## 4 Summary and Conclusions  
This study has presented the trends of the CO and HC conversion efficiency variation in an oxidation catalyst, due to the change of the ambient conditions from standard warm sea-level to extreme operation in cold altitude (2500 m and \(-\)7 \({}^{\circ}\)C). The evaluation of the catalyst performance was carried out in a Euro 6d-Temp diesel engine run under steady-state conditions within a common range of exhaust mass flow and catalyst inlet temperature in both ambient conditions. The CO and HC conversion efficiencies were deteriorated at low temperature, increasing the light-off temperature, but with different trends in CO and HC. The CO conversion efficiency suffered a very marked decrease in the low exhaust mass flow region, where the light-off was delayed approximately 45 \({}^{\circ}\)C. This penalty progressively decreased as the exhaust mass flow increased. The HC abatement was more damaged. Being the HC conversion efficiency above 50% in the entire tested field for warm sea-level, very low conversion efficiency regions were found in cold altitude operation, specially at high mass flow. In this case, the HC light-off was not reached until 150 \({}^{\circ}\)C from 50 kg/h onward in exhaust mass flow. The modeling of the DOC response revealed that the differences that were found in the variation of the CO and HC conversion efficiencies were exclusively due to the role of the adsorption process. This was highly influenced by the change in HC emission at a low temperature. The HC partial pressure in cold altitude became much higher than the one in warm sea-level, despite the lower ambient pressure. Consequently, the adsorption was very high at the beginning of the test. However, once the engine reached the steady-state conditions, the high HC surface coverage penalized the HC conversion efficiency in the low temperature region. Concerning the causes of the oxidation light-off temperature variation, the dwell time and O2 partial pressure were noticed to have a relevant influence in increasing it for both HC and CO in cold altitude operation. While the dwell time is unavoidable, the O2 partial pressure depends on the exhaust temperature mainly due to its relation with the equivalence ratio. These parameters only damaged altitude operation at very low temperature, adding its negative contribution to the light-off temperature increase. However, the pollutants partial pressure were particularly critical in understanding the deterioration of the conversion efficiency at low temperature. The reason was the huge increase of pollutant engine-out emission in altitude operation in this range of working conditions. Consequently, a high increase of the CO and HC oxidation inhibition was produced, thus highlighting the importance of an optimal engine control and ATS matching. Conceptualization, J.R.S. and P.P. ; Formal analysis, P.P. and E.J.S. ; Funding acquisition, J.R.S. ; Investigation, J.R.S., P.P., E.J.S. and B.D. ; Methodology, P.P. and B.D. ; Project administration, J.R.S. and P.P. ; Resources, J.R.S. ; Software, P.P. and E.J.S. ; Supervision, J.R.S. and P.P. ; Visualization, E.J.S. and B.D. ; Writing--original draft, P.P., E.J.S. and B.D. ; Writing--review and editing, J.R.S., P.P., E.J.S. and B.D. All authors have read and agreed to the published version of the manuscript. This research has been partially supported by FEDER and the Government of Spain through project TRA2016-79185-R. Additionally, the PhD candidate [PERSON] has been funded by a grant from the Government of Generalitat Valenciana and FSE (European Union) with reference ACIF/2018/109. The authors declare no conflict of interest.