###### Abstract  
Worldwide emission standards are now required to cover engine operation under extreme ambient conditions, which affect the raw emissions and the efficiency of the exhaust aftertreatment systems. These regulations also target new combustion technologies for decarbonization, such as neat hydrogen (H\({}_{2}\)) combustion or dual-fuel strategies, which involve a challenge to the analysis of exhaust aftertreatment system requirements and performance. This work addresses the impact of high altitude and low ambient temperature conditions on the reactivity of an oxidation catalyst in the presence of H\({}_{2}\). A reaction mechanism is proposed to cover the main conversion paths of CO, HC, and H\({}_{2}\), including the formation and consumption of high-energy surface reaction intermediates. The mechanism has been implemented into a faster-than-real-time reduced-order model for multi-layer washcoat honeycomb catalytic converters. The model was utilized to investigate the effect of H\({}_{2}\) concentration on the reactivity of CO and HC within the catalyst under various operating and ambient conditions. By applying the model and examining the selectivity towards different reaction pathways in the presence of H\({}_{2}\), insights into surface intermediates and reactivity across different cross-sections of the monolith were obtained. This analysis discusses the underlying causes of reactivity changes promoted by H\({}_{2}\) and its relative importance as a function of driving boundary conditions. hydrogen; emissions; altitude; exhaust aftertreatment system; ambient temperature; low temperature; light-off +
Footnote †: journal: Medical  
systems (ATS). The study by [PERSON] et al. [12] revealed that CO, HC, and NOx emissions from a Euro 6d-Temp gasoline engine doubled at \(-\)10 \({}^{\circ}\)C compared to mild ambient temperatures during urban driving scenarios, particularly spiking during cold start conditions. The research by [PERSON] et al. [13] showed that low temperatures significantly impacted emissions from a light-duty diesel engine, with CO, HC, and NOx emissions doubling or even tripling during WLTC at \(-\)7 \({}^{\circ}\)C compared to 20 \({}^{\circ}\)C. Also, research by [PERSON] et al. [14] on Euro 4 internal combustion engines and by [PERSON] et al. [15] analyzing road emissions from three Euro 6d-Temp engines (diesel, gasoline, and hybrid electric gasoline) demonstrated that higher altitudes tend to increase raw emissions of CO and NOx. In lean ICEs, oxidation catalysts (OCs) are essential for reducing pollutants like CO and HC. The efficiency of these catalysts, however, is influenced by environmental conditions. A study by [PERSON] et al. [16], examining a Euro 6d-Temp ICE at various altitudes (0 and 2500 m) and temperatures (20 \({}^{\circ}\)C and \(-\)7 \({}^{\circ}\)C), reported increased raw emissions of CO and HC and decreased OC efficiency at 2500 m and \(-\)7 \({}^{\circ}\)C. Additionally, increased CO and HC raw emissions at higher altitudes led to higher inhibitory effects, damaging OC performance. Historical research, including studies by [PERSON] et al. [17] and [PERSON] et al. [18], has established that high CO and HC concentrations impair the oxidation process, decreasing conversion efficiency. At low temperatures, the inhibitory effects of CO and HC on oxidation increase [19]. Additionally, research by [PERSON] et al. [20], utilizing various diesel fuels and operational settings with different CO, HC, and NOx levels, indicated that increased CO concentrations led to higher CO and HC light-off temperatures. However, at high temperatures, the inhibition effect on oxidation declined because of decreased chemisorption. These observations underscored the competitive nature of CO [21] and HC [22] against O\({}_{2}\) for binding to active sites and the critical role of surface intermediates. Although the presence of H\({}_{2}\) in exhaust gases in lean ICEs is usual, this chemical species has not been given prominence in the study of oxidation catalysts.