Anion exchange membrane fuel cell (AEMFC) is a promising next generation (Pt-free) fuel cell technology. However, AEMFC do not yet demonstrate high performance when running on ambient air: CO2 in air results in a carbonation reaction that impacts membrane conductivity and electrode performance. This contribution analyses and quantifies the effect of CO2 from ambient air on the concentration profiles in the membrane and the anode and assesses the CO2 impact on fuel cell performance via modeling.[1] The physico-chemical model contains chemical and electrochemical reactions, liquid-gas phase equilibria as well as the transport processes in the cell. Results imply that a significant part of the CO2 that enters the cell is absorbed in the cathode and is transported as carbonate ions to the anode. Concentration profiles in the membrane reveal an enrichment zone of CO2 in the membrane close to the anode, negligible hydrogencarbonate and a wide distribution of carbonate across the membrane. The carbonate distribution affects overall anion exchange membrane conductivity. For practically relevant current densities of i > 500 mA cm-2 and typical excess ratios of 1.5 for the hydrogen feed, less than 10% of the anions in the membrane are carbonates. We show that while increasing cell temperature has an ambiguous effect on the carbonation process and on the total effect of CO2 on the cell, current density has a significant effect. The impact of CO2 on AEMFC performance can be significantly decreased when operating the cell at high current densities. A closer look at the transport processes in the membrane reveals that carbonate ions are purged out of the membrane when increasing current density. However, this process is unexpectedly slow. In the talk we will compare simulated and experimental temporal evolution of concentration profiles and conductivity. For validation, data obtained during decarbonation experiments in a setup to determine the true hydroxide conductivity of AEM [2] is used. Ion concentration profiles in the membrane are described quantitatively and it is shown how the differences in ion mobility delay the complete decarbonation process. The model-based insights of the talk aid experimentalists to build and operate well-performing air-fed AEMFCs. [1] U. Krewer, C. Weinzierl, N. Ziv, D. Dekel, Electrochimica Acta 263 (2018) 433-446 [2] A. Zhegur-Khais, F. Kubannek, U. Krewer, D.R. Dekel, J. Memb. Sci. 612, (2020) 118461