0 m and 20 \({}^{\circ}\)C), and Table 3 displays those corresponding to cold high-altitude conditions (2500 m and \(-7\)\({}^{\circ}\)C). Due to the absence of H\({}_{2}\) in these tests, the calibration was restricted to reactions excluding this species. Specifically, it focused on HC adsorption and desorption (\(R_{1}\) and \(R_{2}\)), CO (\(R_{3}\)), and HC (\(R_{4}\)) oxidation reactions in the presence of O\({}_{2}\). The calibration of kinetic constants for these reactions considered the effects of thermal transients on the oxidation and the adsorption-desorption equilibrium, as well as the influence of HC coverage on the reaction rate of the adsorption-desorption dynamics. This methodology facilitated a steady-state representation at test completion, distinguishing between HC oxidation and adsorption conversion efficiencies under varying environmental conditions. In this initial part of the calibration, the objective function, defined by Equation (5) and minimized using the simplex search method by [PERSON] et al. [42], represented the weighted sum of cumulative errors in conversion efficiency (\(C_{eff}\)) for CO and HC. It considered the modulus of the error up to each time point (\(t\)) for each pollutant species (\(p\)). \[\Phi_{OC_{O_{2}}}=\sum_{\text{test}}\omega_{\text{test}}\left(\sum_{t}\varepsilon _{CO}^{t}+\sum_{t}\varepsilon_{HC}^{t}\right) \tag{5}\]  
\[\varepsilon_{p}^{t}=|C_{eff,rep}-C_{eff,mod}| \tag{6}\]  
The results of the CO and HC conversion efficiency of the experimental tests for both warm sea-level and cold altitude cases are shown in the contour maps in Figure 1, which is analyzed in detail in [16]. Figure 1: Experimental results for CO and HC conversion efficiency as a function of exhaust mass flow and catalyst inlet temperature for both warm sea-level and cold altitude conditions: (**a**) CO conversion efficiency at 0 m & 20 \({}^{\circ}\)C; (**b**) HC conversion efficiency at 0 m & 20 \({}^{\circ}\)C; (**c**) CO conversion efficiency at 2500 m & \(-7\)\({}^{\circ}\)C; and (**d**) HC conversion efficiency at 2500 m & \(-7\)\({}^{\circ}\)C [16]. Figure 2 presents the modeling results of the steady-state tests under warm sea-level and cold high-altitude conditions. The comparison of the model results with experimental data, depicted in Figure 1, points out its ability to simulate CO and HC conversion efficiencies accurately. The comparison between Figures 1a and 2a evidences how the model is able to capture the CO conversion behavior at the warm-sea level. The model successfully predicted the low reactivity fashion below 100 \({}^{\circ}\)C OC inlet temperature. Additionally, it accurately reproduced the enhancement in CO conversion efficiency and its sensitivity to variation in exhaust mass flow as temperature increased. Furthermore, the model identified the threshold in mass flow and temperature where 100% CO conversion was achieved. Regarding the CO conversion efficiency under cold high-altitude conditions, as illustrated in Figure 2c, the model reflected the changes in conversion efficiency, caused by modifying environmental conditions. Notably, it reproduced the increase in CO light-off temperature and the significant reduction in CO conversion efficiency at reduced exhaust mass flows, a trend more pronounced under cold high-altitude conditions than its warm sea-level counterpart. The ability of the model to predict HC conversion efficiency under varying environmental conditions was further confirmed by comparing Figure 2b,d with their corresponding experimental data (Figure 1b,d).