## 2 Material and Methods
### Test and Modeling Procedure  
The test campaign was conducted under steady state conditions to obtain the contour maps of the CO and HC conversion efficiencies as a function of the DOC gas inlet temperature and exhaust gas mass flow bounded by two different ambient conditions. Every test was carried out, keeping the engine under constant torque and speed [46], thus ensuring that the DOC boundary conditions were stable once thermal stabilization was reached. Firstly, the conversion efficiency of the pollutants was measured at sea-level and 20 \({}^{\circ}\)C, which will be referred as warm sea-level case and used as the baseline for comparison with the results at 2500 m, i.e., 0.747 bar in ambient pressure and \(-7\)\({}^{\circ}\)C (cold altitude case). To do this, the engine speed was run at 1000 rpm, 1250 rpm, and 1500 rpm in warm sea-level, moving the speed range to higher values in cold altitude to cover similar exhaust mass flow in both cases. The gas temperature at the inlet of the catalyst was also controlled within the same range (100 to 300 \({}^{\circ}\)C) for both atmospheres by acting on the engine pedal. For every tested point, the measurement of the pollutants was carried out continuously following the measurement procedure that is sketched in Figure 2. The example represents the pedal position and the DOC inlet temperature for operating conditions at 1500 rpm and 200 \({}^{\circ}\)C in DOC inlet temperature for sea-level operation. The procedure consisted of an initial preheating stage providing a DOC inlet temperature above 300 \({}^{\circ}\)C. This operating condition was maintained for 10 min. to ensure the desorption of all accumulated HC in the DOC due to previous tests [47]. Afterward, a cooling phase followed for another 10 min., during which the DOC inlet temperature converged to 100 \({}^{\circ}\)C. This way, all of the tests started at the same DOC state. Finally, the engine was run at the desired steady state conditions that aimed to complete the contour maps. The measurements were taken continuously during 15 min. to ensure the exhaust thermal stabilization. The continuous measurement of emissions was required to identify the synergy between the oxidation and adsorption mechanisms on HC abatement. The adsorption is responsible for the high HC conversion efficiency at low temperature. In order to quantify the contribution of each HC abatement mechanism along the test, the modeling task consisted of the simulation of every steady state operating point along the entire test duration. Therefore, the calibration of the kinetic constants, which is summarized in Table 4, considered the influence of the thermal transient (effect on oxidation and adsorption-desorption equilibrium) along with the impact of the gradual HC accumulation (effect on adsorption-desorption reaction rate). This way, a steady state picture was taken at the end of the tests distinguishing HC oxidation and adsorption conversion efficiencies as a function of the ambient condition. Figure 3 compares the experimental and modeling HC conversion efficiency from several operating points that were evaluated at warm sea-level and cold altitude. These \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Kinetic Constants** & \\ \hline  & \(\mathbf{P_{f}}\) [\(-\)] & \(\mathbf{E_{a}}\) [\(\mathbf{J/mol}\)] \\ \hline HC adsorption & 0.67 & 0 \\ HC desorption & \(1\times 10^{9}\) & 110,000 \\ HC oxidation & \(1\times 10^{16}\) & 85,000 \\ CO oxidation & \(9\times 10^{15}\) & 80,000 \\ \hline  & **Inhibition Terms** & \\ \hline  & \(\mathbf{P_{f}}\) [\(-\)] & \(\mathbf{E_{a}}\) [\(\mathbf{J/mol}\)] \\ \hline \(K_{1}\) & 455 & \(-7990\) \\ \(K_{2}\) & \(1\times 10^{3}\) & \(-3\times 10^{3}\) \\ \(K_{3}\) & 2.98 & \(-96\),534 \\ \(K_{4}\) & \(4.79\times 10^{5}\) & 31,036 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Calibration of the reaction rate parameters. Figure 2: The test procedure for every steady state operating point considered to obtain the CO and HC conversion efficiency contour maps.