## 3 Discussion of the Results
### CO and HC Conversion Efficiency
Above 170 \({}^{\circ}\)C in DOC inlet temperature, the HC conversion efficiency increased very slowly and it was only greater than 90% for temperatures that were above \(\sim\)250 \({}^{\circ}\)C.  
The HC was not completely depleted in the tested DOC inlet temperature window, which was most probably due to the existence of very low reactivity HC species [50]. This result was similar for warm sea-level and cold altitude at high temperature. However, higher DOC inlet temperature was required to reach the same conversion efficiency in cold altitude. As example, 200 \({}^{\circ}\)C were required to reach 80% of HC conversion efficiency in cold altitude, i.e., 30 \({}^{\circ}\)C above the conditions in warm sea-level. In fact, the HC conversion efficiency in cold altitude fell fast below this temperature, with a clear dependence on the exhaust mass flow. Whilst the HC conversion efficiency remained around 60% between and 100-150 \({}^{\circ}\)C at a low exhaust mass flow, it sharply decreased as the mass flow increased, reaching 30% at 70 kg/h and 125 \({}^{\circ}\)C.  
The phenomena leading to the decrease of the conversion efficiency are common for CO and HC, as later discussed in Section 3.2. However, as previously stated, the CO and HC conversion efficiency patterns were different because of the HC adsorption. This was contrasted computing the HC conversion efficiencies due to adsorption and oxidation phenomena separately. Figure 6 represents these contributions as a function of the exhaust  
Figure 5: HC conversion efficiency as a function of the exhaust mass flow and catalyst inlet temperature: (**a**) experimental results at 0 m and 20 \({}^{\circ}\)C; (**b**) modeled results at 0 m and 20 \({}^{\circ}\)C; (**c**) experimental results at 2500 m and \(-\)7 \({}^{\circ}\)C; and, (**d**) modeled results at 2500 m and \(-\)7 \({}^{\circ}\)C.  
mass flow and the DOC inlet temperature. Plots (a) and (b) in Figure 6 are related to the HC oxidation conversion efficiency in warm sea-level and cold altitude, respectively. Complementary, plots (c) and (d) in Figure 6 show the adsorption contribution. As observed, the trends in Figure 6a,b, i.e., HC oxidation, were exactly the same as those that were obtained for CO conversion efficiency (comparison with Figure 4) at each tested atmosphere. Table 5 numerically exemplifies the trends in conversion efficiency. The CO and HC conversion efficiency are considered. In the case of HC, the oxidation and adsorption contributions to the total HC conversion efficiency are distinguished. The CO and HC oxidation conversion efficiency increase with the DOC inlet temperature, while the HC adsorption conversion efficiency shows decreasing values. Because the same expressions govern the oxidation reaction rate for CO and HC, the quantitative differences come mostly from the lower HC reactivity and the CO/HC partial pressures to a lesser extent, as forward discussed. Therefore, the adsorption was indeed the main source of discrepancy between CO and HC conversion efficiencies. Noting that the results correspond to the conditions after 15 min. of operation (the adsorption is a time-governed process), the HC adsorption conversion efficiency in warm sea-level was \(\sim\)60% until \(\sim\)125 \({}^{\circ}\)C (point B in Table 5), regardless of the mass flow and it dropping quickly as the DOC inlet temperature increased (\(\sim\)20% at \(\sim\)150 \({}^{\circ}\)C). Equivalently, Table 5 shows how the adsorption conversion efficiency drops by increasing the DOC inlet temperature. In cold altitude, these orders of magnitude were only obtained at very low mass flow (Figure 6d and point F in Table 5). In these conditions, the adsorption HC conversion efficiency was highly penalized as the mass flow increased, in contrast to the warm sea-level (Figure 6c). The adsorption-desorption equilibrium is determined by the HC partial pressure and the HC surface coverage evolution over time, according to Equations (8) and (9). Section 3.2 discusses their impact on the resulting adsorption HC conversion efficiency.